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.

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.

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 either a National or Coop CORS a daily solution will be run for a minimum of 12 days.  After that time the coordinates will enter into the NGSIDB and a coordinate file will be produced.

4. How do I get CORS data ?

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.

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"

8. The data I want are not available on the CORS file server. What do I do ?

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 ?

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?

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?

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.

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?

22. What is OPUS and where can I find it?

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.

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 on your system.

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 "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 2cm !

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.

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.

31. Where can I find out more about a site? What kind of receiver/antenna? Who is the responsible agency for that site? Logfile!

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.

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 it 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.

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 th e 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?

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.

                             4  1
original post-header comments being skipped                 COMMENT

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?

40. I am never able to get OPUS to process my rinex files. Is it the data, the rinex converter, the receiver, or what?

Check the rinex file to see if the data in the C1 and the P2 columns contain the same value. They should not be equivalent. The OPUS programs don't know how to handle this, so you should change the label P2 to T2 (T2 has no meaning, so OPUS ignores it), and add a comment line to the header stating what you did and why. This is how it should look (spacing not to scale):

   4  L1 C1 L2 T2                                     # / TYPES OF OBSERV
   4  L1 C1 L2 P2 (original designation)              COMMENT
   C1 and P2 fields contain the same values           COMMENT

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.

42. What are the criteria for recomputing CORS coordinates?

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 files 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

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.

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?

49. 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.