The National Geodetic Survey Standard GPS Format SP3 is described and documented in the following links:
ftp://igscb.jpl.nasa.gov/igscb/data/format/sp3.txt
ftp://igscb.jpl.nasa.gov/igscb/data/format/sp3_docu.txt

Description of the sp3c format is given in:
ftp://igscb.jpl.nasa.gov/igscb/data/format/sp3c.txt



                      Exchange File for Precise LEO Orbits
                ************************************

                      Proposal 2 February 2001

                           Werner Gurtner


We propose to use the SP3 exchange format, well-known within IGS and used to
distribute the IGS orbit products.


The original format has been modified in 1998 for the IGEX campaign to allow
the inclusion of GLONASS satellites. The modifications were done in
cooperation with the author of SP3, Ben Remondi.

The modifications were desribed in the document

  ftp://igscb.jpl.nasa.gov/igscb/data/format/sp3_glon.txt

also distributed as IGEXMail 0039.

The following modifications were done:

1) Preceed the two-digit satellite numbers by a satellite system character
   (G form GPS, R for GLONASS)

2) Define a formerly unused character field to contain a file type character
   (G for GPS only, R for GLONASS only, M for mixed data)

3) Include a time system indicator in a formerly unused character field
   (GPS for GPS time, UTC for UTC time

4) Change the format version character from "a" to "b"


To store LEO satellite orbits in SP3 files we have to define

a)  A LEO-specific satellite system character. Proposal: L

b)  A 2-digit number for each LEO

    -----------------------------------------
    Satellite             Cospar          SP3
    -----------------------------------------
    TOPEX-POSEIDON        92052A          L01
    GPS/MET               95017C          L02
    CHAMP                 00039B          L03
    SAC-C                 00075B          L04
    GRACE                   ...           L05
    -----------------------------------------

    This Table should be maintained by the IGS Central Bureau.

c)  The interval of subsequent epochs