Subject: [IGEXMail-0045] Remarks on IGEX Data Processing
From: [email-masked] (MARKUS ROTHACHER)
Date: 28 Oct 1998 18:54:04

Precedence: bulk

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IGEX Electronic Mail    28-Oct-1998 18:54:04                   Message No 0045
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Author: [email-masked] (MARKUS ROTHACHER)
Subject: Remarks on IGEX Data Processing

Author: Markus Rothacher, Werner Gurtner

Dear Colleagues,

During the first analyses of IGEX data we detected a few things that might 
be of interest to IGEX participants:

  (1) In our pseudorange processing in the single station mode we determine
      
        station position:           once per session
        receiver clock offsets:     once per epoch
        GLONASS-GPS time offset:    once per session

      using the broadcast satellite orbits and clocks.

      The results for the GLONASS-GPS time offsets typically look like this:

      ***************************************************************
      GLONASS/GPS TIME OFFSETS:  DAY 298, 1998
      ***************************************************************

      FILE  STATION NAME      RCV    TIME OFFSET (NS)  RMS ERROR (NS)
      ---------------------------------------------------------------
        1   BORG 12205M004    R100      1036.915         0.796
        2   CSIR 30310M001    R100      1045.006         0.860
        3   DLFT 13502M006    GG24        90.755         0.736
        4   DLRA 14208M002    R100      1038.114         0.809
        5   GODZ              Z18         88.031         0.809
        6   IRKZ              Z18         85.250         0.758
        7   KHAB              Z18         84.824         0.777
        8   KR0G 10422M001    Z18         69.476         0.714
        9   LINR              R100      1049.974         0.831
       10   MDVZ              Z18         84.844         0.739
       11   METZ 10503M005    Z18         84.485         0.719
       12   REYZ 10202M003    Z18         98.258         0.733
       13   WTZG 14201M012    R101      -713.787         0.873
       14   ZIMZ 14001M005    Z18         85.734         0.755
      ---------------------------------------------------------------
                            
      Although there are still systematic biases in these results (broadcast
      orbit errors, satellite clock errors, ionospheric delays, ...) of the
      order of a few 10 ns, we clearly see, that the R100 receivers show
      differences between GLONASS and GPS time, which are about 1 microsecond 
      larger than those of the Ashtech receivers.

      Currently the correction of UTC(SU) to UTC are of the order of 180 ns
      according to a recent Bulletin T. The broadcast corrections from GLONASS
      system time to UTC(SU) are of the order of -350 ns giving roughly
      a system time difference GLONASS-GPS of +170 ns (the corrections of GPS 
      system time to UTC are of a few ns only).

      The ADDITIONAL offsets estimated with several Asthech Z18 are between 
      zero and say +100 ns whereas the offsets estimated with R100 instruments 
      are about 1 microsecond larger (around 1100 ns).
 
      This seem to indicate that there is a "GLONASS receiver clock" running 
      in the R100 which constantly differs from the GPS one by 1 microsecond.
                                                               -------------
      Another hint that this may be the case is the remark in the Bulletin T:
                                                                                
       "Daily values of C1 are given in the following table. They are obtained
        as follows: the GLONASS data taken at the NMi Van Swinden Laboratorium,
        Delft, The Netherlands, for highest elevation, are smoothed to obtain  
        daily values of [UTC(VSL)-GLONASS time] at 0h UTC; daily values of C1  
        are then derived from them using linear interpolation of [UTC-UTC(VSL)]
    --> A time correction of + 1285 ns is also applied in order to ensure      
        continuity of C1 estimates on 1997, January 1 (MJD = 50449). The global
        uncertainty of daily C1 values is of order several hundreds of         
        nanoseconds."                                                          
                                                                                
      Van Swinden Laboratory is also using a R100 receiver, and they apply 
      this correction to smoothly continue the values determined by another 
      institution (was it Leeds University? Receiver type?) before January 
      1st, 1997.

      The offset of approximately -700 ns seen at Wettzell might stem from the 
      fact that an older receiver (R-101) is used at this location.

  (2) The Z18 receiver (at least the one running in Zimmerwald) generates
      phase observations the absolute values of which obviously increase
      with time. About two weeks after system start the values don't fit
      anymore into the RINEX format field width F14.3. Please check if your
      converter used to generate the RINEX observation files correctly
      re-adjusts the phase values and does not merely increase the format width.

  (3) There are still site logs missing for several IGEX sites. It would 
      help us a lot if these site logs could be made available soon.
      We also noted that the information (MARKER NAME, ...) given in some 
      of the RINEX files are not correct. Please check the header information
      in your RINEX files (it should be consistent with the site log
      information, too).

                                       Markus Rothacher & Werner Gurtner
                                               CODE Analysis Center

--------------------------------------------------------------------------------
Dr. Markus Rothacher        Phone  : ++41-31-631-8591          
Astronomical Institute      Fax    : ++41-31-631-3869
University of Berne         E-mail : [email-masked] 
Sidlerstrasse 5             AFTP   : ftp ubeclu.unibe.ch (130.92.6.18)
CH-3012 Berne                        cd aiub$ftp after login         
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