This Online Positioning User Service (OPUS) provides simplified access to high-accuracy National Spatial Reference System (NSRS) coordinates. Upload a GPS data file collected with a survey-grade receiver and obtain an NSRS position via email.
OPUS requires minimal user input and uses software which computes coordinates for NGS' Continuously Operating Reference Station (CORS) network. The resulting positions are accurate and consistent with other National Spatial Reference System users.
Your computed NSRS position is sent privately via email, and, if you choose, can also be shared publicly via the NGS website. To use properly, please familiarize yourself with the information below.
Handouts: See also OPUS one-pager.
Enter the email address (e.g., email@example.com) where you want OPUS to send your solution report.
|2. DATA FILE|
Provide OPUS a GPS observables data file in any format (for automatic conversion to RINEX format by UNAVCO's teqc converter) or convert it to RINEX yourself first. OPUS also recognizes compressed (UNIX or Hatanaka.yyd) or zipped (gzip or pkzip) files, including multiple data files in a single zip archive.
OPUS accepts receiver epoch rates of 1,2,3,5,10,15 or 30 seconds, all of which are decimated to 30 seconds for processing. Note: Though your data file may already contain survey metadata, including antenna type, height, and mark information; these are IGNORED as we have found they are inconsistenly formatted.
| 3. ANTENNA TYPE|
Select the antenna brand and model you used. This allows OPUS to determine the appropriate antenna calibration model for processing. Take care! Selection of an incorrect or default antenna may result in a height error as large as 10 cm. See antenna calibration to help find an exact match.
|4. ANTENNA HEIGHT|
Enter the vertical height in meters of your Antenna Reference Point (ARP) above the mark you are positioning, as shown in the image above right. The ARP for your antenna type, usually the center of the base or tripod mount, is illustrated at antenna calibration. If you enter a 0.0 antenna height, OPUS will return the position of your ARP.
Press OPTIONS to customize the way your solution is performed and/or reported. Your selections will override the optimized OPUS defaults and should therefore only be employed by experienced users.
How does it work?
OPUS will use either a static or rapid-static process, depending on the duration of your data file.
Static: Files over 2-hours in duration are processed using PAGES static software. Your coordinates are averaged from three independent, single-baseline solutions, each computed by double-differenced, carrier-phase measurements from one of three nearby CORS.
Rapid-static: Shorter data files, under 4-hours, may be processed using RSGPS rapid-static software. Rapid-static processing employs more aggressive algorithms to resolve carrier phase ambiguities, but has more stringent data continuity and geometry requirements; therefore there are some remote areas of the country in which it will not work. See accuracy and availability map.
Under normal conditions, most positions can be resolved to within a few centimeters. Estimating the accuracy for a specific solution is difficult, however, as formal error propagation is notoriously optimistic for GPS reductions. Systematic errors, such as misidentification of antenna type or height, are not detected. Local multipath or adverse atmospheric conditions may also negatively impact your solution.
|Static: Static processing provides
peak-to-peakerrors for each coordinate (X, Y, Z, Φ, λ, h, and H). These describe the error range, the disagreement between the 3 baseline solutions, as shown below.
One advantage of peak-to-peak errors is that they include any error from the CORS reference coordinates. To support a stable national datum, the CORS NAD 83 coordinates are updated less frequently than the ITRF. Your solution will usually show slightly larger errors in NAD 83 than in ITRF.
TO DO: Add discussion of RMS.
see Availability & Accuracy map
Absent any warning messages, the best estimates of coordinate
accuracy are the standard deviations reported by single baseline
analysis. Our experiments indicate that the actual error is less
than these estimated accuracies more than 95 percent of the time.
TO DO: More discussion of the reported accuracy metrics for OPUS-RS is needed here.
|Observe longer: A longer-duration session provides OPUS a better opportunity to accurately fix ambiguities and mitigate multipath error. The graph below shows the correlation between session duration and accuracy.|
|Observe again: A second, independent observation which yields a similar solution is an easy way to increase confidence in your results. Maximize independence by using a different observer, different equipment, on a different day, at a different time of day.|
|Wait a day: OPUS will use the best CORS and orbits available at the time you upload. While most CORS are archived within 30-minutes past the hour, some aren't available until the next day. Rapid orbits, available at 17:00 UTC that next day, may offer a slight improvement in your accuracy.|
|Process the data yourself: Manual data processing with suitable software will include custom cycle slip editing, outlier deletion, experimentation with tropospheric parameters, variable cut-off angle, and different constraints of the carrier phase ambiguities to integer values.|
While there are no absolute rules, most accurate OPUS solutions contain the following:
Since RSGPS uses the double-differenced ionospheric delays at the CORS to interpolate to the delays at the rover, it may perform poorly? or fail? during periods of high ionospheric disturbance. In general, it is best to avoid performing any GPS survey during geomagnetic storms that cause large and variable ionospheric refraction. Geomagnetic storm alerts are issued by NOAA's Space Environment Center, so that the surveyor may avoid collecting data during these unusual events.
Similarly, RSGPS performs a simple geographic interpolation to predict the tropospheric delay at your GPS location. Under normal conditions this works well. However, it may not work well during the passage of a strong weather front, and these situations should be avoided.
Three solution formats are available; standard, extended, and published. Samples are provided below:
NGS OPUS SOLUTION REPORT
9999 OPUS DISCLAIMER OPUS DISCLAIMER OPUS DISCLAIMER OPUS DISCLAIMER
error and warning messages are appended here
| USER: Your.firstname.lastname@example.org
Your email address
| DATE: October 27, 2004
The date and time you used OPUS
Your data file name
| TIME: 18:49:54
Coordinated Universal Time
| SOFTWARE: page5
The software we used
| START: 2004/10/15
The first observation in your data file
| EPHEMERIS: igr12925.eph
The orbit file we used
| STOP: 2004/10/15
The last observation in your data file
| NAV FILE: brdc2880.04n
The navigation file we used
| OBS USED: 8686 / 8804 : 99%
Usable / total observations in your data file
| ANT NAME: ASH700829.3 SNOW
Your selected antenna type
| # FIXED AMB: 41 / 42 : 98%
For static: Fixed / total ambiguities in your data file
For rapid static: quality indicators from network and rover mode solutions (ambiguities are always 100% fixed)
| ARP HEIGHT: 1.295
Your selected antenna height
| OVERALL RMS: 0.020 (m)
For static: The formal statistical root mean square (RMS) error of your solution
For rapid static: a unitless normalized RMS
Accuracies below are reported as either peak-to-peak errors (static) or standard deviation estimates (rapid static)
All initial computations are performed in ITRF. Your NAD83 coordinates are derived by transforming ITRF vectors into the NAD83 reference frame and recomputing the 3 independent and averaged positions (not a direct transformation of the ITRF coordinates; a direct transformation could be considered more accurate, but wouldn't fit your surrounding NAD83 network as well.) For both ITRF and NAD83, the reference coordinates for each CORS are derived from the NGSIDB and are updated using crustal motion velocities from HTDP (Horizontal Time-Dependent Positioning software to your data file's epoch. Your final ITRF coordinates retain this observed epoch, while your NAD83 coordinates are transformed again to the standard epoch date of January 1, 2002.
|REF FRAME: NAD83(CORS96)(EPOCH:2002.0000)||ITRF00 (EPOCH:2004.7887)|
|X: -552474.327(m) 0.015(m)||-552475.001(m) 0.015(m)|
|Y: -4664767.953(m) 0.021(m)||-4664766.631(m) 0.021(m)|
|Z: 4300548.721(m) 0.024(m)||300548.654(m) 0.024(m)|
|ellipsoidal coordinates (latitude, longitude, ellipsoidal height) and accuracies|
|LAT: 42 39 59.51026 0.007(m)||42 39 59.53576 0.008(m)|
|E LON: 263 14 44.18589 0.013(m)||263 14 44.14967 0.013(m)|
|W LON: 96 45 15.81411 0.013(m)||96 45 15.85033 0.013(m)|
|EL HGT: 314.705(m) 0.041(m)||313.753(m) 0.033(m)|
|The North American Vertical Datum of 1988 (NAVD88) orthometric height, if applicable, along with the geoid model used|
|ORTHO HGT: 340.240(m) 0.041(m)||[Geoid03 NAVD88]|
Also reported are the associated zone IDs, meridian convergence, point scale, and combined factor
|UTM COORDINATES||STATE PLANE COORDINATES|
|UTM (Zone 14)||SPC (4002 SD S)|
|Northing (Y) [meters] 4726229.423||43336.983|
|Easting (X) [meters] 684026.367||893325.488|
|Convergence [degrees] 1.52234197||2.46893915|
|Point Scale 1.00001666||1.00004366|
|Combined Factor 0.99996731||0.99999430|
NATIONAL GRID DESIGNATOR: 14TPN8402626229(NAD 83)
The US National Grid coordinates and referenced datum are reported, if applicable
The CORS we used as reference stations and
the nearest mark from the NGS Integrated Data Base (NGSIDB) are reported along with their positions and distances from your position.
|PID DESIGNATION||LATITUDE LONGITUDE DISTANCE(m)|
|AI1569 NLGN NELIGH CORS ARP||N421224.250 W0974743.043 99724.2|
|DF7469 SDSF EROS DATA CORS ARP||N434401.727 W0963718.541 119065.7|
|AH5054 OMH1 OMAHA 1 CORS ARP||N414641.765 W0955440.671 120751.8|
|NEAREST NGS PUBLISHED CONTROL POINT|
|NM0874 D 276||N423846. W0964505. 2286.4|
numerical values for this position solution have
satisfied the quality control criteria of the National
Geodetic Survey. The contributor has verified that
the information submitted is accurate and complete.
Because OPUS is automated and assumes your entries are valid, we add this disclaimer to all solutions.
Publishing helps maintain local ties to the National Spatial Reference System, and, by linking observations, strengthens the models used to translate between modern and legacy mapping products.
High-Quality OPUS Solution