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USING CORS STATIONS WITH TRIMBLE GEOMATICS OFFICE (TGO)

(FOR EASE OF USE, CREATE THE PROJECT IN TGO BEFORE DOWNLOADING – THIS WILL CREATE A “CHECKIN” DIRECTORY WHICH IS WHERE YOU WANT TO SEND THE CORS DATA, BOTH GPS DATA FILES AND NGS COORDINATE SHEETS – TGO ALWAYS LOOKS IN THE “CHECKIN” DIRECTORY FIRST WHEN IMPORTING FILES INTO THE DATABASE.)

To use the CORS stations for horizontal control, execute the following steps:

  1. Log on to the NGS CORS web site ( select the “Maps” options to review and identify the station appropriate for your use. (NOTE: In most cases the CORS stations are significantly distant from your local site – to compensate for the longer distances you should extend your local observation times by at least 1 minute for each kilometer in distance).
  2. After determining the appropriate stations, select the “DOWNLOAD” option. This will bring up selection that allows either the “STANDARD” or “CUSTOMIZED” method. Under the “STANDARD”option you can download hourly files or usually the entire 24 hour file. The “CUSTOMIZED” option is preferable as you can span over several hours or even days but is not available until the day following the observations – the Standard download is usually available with an hour of the closing time frame.

STANDARD DOWNLOAD– this option displays a dialog box listing the available “Sites” (alphabetical by state), the “Option” or data type you required, and the date (BE AWARE THAT THE DATES LISTED ARE GREENWICH, NOT LOCAL). In addition to the GPS data download as described below, this section also allows a “history” of the availability of a particular station – select the “Site” and “Data Availability” to view the reliablility of a CORS site over time.

To download the GPS data, select the “Site”, the “RINEX2” data selection, and the date (the NGS web page holds the CORS data for 30 days, any older data can be obtained by request from the NGS as per the web page instructions). Once you have selected the station and time, select “Find Files” and the screen will display at least two files for the specified date, one with an O extension (this is the raw data), and one with an N extension (this is the ephemeris for the particular data set). You need to download both of these files. NOTE that some CORS stations log hourly (Greenwich time) files, in those cases the observation files will have alphabetical extensions for each hour (a = midnight to 1 am, b = 1 am to 2 am, etc., the file with the 0 (zero) is the 24 hour file).

For example, the 1 hour files for the Table Mountain CORS site in Colorado for 1:00pm to 2:pm MST on May 13, 2022 would have the following name: tmgo133u.02* [tmgo (station identifier)+133 (Julian Day)+u (1pm MDT plus seven hours offset to Greenwich Mean Time equals 20 which equates alphabetically to “u”).02 (the year)+o (the observation or GPS data) or n (the navigation or ephemeris data).

IF YOU CANNOT FIND THE EPHEMERIS (*.0?N) FILE, YOU NEED TO GO TO THE DATA TYPE (“OPTION”) SCREEN AND SELECT “GLOBAL NAVIGATION” – THIS WILL ACCESS AN EPHEMERIS FILE THAT IS VALID FOR ALL CORS STATIONS IN THAT PARTICULAR TIME FRAME. THE GLOBAL FILE FOR MAY 13, 2002 WOULD HAVE THE NAME BRDC1330.02N.

CUSTOMIZED DOWNLOAD – this option asks for the local time, start time, and how many hours of data are required. After selecting these items, select “CONTINUE” and another screen will be displayed offering a selection of CORS stations listed alphabetical by the four character station name as displayed on the map. Select “SUBMIT” and the server will compile a ZIP file for download that contains all the information necessary for processing (the O and N files described above, plus the data sheets (*.ds) and the Precise Ephemeris in SP3 format).

  1. After downloading the above data and ephemeris files (using the Standard Download), select the “Coordinates” option in the data section. This will bring up a data sheet with the station coordinates AND the antenna type -–the antenna type is critical as the software can model the antenna offsets ONLY if the model and type is known. Furthermore, bear in mind that most of the CORS station antennas are surveyed in place so they do not have the normal antenna height – the antenna height should be zero with a “True Vertical”, “Antenna Phase Center”, or Antenna Reference Point (ARP) designation (be sure to use the ARP position of the station). Note the PID number for the station (there are at least two PIDs for each CORS station, be sure to retrieve the “ARP” or monument itself) and go to the NGS data sheet page ( and retrieve the data sheet using the PID option. As above, save the data sheet to the “CHECKIN” directory of your current project.
  2. Once the data (the observation and ephemeris files) have been downloaded, you will have to decompress them for use. These files have been compressed using a UNIX compression algorithm (GZIP). If you have the WinZip utility, it can handle the UNIX format, if not you will also need to download the GZIP utility (also available on the NGS CORS station web page.
  3. After executing the above steps, go to “IMPORT” to bring in the data sheets (using the “NGS data sheet file” option). At this point, edit the station names to suit, note that the symbol for the station will reflect the point quality – most CORS stations will have at least a small triangle to indicate high order horizontal control. Once the data sheets have been imported, import the CORS data itself using the RINEX option. During this process, be sure to edit the point names associated with the CORS GPS data to be EXACTLY THE SAME as the points imported from the data sheets. From this point forward, treat the CORS data as you would your own Trimble *.dat files for processing and adjustment.