[IGEXMail-0052] First Precise GLONASS Orbits from CODE

[rothacher @ ubeclu.unibe.ch (MARKUS ROTHACHER)]

Precedence: bulk

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IGEX Electronic Mail    30-Oct-1998 18:07:59                   Message No 0052
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Author: rothacher @ ubeclu.unibe.ch (MARKUS ROTHACHER)
Subject: First Precise GLONASS Orbits from CODE



Author: Markus Rothacher

Dear Colleagues,

we would like to announce that the first GLONASS precise orbits 
have been computed at the CODE Analysis Center using the data of the 
global IGEX network. The analysis was done by fixing the GPS orbits and Earth
rotation parameters to the CODE final GPS solution and estimating orbital
parameters only for the GLONASS satellites using double differences (including
double differences between GLONASS and GPS satellites). The orbits
were computed for GPS week 0979 (the week before the official start of IGEX)
and stem from overlapping 3-day solutions with 3-day arcs.

The GLONASS/GPS precise orbits are available at the anonymous ftp area of CODE
and can be downloaded for tests as follows:

       ftp 130.92.6.17    (-or- ubeclu.unibe.ch)
       userid: anonymous
       passwd: "your e-mail address"
       cd aiub$ftp
       cd code
       get corWWWWD.eph    (WWWW: GPS Week, D: Day of GPS Week)
       bye

The orbits are given in the SP3 format (modified for GLONASS according to 
IGEXMAIL #0039) and refer to the ITRF96 system. The satellite clock
values included in the precise orbit files are broadcast clock values for
GLONASS (no improvement done so far) and precise CODE estimates for GPS
satellites.

Although only a very few IGEX station were operational at this time, the orbits
are of a quality of about 30 cm, a factor of 8 better than the GLONASS
broadcast orbits. The quality has been assessed using two approaches:

  3-Day Fit:
     An rms of about 20-30 cm is obtained when fitting a 3-day arc through 
     the satellite positions contained in three consecutive SP3 files 

  SLR O-C Check:
     We computed the O-C (observed-computed) values of the SLR 
     data to the GLONASS satellites for this week. The corresponding rms
     is about 30-40 cm.

The table below summarizes the SLR quality check of GLONASS orbits obtained
from the broadcast messages and computed using the GLONASS microwave data:

     DOY     RMS O-C     RMS O-C    
            Broadcast   Microwave

     284       1.71        0.37
     285       2.44        0.39 
     286       1.78        0.38
     287       1.85        0.43
     288       1.55        0.49
     289       1.38        0.26


From now on we will try to deliver precise GLONASS orbits on a weekly basis
with a delay of about 2-3 weeks.

Any feedback about the quality of the orbits (e.g. from regional GLONASS
processing) would be very welcome. We would like to thank the operators
of the IGEX sites and the SLR groups for their efforts.

                                              Markus Rothacher
                                              Tim Springer
                                              CODE Analysis Center

--------------------------------------------------------------------------------
Dr. Markus Rothacher        Phone  : ++41-31-631-8591          
Astronomical Institute      Fax    : ++41-31-631-3869
University of Berne         E-mail : markus.rothacher @ aiub.unibe.ch 
Sidlerstrasse 5             AFTP   : ftp ubeclu.unibe.ch (130.92.6.17)
CH-3012 Berne                        cd aiub$ftp after login         
Switzerland                 WWW    : http://www.cx.unibe.ch/aiub/