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Downloading a Precies GPS Ephemeris

A source for satellite coordinates is required in all GPS data processing. In most projects involving only shorter baselines or having lower accuracy requirements, a broadcast navigation file is adequate. However, when longer baselines are used or any time higher accuracies are required, one should use a precise ephemeris. Typically, this precise ephemeris will be in the SP3 format.

The SP3 format is the distribution standard for the IGS and a 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.

Tutorial

The IGS products are stored at the IGS global data centers. The date arbitrarily selected for these tutorials is February 2, 2001. GPos provides tools to convert between year-doy and week-dow, and those tools will be used in this example.

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.
 
2009-02-15 18:41 UTC