Presentations Library

To meet the needs of the high-precision GNSS community, the National Geodetic Survey (NGS) has constructed an absolute antenna calibration facility which uses field measurements and actual GNSS satellite signals to determine antenna phase center patterns. A pan/tilt motor changes the orientation of the antenna under test, and signals are received at a wide range of angles. The phase center patterns will be publicly available and disseminated in both the ANTEX and NGS formats. We provide the observation models and strategy currently used to generate NGS absolute calibrations, and propose some future refinements. We also show examples of antenna calibrations from the NGS facility. These examples are compared to the NGS relative calibrations as well as absolute calibrations generated by other organizations.

The National Geodetic Survey has established a strategic goal of developing the capability to obtain second-order, class II (FGCS standards) orthometric heights using GPS combined with a high resolution geoid model. As a contribution toward that goal, we are working toward a 1 cm (1 sigma) geoid model for the conterminous United States. Numerous signals effect the geoid at and above the 1 cm level, and one of the most difficult to properly implement has been the consideration of the permanent tidal potential in geoid modeling. In creating G96SSS, it was necessary to determine which tide system each of the following data sets were referred to: terrestrial gravity measurements, ship gravity measurements, altimetrically derived gravity anomalies, digital terrain, and EGM96 coefficients. In some cases, the question was difficult to answer. It was our intention to produce G96SSS in the non-tidal system. From recent knowledge of the parameters of the best fitting global ellipsoid, we know that the G96SSS geoid contains undulations biased from the non-tidal system by 12.0 cm. That is, undulations in the non-tidal system may be obtained from the G96SSS model by removing 12.0 cm from G96SSS. The tide system of GEOID96 was, by definition, dependent on the tide system of the NAD 83 GPS measurements, since GEOID96 was designed solely to convert between NAD 83(86) GPS ellipsoid heights and NAVD 88 Helmert orthometric heights. The GPS measurements were reduced to the non-tidal system, so GEOID96 is also in that system. Finally, the comparison between GPS heights on leveled benchmarks and G96SSS yieled an estimate of the bias of NAVD 88 from global mean sea level. In this comparison, it was essential to know the tide systems of both GPS and G96SSS. With all data reduced to the non-tidal system, our current best estimate of the NAVD 88 bias is -31.4 cm, where the sense of the sign is that the NAVD 88 H=0 reference level is 31.4 cm below global mean sea level.

NOAA’s National Geodetic Survey (NGS) has recently developed an innovative new technique for computing the absolute slant delays to GPS signals, caused by the ionosphere. This method relies entirely on ambiguous carrier phase data, using both the orbital geometry and the spacing of NOAA’s Continuously Operating Reference Station (CORS) network of GPS receivers over the Conterminous USA (CONUS) to solve for delays in absolute space. Using this technique, a set of absolute slant delays between any given CORS receiver and GPS satellite at any epoch can be computed, and these delays then referred to GPS receivers anywhere in CONUS using simple interpolation methods. This eliminates the need for a grid of vertical delays, and all the associated errors that accompany the mapping of vertical delays into slant delays. This method was tested against a variety of GPS positioning software and currently performs at the 1 TECU (about 1 cycle on L1) level in absolute mode and at the 0.01 to 0.1 TECU level in double differenced mode. NGS plans to begin computing daily models of the ionosphere slant delays in fall 2004 and releasing them as an experimental product under the name ICON-1 (for Ionosphere over CONus, version 1). The data will be made freely available for post processing and testing applications. Future plans will be to increase the accuracy of the modeling, reduce the latency of the models and formalize the model as an official NOAA product.

National Geodetic Survey (NGS) Chief Geodesist, Dr. Dru Smith, was interviewed on Federal News Radio 1500AM on Friday, May 21, 2010 following a press release that announced the conclusion of an NGS-hosted Federal Geospatial Summit.

Preliminary Analysis of IGS Reprocessed Orbit and Polar Motion Estimates Jim Ray (Jim.Ray@noaa.gov) Jake Griffiths (Jake.Griffiths@noaa.gov) The Analysis Centers (ACs) of the International GNSS Service (IGS) are reanalyzing the history of global network GPS data collected since 1994 in a consistent way using the latest models and methodology. This is the first reprocessing by the IGS, but it is expected to be repeated in the future as further analysis and reference frame changes occur. All eight final-product ACs are participating, together with three other related groups. First partial results consisting of IGS combined weekly SINEX TRF and EOP combinations have been submitted to the IERS for ITRF2008. A snapshot of the available AC weekly SINEX files was used covering the reprocessed years 2000 through 2007 plus the IGS regular operational solutions for 2008 (from week 1460 onward). Meanwhile, the full reprocessing campaign will continue to completion by about the end of 2009 and will cover the period 1994 to present with long-term consistent, combined SINEX, orbit, and clock products. We have examined the reprocessed AC orbit and polar motion (PM) estimates from the 1024 days (or 1025 for differences) of results till the end of 2007. These parameters are linked since PM is sensed in the GPS modeling as a global diurnal sinusoidal motion of the terrestrial frame relative to the satellite frame. Any similar type errors in the orbital frame can bias the PM and PM rate estimates. For the orbits, each daily AC satellite ephemeris for each pair of consecutive days has been fit to the extended CODE orbit model, extrapolated to the mid-point epoch between the days, and the geocentric satellite position differences computed to give time series of orbit repeatabilities. Occasional data gaps have been filled by linear interpolation, FFT power spectra computed, and the spectra stacked over the full GPS constellation and lightly smoothed. Our analysis reveals considerable diversity among AC orbits. Several show broad semi-annual (probably related mostly to eclipsing) and fortnightly spectral peaks, as well as even harmonics of the GPS draconitic year (1.040 cpy) with varying amplitudes. High-frequency white noise floors can be detected in most AC orbit spectra, with an average sigma of 14 mm and larger. AC PM spectra mostly follow a power law with slope -4 for periods shorter than about 20 d, as expected, except in the few cases when ACs have applied tight day-to-day continuity constraints. Indications of high-frequency white noise are seen in some AC series. Day-boundary discontinuities computed using the AC PM rate estimates can provide a sensitive probe of the quality of the AC modeling, especially for the satellite orbit dynamics. Like the orbit discontinuities, we find the PM discontinuities vary greatly among the ACs. But most spectra of the PM discontinuities show peaks at the annual (broad) and the O1 tidal alias period of 14.19 d (narrow), in addition to odd (rather than even) harmonics of 1.040 cpy. Previously both even and odd harmonics of 1.040 cpy have been found in the spectra of station position time series.

FUTURE IMPROVEMENTS IN DETERMINATIONS OF EARTH ORIENTATION PARAMETERS J.R. Ray NOAA, National Geodetic Survey, Silver Spring, MD, USA The present accuracy of multi-technique combined estimates for Earth orientation parameters (EOPs) can be assessed using the ITRF2005 and other recent experience as a baseline. Both components of polar motion (PM) have daily accuracies of about 30 micro-as (about 1 mm equatorial rotation). GPS observations dominate PM combinations owing to a strong global tracking network and continuous data. VLBI and SLR are weaker due to sparse, non-uniform networks so their PM rotational information is largely exhausted to align their frames with GPS. DORIS PM results are much noisier. GPS daily PM-rates are accurate to about 150 micro-as per day but are subject to prominent systematic errors related to orbit modeling and other effects. Such errors are less obvious in the daily PM offsets. UT1 accuracy is more difficult to evaluate because only VLBI is able to observe it. (Nutation is similar and will not be discussed here.) For VLBI networks designed to monitor EOPs, the daily UT1 accuracy is usually between 4 and 10 micro-s (2 to 5 mm equatorial rotation), though it is sometimes worse. Specialized hour-long, single-baseline UT1 networks yield errors around 25 to 30 micro-s with major systematic errors. LOD measurements from GPS have high-frequency errors of about 4 micro-s after modeling time-varying biases by comparison with the best VLBI UT1 data. Together, both can be combined to yield significantly improved UT1/LOD estimates, though this is not yet done routinely by most EOP services. Stronger future VLBI and SLR contributions will depend largely on the deployment of more robust ground networks; prospects are uncertain and recent trends are mixed. GPS PM is unlikely to be improved much. But addition of data from new GNSSs, with different orbital characteristics, might expose presently undetected systematic errors. GPS LOD performance has hardly changed for more than a decade. But, again, new GNSSs could be important given the strong link with orbital dynamics. At this time it is impossible to foresee how large any future improvements might be. Interest is often expressed in subdaily EOP variations but the main need now is for an updated geophysical model for the diurnal and semidiurnal tidal bands. Mitigation of network-dependent and other biases in the hour-long, single-baseline VLBI UT1 data is also needed. Detection of residual subdaily non-tidal PM variations (in the sub-mm range) remains distant, but progress in monitoring subdaily non-tidal UT1 will be a challenging possibility.

The National Geodetic Survey (NGS), part of the National Oceanic and Atmospheric Administration (NOAA) is responsible for defining, maintaining and providing access to the National Spatial Reference System (NSRS) for the United States. The NSRS is the official system to which all civil federal mapping agencies should refer, and contains, amongst other things, the official vertical datum (NAVD 88), horizontal datum (NAD 83) and great lakes datum (IGLD 85). Although part of the United States NSRS, all three of these datums have been created through international partnerships across North America. Unfortunately, time has shown both the systematic errors existent within these datums, as well as the inherent weaknesses of relying exclusively on passive monuments to define and provide access to these datums. In recognition of these issues, the National Geodetic Survey has issued a “10 year plan”, available online, which outlines steps which will be taken to update NAD 83, NAVD 88 and IGLD 85 concurrently around the year 2018. The primary source of success will be in the refinement of the CORS network and the upcoming execution of the GRAV-D project (Gravity for the Re-definition of the American Vertical Datum). Conversations are ongoing with colleagues in Canada, Mexico, Central America and the Caribbean in order to coordinate all of these efforts across the entire continent. The largest changes expected to occur are the removal of over 2 meters of non-geocentricity in NAD 83; the removal of decimeters of bias and over a meter of tilt in NAVD 88; the addition of the ability to track motions (subsidence, tectonics, etc) in the datums; the removal of leveling as a tool for long-line height differencing; the use of a “best” geoid as the orthometric height reference surface; the addition of datum velocities (motions of the geometric frame origin and motions of the geoid); and the use of GNSS technology as the way to access both orthometric and dynamic heights in the vertical datum. This talk will outline the broad plan of action and invite further collaboration along these lines.

Status and Prospects for IGS Polar Motion Measurements Jim Ray (1), Remi Ferland (2) (1) U.S. NOAA/National Geodetic Survey, USA (2) Geodetic Survey Division, NRCan, Canada GNSS-based measurements of polar motion by the International GNSS Service (IGS) dominate modern multi-technique combinations. This can be attributed to a very strong global tracking network and continuous data. IGS Final polar motion coordinates have daily accuracies of about 30 micro-as (about 1 mm of equatorial rotation), or perhaps a bit better for the most recent results. The daily polar motion rate measurements have larger errors, about 160 micro-as/day, largely due to greater sensitivity to errors in the IERS model for subdaily EOP tidal variations. Orbit modeling errors also affect the polar motion rates and perhaps also the polar motion offsets at a subtle level. IGS Ultra-rapid products provide reduced latency and more frequent updates with only modestly increased errors.

The National Geodetic Survey (NGS) of the United States, in collaboration with the National Aeronautics and Space Administration (NASA) and the International Earth Rotation Service (IERS), recently conducted a site survey of four space geodesy techniques: Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Doppler Orbitography and Radio-positioning Integrated by Satellite instrument (DORIS), and the Global Positioning System (GPS), collocated at the Goddard Geophysical and Astronomic Observatory (GGAO) in Greenbelt, MD. This survey sought to determine, at the highest levels of accuracy, the geometric vectors which connect multiple geodetic techniques at a terrestrial site where these techniques are located near to one another. Realization of the International Terrestrial Reference Frame (ITRF) is enhanced by combining and comparing different space geodetic techniques through local ties referred to as site surveys. Because these four techniques all contribute their own strengths to the determination of the ITRF, the quality of the connectivity between the techniques is a direct contributor to the accuracy of the ITRF itself. Site surveys of the GGAO space geodesy techniques have been carried out periodically in the past. The site survey carried out by the NGS allowed for an independent comparison against results from previously conducted site surveys. The ultimate goal of this site survey was to improve the overall accuracy of local ties between space geodesy techniques by employing the latest surveying technology, as well as re-engage NGS in the field of IERS site surveys. Results of the survey will be presented, as well as discussion of future surveys and possible collaborations with other international surveying teams to improve the overall connectivity of techniques used in defining the ITRF.

Status and Prospects for Combined GPS LOD and VLBI UT1 Measurements Ken Senior (1), Jan Kouba (2), Jim Ray (3) (1) U.S. Naval Research Laboratory, USA (2) Geodetic Survey Division, NRCan, Canada (3) U.S. NOAA/National Geodetic Survey, USA Our Kalman filter combines irregular VLBI UT1-UTC with daily GPS LOD by handling correlated GPS errors with a Gauss-Markov plus fortnightly sinusoid model added to the random walk excitation. Evaluated against (AAM + OAM), this filter gives the lowest residuals and highest correlations. Optimal UT1+LOD results exclude UT1 from all Intensives plus other VLBI sessions with formal errors >5 microsec. Rescaling VLBI formal errors is not fully effective for the heterogeneous VLBI data. GPS LOD esimates are more uniform, but biased; but properly modeled, the LOD residuals are ~4 microsec. Addition of GPS LOD significantly improves combined UT1 series. Prediction services could benefit further using the IGS Ultra-rapid LOD values reported four times daily with 15 hr delay.

Preparations for the 2nd IGS Reprocessing Campaign Jim Ray The Analysis Centers (ACs) of the International GNSS Service (IGS) are now completing their first collective reanalysis of the history of global network GPS data collected since 1994. A consistent set of the latest models and methodology is being used to generate GPS orbits, Earth orientation parameters (EOPs), station coordinate time series, and station and satellite clocks. These results have been contributed to the new ITRF2008 multi-technique terrestrial reference frame and EOP combination. Preparations will begin during 2010 for the next IGS reprocessing effort. Despite the major progress made in the first IGS reanalysis, further analysis improvements remain to be implemented. The list includes: add GLONASS as well as GPS observations; adopt a new reference frame based on ITRF2008; update the IGS antenna calibrations based on the first reprocessing results and other sources; use the new EGM2008 geopotential model with perhaps revised time-varying coefficients; implement a model for previously neglected higher-order ionospheric effects; consider the satellite dynamical effects of Earth albedo reflection and re-radiated thermal emissions; apply various refinements in modeling tropospheric delays; include station displacements due the S1 and S2 atmospheric pressure tides; use a new model for the subdaily EOP tidal variations, if available; reconsider the handling of EOP constraints and a prioris by ACs; incorporate all high-order relativistic effects; and revisit the treatment of all analysis constraints to remove as many as possible and to understand better the effects of those that remain. Other operational aspects need to be evaluated also, such as how best to treat non-tidal loading station displacements, whether to continue forming weekly SINEX solutions or to move instead to daily integrations, and more consistent and rigorous methods to combine AC solutions.

Results from the New IGS Time Scale Algorithm (version 2.0) Ken Senior (1), Jim Ray (2) (1) U.S. Naval Research Laboratory, USA (2) Geodetic Survey Division, NRCan, Canada Since 2004 the IGS Rapid and Final clock products have been aligned to a highly stable time scale derived from a weighted ensemble of clocks in the IGS network. The time scale is driven mostly by Hydrogen Maser ground clocks though the GPS satellite clocks also carry non-negligible weight, resulting in a time scale having a one-day frequency stability of about 1E-15. However, because of the relatively simple weighting scheme used in the time scale algorithm and because the scale is aligned to UTC by steering it to GPS Time the resulting stability beyond several days suffers. The authors present results of a new 2.0 version of the IGS time scale highlighting the improvements to the algorithm, new modeling considerations, as well as improved time scale stability.

STATUS OF IGS CORE PRODUCTS The core products of the IGS consist of global terrestrial frames, GNSS orbits, Earth rotation parameters (ERPs), and clocks. Three main product series are issued with varying latency, accuracy, and completeness: Ultra-rapids (predictions and observations) for real-time and near real-time applications; later Rapids for near-definitive results; and still later Finals for the most complete and accurate uses. The accuracies of all IGS products are heavily dominated by systematic errors. Beginning with the orbits, rotational errors greatly exceed random noise probably due to limitations of current once-per-rev empirical parameterizations and errors in the IERS subdaily tidal ERP model. These error components alias into longer-period effects, including draconitic harmonics, and then propagate into all other products. Having said that, however, it is nevertheless true that the accuracy and utility of IGS GNSS products exceed that of any other sources, usually by large amounts, for all but a handful of observables, UT1 and the terrestrial frame scale being probably the most notable exceptions. The overall IGS quality has steadily improved over time, though we are probably near an asymptotic level now. A key responsibility of the IGS must be to identify the residual error sources in its products, develop methods to mitigate them, and advise users how best to avoid misinterpretation of spurious effects. This becomes an ever more challenging task as both the product generation and usage progressively move toward fully automated and routine operations. The current status of IGS products will be reviewed and suggestions offered for some operational changes.

Dependence of IGS Products on the ITRF Datum J. Ray (1) P. Rebischung (2) R. Schmid (3) (1) NOAA/National Geodetic Survey, USA (2) Institut Geographique National, France (3) Technische Universitaet Muenchen, Germany Throughout its history of nearly two decades, the International GNSS Service (IGS) has attempted to align its products as closely as possible to the successive realizations of the International Terrestrial Reference Frame (ITRF). This has been disruptive for IGS users at times, especially during the 1990s when some radial ITRF datum definitions were adopted. During the past decade IGS impacts due to ITRF updates have been smaller and mostly been caused by random and systematic errors in the results from the contributing space geodetic techniques. As with all techniques, frame rotational orientations are purely conventional and so the IGS relies on the ITRF via a subset of reliable, globally distributed stations with no significant problems. As regards the origin, the IGS in principle could be self-reliant, or contributory, in determining a frame origin aligned to the long-term center of mass of the entire Earth system. In practice, however, GNSS-based results have been less reliable than those from satellite laser ranging (SLR) to LAGEOS. So the ITRF origin, based on SLR only, has been adopted historically. Until the transition from ITRF2005 to ITRF2008 there have sometimes been significant shifts associated with this practice as SLR results have evolved. However, the present stability of the ITRF origin may finally have reached the ~1 mm level, though that remains to be verified. In many respects, the IGS dependence on the ITRF scale is most subtle and problematic. In addition to an overall Helmert alignment of the IGS frame to match the ITRF scale (and other datum parameters), since 2006 the IGS calibration values for the GNSS satellite antenna z-offsets depend directly on the same ITRF scale (due to high correlations if the IGS frame scale is not fixed). We therefore face a non-linear situation to maintain full consistency between all IGS products and the ITRF scale: each IGS frame contribution to ITRF based on one set of antenna calibrations must be used, together with frames from other techniques, to determine an updated ITRF and new antenna calibrations, which are then no longer strictly consistent with the starting IGS frame. One can hope that the process will iteratively converge to a sufficient accuracy eventually. But potentially large shifts in the ITRF scale, such as the -0.94 ppb (about -6 mm in height) change from ITRF2005 to ITRF2008, are highly disruptive, much more so than the associated rotational or translational shifts. Only SLR and very long baseline interferometry (VLBI) have been considered reliable and accurate enough to be used for the ITRF scale. But experience and theoretical studies have shown that neither is accurate to better than about 1 ppb. Note in particular that the formal 2 ppb error of GM (which should probably be closer to 1 ppb based on recent results) fundamentally limits the possible SLR/VLBI scale agreement to no better. Consequently, the IGS strongly urges that the ITRF scale hereafter be fixed conventionally to the ITRF2008 scale indefinitely in the future until it is convincingly shown that VLBI and/or SLR can determine the ITRF scale within 0.5 ppb. If this is not done, the IGS may maintain its own frame aligned to the ITRF2008 scale in order to minimize operational disruptions.

Current Accuracy of Terrestrial Positions from Space Geodesy In the International System (SI) of units, length is specified in terms of a corresponding time interval via an adopted value for the speed of light. Moreover, the SI second can be realized more accurately than any other base unit. So, since all space geodetic positioning systems rely inherently on high-accuracy timing measurements, one might suppose that position determinations can be traced directly and accurately to the SI meter. In fact, such metrological assessments are very limited. This is mainly because it is not practical to materialize metrological standards over distances longer than about 1 km, for which the best GPS accuracies demonstrated have been about 0.3 mm RMS. Relating such highly localized accuracies to global measurements is extremely problematic, however, for many reasons. Generally, errors unrelated to thermal measurement noise processes grow rapidly as the spatial scale increases and quickly dominate. The classical metrics most often used as a substitute for true global geodetic accuracy are: 1) repeatability of a given parametric estimate to gauge internal precision; and 2) differences among independent techniques to estimate external relative accuracy. Clearly, repeatability can only set a lower limit to true absolute accuracy since systematic biases are normally undetectable by this method. External comparisons, even when feasible, may be skewed unfavorably if it is necessary to use intermediate measurements, which have their own errors (such as local ties to relate station coordinates among co-located observing systems). When direct external comparisons are possible (such as for polar motion), the results can be optimistic if systematic errors are not genuinely independent (such as relying on common data reduction and geophysical models). When one system is clearly superior to any others, it is especially difficult to estimate its accuracy in any reliable absolute sense. Despite the difficulties involved, accuracy assessments can be useful to guide technique improvements by identifying limiting errors and spurring efforts to better control them. This paper will review current accuracy estimates for some space geodetic positioning measures.

Using the Vermont CORS Network to Access the National Spatial Reference System In 2006, the Vermont Agency of Transportation began to install a network of Continuously Operating GNSS Reference Stations (CORS). Now, nearly four and a half years later the network is almost complete. The use of the VT network by positioning professionals has been steadily increasing, and it is anticipated that its use will begin to grow exponentially over the next couple of years. Topics covered in this webinar will include: • A brief description of the VT CORS network and how it fits into the National CORS Network • Current capabilities and usage of the network • A description of current methods and techniques of accessing the NSRS using the VT CORS • Future trends for accessing the NSRS

The On-line Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. The OPUS BETA website offers several notable enhancements. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and “one click” publishing for an entire project. OPUS-S uses a new processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2008. These and other new developments will be described.

The On-line Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. The OPUS BETA website offers several notable enhancements. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and “one click” publishing for an entire project. OPUS-S uses a new processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2008. These and other new developments will be described.

The On-line Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. The OPUS BETA website offers several notable enhancements. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and “one click” publishing for an entire project. OPUS-S uses a new processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2008. These and other new developments will be described.

The On-line Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. Several notable enhancements are pending or in development. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and “one click” publishing for an entire project. OPUS-S uses a new processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2000. These and other new developments will be described.

The On-line Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. Several notable enhancements are pending or in development. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and “one click” publishing for an entire project. OPUS-S uses a new processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2000. These and other new developments will be described.

This webinar focused on describing the forthcoming change in coordinates (position and velocities) for CORS sites from ITRF2000 epoch 1997.0 and NAD 83(CORS96)epoch 2002.0. The new coordinates will be available in NGSTRF08 epoch 2005.0 and NAD 83(2011) epoch 2010.0. The underlying datum for IGS08 is based on ITRF2008, but the positions are calibrated for the impending release of IGS08 (+igs08.atx). In this webinar, we will describe the: - methodology used to establish the new coordinates - magnitude of the coordinate changes - implications for post-processing applications outside of NGS.e.g. holding CORS with fixed positions and use of absolute (vs. relative) antenna calibrations - time-frame for the distribution of coordinates

The mission of NOAA's National Geodetic Survey is to define, maintain and provide access to the National Spatial Reference System (NSRS), including vertical control for measuring accurate elevations. Since 2000, NGS has been implementing an initiative to improve access to the vertical component of the NSRS through technologies like Global Navigation Satellite Systems (GNSS). The program has been implemented through a state-by-state approach funded by Congressional earmarks, but in 2006 NGS began a more comprehensive approach to ensure consistency across the nation was achieved. NGS also began investigating a new approach to defining the vertical datum through a high accuracy gravimetric geoid. Details can be found in the project plan, Gravity for the Re-definition of the American Vertical Datum (GRAV-D), and in the NGS 10-year plan. This workshop will describe how each of these programs have a role to play in improving the vertical reference frame of the United States.

Results from the New IGS Time Scale Algorithm Since 2004 the IGS Rapid and Final clock products have been aligned to a highly stable time scale derived from a weighted ensemble of clocks in the IGS network [Senior et al., 2003]. The time scale is driven mostly by Hydrogen Maser ground clocks though the GPS satellite clocks also carry non-negligible weight, resulting in a time scale having a one-day frequency stability of about 1E-15. However, because of the relatively simple weighting scheme used in the legacy time scale algorithm and because the scale is aligned to UTC by steering it to GPS Time the resulting stability over shorter intervals and beyond several days suffers. A new time scale algorithm (version 2.0) has been implemented to address these limitations. The algorithm has been evaluated to a subset of data in the IGS REPRO1 reprocessing campaign, presented here. Repro1 products are currently being re-aligned using this new timescale algorithm.

This program discusses the foundational elements of the National Spatial Reference System (NSRS), including: Fundamental geodetic concepts of horizontal and vertical datums such as NAD 83 and NAVD 88; reference ellipsoids and geoid models. Discussions will include the realization of the datums in the form of GPS High Accuracy Reference Networks (HARNs) and the international densification of the Continuously Operating Reference Stations (CORS) network and their impact on the design and implementation of local geodetic systems, release of a new national geoid model, enhancements to the On-Line Positioning User Service (OPUS) suite of programs, and the major elements of the NGS ten-year plan for modernization of NSRS.

The GRAV-D (Gravity for the Redefinition of the American Vertical Datum) Project of the U.S. National Geodetic Survey plans to collect airborne gravity data across the entire U.S. and its holdings over the next decade. The goal of the project is to create a gravimetric geoid model to use as the vertical datum for the U.S. by 2021. Airborne gravity survey work began more than two years ago, with Alaska as a high priority for new data collection. Data collection there is underway and will be ongoing for several more years, but two roughly 400 km x 400 km surveys have been completed: in 2008 (centered over Cook Inlet near Anchorage) and in 2009 (centered over the Interior, to the north of the Alaska Range and west of Fairbanks). The gravity data for both surveys was collected with a MicroG LaCoste TAGS system but each survey utilized a different aircraft and survey layout. The 2008 survey was flown at 35,000 ft with the NOAA Cessna Citation jet, with 10 km data line spacing and 60 km cross lines spacing. The 2009 survey was flown at 12,500 ft with the Naval Research Lab King Air (RC-12) turboprop, with 7.5 km data line spacing and 37.5 cross line spacing. The 2008 data reveal the > 20 km resolution gravity effects of all the near-trench features (from accretionary prism to volcanic arc) for a 400 km stretch of the active plate boundary. In comparison, the 2009 gravity data allow a slightly better resolution (>15 km) view of the distal deformation to the north of the Alaska Range. The free-air gravity disturbances for each survey were computed and then complete (terrain-corrected) Bouguer gravity anomalies were calculated with Gauss-Legendre Quadrature integration (von Frese, et al., 1999) using standard density assumptions. Topography used to calculate the corrections came from the freely-available GTOPO30 (USGS, online) and bathymetry from the Smith and Sandwell (1997) altimetry-derived data. Interpretations of the complete Bouguer gravity anomalies will be made in the context of the tectonic activity in southern Alaska.

The U.S. National Geodetic Survey’s mission is to define and maintain the spatial reference system of the United States. Official policy, adopted in 2008, calls for the definition a new national vertical datum based on a gravimetric geoid by 2018 and for its maintenance into the future. The project that will accomplish data collection and analysis tasks toward that goal is called GRAV-D (Gravity for the Redefinition of the American Vertical Datum). The project is underway to collect new airborne gravity data across the entire U.S. To date, GRAV-D has collected nearly 1 million sq km of high-altitude airborne gravity data at 12,500 ft to 35,000 ft. Data sets exist in Alaska, Puerto Rico and the Virgin Islands, and the coastal Gulf of Mexico from the Florida panhandle to the Mexican border. In support of the GRAV-D mission, information about the geologic setting of the data sets and geophysical interpretations of the gravity data are necessary. For instance, geodetic concerns about knowledge of an area’s density structure for completing geoid calculations within topography can be addressed with geophysical interpretation techniques. Here we examine GRAV-D data located in a tectonically and topographically complex area of the country near Anchorage, AK and Fairbanks, AK. We assess the contribution of information gained from gravity analysis techniques, combined with information from geologic studies, for geodetic application in the area.

The mission of NOAA's National Geodetic Survey (NGS) is to "define, maintain and provide access to the National Spatial Reference System" (NSRS). NAVD 88 (North American Vertical Datum of 1988) provides the vertical reference for the NSRS. Comparisons with the Gravity Recovery and Climate Experiment (GRACE) satellite gravity data have demonstrated significant problems with NAVD 88. As repairing NAVD 88 through a massive leveling effort is impractical, NGS has decided to establish a gravimetric geoid as the vertical reference. The linchpin in NGS's effort is the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) program, which will ultimately incorporate satellite, airborne and terrestrial gravity data to build the geoid accurate to 1-2 cm that the U.S. surveying public requests. The GRAV-D program has two thrusts. First, a "high resolution snapshot" one-time measurement campaign with dense spatial sampling but short temporal span would be used to repair and improve existing gravity holdings. This campaign would involve airborne gravity surveys conducted over coastal areas first and interior areas later for the entire US and its holdings. Second, a "low resolution movie" will monitor temporal changes to the gravity field by tracking low order and degree changes in GRACE gravity data (Gravity Recovery and Climate Experiment satellite) augmented with a recurring terrestrial survey in areas of most rapid temporal changes. This effort would involve time series of absolute and relative terrestrial gravity measurements at these areas to help update the geoid over time. Initial data collection supporting GRAV-D was completed in July 2008. An airborne survey based out of Anchorage, AK covered an area 500 x 400 km over Cook Inlet and Kachemak Bay in 24 flights and about 100 flight hours. Funding from the US Army Corps of Engineers has facilitated surveying along the Coast of the Gulf of Mexico during the fall and winter, and a region covering Puerto Rico and the Virgin Islands. We present our project plan and most recent results.

The mission of NOAA's National Geodetic Survey (NGS) is to "define, maintain and provide access to the National Spatial Reference System" (NSRS). NAVD 88 (North American Vertical Datum of 1988) provides the vertical reference for the NSRS. Comparisons with the Gravity Recovery and Climate Experiment (GRACE) satellite gravity data have demonstrated significant problems with NAVD 88. As repairing NAVD 88 through a massive leveling effort is impractical, NGS has decided to establish a gravimetric geoid as the vertical reference. The linchpin in NGS's effort is the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) program, which will ultimately incorporate satellite, airborne and terrestrial gravity data to build the geoid accurate to 1-2 cm that the U.S. surveying public requests. The GRAV-D program has two thrusts. First, a "high resolution snapshot" one-time measurement campaign with dense spatial sampling but short temporal span would be used to repair and improve existing gravity holdings. This campaign would involve airborne gravity surveys conducted over coastal areas first and interior areas later for the entire US and its holdings. Second, a "low resolution movie" will monitor temporal changes to the gravity field by tracking low order and degree changes in GRACE gravity data (Gravity Recovery and Climate Experiment satellite) augmented with a recurring terrestrial survey in areas of most rapid temporal changes. This effort would involve time series of absolute and relative terrestrial gravity measurements at these areas to help update the geoid over time. Initial data collection supporting GRAV-D was completed in July 2008. An airborne survey based out of Anchorage, AK covered an area 500 x 400 km over Cook Inlet and Kachemak Bay in 24 flights and about 100 flight hours. Funding from the US Army Corps of Engineers has facilitated surveying along the Coast of the Gulf of Mexico during the fall and winter, and a region covering Puerto Rico and the Virgin Islands. We present our project plan and most recent results.

This presentation gives a short overview of gravity in relation to geodesy, as well as the instruments, processing, and interpretation involved in conducting airborne gravity studies. The slides are meant for a general audience or non-geology undergraduate student level.

Starting in 2008, airborne observations were made by the National Geodetic Survey over the state of Alaska. These data have been refined into aerogravity for use in developing geoid height models to serve as a future vertical datum. While the collection over the state is far from complete, the data processed thus far demonstrate the expected changes brought about by a consistent and seamless aerogravity campaign as a part of the Gravity for the Redefinition of the American vertical Datum (GRAV-D) project. This work shop will cover aspects of the collection as well as the net change to the gravimetric geoid model for some regions of Alaska. This is not yet a complete look, since not all of the data have been collected yet. However, it should provide insight into what the final model will look and the expected reliability of a future Alaskan vertical datum.

Starting in 2008, airborne observations were made by the National Geodetic Survey over the state of Alaska. These data have been refined into aerogravity for use in developing geoid height models to serve as a future vertical datum. While the collection over the state is far from complete, the data processed thus far demonstrate the expected changes brought about by a consistent and seamless aerogravity campaign as a part of the Gravity for the Redefinition of the American vertical Datum (GRAV-D) project. This work shop will cover aspects of the collection as well as the net change to the gravimetric geoid model for some regions of Alaska. This is not yet a complete look, since not all of the data have been collected yet. However, it should provide insight into what the final model will look and the expected reliability of a future Alaskan vertical datum.

Starting in 2008, airborne observations were made by the National Geodetic Survey over the state of Alaska. These data have been refined into aerogravity for use in developing geoid height models to serve as a future vertical datum. While the collection over the state is far from complete, the data processed thus far demonstrate the expected changes brought about by a consistent and seamless aerogravity campaign as a part of the Gravity for the Redefinition of the American vertical Datum (GRAV-D) project. This work shop will cover aspects of the collection as well as the net change to the gravimetric geoid model for some regions of Alaska. This is not yet a complete look, since not all of the data have been collected yet. However, it should provide insight into what the final model will look and the expected reliability of a future Alaskan vertical datum.

The use of the Global Positioning System (GPS) for positioning and mapping has been steadily increasing since its introduction in the late 1980’s. In the last few years, the use of GPS has exploded, primarily due to the establishment of regional or state-wide CORS networks that provide real-time correction data. The Vermont Agency of Transportation (VTrans) is in the process of building such a network with its primary purpose to support accurate positioning and mapping along Vermont’s Interstate corridors and other major highways. CORS are geodetic quality GPS receivers and antennas that are permanently installed. These stations collect GPS data continuously, and transmit data via the Internet to a central server. At the server, the data is archived for future use, and made available for download by any user. The incoming data is also processed at the server to generate corrections which are made available over the Internet to users in real-time. The Vermont Network has been named VECTOR (Vermont Enhanced CORS and Transmission Of Real-time corrections) to emphasize the expanded range of products available. The VT CORS Network has provided significant benefit to VTrans users and the tax payers of VT, and supports a variety of different applications from a diverse user community outside of VTrans. The information in this report shows that the direct savings VTrans has realized will likely pay for the system by the end of 2009, indicating a three-year return on investment. When considering the total savings realized by all users, the system paid for itself many times over in 2008 alone.

A presentation showing the current status of the vertical network in New England, and highlighting the need to modernize. The presentation discusses the rational for modernizing as discussed in the 10-year plan.

Presentation given to NGS employees and stakeholders at the 2010 NGS Convocation in Silver Spring, MD. The presentation goes over the NGS County Scorecard web survey results from a 2010 survey of over 500 members of the National Association of County Engineers or NACE. An overview is given of the current NGS Government Performance and Results Act or GPRA performance measure that includes the County Scorecard web survey as well as plans for a new replacement GPRA measure which tracks the progress of NGS' Gravity for the Redefinition of the American Vertical Datum (GRAV-D) initiative.

This is a .wav file of the NGS Director's opening remarks to congressional staffers and stakeholders during the rollout to Congress of a 2009 socio-economic benefits study of the NSRS, CORS and GRAV-D components. The full study is available here: http://www.ngs.noaa.gov/PUBS_LIB/Socio-EconomicBenefitsofCORSandGRAV-D.pdf A one page overview of the study is available here: http://www.ngs.noaa.gov/INFO/OnePagers/socio_eco_handout.pdf

This presentation is from the the keynote speech given by the NGS Director at the 2009 ESRI Survey & Engineering GIS Summit. Discusses the history, importance and future of the National Spatial Reference System (NSRS).

Brief NGS 101 and overview given by the NGS Director to new Hydrographic Services Review Panel (HSRP) members

Acting NGS Director, Ronnie Taylor, presents an update of Federal Geodetic Control Subcommittee(FGCS)activities to the Federal Geographic Data Committee (FGDC). FGCS website is: http://www.fgdc.gov/participation/working-groups-subcommittees/fgcs

Acting NGS Director, Ronnie Taylor, presentation to the Transportation Research Board 90th Annual Meeting. Provided an update on NGS activities including: new geopotential and vertical datums efforts, VDatum,GRAV-D, OPUS, LOCUS, Height Modernization, and Aeronautical Survey Program.

Acting NGS Director, Ronnie Taylor, update on NGS activities and 2010 accomplishments to the HSRP, including: Top 2010 Accomplishments, CORS, Shoreline Mapping, VDatum, County Scorecard, FY 2011 President's Budget and planned 2011 milestones.

NGS Director, Juliana Blackwell, provides an update to the HSRP on NGS activities including FY2010 performance measures, shoreline mapping ARRA funding, VDatum, Height Modernization, GRAV-D, Haiti earthquake response, Deepwater Horizon response, NGS 2010 milestones, FY2011 President's Budget and the Federal Geospatial Summit.

The California advisor and newly-appointed SW Region advisor will give updates on some of NGS' more popular programs, including the CORS network, OPUS, and DSWorld software. In response to requests, a review of the Datasheet fields and clarification of textual metadata will be provided. In light of increasing concerns about planning for Sea Level Rise, there will be a brief primer on tidal datums and usage of VDATUM software, which incorporates geodetic datums. Looking to the future, we will discuss the adoption of ITRF2008 for CORS coordinates, the shift to absolute antenna calibrations, and the migration-- in a decade or so-- to completely new horizontal and vertical datums.

The California advisor will give updates on some of NGS' tools and programs, including OPUS (data submission), as well as visualization tools such as DSWorld software and the Advisor's mapping website. A review of the Datasheet fields and clarification of textual metadata will be provided. In light of increasing concerns about planning for Sea Level Rise, there will be a brief primer on tidal datums and usage of VDATUM software, which incorporates geodetic datums. You will be informed about proposed changes in geodetic reference systems-- the adoption of ITRF2008 and the shift to absolute antenna calibrations impacting CORS coordinates this Spring, and the migration in a decade or so to completely new horizontal and vertical datums.

"Survey of the Coast", a predecessor of the National Ocean Service within the National Oceanic and Atmospheric Administration, is the oldest American civilian scientific agency established in 1807, to survey and map the nation's coastline. This organization also became the first agency to collect masses of geographic information (geodetic control, tidal, shoreline, soundings, geomagnetic, etc.) and processed this information to produce products for the safety and welfare of our citizens. Today the National Geodetic Survey continues to provide products and standards, including the national shoreline and derivatives including ortho imagery, processed lidar, and FGDC metadata to meet our nation's economic, social, and environmental needs. The National Geodetic Survey also provides emergency response imagery and lidar to support homeland security and emergency response requirements. The data is available through GeoSpatial One Stop, Digital Coast, and NOAA Shoreline Data Explorer applications.

NOAA's National Geodetic Survey (NGS) is mandated to map the National Shoreline, the legally-recognized shoreline depicted on NOAA nautical charts. While the primary application of this shoreline is in support of safe navigation, the data are now being used for an increasingly wide range of coastal science applications, including understanding and responding to threats of climate change. Over the past decade, NGS has collaborated with academic, government, and private sector partners to develop and implement new lidar-based shoreline mapping procedures. However, while NGS' lidar shoreline mapping workflow is now beginning to be used operationally, rigorous methods of assessing the uncertainty in the lidar-derived shoreline position have lagged behind in development. The study presented here aims to address this issue through development and comparison of two new methods of assessing uncertainty in NOAA lidar-derived shoreline: an empiric

NGS Director, Juliana Blackwell, provides an update to the HSRP on NGS activities including FY2011 and FY2012 information on performance measures, significant activities, milestones and budget.

IGS08: Elaboration, consequences and maintenance of the IGS realization of ITRF2008 The International GNSS Service (IGS) has designated its own realization of ITRF2008, known as IGS08, as the basis of its products starting in early 2011 and for the next full reprocessing campaign. The philosophy generally follows IGS practice since 2000 when the IGS97 realization of ITRF97 was adopted. However, unlike frames IGS97 through IGS05, IGS08 was initially intended to be a direct subset of well performing, stable GNSS stations from ITRF2008 rather than a separate GNSS-only frame solution. But, while the IGS contribution to ITRF2008 was computed using the original set of “absolute” GNSS antenna calibrations (igs05.atx), IGS08 had to be consistent with the latest set of calibrations (igs08.atx) that includes new determinations for some existing antennas. Coordinate corrections due to the antenna calibration updates were thus estimated and applied when possible to the ITRF2008 coordinates of 64 affected stations (out of a total of 232 stations in IGS08). As regards GNSS, the scale of the terrestrial frame is highly correlated with the satellite phase center offsets (PCOs) in the radial Earth direction. As the ITRF2008 scale differs by about -1 ppb from ITRF2005, new satellite PCOs consistent with ITRF2008 and IGS08 had to be derived for igs08.atx. They were obtained by back-solving the reprocessed solutions of five IGS analysis centers, while fixing their scales to the ITRF2008 scale. In order to satisfy regional users, many reference stations were selected in areas with dense GNSS coverage, such as Europe. This led to density heterogeneities in the IGS08 network, which is not optimal for the alignment of global frames. So a smaller, well distributed core network was additionally defined and recommended for global applications (such as for the IGS core products). Simulations show that using this core network instead of the full IGS08 set as reference frame indeed significantly reduces the “network effect”. Transformation parameters from IGS05 to IGS08 are unsurprisingly close to those from ITRF2005 to ITRF2008. Rotations are at the level of 0.01 mas so that the IGS orbits and Earth orientation parameters should be marginally affected by the switch from IGS05 to IGS08. But the scale difference of ~ -1 ppb and the Z translation of ~6 mm will result in changes in station positions by several millimeters. IGS08 is already beginning to suffer from continuous loss of reference stations due to earthquakes and mainly antenna changes, as was an even more critical problem for IGS05. To avoid a future crisis situation for the IGS products, it might be necessary to consider regular updates of the IGS08 reference frame before the next ITRF release. Such updates would require updated, post-discontinuity IGS08 coordinates to be estimated. A method to obtain such updated reference coordinates, based on the IGS operational cumulative solution, will be proposed.

Status of IGS Orbit Modeling and Areas for Improvement J.R. Ray (jim.ray@noaa.gov) J. Griffiths (jake.griffiths@noaa.gov) NOAA/NGS, Silver Spring, Maryland, US While the overall mean inaccuracy of the recent Final orbits of the International GNSS Service (IGS) is estimated to be about 2 cm (1D RMS), three aspects of the orbit modeling can probably be significantly improved: 1) ensure consistent and accurate modeling of satellite attitude variations; 2) mitigate spurious rotations of the constellations; and 3) add accelerations due to Earth radiation pressure. The errors associated with these are all highly systematic, not random. Reliable models for the attitude control of the older GPS satellites have been published for some years. Recently new models have been developed for GLONASS and the newest generation of GPS satellites as well. However, the implementation of these models among IGS Analysis Centers (ACs) is not consistent. Partly this is probably because the GPS Block IIR spacecraft were designed in such a way that attitude effects were nearly benign, so the major analysis errors were for the older, dwindling generations. However, with newer constellations and GPS Blocks the attitude variations probably cannot be treated so simply for high-accuracy results. The impact on user products is mostly on satellite clock variations and therefore on precise point positioning (PPP) results. So it is vital to ensure overall consistency by the IGS ACs adopting common models to generate combined products and by users implementing the same models in their PPP solutions. A leading error in the current IGS orbits is spurious net rotations of the constellation. It was learned in the early years of the IGS that once-per-revolution empirical parameters (or similar) were needed to model subdaily effects of solar radiation pressure. Failing to do so caused mainly large translational offsets in the Y component of the GPS orbit origin. But even with the higher-order parameterizations, much smaller rotational errors remain. The spectral features of these seem strongest near odd multiples of the GPS draconitic frequency (1.04 cycles per year) and probably also near fortnightly periods. Deficiencies in the widespread once-per-rev empirical modeling are likely to be responsible for these rotational errors. Most IGS ACs neglect the satellite accelerations due to reflected and thermally emitted radiation from the Earth as well as recoil thrust from the GNSS transmitters, largely because an accepted model for GNSS spacecraft is not yet available. Studies indicate that including at least the Earth albedo effect could remove most of the observed 2 cm bias between IGS orbits and satellite laser ranging. So developing an acceptable model and implementing it should be a high near-term priority for the IGS.

NGS Geodetic Tool Kit; a 2-hour Seminar The NGS Geodetic Tool Kit includes several geodetic computational utilities including the geoid computation utilities, HTDP, LVL_DH, the geodetic inverse and forward utilities, OPUS, gravity prediction utilities, NADCON, VERTCON, and the XYZ to latitude, longitude, height conversion utility. Some utilities you use every day and some you don’t. You probably don’t need to compute a geodetic inverse very often, or convert from geocentric coordinates X, Y, and Z to latitude, longitude, and ellipsoid heights on a daily basis, so you probably don’t have these utilities on your personal computer. But, when you need them they can be found in the NGS Geodetic Tool Kit! This workshop will highlight many of these utilities and describe their expected output and usage.

On-line Positioning User Service (OPUS); a 2-hour Seminar The National Geodetic Survey (NGS) operates the On-line Positioning User Service (OPUS) as a means to provide GPS users easier access to the National Spatial Reference System (NSRS). OPUS allows users to submit their GPS data files to NGS, where the data will be processed to determine a position using NGS computers and software. The position for your data will be reported back to you via e-mail in both the International Terrestrial Reference Frame (ITRF) and NAD83 coordinates as well as Universal Transverse Mercator (UTM), U.S. National Grid (USNG) and State Plane Coordinates (SPC) northing and easting. Recent and proposed developments regarding the various OPUS utilities will be discussed.

GPS-Derived Heights ; a 4-hour Seminar GPS-Derived Heights is a 1/2-day seminar detailing heights, height systems, their relationships, and development through the use of the Global Positioning System (GPS). Discussion describes the use of NGS GPS-Derived Ellipsoid and Orthometric Heights Guidelines. Development of sample projects and analysis of example baseline processing and project adjustments illustrate the basic concepts outlined. This seminar addresses both large scale and "local" projects, the use of the Continuously Operating ReferenceStations (CORS), On-line Positioning User Service (OPUS) and other NGS products and services.

Geodetic control is one of seven core framework themes in the California Geospatial Framework Data Plan document. Discussions are underway within a Work Group under the auspices of the CA GIS Council with regard to what type of geodetic control should be considered 'framework' points, who in California would be the steward for maintenance, and whether and how to incorporate real-time data into this theme. National Geodetic Survey has been responsible for providing an accurate National Spatial Reference System and the NGS Geodetic Advisor for California, an active member of the Work Group, will provide updates on the process of defining, populating, and maintaining the geodetic control framework layer in this state that has very active Earth surface processes, both horizontally and vertically.

This presentation provided information about NGS accomplishments in the past year and near-term future activities and was organized into 3 sections. 1)Priority programs: CORS, OPUS, GRAV-D with a subsection on GEOID09 in CA and OPUS-DB, and Ht Mod; 2)Updates of activities in many of the branches within the 6 Divisions; and 3)NGS as part of NOAA, particularly collaboration with CO-OPS.

A review of GPS and GRACE estimates of surface mass loading effects Tonie van Dam, Xavier Collilieux, Zuheir Altamimi, and Jim Ray Since its launch in 2002, many authors have compared GPS height coordinate residuals with radial displacements predicted from the Gravity Recovery and Climate Experiment (GRACE). Most comparisons have demonstrated significant annual correlations at perhaps 50% - 75% of the sites. At the other sites, there exists little to no correlation between the GPS observed and GRACE predicted heights. The disagreement is often attributed to problems in the GPS data analysis, e.g. ocean tide aliasing, seasonal monument motion, or reference frame effects. In this presentation, we revisit the GPS/GRACE comparison using GPS height residuals, GRACE data, and an environmental loading model in an attempt to better explain the discrepancy between the signals. We will also compare the degree-1 in the GPS time series with that from the environmental loading model.

Abs. To access the quality of NGS airborne gravity system and the impact of the flight altitude, NGS performed test flights at 1700, 6300 and 11000 meters altitudes in 2008, with track spacing 10 km for the two lower flights and 5 km for the highest flight. The flights provide not only important data sets for testing the precision and accuracy of the NGS airborne gravity system, but also the impact of the flight altitude to the collected gravity field. Results show that the system is stable and delivers high quality gravity data at three altitudes. The gravity collected at three altitudes agrees with each other from 1.4 to 3.3 mGal (RMS) at 11000 m flight altitude. If the biases are removed, the agreement is better than 1 mGal. At the ground, the downward continued gravity anomalies from the three altitudes agree within 1.9 to 3.8 mGal (RMS). After the biases removed, the agreement is better than 1.7 mGal. The similar results are obtained in comparison with NGS2008 surface gravity. The overall agreement between the downward continued airborne gravity at three altitudes and the NGS2008 surface gravity are better than 1.7 mGal, after the mean values are removed. The flight altitude has a direct impact on the airborne gravity accuracy. The comparisons show that the gravity collected at 11000 m altitude performance the worst, probably due to smaller signal/noise ratio and larger downward continuation effect. Due to 10 km track spacing, the gravity collected at 1700 m altitude does not outperform those collected at 6700 m altitude. Gravity at these two altitudes estimated to have an accuracy ±2 mGals at the ground.

The United Sates adopted the North American Vertical Datum of 1988 (NAVD 88) for its official vertical datum in the 1990s. Canada has been using the Canadian Geodetic Vertical Datum (CGVD28) for its height applications since the 1930s. The use of the different datums causes inconsistent heights across the border between the two countries, and the topographic height data from the two countries are not compatible. Both datums rely on passive control and significant pre-modern survey data, yielding not only misalignment of the datums to the best known global geoid at approximately 1-2 meters, but also local uplift and subsidence issues which may significantly exceed 1-2 meters in extreme cases. Today, the GNSS provides the geometric (ellipsoidal) height to an accuracy of 1-2 centimeters globally. Because of this, users have begun to demand a physical height system that is closely related to the Earth’s gravity field to a comparable accuracy. To address this need, government agencies of both countries are preparing the next generation of vertical datums. Even if the new datums are based on the same concepts and parameters, it is possible to have inconsistent heights along the borders due to the differences in the realization of the datums. To avoid inconsistency, it is in the interest of both countries to have a united, seamless, highly accurate vertical datum. The proposed replacements for CGVD28 and NAVD88 shall be based on GNSS positioning and a high accuracy gravimetric geoid that covers the territories of the United States, Canada, Mexico and the surrounding waters (to include all of Alaska, Hawaii, the Caribbean and Central America). To account for the effect of the sea level change, postglacial rebound, earthquakes and subsidence, this datum will also provide information on these changes. Detailed description of the definition, realization and maintenance of the datum is proposed. The challenges in realization and maintaining the datum are also discussed.

The geoid over the US Rocky Mountains is computed using two different procedures. The NGS group bases its geoid computations on Helmerts 2nd condensation method and presents an approximate and a precise version of it. The former is based on the Faye anomaly, which involves approximations of the terrain effect and the downward continuation of the surface gravity residuals to the geoid. The precise version is based on accurate account of the topographic masses and downward continuation. The second procedure leans upon the European geoid modeling; it uses the Molodenskii theory and converts height anomalies to geoid heights. Both procedures employ EGM08 as a reference field and SRTM-DTED1 (3") elevations. However, the two procedures use different maximal degrees of EGM08 and utilize its low degrees differently. The results are compared to GPS Bench Marks and the resulting deflection models to astro-geodetic deflections.

The biggest obstacle in solving the geodetic interior boundary value problem is the unavailability of the mass density distribution of the Earth. However, if the density of the topography is known, solutions valid only in the topographic masses can always be found after some mathematical manipulations. The disturbing potential inside the topographic masses is “harmonized” by subtracting a non-harmonic potential field. The “harmonized” potential field is then solved by analytical downward continuation. A solution for the non-harmonic potential is presented for the special case where the mass density of the topography is a constant.

Based on the assumption that the ultra-high frequencies of the gravity field are produced by the topography variations, we compute the omission errors by using 3 arc-second elevation data from the Shuttle Radar Topography Mission (SRTM). It is shown that the omission errors to the geoid are in the range of dm, cm and sub-cm level for grid sizes of 5', 2' and 1' over the contiguous United States (CONUS), respectively. The results suggest that a 1 arc-minute grid size is sufficient for the 1-cm geoid, even for areas with very rough topography and high gravity variations. The results also show that the omission errors to gravity are significant even for 1' grid size, at which the smoothed-out gravity still reaches tens of mGals. The omission errors to gravity at a 5' grid size peaks above 100mGals, demonstrating the importance of correction of residual terrain to gravity observations in data gridding or block mean value computations. The results are also compared with those based on Kaula’s rule. While the omission errors based on Kaula’s rule are 0.5 and 3.0cm for 1' and 5' grid size, respectively, the RMS values of the omission error in this paper are 0.1 and 1.1cm. The differences suggest Kaula’s rule may overestimate the power of the gravity field at the ultra-high frequency band, which renders the convergence studies of the spherical harmonic series based on Kaula’s rule questionable.

Since its official unveiling at the 2008 General Assembly of the European Geosciences Union, EGM2008 has demonstrated that high-degree harmonic expansions constitute a useful and effective final representation for high-resolution global gravitational models. However, such expansions also provide a versatile means of capturing (modeling), inter-comparing, and optimally combining local and regional high-resolution terrestrial data sets of different types. Here we present a general recipe for using high-degree expansions to capture, downward-continue and assimilate airborne survey data. This approach relies on the production of two ‘competing’ high-degree expansions. A first, ‘terrestrial-only’ expansion incorporates EGM2008 globally, and high-resolution terrestrial gravimetry regionally. This expansion can be used to upward-continue the regional terrestrial data to the flight level of the airborne survey, such that the terrestrial gravimetry outside the survey area can be merged with the airborne data inside the survey area, all at flight level. Harmonic analysis of this merged data set, also at flight level, yields a second ‘airborne-augmented’ expansion, which closely matches the ‘terrestrial-only’ expansion outside the survey area, but which also closely reproduces the airborne survey data inside the survey area. Capturing the airborne and terrestrial data in this way means that downward-continuation of the airborne data, as well as spectral/spatial comparison (and ultimate combination) of the airborne data with the terrestrial (and satellite) data, can all be achieved through spherical- and ellipsoidal-harmonic synthesis of these two competing expansions, and their spectral combination. This general approach is illustrated with a worked example.

The workshop will consists of: 1. General overview digital leveling - NGS way 2. WINDESC program - with hands on examples 3. Translev program - with hands on examples 4. LOCUS 5. Project reports. The goal is at the end of the 8 hours a small leveling project will be completed. Plus, the workshop will be alot on hands with examples and problems so everyone will need to have their own laptop.

Reference frame changing for CORS; Adjustment/Datum tag changing for passive; CORS what's different?; Want to keep up?; HTDP what's different?; RTN Guidelines; GC Mapping tools; Learning Resources

CBL Process CBL Site Characteristics Adoption of new reference frame for CORS Subsequent adjustment for passive stations HTDP CGAR Developments of various NGS products

The U.S. Army Corps of Engineers (USACE) National Oceanic and Atmospheric Administration (NOAA) and the Federal Emergency Management Agency (FEMA) are in the early stages of developing a tri-agency initiative entitled SAGE, which stands for Systems Approach to Geomorphic Engineering. The purpose of this tri-agency initiative is to pursue and advance a comprehensive view of shoreline change and to utilize integrated methodologies for coastal landscape transformation to slow/prevent/mitigate/adapt impacts to coastal communities from the consequences of climate change. This concept will utilize a holistic approach in exploring the idea of hybrid engineering, linking "soft" ecosystem-based approaches with "hard" infrastructure approaches, to develop innovative techniques and solutions to aid in the adaptation of our changing coastlines. This presentation is an overview of National Ocean Service (NOS) contributions to understanding essential integrated data needs for inporming SAGE. It was presented by Juliana Blackwell, Director of NOAA's National Geodetic Survey.

In North America, the last attempt to unify the vertical datum, the North American Vertical Datum of 1988 (NAVD88), was only adopted by the USA and Mexico, while Canada elected to remain on the Canadian Geodetic Vertical Datum of 1928. Furthermore, no attempt to provide a unified datum to the Central American and Caribbean nations was part of NAVD88. This has led to continued cross-border height issues on the continent. Recently, Canada and the United States have initiated policy decisions to replace their respective datums with new datums realized primarily through GNSS technology and a gravimetric geoid model. The USA has decided to wait until the GRAV-D campaign is complete (2022) to make this change. Canada will likely adopt sooner in 2013, with a possible update in 2022 aligned with the USA. Although Mexico has not yet adopted a policy of replacing NAVD88, they are participating in all coordinated efforts towards obtaining a common datum. Government geodetic agencies in the USA, Canada and Mexico are working toward the joint computation and adoption of a single gravimetric geoid model covering all of North America, including all parts of Alaska, Hawaii, Central America and the Caribbean. The adopted geoid model will be coordinated with (but may not be identical to) the IAG's adoption of a unified World Height System. The North American unified vertical datum will be accessible to all nations in this region through GNSS technology. This paper discusses progress toward the unification of a vertical datum for the entire continent.

We have developed a block model of tectonic deformation of North America west of longitude 100° W and between latitudes 30°N and 49°N. NOAA's National Geodetic Survey (NGS) is in the process of incorporating this model into version 3.1 of the horizontal time dependent positioning (HTDP) software tentatively scheduled for release in early 2011. By estimating the horizontal linear velocity for any point on the ground, HTDP enables surveyors and others to update (or backdate) positional coordinates measured on one date to corresponding coordinates that would have been measured on another date. The model consists of 59 crustal blocks with 46 independent rotation poles and 38 independent strain rate tensors. The model also includes elastic coupling coefficients on faults that bound adjacent blocks. NGS recently updated estimates for model parameters by using 6,063 GPS-derived velocity vectors (including vectors from the 2009 Plate Boundary Observatory (PBO) solution and the NGS Multiyear CORS) and 330 geological measurements of fault slip and/or fault orientation. In general, the fault slip rates and the interseismic coupling coefficients are consistent with the results of previous studies; however, because of the comprehensive nature of this model, we are able to quantitatively map deformation rates over the entire deforming region. Slip rates on the faults range from over 30 mm/yr for the Cascadia subduction zone and parts of the San Andreas system to near zero for faults adjacent to stable North America. In particular, our study confirms very low deformation rates across eastern Nevada and western Utah.

Are the National Geodetic Survey's surface gravity data sufficient for supporting a 1cm-accurate geoid? We evaluate the errors of these surface data and their effect on the geoid. Long wavelength errors are derived by comparison to synthetic GRACE gravity and high-frequency errors by crossover analysis and K-Nearest-Neighbor predictions.

Discussion of the new geometric and vertical datums NGS intends to deliver on or about 2012.

Heights (elevations) are complicated. Some are reckoned as straight lines perpendicular to a reference surface (ellipsoid heights), some are curved lines parallel to gravity at every point (orthometric heights), and some 'heights' have no geometric meaning at all yet tell you where water will go (dynamic heights). The datum to which heights refer can be a mathematical ellipsoid surface, which in turn is referenced to one of several different global or regional frames. It can be a vertical datum defined by a geodetic leveling network, or by a geoid, a gravitational equipotential surface more-or-less representing global mean sea level. It can be defined by a particular tide gage as local mean sea level, or mean high water, or mean lower low water, etc. Adding to the complexity, heights are measured by a wide variety of equipment, each with corrections, models, and methodologies that yield particular types of heights referenced to various datums over a broad range of accuracies.

NOAA has created two tools, VDatum and LOCUS, to help geospatial professionals make better use of height data and measurements. VDatum is free software developed jointly by NOAAs National Geodetic Survey (NGS), Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS). It vertically transforms geospatial data among a variety of tidal, orthometric, dynamic, and ellipsoidal height systems. This allows users to convert their data from different vertical (and horizontal) references into a common system and enables the fusion of diverse geospatial data into a uniform reference system. For example, VDatum can be used to combine a bathymetric survey referenced to a local tidal datum with a digital elevation model based on the North American Vertical Datum of 1988 (NAVD 88) into a single seamless surface model.

LOCUS (Leveling Online Calculations User Service) is a free Internet-based NGS service that checks, corrects, and adjusts leveling data submitted by users and provides heights with respect to published NGS vertical control. Philosophically, it is similar to the popular NGS Online Positioning User Service (OPUS) used for GPS data. As with OPUS, the intent is to make it as simple as possible for users to get correctly adjusted heights by uploading their leveling data via the Internet and immediately receiving results. For example, if a user wants to level with respect to NGS NAVD 88 benchmarks, LOCUS will apply the appropriate corrections (including the gravity model) to convert the observed leveled heights differences to NAVD 88 orthometric heights.

Whether combining existing vertical datasets from a variety of sources (VDatum) or correcting and adjusting precise vertical measurements (LOCUS), NOAA has developed tools that will assist users in achieving great heights.

This presentation follows previous talks that focused on outreach and education of surveyors. It is designed to update surveyors on the latest research related to the development of geoid height models as tools for datum transformation between NAD 83 and NAVD 88. All previous geoid height models have relied upon control data existent in the National Geodetic Survey Integrated Database (NGSIDB) to develop the conversion surface between the datums. In GEOID09, nearly 20,000 such points were available. However, about half of these points were located in only four of the lower 48 states. Most of the nearly 500,000 bench marks that exist in the NGSIDB have never been occupied with a GPS receiver. Hence, the bulk of the points are unused in determining the conversion surface. Given the disparate distribution of the few points that were occupied with GPS and the desire to supplement these in sparsely covered regions, alternative control data is desirable. The Online Positioning User Service Database (OPUS-DB) is where surveyors have the option of storing their observations for the use of others. Many of these marks were obtained on leveled bench marks. In November of 2010, there were about 422 points pulled from OPUS-DB with 285 representing new bench marks and providing supplemental control not previously available. These points are spread across the country and provide significant improvement in many regions, especially the sparsely covered western states. The errors resulting from interpolation over hundreds of kilometers can result in dm to multi-dm level errors in the resulting geoid height model. The OPUS-DB determined points then supplement the existing coverage from the NGSIDB. Many of these gaps were filled, and this reduced the interpolation error for those regions. More over, a campaign can be put in place to identify the sparse regions and likely candidate bench marks to target and fill these gaps. State Advisers/Coordinators and various state surveying groups have begun to organize efforts to collect GPS observations on the previously unoccupied bench marks and store them in OPUS-DB. For example, the distribution of control data from the NGSIDB points in Arizona have been examined, and that State Adviser has made known regions that require supplemental information in the OPUS-DB. These points will be examined as they become available and a determination made as to whether to incorporate them into future geoid height models. Preliminary analysis does indicate that there is some slight inferiority to the quality of OPUS-DB data in that the apparent error signal (noise) is generally about double that of NGSIDB data. However, noisier data can be accounted for using least squares collocation - missing signal due to gaps cannot be easily overcome. Use of OPUS-DB to supplement coverage shows great promise as a means of readily collecting information without the need for following the Bluebook, but doing so where it can provide the most improvement to future geoid height models for datum transformations.

Extensive airborne gravity surveys have been conducted as a part of the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project by the U.S. National Geodetic Survey. The intent is to capture the mid-wavelengths of the gravity field over all of the U.S.A. GRAV-D bridges the gap between long wavelengths determined by satellite gravity missions and shorter wavelengths determined from surface observations as well as forward-modelling of surface density and elevation models. The GRAV-D surveys were collected in 10-km spaced profiles covering 400-500 km patches, which are adequate in scale to compare to Earth Gravity Models through about degree 90. Initial comparisons with EGM2008 revealed no detectable slope across these patches, supporting the idea that no significant long wavelength differences exist between the aerogravity and the GRACE satellite data on which EGM2008 is based. However, only half the signal in EGM2008 is from satellite data at degree 95; indicating that a strict comparison of GRAV-D to the GRACE gravity field is less than exact. With the release of the GOCE data, higher resolution EGM's based entirely upon satellite gravity models make this comparison a more direct assessment of the long to intermediate quality of the aerogravity. This will enable generation of an EGM with 20 km resolution (approximately degree 2000). In turn, this model will be combined with the existing terrestrial data to build a higher resolution model towards defining a cm-level accurate gravimetric geoid model to be used as a new vertical datum for the United States.

NOAA's National Geodetic Survey (NGS) is responsible for maintaining the National Spatial Reference System. This includes the national geometric and geopotential datums, which are the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88), respectively. For NAVD 88, Helmert orthometric heights were defined in a block adjustment of over 500,000 geopotential differences at bench marks. As an alternative to leveling from established bench marks, NGS provides geoid height models such as GEOID09 (Roman et al. 2011) that transform between NAD 83 and NAVD 88. This provides the ease of calculating your position with GPS but yields more practical orthometric heights. To develop such models requires that both the GPS-derived ellipsoidal and leveling-derived orthometric heights on Bench Marks (GPSBM's) be known. Because of the requirement for both heights on a bench mark, the pool of control points is much smaller (only about 18,000) and not very equitably distributed (with potential dm-level interpolation errors). This paucity of points is driven by the rigorous processes ("Bluebooking") required to enter data into the NGS Integrated Database (NGSIDB). To mitigate this, the Online Positioning User Service Database (OPUS-DB) was explored as a source for supplemental data. This nascent database is rapidly being accepted by the broader surveying community and can even be used to target significantly deficient areas. A pull of OPUS-DB in November 2010 yielded 422 points. While this number is small in comparison to the overall NGSIDB data, the potential for growth is significant. These points fell into three categories: (1) 80 that were common to both databases and were used in making GEOID09, (2) 57 that were common to both but not used in making GEOID09, and (3) 285 with new geometric observations for points not previously observed with GPS (i.e., new control points). Residual values were formed by removing the same geoid and orthometric heights from ellipsoid heights obtained from NGSIDB and OPUS-IDB. Smaller values imply a better fit and less noise. For the first group, OPUS-DB was noisier (SD 0.031 m (one sigma)) than NGSIDB (SD 0.015 m (one sigma)). For the second group, OPUS-DB was less noisy (0.037 m (one sigma)) than the NGSIDB (0.043 m (one sigma)). For the last group, only OPUS-DB data were available and they were a little worse (0.047 m (one sigma)) than before. This is consistent with the level of agreement seen when forming residuals between ellipsoidal heights from NGSIDB and OPUS-DB in groups 1 and 2 (SD of 0.028 m and 0.044 m (one sigma), respectively). Overall, OPUS-DB demonstrated very good agreement with more rigorously determined NGSIDB data, provided expanded coverage into regions with poor coverage, and demonstrated a significant potential for use in future geoid modeling.

COOL GEODETIC RESOURCES FOR YOUR PROJECT Nearly every effort that involves planning, protecting, or monitoring our Nation's coasts and Great Lakes relies on knowing or establishing geographic positions and elevations of objects or locations-of-interest in the project area. A recommended method to accomplish that is to reference the project to the well-known and respected reference system, the National Spatial Reference System (NSRS). The NSRS, defined and maintained by NOAA's National Geodetic Survey (NGS), is a consistent National coordinate system that specifies latitude, longitude, height, scale, gravity, and shoreline throughout the Nation. It is NGS' mission to develop and provide access to the NSRS "to meet our Nation's economic, social, and environmental needs." This panel discussion will improve attendees' knowledge of methods they can use to obtain and use geodetic control. It will also update the audience about NGS' goals for improving the definition and delivery of horizontal and vertical datums. This session will inform the audience about new methods developed by NGS to facilitate easy access to data for approximately 1.5 million geodetic control marks that exist in the NGS database, with a brief description of software tools such as DSWorld, which works with GoogleEarth. It will inform the audience about the latest Online Positioning User Service (OPUS) utilities, which can be used to establish, or check, coordinates and elevations for projects referenced to the NSRS, enabling users to complete their work more efficiently. Also to be covered will be information about resources that are available to anyone needing assistance with locating, using, or producing geodetic information about their projects. Attendees will learn about NGS' Geodetic Advisors, who are located around the United States with the purpose of educating and assisting people in utilizing NGS products and services in their projects or applications.

Nearly every effort that involves planning, protecting, or monitoring our Nation's coasts and Great Lakes relies on knowing or establishing geographic positions and elevations of objects or locations-of-interest in the project area. A recommended method to accomplish that is to reference the project to the well-known and respected reference system, the National Spatial Reference System (NSRS). The NSRS, defined and maintained by NOAA's National Geodetic Survey (NGS), is a consistent National coordinate system that specifies latitude, longitude, height, scale, gravity, and shoreline throughout the Nation. It is NGS' mission to develop and provide access to the NSRS "to meet our Nation's economic, social, and environmental needs." This panel discussion will improve attendees' knowledge of methods they can use to obtain and use geodetic control. It will also update the audience about NGS' goals for improving the definition and delivery of horizontal and vertical datums. This session will discuss the relationship of geodetic and tidal vertical datums, and the necessity of understanding how to describe the relationship. The software program VDATUM was developed jointly by NOAA's NGS, Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS) to enable users to easily transform data from various vertical and/or horizontal datums into another datum for a location that is on the tidally influenced coastline. The input data can be elevations or (bathymetric) soundings, and batch files can be submitted. A brief tutorial will illustrate how to use the software, the accuracy associated with the conversions, and some of the common errors that users make.

COOL GEODETIC RESOURCES FOR YOUR PROJECT Nearly every effort that involves planning, protecting, or monitoring our Nation's coasts and Great Lakes relies on knowing or establishing geographic positions and elevations of objects or locations-of-interest in the project area. A recommended method to accomplish that is to reference the project to the well-known and respected reference system, the National Spatial Reference System (NSRS). The NSRS, defined and maintained by NOAA's National Geodetic Survey (NGS), is a consistent National coordinate system that specifies latitude, longitude, height, scale, gravity, and shoreline throughout the Nation. It is NGS' mission to develop and provide access to the NSRS "to meet our Nation's economic, social, and environmental needs." This panel discussion will improve attendees' knowledge of methods they can use to obtain and use geodetic control. It will also update the audience about NGS' goals for improving the definition and delivery of horizontal and vertical datums. This session will provide information about the efforts underway to produce a new International Great Lakes Datum (IGLD), with a goal for release in 2015. Being discussed are the reasons why a new IGLD is desirable, the data collection and analysis effort to update the datum, and examples of the impact of the new datum on the Great Lakes region. The international GPS campaign related to the new IGLD, completed in the Great Lakes region in 2010 by Natural Resources Canada's Geodetic Survey Division and NOAA's NGS, will be described.

Global Navigation Satellite System (GNSS) data processing requires many different numerical models to describe the physical processes affecting the positions of the satellites and points on the ground as well as the propagation of the GNSS signals from the satellites to the user. Some of these models are commonly known: satellite orbits, tropo and antenna corrections are examples from this group. Others are probably less well known: phase wrapping, atmospheric gradients and solid Earth tides are examples from this group. In this presentation, many of these models will be described with a focus on broader conceptual understandings rather than detailed technical descriptions. Real examples from data processing will be included whenever possible thereby adding the magnitudes of these physical processes to your understanding. The ultimate goal is to give you a better awareness of what your GNSS processing software should be doing to give you the accuracy necessary for your needs.

The National Geodetic Survey, NGS, traces its history back to the early 1800's. Although its mission has adapted to changing times and technology, this two hundred year history is alive in the NGS today. Using an overview of the NGS as an organizing framework, some NGS activities of particular interest to the surveying communities will be highlighted. Among the activities highlighted in this presentation are: improved coordinates for the CORS and a large subset of the passive mark networks (NAD 83(2011) and NA2011); the related work to define a new hybrid geoid model providing improved consistency with these new coordinates and velocities (GEOID12); the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) mission to create a snapshot of gravity across the United States in unparalleled detail; the Online Positioning User Service (OPUS) providing virtually hands-off, high-accuracy GNSS data processing; and the creation of guidelines to help real-time network providers more rigorously tie their networks to the global and national datums.

The Online Positioning User Service (OPUS) is a National Geodetic Survey tool that provides you with a National Spatial Reference System coordinate via email in seconds using your own GPS data file. Several notable enhancements have been implemented or are pending for OPUS. OPUS-Projects is a new option providing tools to handle GPS projects involving several sites occupied over several days. OPUS-Projects includes project visualization and management tools, enhanced processing options, and one click publishing for an entire project. OPUS is testing a new static processing strategy. By including more CORS at various distances and more sophisticated geophysical models, this new strategy improves the reliability of the results without sacrificing flexibility. OPUS-RS also offers a new CORS selection strategy which improves reliability and expands the regions in which this is a viable processing option. Underlying these enhancements are new CORS coordinates derived from a recently completed global GNSS network solution. This solution provides improved coordinates for all included CORS that are consistent with recognized reference systems such as the ITRF2008. These and other new developments will be described.

The California Geodetic Advisor will discuss: NEW Coordinates and epoch for the NSRS CORS that were published on Sept 6; In-progress adjustment of nearly 80K passive stations nationwide; Development of GEOID12 based on new ellipsoid heights; Changes to the Datasheet format/content; and DSWorld: a GoogleEarth mapping tool and more.

The National Oceanic and Atmospheric Administration's (NOAA), National Geodetic Survey (NGS) has embarked on a ten year project called GRAV-D (Gravity for the Redefinition of the American Vertical Datum). The purpose of this project is to replace the current official vertical datum, NAVD 88 (the North American Vertical Datum of 1988) with a geopotential reference system based on a new survey of the gravity field and a gravimetric geoid.
As part of GRAV-D, NGS plans to execute a set of "geoid validation surveys" at various locations of the country. These will be surveys designed to independently measure the geoid to provide a check against both the data and theory used to create the final gravimetric geoid which will be used in the geopotential reference system.
The first of these surveys, known as the Geoid Slope Validation Survey of 2011 (GSVS11) was executed between July and October, 2011 in the central region of Texas. The survey took place over a 325 kilometer line running more or less north-south from Austin to Corpus Christi, Texas. Measurements were taken at 218 marks (one per mile) and included static GPS, RTN GPS, geodetic leveling, astro-geodetic deflections of the vertical using the Swiss DIADEM camera, absolute gravity, gravity gradients and LIDAR. This region was chosen for many factors including the availability of GRAV-D airborne gravity over the area, its relatively low elevation (220 meter orthometric height max), its geoid slope (about 130 cm over 300 km), lack of significant topographic relief, lack of large forestation, availability of good roads, clarity of weather and lack of large water crossings.
This talk will outline the initial results of the survey, specifically the comparison of various geoid slopes over this region: gravimetric geoid models (with and without airborne gravity), minimally constrained GPS and leveling and from astro-geodetic deflections of the vertical.

Consistency of Crustal Loading Signals Derived from Models and GPS: Inferences for GPS Positioning Errors
After applying corrections for surface load displacements to a set of station position time series determined using the Global Positioning System (GPS), we are able to infer precise error floors for the determinations of weekly dN, dE, and dU components. The load corrections are a combination of NCEP atmosphere, ECCO non-tidal ocean, and LDAS surface water models, after detrending and averaging to the middle of each GPS week. These load corrections have been applied to the most current station time series from the International GNSS Service (IGS) for a global set of 706 stations, each having more than 100 weekly observations. The stacking of the weekly IGS frame solutions has taken utmost care to minimize aliasing of local load signals into the frame parameters to ensure the most reliable time series of individual station motions. For the first time, dN and dE horizontal components have been considered together with the height (dU) variations.
By examining the distributions of annual amplitudes versus WRMS scatters for all 706 stations and all three local components, we find an empirical error floor of about 0.65, 0.7, and 2.2 mm for weekly dN, dE, and dU. Only the very best performing GPS stations approach these floors. Most stations have larger scatters due to other non-load errors. These global error floors have been verified by studying differences for a subset of 119 station pairs located within 25 km of each other. Of these, 19 pairs share a common antenna, which permits an estimate of the fundamental electronic noise in the GPS estimates: 0.4, 0.4, and 1.3 mm for dN, dE, and dU. The remaining 100 close pairs that do not share an antenna include this noise component as well as errors due to multipath, equipment differences, data modeling, etc, but not due to loading or direct orbit effects since those are removed by the differencing. The WRMS dN, dE, and dU differences for these close pairs imply station error floors of 0.8, 0.9, and 2.1 mm, respectively, almost the same as the error floors inferred from the global results where orbit errors should be fully expressed. This match implies that GPS orbit errors are only a minor part of the IGS weekly position uncertainties. A similar comparison for periodic signals implies that about a third of the GPS draconitic harmonics probably arise from sources local to the stations whereas the remaining two-thirds comes from orbital effects.

Subdaily Alias and Draconitic Errors in the IGS Orbits
Harmonic signals with a fundamental period near the GPS draconitic year (351.2 d) and overtones up to the 8th multiple have been observed in the power spectra of nearly all products of the International GNSS Service (IGS), including station position time series [Ray et al., 2008; Collilieux et al., 2007; Santamaria-Gomez et al., 2011], apparent geocenter motions [Hugentobler et al., 2008], and orbit jumps between successive days and midnight discontinuities in Earth orientation parameter (EOP) rates [Ray and Griffiths, 2009]. Ray et al. [2008] suggested two mechanisms for the harmonics: mismodeling of orbit dynamics and aliasing of near-sidereal local station multipath effects. King and Watson [2010] have studied the propagation of local multipath errors into draconitic position variations, but orbit-related processes have been less well examined.
Here we elaborate our earlier analysis of GPS orbit jumps [Griffiths and Ray, 2009; Gendt et al., 2010] where we observed some draconitic features as well as prominent spectral bands near 29, 14, 9, and 7 d periods. Finer structures within the sub-seasonal bands fall close to the expected alias frequencies of subdaily EOP tide lines but do not coincide precisely. While once-per-rev empirical orbit parameters should strongly absorb any subdaily EOP tide errors due to near-resonance of their respective periods, the observed differences require explanation. This has been done by simulating known EOP tidal errors and checking their impact on a long series of daily GPS orbits. Indeed, simulated tidal aliases are found to be very similar to the observed orbital features in the sub-seasonal bands. Moreover and unexpectedly, some low draconitic harmonics were also stimulated, potentially a source for the widespread errors in most IGS products.

How do we achieve confidence with our Real Time (RT) work? What pitfalls should we avoid? Are there guidelines to follow to help us with our procedures? How accurate are the Real Time Networks? Should I be using Glonass?
Those are just some of the common questions surveyors, engineers and other geospatial professionals ask when they go to the field to obtain RT GNSS positional coordinates. Because so much of RT GNSS positioning is transparent to the user and is entirely dependent on the field technician to bring back good data, it is incumbent on that technician to follow correct procedures and certain criteria to ensure a successful campaign. This webinar will show the recommended specific criteria to achieve 4 different grades of precision at the 95% confidence level, and will present the critical areas that can affect our RT data collection.

Evaluation of GPS Orbit Prediction Strategies for the IGS Ultra-rapid Products
To serve real-time and near real-time users, the International GNSS Service (IGS) produces Ultra-rapid GPS & GLONASS orbit product updates every 6 hr. Each is composed of 24 hr of observed orbits, with an initial latency of 3 hr, together with propagated orbits for the next 24 hr. We have studied how the orbit prediction performance varies as a function of the arc length of the fitted observed orbits and the parameterization strategy used to estimate empirical solar radiation pressure (SRP) effects. To focus on the dynamical aspects of the problem, nearly ideal conditions have been adopted by using IGS Rapid orbits as observations and known Earth orientation parameters (EOPs). Performance was gauged by comparison with Rapid orbits as truth by examining WRMS and median orbit differences over the first 6 hr and the full 24 hr prediction intervals, as well as the stability of the Helmert alignment parameters. Note that the actual IGS Ultra-rapid accuracy is limited mostly by rotational instabilities, especially about the Z axis due to errors in near real-time and predicted UT1 values.
We found that observed arc lengths of 40 to 44 hr produce the most stable and accurate predictions during 2010. Two versions of the extended SRP orbit model by the Centre for Orbit Determination in Europe (CODE) were tested. Adjusting all 9 SRPs (offsets plus once-per-rev sines and cosines in each D,Y,B component) for each satellite shows smaller mean subdaily, scale, and origin translation differences. On the other hand, when the 4 once-per-rev SRPs in the D and Y directions are held fixed, then smaller, more stable rotational differences are obtained. A combined strategy of rotationally aligning the 9-SRP results to the 5-SRP frame should give optimal predictions with about 15 mm mean WRMS over the first 6 hr and 35 mm over 24 hr. Actual Ultra-rapid performance will be degraded due to larger errors in the available near real-time observed orbits and EOP predictions.

Geodetic GNSS applications routinely demand millimeter precision and extremely high levels of accuracy. To achieve these accuracies, measurement and instrument biases at the centimeter to millimeter level must be understood. One of these biases is the antenna phase center, the apparent point of signal reception for a GNSS antenna. It has been well established that phase center patterns differ between antenna models and manufacturers; additional research suggests that the addition of a radome or the choice of antenna mount can significantly alter those a priori phase center patterns. For the more demanding GNSS positioning applications and especially in cases of mixed-antenna networks, it is all the more important to know antenna phase center variations as a function of both elevation and azimuth in the antenna reference frame and incorporate these models into analysis software.
To help meet the needs of the high-precision GNSS community, the National Geodetic Survey (NGS) now operates an absolute antenna calibration facility. Located in Corbin, Virginia, this facility uses field measurements and actual GNSS satellite signals to quantitatively determine the carrier phase advance/delay introduced by the antenna element. The NGS facility was built to serve traditional NGS constituents such as the surveying and geodesy communities, however calibration services are open and available to all GNSS users as the calibration schedule permits. All phase center patterns computed by this facility will be publicly available and disseminated in both the ANTEX and NGS formats.
We describe the NGS calibration facility, and discuss the observation models and strategy currently used to generate NGS absolute calibrations. We demonstrate that NGS absolute phase center variation (PCV) patterns are consistent with published values determined by other absolute antenna calibration facilities, and compare absolute calibrations to the traditional NGS relative calibrations.

Although originally designed to enable accurate positioning and time transfer, the Global Positioning System (GPS) has also proved useful for remote sensing applications. In this study, GPS signals are used to measure snow depth via GPS interferometric reflectometry (GPS-IR). In GPS-IR, a GPS antenna receives the desired direct signal as well as an indirect signal which reflects off of the ground or snow surface. These two signals interfere, and the composite signal recorded by the GPS receiver can be post-processed to yield the distance between the antenna and the reflecting surface, that is, distance to the snow surface.
We present the results of a new snow depth product for the state of Minnesota over the winter of 2010-2011. Although single-station examples of GPS snow depth measurements can be found in the literature, this is one of the first studies to compute GPS snow depth over a large regional-scale network. We chose Minnesota because the state Department of Transportation runs a network of continuously operating reference stations (CORS) with many desired characteristics: freely available data, good GPS station distribution with good proximity to COOP weather stations, GPS stations located adjacent to farm fields with few sky obstructions, and receiver models known to have sufficient data quality for GPS-IR.
GPS-IR with CORS has many advantages over traditional snow depth measurements. First, because we leverage existing CORS, no new equipment installations are required and data are freely available via the Internet. Second, GPS-IR with CORS measures a large area, approximately 100 m2 around the station and 20 m2 per satellite.
We present snow depth results for over 30 GPS stations distributed across the state. We compare the GPS-IR snow depth product to COOP observations and SNODAS modeled estimates. GPS-IR snow depth is one of the few independent data sources available for assessment of SNODAS. Ideally snow depth via GPS-IR will be available for ingestion into operational systems such as NOAA-NWS streamflow, weather and climate forecast systems at other locations across the US in the not too distant future.

As part of the National Geodetic Survey's 10 year plan for the modernization of the National Spatial Reference System (NSRS), entirely new horizontal and vertical datums will be developed to replace the existing NAD 83 and NAVD 88. The changes in these datums will have a significant impact on the users of geodetic data nation wide. This presentation will describe the existing components of NSRS and the rational for the need to adopt new reference frames.

As part of the National Geodetic Survey's 10 year plan for the modernization of the National Spatial Reference System (NSRS), entirely new horizontal and vertical datums will be developed to replace the existing NAD 83 and NAVD 88. The changes in these datums will have a significant impact on the users of geodetic data nation wide. This presentation will describe the existing components of NSRS and the rational for the need to adopt new reference frames.

Quantifying Load Model Errors by Comparison to a Global GPS Time Series Solution Various space geodetic studies over the past two decades have shown that temporal variations in the distribution of ocean, atmospheric, and continental water masses cause detectable vertical displacements of the Earth's surface. Unlike most past research that focused on a single load component for only vertical motions, we have included the horizontal, as well as vertical, components and considered atmosphere, non-tidal ocean, surface water load models. Our geodetic solution is the most current reprocessed station time series from the International GNSS Service (IGS) for a global set of 706 stations, each having more than 100 weekly observations. The long-term stacking of the weekly frame solutions has taken utmost care to minimize aliasing of local load signals into the frame parameters to ensure reliable time series of individual station motions. Our reference load model consists of components from NCEP atmosphere (corrected for high-resolution topographic variations), ECCO non-tidal ocean, and LDAS surface water (cubic detrended over 1998 to 2011 to remove inter-annual artifacts), then combined, linearly detrended, and averaged to the middle of each GPS week as a posteriori corrections. This reference model reduces the WRMS scatters of about 72, 63, and 87% of GPS station dN, dE, and dU components, respectively. Alternative load models, for individual components or the total, can be tested against the same set of GPS time series to determine their relative accuracy. For example, not removing a cubic trend from the LDAS surface water loads causes a global average quadratic increase in WRMS scatters of about 0.1, 0.1, and 0.5 mm in dN, dE, and dU. The method is sensitive to load model error differences at the level of about 0.1 mm in the horizontal components and about 0.2 to 0.3 mm in the vertical due to residual load aliasing in the GPS time series. We will report relative accuracy differences for a range of load model pairs.

ICON (Ionosphere over CONus) was an experimental ionosphere model developed at NGS between 2002 and 2005. It relies solely on ambiguous phase data, and uses a mathematical truth to arrive at absolute TEC values. This presentation discusses ICON, and another NGS-sponsored model, called MAGIC (which ultimately became the SWPC's USTEC engine). Unlike MAGIC, which was a 3-D (plus time) model, ICON was a 2-D (shell, plus time) model. ICON never left the experimental stages, due to a variety of instabilities and the generally better performance of MAGIC. However the mathematical solution to arrive at UNambiguous TEC from ambiguous phase data remains valid and may prove useful in the future.

This presentation shows the original work that led to VDatum and the pilot projects it supported.

Consistency of Crustal Loading Signals Derived from Models and GPS: A Re-examination
Various space geodetic studies over the past two decades have detected vertical displacements of the Earth's surface caused by temporal variations in the distribution of ocean, atmospheric, and continental water masses. Most past research has focused on a single component of the mass load and till now only vertical motions have been examined. Successively stronger correlations have been seen as improvements have been made in the load models as well as in measurements by the Gravity Recovery and Climate Experiment (GRACE) of surface mass changes and by the Global Positioning System (GPS) of station height variations. Initial comparisons of modeled surface mass load displacements with GPS heights were most successful for areas with large signals, such as the Amazon River basin, where the amplitude of annual height variations reaches ~13 mm. Following large-scale efforts to reprocess historic GPS data series with modern analysis methods, the most recent results find that mass load corrections reduce the WRMS scatter of GPS verticals for ~77% of global networks of more than 100 stations.
We have re-examined this problem but included the horizontal, as well as vertical, components and used the most current station time series from the International GNSS Service (IGS) for a global set of 706 stations, each having more than 100 weekly observations. The long-term stacking of the weekly frame solutions has taken utmost care to minimize aliasing of local load signals into the frame parameters to ensure the most reliable time series of individual station motions. Using a combination of NCEP atmosphere, ECCO non-tidal ocean, and LDAS surface water load models (averaged to the middle of each GPS week) as a posteriori corrections, the WRMS GPS scatters are reduced for 72, 63, and 87% of station dN, dE, and dU components, respectively. Fitted annual amplitudes are correspondingly reduced for similar fractions of stations. The weighted mean dU annual amplitude drops from 3.9 to 1.7 mm with load corrections applied. An absence of dU improvement is nearly only limited to island and coastal sites with small load effects, but degradations for dN and dE are more widespread. Some stations are clearly exceptional due to data problems. The quality and global coverage of current GPS time series has reached a point that they can be used as an independent reference to precisely evaluate the relative performance of competing load models.

To achieve the best airborne gravity data accuracy possible, the GPS position solutions must provide not just accurate and precise positions, but accurate and precise velocities and accelerations to be used in calculating gravity corrections. To our knowledge, no head-to-head comparisons have been done of available kinematic processing techniques with a focus on producing good airborne gravity results.
ORIGINAL GOALS:
1. 0.5 mGal or less two-sigma values (precision) for free-air disturbances calculated with different GPS position solutions, but the same gravity processing.
2. Close comparisons to EGM08 (accuracy), the best available global gravity model (Pavlis, et al., 2010), in an area where EGM08 is well-constrained.
Thus, in Fall 2010 the National Geodetic Survey announced the “Kinematic GPS Challenge” to the entire GPS community. The Challenge solicited position solutions for two GRAV-D airborne gravity flights done in Louisiana in Fall 2008. The flights are described in the data section. The response of the community was outstanding, with some groups submitting multiple solutions: Participating groups: 10; Solutions for each flight: 16; Total solutions (for 4 gravity lines): 64. GPS processing types span the range of differential and PPP solutions, with different methods developed by each group. The results are presented anonymously here (each solution presented with a unique f## designator rather than the software’s & developer’s names), protecting the GPS participants while they discuss these results but allowing the airborne gravity community to benefit from the early conclusions.

This presentation provides an overview of why geodesy and datums are important to GIS. Various tools and products are covered so the user is aware of how they can use and access the NSRS and its data.

National Geodetic Survey Director, Juliana Blackwell, gave a presentation on NGS’s Gravity for the Redefinition of the American Vertical Datum (GRAV-D) program at the Management Association for Private Photogrammetric Surveyors (MAPPS) winter conference held January 22-23, 2012. MAPPS is an association of photogrammetry, mapping, and geospatial firms, and this briefing discussed the importance of GRAV-D and how MAPPS members and NGS can work together and support one another towards common goals. The new vertical datum in development by NGS will be important to both the Geographic Information Systems (GIS) and MAPPS communities, which rely on accurate elevations to perform their missions.

New Revised CORS Coordinates in IGS08 epoch 2005.00 and NAD 83 (2011/MA11/PA11) epoch 2010.00

As part of the continuing efforts to improve the National Spatial Reference System (NSRS), NOAA's National Geodetic Survey (NGS) has performed the National Adjustment of 2011 (NA2011). This adjustment yielded updated North American Datum of 1983 (NAD 83) coordinates on passive control stations with positions determined using Global Navigation Satellite System (GNSS) technology. It is a simultaneous least-squares adjustment of nearly 80,000 passive control stations using a nationwide network of over 400,000 GNSS vectors that represent over 4100 survey projects spanning from the mid 1980s to August 2011. NA2011 is constrained to current NAD 83 coordinates of the NGS Continuously Operating Reference Station (CORS) network, which is a GNSS-based “active” control system and the geometric foundation of the NSRS. These NAD 83 CORS coordinates were determined in the NGS Multi-Year CORS Solution (MYCS) through a combined solution of all CORS GNSS data from 1994 to April 2011. Constraining NA2011 to the MYCS optimally aligns the GNSS passive control with the active control, providing a unified reference frame. The resulting realization gives positions at an epoch date of January 1, 2010, and it is formally designated as NAD 83(2011) epoch 2010.00. To ensure consistency in the NSRS, NGS developed a new hybrid geoid model (GEOID12) for concurrent release with NA2011. GEOID12 is based on a new gravimetric geoid model (USGG2012) modified using NAD 83(2011) ellipsoid heights on vertical bench marks. To improve access to the NSRS, NGS developed a new datasheet format that will provide some of the new information associated with NA2011, such as detailed accuracy information. The NGS “Bluebook” process for submitting control surveys for publication has also been improved, including a new version of the ADJUST least-squares adjustment program. This workshop describes the methods and results of NA2011, as well as the MYCS that provides the control for NA2011. The workshop also gives an overview of GEOID12, the new NGS Datasheet format, and other updates, such as the new version of ADJUST and new GIS-compatible products and services. NA2011 improves how NGS meets its wide range of customer needs in providing the basis for accurate and reliable georeferencing throughout the US and its territories. Completion of NA2011 – together with related products and services – represents a significant step toward a more integrated NGS, in terms of both better positions and improved access to the NSRS.

Juliana Blackwell, Director, NOAA’s National Geodetic Survey (NGS), presented an overview of why geodesy and datums are important to geographic information systems at the National States Geographic Information Council (NSGIC) meeting held February 29 to March 1 in Annapolis, MD. Included in the presentation was a background of NGS’ modernization efforts for the National Spatial Reference System (NSRS) that are currently underway, as well as key products and services of interest to the GIS (geographic information systems) community, including NGS’s Gravity for the Redefinition of the American Vertical Datum (GRAV-D) initiative, the National Adjustment of 2011 (NA2011) project, the new GEOID12 gravity model, and new GIS tools and datasets for display and analysis of survey data. An overview of the socio-economic benefits of NOAA/NGS products and services will also be provided.

Surveyors use Real Time GNSS (RT) technology becauseit’s a fast, efficient (labor saving) positioning tool that can yield survey grade coordinates used in a wide variety of applications. But, how good do you feel about the data you are producing with your real time gear? It is all “black box” technology after all. Are the data collector position quality values displaying precision or are they displaying accuracy and at what confidence level? What position deltas would you expect if you get another shot at a different time or with different weather? What are the factors that might be affecting your data, anyway? Would it be better to use a new real time network (RTN)to get your data? Is there any way to have real confidence with a RT established position? As we can see, there are a lot of questions with this technology and the answer to all of them is: “It depends”. That’s why NOAA’s National Geodetic Survey (NGS) has produced a set of single base RT user guidelines that recently left draft status to become an official NGS document. This workshop will discuss how you can have real confidence with real time work and go over the important criteria for the surveyor to achieve successful field campaigns based on the guidelines. Topics of discussion for best methods for the field include: constraining local monumentation, DOP, weather conditions, data collection parameters,multipath, how RT works, how RT doesn’t work, and many others emphasizing the attendees’ area of interest.

Recent and near-future changes in the geodetic datums and other developments being done by NGS are presented by the California Geodetic Advisor. She will: - Discuss how the new geodetic reference frame--IGS08-- was realized, - Detail the process for the nationwide adjustment of ~80,000 passive stations, - Explain the development of the upcoming GEOID12 gravity model - Introduce the RTN Validation initiative. Looking to the far future (10 years), you will learn about the developments that have been initiated to: - totally revamp the horizontal and vertical datums that define the NSRS, - obtain airborne gravity measurements to develop a geoid model accurate to 1 cm, - collect and analyze data for a Gravity Slope Validation Survey. There will be demonstrations of NGS software and online tools: - DSWorld for mapping NGS control locations in Google Earth, - DSWorld for submitting database corrections and updates (recovery notes), - VDATUM for relating tidal datums to geodetic datums - HTDP for getting different epoch coordinates

NGS Director, Juliana Blackwell, gave the keynote address at The Hawaii Pacific Geographic Information Systems (GIS) Conference 2012 held in Honolulu, HI March 5 to 9, 2012. Ms. Blackwell’s presentation highlighted NGS’ and NOAA’s geospatial activities in the Pacific region. Topics covered included an overview of selected NOAA geospatial data and services supporting mapping and charting, comprehensive ocean and coastal planning, new approaches for visualizing and using NOAA data, including the latest mobile applications, and the development of a new NOAA Geospatial Platform for access to the breadth of NOAA's geospatial data, services, and applications.

Understanding the relationships between vertical datums is important, especially when converting elevations from one datum to another. Unfortunately, the various datums are not very far apart and it is easy to make mistakes adding or subtracting datum shifts. The presentation will show the basic relationships among common datums and advocate an easy to understand method of datum conversion.

The National Geodetic Survey has readjusted the passive mark network throughout the United States to reflect the most accurate positions of the CORS network. The adjustment results and their impacts on Minnesota will be discussed.

Technology has changed the face of surveying and mapping, and the National Geodetic Survey (NGS) is at the forefront in the implementation of many of these technologies to provide the Nation with a consistent and accurate reference system. NGS produces the National Spatial Reference System (NSRS) ensuring projects have the consistency and accuracy desired. There are many tools available to access the NSRS and these will be highlighted during this session. In particular, DS-World, CORS and the Online Positioning User Service (OPUS) will be discussed. Using Google Earth, DS-World, makes it possible for users to display the million-plus geodetic survey marks and the GPS Continuously Operating Reference Stations (CORS) that make up the NSRS. This useful tool can display all the survey marks available in a particular geographic area and the associated information about each point, including its description, position, and other information gathered when the mark was set. NGS’ OPUS program is highly automated and requires minimal user input accessing the network of CORS for determining ones position. With OPUS, users can obtain high-accuracy NSRS coordinates, using only a clear view of the sky and a survey-grade GPS receiver. OPUS processes GPS data files along with CORS coordinates to provide results consistent with those of other users. Many variations have and are evolving. There are many other developments occurring in NGS that will be presented. These include: NGS role with the development of real time GPS; Modernization of the NSRS; the new adjustment (to be completed by 2011); and the GRAV-D program and how it may change the way we obtain vertical heights.

Presentation is a 2-hour talk on the background and components of the NGS National Height Modernization Program, including how it fits into NGS' Ten-Year Plan.

Changing Geodetic Advisor Program LEARNING/CONT EDUCATION: NGS Resources Non-NGS CGPS Data/Info—PBO and CSRC OPUS: S, RS, DB, Projects HTDP Datasheet Format/Content Changes VDATUM DSWORLD Demonstration

NSRS Ref frame improvements: For CORS, for passive, Geoid model; NSRS Evolution: GRAV-D, GSVS, NSRS revolution; NGS FY12 Budget & Geodetic Advisor Program; Coastal Products & Services: Shoreline Change, VDATUM, SLR, GLOSS, CSC; Lightning topics

This presentation discusses the status of the national Continuously Operating Reference Station (CORS) network and the recent multi-year CORS solution (new published CORS coordinates), the status of the National Adjustment of 2011 (forthcoming new published coordinates on passive network stations), and the next generation (in about a decade) of national horizontal and vertical datums.

This presentation discusses the NGS Continuously Operating Reference Station (CORS) network, the recent multi-year CORS solution (new CORS coordinates), the NGS Online Positioning User Service (OPUS), the National Adjustment of 2011 (forthcoming new coordinates for passive control network), the NGS Ten-year Plan / plans for new datums (in about a decade), DSWorld software, forthcoming changes to NGS datasheets, the Geoid Slope Validation Survey of 2011, and other topics.

On September 6, 2011, the National Geodetic Survey (NGS) published new coordinates for the nationwide continuous GPS network called CORS, which had not been updated in 9.5 years. The official datum of the National Spatial Reference System is still NAD83 but the realization of the datum changes with each adjustment that generates new coordinates. This talk will explain what the metadata should include, particularly in California, and give a few examples of the differences among coordinates, between 1986 and present, even if all are referenced to “NAD83.” NGS anticipates that, in 10 years, the NAD83 datum will be replaced by a truly geocentric datum that will be related to the International Terrestrial Reference Frame datum. Although the horizontal position (latitudes and longitudes) will of course change, it is notable that the ellipsoid heights could differ by 0.5 meters.

Conclusions • IGS produces Ultra-rapid ERPs with subdaily resolution & high accuracy – observed ERPs updated every 6 hr with 15 hr latency – PM accuracy roughly similar to IGS Finals: 25 to 30 μas – IGS Rapid PM accuracy may be even better: 15 to 16 μas – IGU dLOD accuracy may be better than IGS Finals: ~5 μs – further study needed to assess accuracy of IGS ERPs • Main (systematic) error components are probably: – errors in IERS subdaily EOP tide model – orbit mis-modeling (draconitic signals) – instabilities in terrestrial reference frame (though none detected directly) • IGS Ultra-rapid PM predictions better than operational services – IGU dLOD predictions similar to operational service • IGS Ultra-rapid ERP observations & predictions should be assimilated by operational EOP prediction services !

The International GNSS Service (IGS) is preparing for a second reanalysis of the full history of data collected by the global network using the latest models and methodologies. This effort is designed to obtain improved, consistent satellite orbits, station and satellite clocks, Earth orientation parameters (EOPs) and terrestrial frame products using the current IGS framework, IGS08/igs08.atx. It follows a successful first reprocessing campaign, which provided the IGS input to ITRF2008. Likewise, this second campaign (repro2) should provide the IGS contribution to the next ITRF. We will discuss the analysis standards adopted for repro2, including treatment of and mitigation against non-tidal loading effects, and improvements expected with respect to the first reprocessing campaign. International Earth Rotation and Reference Systems Service (IERS) Conventions of 2010 are expected to be implemented. Though, no improvements in the diurnal and semidiurnal EOP tide models will be made, so associated errors will remain. Adoption of new orbital force models and consistent handling of satellite attitude changes are expected to improve IGS clock and orbit products. A priori Earth-reflected radiation pressure models should nearly eliminate the ~2.5 cm orbit radial bias previously observed using laser ranging methods. Also, a priori modeling of radiation forces exerted in signal transmission should improve the orbit products. And use of consistent satellite attitude models should help with satellite clock estimation during Earth and Moon eclipses. Improvements of the terrestrial frame products are expected from, for example, the inclusion of second order ionospheric corrections and also the a priori modeling of Earth-reflected radiation pressure. Because of remaining unmodeled orbital forces, systematic errors will however likely continue to affect the origin of the repro2 frames and prevent a contribution of GNSS to the origin of the next ITRF. On the other hand, the planned inclusion of satellite phase center offsets in the long-term stacking of the repro2 frames could help in defining the scale rate of the next ITRF.

The gravity field is directly related to the structure of the Earth and how its mass is distributed. Every piece of mass creates a potential of gravity (geopotential) that drops off with distance. The cumulative effect of all these produces the Earth's gravity field. This session will focus on the relationship between various aspects of the Earth's gravity field such as the geoid, geopotentials, gravity, deflections of the vertical, and physical heights (e.g., above mean sea level). It covers different means of observing the gravity field and how they are combined to produce models for height determination both at global scales, such as the World Height System, and locally for National Vertical Datums.

The National Geodetic Survey (NGS) is responsible for maintaining both the horizontal and vertical datums within the U.S. National Spatial Reference System (NSRS), which are the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88), respectively. NGS periodically produces hybrid geoid height models that transform between these datums to facilitate GPS/leveling in surveying as well as other engineering and scientific activities. The GEOID12 model represents the latest effort in this series. However, both NAD 83 and NAVD 88 have significant systematic problems, which the current hybrid geoid height models faithfully replicate. While the datums remain internally consistent (i.e., precise) they are inconsistent with other reference systems at the meter level (i.e., not accurate). Comparisons at tide gauges and with global satellite gravity field models demonstrated a meter level cross-continent trend in NAVD 88 likely due to accumulated leveling errors in the adjustment that created it. Likewise NAD 83 is known to have a 2.2 meter offset from IGS 2008. The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project was started to realize a new vertical datum that is both accurate and precise. The aim of the project is to produce a gravimetric geoid height model that is accurate to cm-level and can be combined with an improved geometric reference frame to produce similarly improved physical heights, removing entirely the need to have “hybrid” geoid models which absorb systematic datum errors. Several factors are critical to ensuring this works. In anticipation of its adoption in 2022 at the completion of GRAV-D, an optimum geopotential surface (geoid) was recently selected based on comparisons with tidal bench marks around Canada and the United States and some portions of the Caribbean. A geopotential value of 62,636,856.00 m2/s2 best fit available data. This value is the same as that adopted by the International Astronomical Union (IAU) & the International Earth Rotation and Reference Systems Service (IERS) and is one of many that have been offered as the best representative of global mean sea level (MSL). Comparisons all around the North American continent with tide gauges, satellite altimeter measurements of the ocean surface, and models of ocean height variability all support adoption of this number as the best estimate of MSL for North America and future vertical reference systems defined for the United States and Canada. Canada will be adopting this value and a geoid height model based on it as their official vertical datum in 2013 while the United States will follow suit in 2022. The United States continues to collect aerogravity to remove systematic errors in the terrestrial gravity data holdings to ensure that a cm-level of accuracy is achieved. This is on track and should be accomplished as planned in 2022 with a new vertical datum realized by a gravimetric geoid height model and GNSS observations.