Format Modifications for Mixed GPS/GLONASS SP3 Files for the IGEX98 Campaign
----------------------------------------------------------------------------

                             Revised 27-Oct-1998
                                           

For the global IGEX98 GPS/GLONASS campaign we have to slightly modify the SP3
orbit format to allow for GLONASS only or mixed GPS/GLONASS orbit files.

Our proposal which closely follows the RINEX standard is as follows:

1. Satellite System Indicator
-----------------------------

Replace the " nn" by "Gnn" for GPS satellites (nn being the GPS satellite prn)
and "Rnn" for GLONASS satellites (nn being the slot number of the GLONASS
satellite) in all records where the satellite number appears (list of
satellites in the header, position and velocity records).

Satellite list:

+   32   G01G02G04G05G06G07G09G12G14G15G16G17G18G19G20G21G22   <--- Gnn or Rnn
+        G23G24G25G26G27G28G29G31R01R02R03R04R05R06R07  0  0

Positions etc:

*  1998  9 17  0  0  0.00000000
PG01  16258.524750  -3529.015750 -20611.427050    -62.540600   <--- Gnn or Rnn
PG02 -21998.652100  -8922.093550 -12229.824050   -131.326200
PG03 -26019.547600   4809.810900  -2508.578200      3.544600
PG05   7014.950200  21130.960300 -14387.334650     79.692800
   .
   .
   .
PR05  13204.937750 -20485.533400  10794.787000     55.200800
PR06  -1638.431050 -24391.479200  10455.312650      3.690300
PR07   6265.255800 -25687.986950   -753.359000     70.830800

2. File Type                                              (modified: 27-Oct-98)
------------

To better indicate the contents of the file we define the first
one-character spare field in line 13 of the SP3 header as "file type field":

%c ft cc ccc ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc   <--- G, M or R

   ft: "G " for GPS only files                            
       "M " for mixed files
       "R " for GLONASS only files

       - single character left-justified in the two-characters field
       - No default value, either "G ", "M ", or "R " is required

3. Time System Indicator
------------------------

In order to remove ambiguities with respect to the time system used for the
satellite positions (GPS time versus UTC) we define the first three-character
spare field in line thirteen of the SP3 header portion as "time system field":

%c M  cc tsf ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc   <--- GPS or UTC

         tsf: "GPS" for GPS time, "UTC" for UTC time

         - No default value, either "GPS" or "UTC" is required
         - For the IGEX98 campaign we request GPS time for all SP3 orbit files

4. Format Version
-----------------

To distinguish files with these new definitions from old files we change the
format version character in the first header line from "a" to "b":

#bP1998  9 17  0  0  0.00000000     126     d ITR96 FIT  COD   <--- b

This proposal has been reviewed by Dr. Ben Remondi, the primary author of the
SP3 format. Unfortunately no feedback could be obtained from the co-author of
the SP3 format documentation (see ftp://igscb.jpl.nasa.gov/igscb/data/format)
till now.

We plan to have this proposal also accepted by IGS for future use.

Best regards,

Werner Gurtner                                    Markus Rothacher
IGEX98 Steering Committee                         CODE Analysis Center