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1.Creating A Work Area
2.Downloading GPS Data
3.Downloading an Ephemeris
4.Selecting Sites To Hold Fixed
5.Creating The POM Files
6.Running run_survey
7.Manually Modifying The Processing

Description

A source for satellite coordinates is required. Most commonly, one would use a precise ephemeris. run_survey can use a nav file, an SP1 or SP3 ASCII format file, or a GAMIT binary format file used internally in NOAA for orbit processing (the latter will be excluded from this discussion). The U.S. Coast Guard distributes precise ephemerides in ECF18 binary format but provides programs which convert from ECF18 to SP3. The SP3 format is the distribution standard for the IGS. A standard naming convention for the ephemerides is used: CCCWWWWD.sp3 where
  CCC is a three letter ID for the center which generated the file.
  WWWW is the four digit GPS week number.
  D is the single digit day-of-week (dow): 0 = Sunday, 1 = Monday, ...
Occasionally, an extension of .eph will be used but this is largely avoided because of the possible confusion when selecting a file of the desired format.

In some circumstances, verifying data in the field for example, one may wish to proceed without a precise ephemeris. As mentioned, run_survey can be used to process data with a broadcast navigation file as the source of satellite coordinates. Later, a precise ephemeris can be swapped in with minimal reprocessing.

Tutorial

At the end of the last tutorial, the current directory was "~/tutorial/01_033/".

We will use an IGS precise ephemeris. These IGS products are stored at the IGS global data centers so we'll access the CDDIS again. Tools are available as part of the software distribution to convert between year-doy and week-dow. Scripts are available to facilitate downloading ephemerides.

Convert year-doy to GPS week

gpswk 01 033
1099
Convert year / doy to dow
dow 01 033
5


Download a precise ephemeris from the CDDIS data center. The ephemerides are stored in a different directory than the data so the steps will be similar but not identical to downloading data.

 
  ftp cddis.gsfc.nasa.gov
  Connected to cddisa.gsfc.nasa.gov.

  ... welcome message ...

  Name (...): anonymous
  331 Guest login ok, send your complete e-mail address as password.
  Password: me@my.home.address
  ftp> bin
  200 Type I ok.
  ftp> prompt
  Interactive mode off.
  ftp> cd gps/products/1099
  250 "/gps/products/1099" is new cwd.
  ftp> get igs10995.sp3.Z
  local: igs10995.sp3.Z remote: igs10995.sp3.Z

  ... transfer size and time info ...

  226 Transfer completed.
  ftp> bye
  221 Goodbye.
 

Notes

This task could have been done while connected to the CDDIS in the previous tutorial. It has been separated to better distinguish the different file types required for processing.

The directory "~/tutorial/01_033/" should now contain five files:

ls
brdc0330.01n.Z gode0330.01o.Z igs10995.sp3.Z sol10330.01o.gz usna0330.01o.Z
 
 
2009-02-15 18:41 UTC