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Binary Ephemeris File

Overview

The principal product of arc is the GPS satellite ephemeris. Historically called the "u_file", this is a binary file to minimize file size and time reading this file. arc is capable of generating strictly the ephemeris or, as it is more commonly used, the ephemeris with the appropriate partial derivatives for adjusting the ephemeris.

The u_file begins with a few lines of descriptive information which includes:

1. information about the ephemeris itself:
   • an 80 character descriptive string,
   • the integer year, month and day of the initial conditions used to generate this ephemeris,
   • the double precision hour, minute and second of the initial conditions,
   • the integer year, month and day of the ephemeris' starting epoch,
   • the double precision hour, minute and second of the starting epoch,
   • the integer year, month and day of the ending epoch,
   • the double precision hour, minute and second of the ending epoch,
   • the double precision tabular interval, in seconds, of the ephemeris,
   • the integer number of epochs in the ephemeris,
   • the double precision UT1 - UTC value in seconds,
   • the double precision X pole value in arcseconds,
   • and the double precision y pole value in arcseconds.
2. information about the satellites and the initial conditions:
   • three 80 character comment lines,
   • the integer number of satellites contained in this ephemeris,
   • the integer number of terms (coordinates and partials) for each satellite entry,
   • and in sets, one for each satellite in the ephemeris:
     • a 16 character name for each satellite,
     • the initial condition vector.

   This is followed by the epoch-by-epoch information of the ephemeris. Each line contains all terms for all satellites. No time or descriptive information is include on these lines since all pertinent information was given in the header lines. All values on these lines are double precision and all terms for a satellite are grouped together. Remember that all information for each epoch is held on a single binary line. The FORTRAN pseudocode to read one of these lines would look something like

         read( lu ) (( vector(i,j), i= 1, "terms per satellite",), j= 1, "satellites")
   

   The terms are, in this order:
   1. X
   2. Y
   3. Z
   followed by the partials, if requested:
   4. parital of X with respect to X0,
   5. parital of X with respect to Y0,
   6. parital of X with respect to Z0,
   7. parital of Y with respect to X0,
   8. parital of Y with respect to Y0,
   9. parital of Y with respect to Z0,
   10. parital of Z with respect to X0,
   11. parital of Z with respect to Y0,
   12. parital of Z with respect to Z0,
   13. parital of Xdot with respect to X0,
   14. parital of Xdot with respect to Y0,
   15. parital of Xdot with respect to Z0,
   16. parital of Ydot with respect to X0,
   17. parital of Ydot with respect to Y0,
   18. parital of Ydot with respect to Z0,
   19. parital of Zdot with respect to X0,
   20. parital of Zdot with respect to Y0,
   21. parital of Zdot with respect to Z0,
   22. parital of radiation pressure term 1 with respect to X0,
   23. parital of radiation pressure term 1 with respect to Y0,
   24. parital of radiation pressure term 1 with respect to Z0,
   25. parital of radiation pressure term 2 with respect to X0,
   26. parital of radiation pressure term 2 with respect to Y0,
   27. parital of radiation pressure term 2 with respect to Z0,
   and so on for all remaining state vector components.

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   u_file.html
   January 14, 2000
   Steve Hilla