CORS FAQs
0. What is going on with the navigation (n) file? My software says it can't compute a position due to missing nav file(s)?
We no longer supply a navigation file for each site. We instead ask the user to download the global navigation or broadcast file. This global nav file can be used for any site around the globe since it contains all broadcast messages for the entire 24 hour period. You can rename the file if your software requires that the Obs file and Nav file have the same 4 character site designation.
Get Global Navigation from http://www.ngs.noaa.gov/CORS/standard1.shtml
Others thoughts....... Have you tried using the precise orbit to improve your position?
1. Why doesn't gzip386.exe work correctly on an IBM PC or compatible when trying to uncompress files created by gzip, UNIX compress or pack?
If you get the error message "DMPI: Not enough memory", you are using a memory manager which allocates physical memory immediately instead of allocating on demand when pages are used for the first time. This problem occurs only when using DMPI. (Try under plain DOS without loading any memory manager in config.sys.)
Read the document gzip.doc, in particular the description of the -N option which is very useful for MSDOS to restore the original file names that have been truncated. You can set it by default by adding
set GZIP= -N
in your autoexec.bat file.
2. Why do the utility programs downloaded from the directory cors/utilities on the CORS file server not work on the receiving machine?
If you download executable files from a remote machine using the FTP (File Transfer Protocol), make sure to set the transfer to binary mode by typing bin or binary at the FTP prompt. Executable files should be transferred in binary mode and ASCII files transferred in ascii mode. Also, make sure to download the appropriate executable for your specific platform (computer).
3. The position file for site SSSS is not available. When will the positions be available to the public ?
Once a site has passed the test to become an NGS (National) CORS, a daily solution will be run for a minimum of 12 days. After that time the coordinates will entered into the NGSIDB and a coordinate file will be produced.
4. How do I get CORS data ?
CORS data are available on a file server at the National Geodetic Survey
To get the Data go here -
http://www.ngs.noaa.gov/CORS
or our alternate server here -
http://alt.ngs.noaa.gov/CORS
You can also obtain customized data sets by going to User Friendly CORS UFCORS
5. Can I use PKUNZIP to uncompress the CORS data ?
PKZIP / PKUNZIP Version 2.04 will not be able to uncompress the CORS data. The CORS data are compressed with the utility gzip. gzip is more widely used on the various platforms, offers tighter compression and files compressed with gzip can be transferred to various platforms. PKZIP is predominantly used on the Intel platforms.
gzip is available for the Intel x86 platform. Check here for more information.
6. What does the extension ".gz" mean at the end of the data files ?
The extension ".gz" indicates the file was compressed with the utility gzip on a UNIX platform. To uncompress a file with a ".gz" extension on an Intel platform, download the file to your working directory. The file name should be renamed so the last character is the letter 'z'. Then run the command
gzip -d -N "filename.z"
See the answer to question #1.
7. Where do you store the US Coast Guard DGPS correctors ?
The National Geodetic Survey does not store DGPS correctors from the U.S. Coast Guard. Only RINEX-2 format data from the CORS sites are available for post-processing applications.
8. The data I want are not available on the CORS file server. What do I do ?
We now keep all of the data that we have online. It is possible that we were not able to get some data due to many reasons. Since many of our sites are owned and operated by other agencies, it may be possible to retrieve data from those agencies. You can find the responsible agency from the site's Logfile. Go to the map and select your site.
9. I downloaded CORS data but it does not work with my reduction software. What is wrong with the data ?
The CORS data has been compressed to save space. When it is retrieved via FTP it must be treated as if it is binary data and it must be uncompressed. (WEB transfer software usually recognizes binary data and sets the proper mode.) Thus, before the transfer the user must set the transfer to binary mode (bin or binary, see answer 2). After transfer and decompression, the file should be a readable ASCII file in RINEX format. If you cannot open the file and recognize numbers and words in the file then the transfer and decompression has not worked properly. If the file can be opened and read easily, then the reduction software may not be expecting a RINEX 2 file. Ensure that you are importing the data properly. If all else fails contact the manufacturer of your software.
10. I used your data with my reduction software and my positions are 30 meters (100 feet, 100 m, etc.) different from yours. What is wrong with your position ?
This problem may result from several causes but the chance the NGS CORS position is seriously in error is very small. First, you must ensure that you are comparing NAD 83 positions with NAD 83 positions or the equivalent. Different datums can introduce over 100 meters in position differences, e.g. NAD 27 to NAD 83. If the datums are the same, are you trying to compare an uncorrected point position (yours) with an NGS published position? An uncorrected point position can be in error by as much as 100 meters (horizontally when SA was on) 95% of the time due to Department of Defense signal dithering. If that is not the problem, have you checked to ensure that the reference position you think you are using in your computations is actually being used by the reduction software? The positions in the RINEX files are ONLY APPROXIMATE and may be tens of meters or more in error. You must enter into your reduction software an accurate position from the appropriate coordinate file. There are other possible problems, but these are the most obvious and most common.
11. Where do I get the NGS precise orbits ?
Precise orbits may be obtained from the NGS Orbits Web Page.
12. How do I receive your CORS Newsletter on a weekly basis ?
To receive the CORS Newsletter on a weekly basis, please send a request to CORS Info.
13. Will RINEX-2 data work with my reduction software ?
In general, we believe the answer is yes. If you are unsure, we advise you to contact a technical advice person for your GPS receiver manufacturer. Some general phone numbers for GPS receiver manufacturers are as follows:
Allen Osborne Associates - (805) 495-8420
Ashtech, Inc. - (408) 615-5100
Corvallis Microtechnology, Inc. - (503) 752-5456
Garmin International - (913) 599-1515
Leica - (310) 791-5300
Magellan Systems Corp. - (909) 394-5000
NovAtel Communications Ltd. - (403) 295-4900
Sokkia Corporation - 1-800 257-2552
Topcon America Corporation - (201) 261-9450
Trimble Navigation Ltd. - (408) 481-8000
This list is not inclusive and represents only a few of the 50 or more manufacturers currently available. Mention of this group does not infer endorsement or recommendation of their products.
14. I have seen the CORS map with concentric circles. What do they represent?
The circles are in increments of 250 kilometers from the published position.
15. What is the difference between National CORS and Cooperative CORS?
Cooperative (Coop) CORS has been merged into National CORS.
Check out the page ...
http://www.ngs.noaa.gov/CORS/Establish_Operate_CORS.shtml
16. What does the name of the directory 1996/044 stand for ? Does 044 represent April 4th ?
The directory 1996/044 contains data for the 44th day of the year (1996). The format is YYYY/DDD.
17. I plotted the coordinates of a CORS site and it is a long way from the town or city of that name. Why is the name in error?
There are two types of CORS sites which fit this naming problem. Those that are in a suburb of some large city where the major city name was used and those sites which were funded by the US Army Corps of Engineers (now by the U.S. Coast Guard). These are named after the local district headquarters.
Some sites include:
Louisville, KY (LOU1 & LOU2) - Taylorsville, KY
Memphis, TN (MEM1 & MEM2) - Frenchman's Bayou, AR
Kansas City, MO (KAN1 & KAN2) - Perry, KS
Omaha, NE (OMH1 & OMH2) - Pisgah, IA (decommissioned)
Rock Island, IL (RIS1 & RIS2) - Teeds Grove, IA
St Louis, MO (STL3 & STL4) - Summerfield, IL
St Paul, MN (STP1 & STP2) - Alma, WI
18. I'm trying to use data from a CORS site but there is only the observation file (99o). I would like to process my GPS data using this site. Why is the navigation file (99n) missing?
We supply one "global" navigation file which can be used for all GPS processing.
ftp://www.ngs.noaa.gov/cors/rinex
select a year, select a day, then select the file brdc----.--n.gz
20. Should I use the positional coordinates for the L1 phase center or those for the antenna reference point (ARP)?
The L1 phase center is the theoretical point in space where the L1 carrier phase is received "on average." The actual location where this signal is received, however, varies as a function of the direction of the incoming GPS signal, and hence an averaging process is required.
The antenna reference point (ARP) is a specified physical point on the antenna.
The relative spatial relationship between these two points is determined via a calibration process in a laboratory-type environment. This process involves collecting and processing several hours of GPS data, and it involves several assumptions about antenna characteristics.
The National Geodetic Survey (NGS) has calibrated many antennae and has determined the average spatial relationship between these two points for each of several classes of antennae. See http://www.ngs.noaa.gov/ANTCAL
For sites in the CORS system, NGS provides spatial coordinates for both the L1 phase center and the ARP, as different GPS-processing software packages may use one or the other of these points or possibly even some other reference point.
To find out whether your GPS-processing software uses the L1 phase center or the ARP, you need to contact the company that produced this software.
NGS encourages GPS companies to have their software use the ARP, but our agency can not require this practice.
21. I see that you have a 24 hour file out there for the site that I need, but I only need 1 hour of that data. Is there an easier way to get what I need?
Try our User Friendly CORS (UFCORS) to get customized data sets.
22. What is OPUS and where can I find it?
OPUS or Online Positioning User Service is a tool used to determine a position for you based on the submitted GPS data. For more information visit OPUS
23. How is the UFCORS different from the way I was retrieving data?
The National Geodetic Survey has designed UFCORS to facilitate the retrieval of GPS data from the National CORS (continuously operating reference stations) network. In particul ar, UFCORS enables users to:
* obtain CORS data for an exact time interval specified in international or local time,
* choose a sampling rate for the requested data,
* specify how the data should be compressed to speed data transfer to the user,
* receive the requested data within a few minutes,
* obtain pertinent descriptive text (log files and datasheets) for CORS sites
automatically,
* retrieve adopted NAD83 and ITRF positions and velocities for CORS sites automatically,
* obtain meteorological data for those CORS sites that have "met" sensors automatically, and
* retrieve both the highly accurate GPS satellite orbits calculated by the International GNSS Service for Geodynamics (IGS) and the "broadcast" GPS orbits aut
omatically.
A utility connected with UFCORS automatically provides the CORS data exactly as requested, saving users from tedious data manipulation and file management tasks.
The National Geodetic Survey welcomes suggestions for improving UFCORS. Please forward suggestions to us, here.
24. On the UFCORS Page there are several ways to compress the data. Can you explain the different methods?
Data files may be compressed in one of the following formats.
- gzip - Files have the .gz extension. To unzip, use the following command line.
gzip -d filename.gz
For help on gzip, type gzip --help.
- pkzip - Files have the .zip extension. These files can be unzipped by pkunzip and Winzip. To unzip using pkunzip, use the following command line.
pkunzip filename.zip
For help on pkzip, type pkzip
For Winzip, copy the zipped file to the Winzip icon and the files should be extracted.
- compress Files have the .Z extension. To uncompress these files on a UNIX machine, use the following command line.
uncompress filename.Z
25. On the UFCORS Page I do not know which time zone to select. Do I have to convert to UTC (Universal Time Coordinated)?
The following timezones are supported by the User Friendly Cors Page (UFCORS):
UTC
Atlantic (New Brunswick)
Eastern Standard
Eastern Daylight Savings
Central Standard
Central Daylight Savings
Mountain Standard
Mountain Daylight Savings
Pacific Standard
Pacific Daylight Savings
Alaska
Hawaii
If you are requesting data for your time zone, the UFCORS page will automatically do the conversion to UTC for you. You do not need to do the conversion and have that represented in your start time. For example, if you want data starting at 11:00 am Eastern Standard Time, simply enter 11:00 for "Start time", and Eastern Standard Time for "Time Zone".
You must select daylight savings time if your timezone supports it and you are currently in daylight savings time. Daylight savings time begins when clocks are set from 2:00 a.m. to 3:00 a.m. Beginning in 2007, most of the United States begins Daylight Saving Time at 2:00 a.m. on the second Sunday in March and reverts to standard time on the first Sunday in November. In the U.S., each time zone switches at a different time. Hawaii, Arizona, and most of Indiana do not use daylight savings time. Remember "Spring Forward", "Fall Back".
The RINEX file times are all in UTC (actually GPS time) no matter which time zone you select on the UFCORS page.
26. I have heard that I can improve my results by applying the "precise" orbits? Where can I find these orbits?
The "Broadcast" orbit (navigation file) information is sent by the satellites. Since the satellite's clock may be off by as much as 7 nanosecs, its position could be off as much as 210 cm. The precise orbit is calculated after the fact by several analysis centers around the world ( the National Geodetic Survey has its own analysis center located in Silver Spring, MD). A combination of the precise orbits yield a global precise called the IGS orbit and is good to less than 2 cm !
Check http://igscb.jpl.nasa.gov/components/prods.html> for more information on GPS orb its.
27. Why can't I find the site I want in the choice list for UFCORS?
If the four-character identifier for a particular site does not appear in the list, then either the National Geodetic Survey (NGS) does not distribute GPS data for that site or NGS does not have GPS data for the requested time period for that site. Using the starting date, starting time, and duration that you specify, the UFCORS software looks at the records for each site to determine whether or not GPS da ta are available for the corresponding time period. If GPS data are available, then the site's name is added to the list.
The CORS Newsletter provides a somewhat comprehensive list of sites for which NGS distributes GPS data. The current CORS Newsletter may be accessed via the COR S web site: http://www.ngs.noaa.gov/CORS/news.shtml
A data sheet is the principal information product for NGS. It combines control-point position and height information in a single format. Data sheets are retrieved directly from the NGS database by specifying a PID (Permanent Identifier), control point name, area, or survey project identifier. For the CORS sites NGS provides a data sheet for both the L1 phase center and the antenna reference point (ARP). Examples of data sheets for these points are included.
1 National Geodetic Survey, Retrieval Date = MAY 13, 1999 AB6375 *********************************************************************** AB6375 CORS - This is a GPS Continuously Operating Reference Station. AB6375 DESIGNATION - ANNETTE ISLAND 1 CORS L1 PHASE CENTER AB6375 CORS_ID - AIS1 AB6375 PID - AB6375 AB6375 STATE/COUNTY- AK/KETCHIKAN GATEWAY BOROUGH AB6375 USGS QUAD - KETCHIKAN A-5 AB6375 AB6375 *CURRENT SURVEY CONTROL AB6375 ___________________________________________________________________ AB6375* NAD 83(CORS)- 55 04 08.64567(N) 131 35 58.25576(W) ADJUSTED AB6375* NAVD 88 - AB6375 ___________________________________________________________________ AB6375 EPOCH DATE - 1997.00 AB6375 X - -2,430,153.226 (meters) COMP AB6375 Y - -2,737,194.074 (meters) COMP AB6375 Z - 5,205,816.416 (meters) COMP AB6375 ELLIP HEIGHT- 32.30 (meters) GPS OBS AB6375 GEOID HEIGHT- -5.86 (meters) GEOID96 AB6375 AB6375 HORZ ORDER - SPECIAL (CORS) AB6375 ELLP ORDER - SPECIAL (CORS) AB6375 AB6375.ITRF positions are available for this station. AB6375.The coordinates were established by GPS observations AB6375.and adjusted by the National Geodetic Survey in April 1996. AB6375.The coordinates are valid at the epoch date displayed above. AB6375.The epoch date for horizontal control is a decimal equivalence AB6375.of Year/Month/Day. AB6375 AB6375 AB6375.The XYZ, and position/ellipsoidal ht. are equivalent. AB6375 AB6375.The ellipsoidal height was determined by GPS observations AB6375.and is referenced to NAD 83. AB6375 AB6375.The geoid height was determined by GEOID96. AB6375 AB6375 SUPERSEDED SURVEY CONTROL AB6375 AB6375 NAD 83(CORS)- 55 04 08.64567(N) 131 35 58.25576(W) AD(1996.00) c AB6375 ELLIP HT - 32.30 (m) GP(1996.00) c c AB6375 AB6375.Superseded values are not recommended for survey control. AB6375.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums. AB6375.See file dsdata.txt to determine how the superseded data were derived. AB6375 AB6375_STATION IS THE L1 PHASE CENTER OF THE GPS ANTENNA AB6375 AB6375 STATION DESCRIPTION AB6375 AB6375'DESCRIBED BY NATIONAL GEODETIC SURVEY AB6375'STATION IS A GPS CORS. LATEST INFORMATION INCLUDING POSITIONS AND AB6375'VELOCITIES ARE AVAILABLE IN THE COORDINATE AND LOG FILES ACCESSIBLE AB6375'BY ANONYMOUS FTP OR THE WORLDWIDE WEB. AB6375' FTP CORS.NGS.NOAA.GOV: CORS/COORD AND CORS/STATION_LOG AB6375' HTTP://WWW.NGS.NOAA.GOV UNDER PRODUCTS AND SERVICES. 1 National Geodetic Survey, Retrieval Date = MAY 13, 1999 AF9530 *********************************************************************** AF9530 CORS - This is a GPS Continuously Operating Reference Station. AF9530 DESIGNATION - ANNETTE ISLAND 1 CORS ARP AF9530 CORS_ID - AIS1 AF9530 PID - AF9530 AF9530 STATE/COUNTY- AK/KETCHIKAN GATEWAY BOROUGH AF9530 USGS QUAD - KETCHIKAN A-5 AF9530 AF9530 *CURRENT SURVEY CONTROL AF9530 ___________________________________________________________________ AF9530* NAD 83(CORS)- 55 04 08.64571(N) 131 35 58.25572(W) ADJUSTED AF9530* NAVD 88 - AF9530 ___________________________________________________________________ AF9530 EPOCH DATE - 1997.00 AF9530 X - -2,430,153.192 (meters) COMP AF9530 Y - -2,737,194.036 (meters) COMP AF9530 Z - 5,205,816.345 (meters) COMP AF9530 ELLIP HEIGHT- 32.21 (meters) GPS OBS AF9530 GEOID HEIGHT- -5.86 (meters) GEOID96 AF9530 AF9530 HORZ ORDER - SPECIAL (CORS) AF9530 ELLP ORDER - SPECIAL (CORS) AF9530 AF9530.ITRF positions are available for this station. AF9530.The coordinates were established by GPS observations AF9530.and adjusted by the National Geodetic Survey in April 1996. AF9530.The coordinates are valid at the epoch date displayed above. AF9530.The epoch date for horizontal control is a decimal equivalence AF9530.of Year/Month/Day. AF9530 AF9530 AF9530.The XYZ, and position/ellipsoidal ht. are equivalent. AF9530 AF9530.The ellipsoidal height was determined by GPS observations AF9530.and is referenced to NAD 83. AF9530 AF9530.The geoid height was determined by GEOID96. AF9530 AF9530 SUPERSEDED SURVEY CONTROL AF9530 AF9530 NAD 83(CORS)- 55 04 08.64571(N) 131 35 58.25572(W) AD(1996.00) c AF9530 ELLIP HT - 32.21 (m) GP(1996.00) c c AF9530 AF9530.Superseded values are not recommended for survey control. AF9530.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums. AF9530.See file dsdata.txt to determine how the superseded data were derived. AF9530 AF9530_STATION IS THE ANTENNA REFERENCE POINT OF THE GPS ANTENNA AF9530 AF9530 STATION DESCRIPTION AF9530 AF9530'DESCRIBED BY NATIONAL GEODETIC SURVEY 1996 AF9530'STATION IS A GPS CORS. LATEST INFORMATION INCLUDING POSITIONS AND AF9530'VELOCITIES ARE AVAILABLE IN THE COORDINATE AND LOG FILES ACCESSIBLE AF9530'BY ANONYMOUS FTP OR THE WORLDWIDE WEB. AF9530' FTP CORS.NGS.NOAA.GOV: CORS/COORD AND CORS/STATION_LOG AF9530' HTTP://WWW.NGS.NOAA.GOV UNDER PRODUCTS AND SERVICES.
29. What is a coordinate file?
A "coordinate" file for a CORS contains the officially adopted position and velocity for the site's antenna reference point (ARP) as well as for its L1 phase center. Hence, this file is also called a "position-velocity" file. This file may also contain officially adopted positions and velocities for selected geodetic monuments located near the CORS. Note that the ARP usually corresponds to the center point on the bottom of the pre-amp on the CORS antenna.
A coordinate file presents positions and velocities in both the International Terrestrial Reference Frame (ITRF) and the North American Datum of 1983 (NAD 83). Also, it presents positions in both geocentric cartesian coordinates (X, Y, Z) and in geodetic coordinates (latitude, longitude, and ellipsoidal height) for the GRS80 ellipsoid. NAD 83 positions given in these files are identical to those contained in the National Geodetic Survey's Integrated Database.
30. Can you explain the benefit of "Interpolation"?
The National Geodetic Survey (NGS) provides GPS observation data at certain sites with a 30 second sampling rate. This rate may too slow for some applications. When a non-default sampling rate is requested, NGS will use the 30-second data to generate a new RINEX-format file with interpolated GPS satellite observations at the user-selected rate. New code range (pseudorange), Doppler, carrier phase, and receiver clock offsets (if available) are interpolated. Error of interpolated code range data seldom exceeds 10 cm. Due to the possibility of cycle slips, interpolated carrier phase must be considered experimental.
For more detailed discussion of the accuracy of interpolated data, the applicability of interpolated data to positioning, and the interpolation algorithm, please refer to the user documentation for the interpo program.
ftp://www.ngs .noaa.gov/cors/utilities/Dos/interpo.txt
31. Where can I find out more about a site? What kind of receiver/antenna? Who is the responsible agency for that site? Logfile!
We have that information in the site's station log. Enter the 4 char Id in the box in the upper left of this page.
32. What are the contents and format of a Met data sheet?
Each Met file basically contains data from one met site and one session. The meteorological data for a particular site consists of the Pressure(PR) in millibars , Dry temperature (TD) in degree Celsius , the relative humidity (RH) in percent and the Wet zenith path delay (ZW) in millimeters. The above observations are carried out with the Met Sensor. The details of the Met Sensor such as the name of the manufacturer, the type and the accuracy are also included in the Met Data sheet.
The Met file consists of a header section and a data section. The header section contains global information for the entire file and is placed at the beginning of the file. For details of the actual format as well as examples, click on Rinex-2.11 format.
33. I have experienced some trouble retrieving gzip compressed files from your site. I download it but when winzip runs it says it is not in gzip format. Any ideas?
When I click on the file that is online and gzipped, my PC responds with the program "Winzip" and opens it up. When I then save it to my disk, it saves it WITH the .gz extension. What I should have done is save it with another name and not let the PC decide for me.
For example, I download gait1230.99o.gz , Winzip opens it then asks me to save it to disk.... Do you want to save file gait1230.99o.gz? I should save it as gait1230.99o (since it is already uncompressed). If I save it with the .gz ext. Winzip tries to unzip a file that is already unzipped....
34. I use WINZIP to uncompress your files. Last year it worked fine but since the new year (2000), my software tells me your data is from a different year like 1970 or 2007. Any ideas?
The data are not corrupt. This irritation is a WinZip Y2K problem and only affects the time stamp of the file and not the data.
- From WinZip Technical Support
This is known behaviour with the current version of WinZip and should be
addressed in a future release. WinZip is simply using the wrong date
format when performing the conversion, resulting in dates in the 2007-2008
range.
35. Can I improve the position I get from my handheld GPS unit?
This question generally refers to those handheld GPS units designed for outdoor activities, such as, cycling, orienteering, and hiking which are in the $500 or less price range.
When a position is determined using one GPS receiver without the benefit of corrections from a secondary source, it is known as point positioning; that is, the GPS receiver is working in autonomous mode. Due to various error sources, the largest WAS the Department of Defense (DOD) intentional signal degradation (i.e., selective availability - SA), the typical accuracy is (was) usually better than 100 meters of the true position. Also, by averaging several readings your accuracy may be improved.
To increase the accuracy of autonomous GPS, differential corrections can be applied to the GPS observations. Differential corrections eliminate or reduce errors associated with the measurements made to each satellite (pseudo-range) that was used in determining the point position. There are two common methods for applying these corrections; real-time differential GPS and post-processed differential GPS. Both methods require information from a secondary source (e.g., a second GPS receiver located at a known point).
Real-time differential GPS requires a radio link between the rover receiver (i.e., your handheld unit) and a base receiver (i.e., the GPS receiver located at a known point). Via this radio link corrections to the pseudo-ranges are made before the point position is computed. Once the position is computed all the pseudo-ranges and pseudo-range corrections are not usually saved. Even if this information were saved, it would be somewhat inappropriate to further correct the data. Depending on the equipment, distance from the base station, and a host of other variables your positional accuracy can be improved to the sub-meter level.
Post-processed differential GPS requires that GPS data be collected simultaneously at both the rover and base receivers. These data must include pseudo-range measurements to common satellites and precise time stamps for each measurement. After the measurements are made, the base and rover station data are combined via a computer with post-processing software and corrections are applied to the rover data. Depending on the equipment, the distance from the base station, and a host of other variables; this method can also improve your positional accuracy to the sub-meter level.
For both real-time and post-processed differential GPS, you are required to have additional hardware. Real-time GPS requires that your handheld GPS receiver includes an attachment capable of receiving radio signals containing differential corrections. Post-processed GPS requires that your receiver can output appropriate data in a proper format (e.g., RINEX format). Post-processing also requires software that will allow you to combine data from the rover and base stations, and a base station collecting appropriate data in a proper format.
A general rule of thumb is that if a receiver is real-time or post-processed capable, then the manufacture will make accessory equipment (e.g., real-time rover radio receiver) and software (e.g., post-processing software) available, although usually at an additional cost. The good news is that numerous base stations, supporting real-time and/or post-processed, differential GPS are available throughout the United States. The U.S. Coast Guard provides real-time differential correction free of charge to anyone within the range of their broadcast beacons. Also, the National Geodetic Survey provides GPS data for post-processing activities from about 900 continuously operating reference stations (CORS) throughout the United States. Users may freely download these CORS data via the Internet at http://www.ngs.noaa.gov/CORS
36. When I post-process GPS phase data with my manufacturer's software, which coordinate should I use for CORS -- the L1 phase center (L1PC) or the Antenna Reference Point (ARP)?
You should use the coordinate and the method that minimizes the confusion and the amount of work required of you. Most people find that the ARP coordinate fits that description.
37. Is there a list of all the nationwide Differential GPS (DGPS) sites, operational, planned and proposed?
Visit the U.S. Coast Guard Navigation Center's Page
38. I think there is a problem with some of the rinex files for several cors stations. The files seem to have trash in them that is causing many errors in my software.
Is this what you mean??4 1 original post-header comments being skipped COMMENT
These lines ARE acceptable according to rinex standards. We are now joining our hourly rinex files with the program "Teqc" developed by UNAVCO. The "4 1" says files have been joined and there is 1 comment line to follow. We do suppress several lines of comments with some options but this is the best we can do. If this is causing your software to blow up, you must correct your software. Check Rinex-2.11 description for more information. Specifically Table A2, EVENT/epoch FLAG.
39. I've noticed that one of your sites has varying coordinates for the base station. The Rinex file displays one coordinate while the property box in my manufacturer's software displays another with the WGS as a reference frame. I thought I could find the correct coordinate by visiting the CORS website and looking at the data sheet, but that gives me another coordinate. Which is correct?
We supply 2 positions for our CORS users. The L1 Phase Center (L1PC) and the center of the bottom of the antenna which we call the antenna reference point (ARP). We supply these positions in 2 reference frames, NAD83
(epoch 2002.0) and ITRF00 (epoch 1997.0) . We do not use the WGS84 to position points. You can read more about WGS here ......
http://www.ngs.noaa.gov/CORS/Articles/Reference-Systems-Part-3.pdf
Here is the file (coordinate file) which shows the positions for EPRT
ftp://www.ngs.noaa.gov/cors/coord/coord_00/eprt.
You can get any coordinate file by changing the site_ID in that link.
The datasheet will only show positions in NAD83 (not ITRF) and may show other control marks. The datasheet lists each mark with a PID. The PID for EPRT's L1 phase center is AH5047. The PID for the ARP is AH5046 . roundoff error of about 1 mm)
Consider the coordinate in the rinex file header and station logs as APPROXIMATE. Always use the Coordinate file (Position Velocity file) position. It is the policy of NGS to overwrite the appoximate position in the header with the published NAD83 position, but some older files may not have these corrections.
40. I am never able to get OPUS to process my rinex files. Is it the data, the rinex converter, the receiver, or what?
Visit the OPUS Faq web page
41. We are looking at installing CORS sites around the county and we would like to know about the different ways the antennas can be mounted. We have a variety of places to put them such as on buildings and in open areas. We would like to know if there are any types of standard mounts that we should use.
Check out the CORS Guidelines at http://www.ngs.noaa.gov/PUBS_LIB/CORS_ guidelines.pdf and http://www.ngs.noaa.gov/CORS/Adaptors
42. What are the criteria for recomputing CORS coordinates?
Visit http://www.ngs.noaa.gov/INFO/Policy/cors_coordinate.htm
43. I noticed that the NGS and IGS precise ephemeris files are now both 15 day latency. Is this a permanent situation? I just wanted to confirm that 8 day files are no longer available.
This is true. NGS no longer produces the orbit with overlap of data on both sites of the day. The NGS precise orbit is in exactly the same format as the IGS orbit. Since NGS is a contributor to the IGS, you should use the IGS precise. If that is not available, use the IGS rapid (igr).
44. I downloaded one of the precise orbit files (igr13600.sp3c.gz) from the NGS website and was wondering at the format of the data. The SP3 format link states that the data is either position or velocity, which is indicated by either a "P" or a "V" at the beginning of each line, followed by 4 values. This file has a "PG" at the beginning of each line. I assume that this is position. However, there are 4 integers after the clock bias (or drift for velocity) term. What are these numbers? I was also wondering what the units are when the data is velocity. The SP3 format link gives the units for position data only.
You are looking at the new format which we (NGS) label "sp3c" for the time being. This orbit file will include glonass satellites. The P represents position, the G represents GPS satellite and R for glonass. Eventually, like the IGS, this "sp3c" will be the new "sp3" file.
Here is a line from an "sp3c" file showing "base" numbers...
%f 1.2500000 1.025000000 0.00000000000 0.000000000000000
1.25 is the base number for sdevs of X, Y, and Z
1.025 is the base number for sdev of the clock offset
| kilometers from earth center of mass | microsecs | (base**n mm) base is 1.25 | (base**n picosecs) base is 1.025 | |||||
|---|---|---|---|---|---|---|---|---|
| Position GPS PRN1 | X | Y | Z | PRN clock offset | X sdev n | Y sdev n | Z sdev n | clock sdev n |
| PG01 | -17277.867518 | -15136.238599 | 13567.996481 | 44.522498 | 5 | 8 | 8 | 149 |
If we had a program to convert sp3c to add velocity then the epoch showing velocity would look like ....
| Velocity (dm/sec) | microsecs/sec | (base**n * 10**-04 mm/sec)
base is 1.25 | (base**n* 10**-04 picosecs/sec) base is 1.025 | |||||
|---|---|---|---|---|---|---|---|---|
| Velocity GPS PRN1 | X | Y | Z | PRN clock offset | X sdev n | Y sdev n | Z sdev n | clock sdev n |
| VG01 | 20298.880364 | -18462.044804 | 1381.387685 | -4.534317 | 14 | 14 | 14 | 191 |
Check out .... http://igscb.jpl.nasa.gov/igscb/data/format/sp3c.txt for a description of the "new" sp3 ( sp3c ) format.
45. Request timed out. I cannot connect to your ftp server.
Unexpected Closed Connection when attempting to retrieve CORS data via FTP
This may be indicative of an issue with your local DNS configuration. NGS does not block any IP addresses for FTP access. One thing that can be used to test connectivity is a proxy server ( such as www.anonymouse.org ).
Then go to: ftp://ftp.ngs.noaa.gov/
If that works it is most likely a local DNS configuration problem. If the Internet Service Provider has not configured their Domain Name Server to have a matching PTR and an A record in DNS, the FTP server may close the connection.
Another link to help describe the problem......
http://oit.nd.edu/email/dnsmismatch.shtml
46. You insist that all antennas must be oriented toward True north but some antennas like Brand X mention that there is no need to orient their antenna towards north.
We agree that the Brand X antennas are fairly symmetric but we still require the level and orient device. Basically the environment around the antenna will bias this calibration a bit and we want to ensure that over time other antennas which may or may not be symmetric, have the correct phase center applied, otherwise we can get a 1+ cm of distortion. In addition the orient and level device allows the antenna to always be returned to the same position in 3-D space on the antenna which is critical.
47. I see in the directions for filling out a station log that it asks for
Alignment from True N : (deg; + is clockwise/east)
How do I get this information. I am in Flemington, NJ and my coordinates are
40 30 6 N
74 54 4 W
Here is the value for your coordinates as computed at ....
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
Declination = 12º 32' W changing by 0º 1' E/year
So if you put the antenna aligned to magnetic north then the log file should read -12.53 deg . The best solution is when first putting up the antenna, to always align it with true north, that is, align it with magnetic north then turn it (at this location) 12.53 degrees clockwise. However, it is best not to touch the antenna once it is set it place and collecting data.
48. Under Antenna Reference Point for CORS info - To what is BPA referring?
For over 95% of the antennas in use for geodetic surveying, the BPA (Bottom of PreAmp) is the same as the ARP (Antenna Reference Point), or the bottom centermost point on the antenna, or where the tripod mounts to the antenna. All of our antenna calibrations are referenced to this point. For a list of antenna diagrams, see http://igscb.jpl.nasa.gov/igscb/station/general/antenna.gra .
49. Should I add a leveling/orienting device to my existing National CORS site even though I did not have one when it was first accepted?
NO!
Your site is an existing site with a published coordinate
for the ARP (antenna reference point or the bottom of
the antenna). When the ARP is relocated in 3-D space,
such as either by reconstructing the monument that supports
the antenna or by adding an adapter between the monument
and the antenna, then the prior location in 3-D space has been
destroyed. The antenna will have moved too much to simply
ignore the movement and continue to use the same published
coordinate.
When your site was accepted, it was not required to have an orientation device such as a Seco adapter. You could simply spin a replacement antenna onto an existing bolt until the antenna was tight.
The action of spinning an antenna onto a bolt is inferior, however, in that the installed antenna is likely aligned or pointed to some random azimuth based on the count and orientation of the male treads on the bolt and the female threads inside the antenna. When an antenna is pointed in azimuth other than true north, the calibration fails enough such that the differencing of the L1 and L2 frequencies fails to provide an accurate measure of the amount of water vapor in the atmosphere, which affects the refraction or bending of the signals. Incorrect ellipsoid heights of up to several centimeters are the direct result.
Antennas are calibrated when they are oriented to true north--or in alignment with the reference frame of the orbiting satellites.
If software is to correctly remove the refraction of the signals as they pass through the water vapor in the atmosphere, your antenna must likewise be pointed to azimuth true north--same as that expected by the calibration.
For this reason, NGS now requires that all new sites include an orientation device, such as a Seco adapter.
That means new sites only, not existing sites.
If you retro-actively install a Seco adapter on an existing site, you destroy the existing 3-D point in space, requiring the decommissioning of the prior site.
No, you cannot alter the monument of any existing CORS. You cannot add or remove a Seco adapter or any device or spacer after a CORS has been established with this agency. You cannot move or shift the pipe or monument in any way for an existing CORS. The ARP of the original antenna and of all replacement antennas must be installed at the exact same point in 3-D space relative to the Earth's center of mass for all RINEX data archived at this agency.
50. What's the story with AIS5 and 6? Is the problem with all of the U.S. Coast Guard sites? Which is the right station to use? Is the station currently operational? When will data be available for downloading for differential correction?
To ensure safety of life operations, the US Coast Guard operates two independent sets of GPS equipment at each of its eighty-plus installations. The locations broadcast pseudorange correctors in real time for maritime navigation, but as a convenience also route GPS data to this agency for applications that correct the observables through post-processing methods.
The two towers are normally separated by 30 meters. Atop each tower is a GPS reference station antenna and an identical integrity monitor antenna, separated by about three feet. Cables from the four antennas (two on each tower) are routed inside a building where the electronics are stored with environmental controls and generator backup.
Many of the eighty-plus USCG installations have operated for a dozen years under the Ashtech brand of GPS equipment, a company that has been bought and sold at least twice during those years. The USCG currently is modernizing their equipment from Ashtech to Trimble. The first equipment to be replaced is the antennas, because they have suffered the most from exposure to the elements.
When the Ashtech antennas are removed, a stainless steel post and mounting apparatus are added to accommodate the differently sized Trimble antennas. The added post and mount substantially change the location in 3-D space of the antenna reference point, especially in height, so we (NGS) rename the data to reflect the change in location, with odd-numbered sites such AIS1 becoming AIS5 and even-numbered sites such as AIS2 becoming AIS6.
For our own continuity of operations, this agency (NGS) usually collects data from the odd-numbered USCG sites at our headquarters in Maryland. Data from the even-numbered sites are collected by our bare-bones backup computers in Colorado. We hope someday to have sufficient resources to collect data from odd and even USCG sites at both of our computer facilities, but that remains impossible today.
Polished web-distribution utilities such as User Friendly CORS are unavailable from our bare-bones Colorado computers.
You may download data from Colorado for AIS6 by anonymous file transfer protocol (FTP): >ftp alt.ngs.noaa.gov (web address) >anonymous (login) >your email address (password) >bin (binary format transfer) >cd /bcors/rinex/2008/161/ais6 (change directory) >get ais61610.08o.gz (request the file) >quit (logoff, also "bye")
The FTP method with its typed commands is more convenient for data retrieval by automated software than by human "clickable" interaction.
Alternatively, you may simply point your web browser to URL (uniform resource locater): ftp://alt.ngs.noaa.gov/bcors/rinex/2008/161/ais6
On the day of observations, RINEX files are accumulated hourly (a,b,c,...) beginning at 00:00 UTZ. At the end of the day, the string of hourly files is combined into a daily file and the hourly files are deleted.
On the day following the day of observation, two versions of the 24-hour daily file are available. The file with compression extension "gz" contains 24 hours of data at the native collection rate, such as 5-second epochs. The file with compression extension "g.Z" is double compressed and decimated to the 30-second rate, for quicker data transmission although the double compression adds an additional layer of complexity upon receipt.
The Annette Island location is remote. The USCG is aware that data transmission from AIS5 has failed. According to the USCG, the data logger is not connected.
A wealth of information is available to prove that a reasonable choice of CORS is immaterial for most post-processing applications. For example, the method of correcting pseudorange observables that is employed by Trimble's Pathfinder software will yield rover coordinates that are indistinguishable within the error allowances of the method (~1m) for any CORS within 300 kilometers (180 miles) of the rover location. That means that Biorka Island (BIS1) and Level Island (LEV5) are equally applicable, if you prefer access to our "clickable" web-distribution utilities such as User Friendly CORS. Additional sites near Juneau (JNU1) and Gustavus (GUS2) are likely to betray a slight (~1-2 feet) degradation in rover positioning, because the Pathfinder method is based on the simultaneous arrival of the signals at the speed of light at both the base and rover locations.
51. We are establishing a CORS with RTK and we would like to keep the RTK antenna near the CORS antenna. Will this cause a problem to becoming an NGS CORS?
Yes. This potential site will be rejected.
The RTK antenna needs to be moved far enough away so that it doesn't block signals from the satellites. The guidelines state that the distance should exceed 3 meters or farther to eliminate blockages above 0 degrees.
Read section B.2 of the guidelines.
The satellites travel at 4 km/sec, thus a blockage will not be apparent in a 24-hour file if the ground-based antenna phase centers, the RTK broadcast antenna, and the fast-moving satellites are never in alignment at the precise epoch of sampling. A higher sampling rate increases the odds of alignment and therefore of being evident as a cycle slip. The blockage is still there, but it just isn't apparent given the speed of the satellites and the slowness of the receiver's sampling rate. Slips may not occur during a week or even a month until the 4-minute advance in sidereal versus clock time eventually allows the alignment to occur and repeat at regular intervals before again "disappearing." The blockage is thus perceived as transitory, although, if the receiver's sampling rate were continuous rather than episodic, the blockage would be readily in evidence any day and every day.
In the method of double-differences to remove clock errors, a cycle slip at a CORS will result in a data loss for every GPS receiver involved. OPUS-RS uses up to nine CORS to model the ionosphere and atmosphere at the rover--a total of ten sites. A 15-minute OPUS-RS session results in only 30 total epochs at the 30-sec sampling rate, thus you throw away 3.3 percent of the data per baseline for every slip.
Slips are a non-trivial fault when observing sessions become shorter and shorter, which explains why manufacturers are ever hungry for more and more satellites, such as Glonass and Galileo. Slips at CORS are costly indeed.
The frequency of the RTK broadcast is not expected to interfere with the GPS frequencies, else the manufacturer would have shot himself in the foot. UNAVCO, however, does report interference with the Trimble R9 and Leica GR-10 receivers and Iridium communication modems because of the increased spectral wavelength of these new all-constellation receivers. They find interference at a distance of more than 35 meters. See:
Update: GPS/GNSS Interference From Iridium Data Transmissions
