[IGSMAIL-0906] Wk 0782-785 IGS Final Orb.

[AC]

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IGS Electronic Mail      Tue Mar 14  8:03:33 PST 1995      Message Number 0906
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Author: AC Coordinator
Subject: Wk 0782-785 IGS Final Orb.


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IGS Final orbits/clocks completed and put onto CDDIS (Weeks 782 - 785)
                                                     (Jan. 01 - Jan. 28, 1995)
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According to the recommendations of the Analysis Centre Workshop the IGS
Rapid Orbits were recomputed using up to date orbits and the final EOP values
of Bulletin B, published by IERS. ( Daily values, page 2 of Bull. B )
The EOP were corrected for the difference between the IERS pole series 
and ITRF as published in the IERS 1993 Annual Report (p. II-19, Table II-3),
so both the IGS orbits and the associated ERP files are consistent with ITRF93
(current epoch).

We have downloaded all the submitted ephemeris and ERP files from CDDIS before
the final orbit/clock combination to ensure that the sets are complete and up
to date. In the final combination we have largely followed the actions taken
during the Rapid orbit generation and since there were very few orbit
resubmissions, both the IGS Rapid and Final orbits are virtually the same except
for the the orientation difference (the current Bull.A and the final Bull.B).
Orbit evaluation are also nearly the same, since we used centre specific ERP
in the long arc evaluations. For more information see the corresponding
summary files on CDDIS.

Starting with the GPS week 742 a new clock combination scheme was put in place
for the final orbit/clock combinations. The clock solution alignment and centre
clock weight assignment are based on non SA broadcast  satellite corrections
only (PRN12, 15, 20). This should result in more realistic and objective clock
weighting than employed before, provided that the non SA satellites are
representatives of centre clock solution quality. Starting with the GPS
week 756 the new IGS erp format (see IGSMAIL# 662) is used by all Analysis
Centres and for the IGS erp files. ERP rates, whenever given, are also utilized
in both orbit combination and the long arc evaluation.

PRODUCTS GENERATED:

The resulting final IGS ephemeris and clocks are in daily files in the SP3
format. The sat. orb. accuracy codes displayed in the headers are based on
weighted RMS for each satellite. Note that each file contains all the submitted
satellites. If satellite or centre had to be excluded from the combination it
is noted in the summary file along with the reason for doing so.

The IGS FINAL ORBIT files generated and uploaded into CDDIS @ NASA/GSFC are:

IGS07820-6.SP3   IGS GPS orbit/clock files for days 0-6, week 0782
IGS07830-6.SP3   IGS GPS orbit/clock files for days 0-6, week 0783
IGS07840-6.SP3   IGS GPS orbit/clock files for days 0-6, week 0784
IGS07850-6.SP3   IGS GPS orbit/clock files for days 0-6, week 0785

IGS07827.ERP    EOP associated with the above orbits (Bull. B aligned to ITRF)
IGS07837.ERP    EOP associated with the above orbits (Bull. B aligned to ITRF)
IGS07847.ERP    EOP associated with the above orbits (Bull. B aligned to ITRF)
IGS07857.ERP    EOP associated with the above orbits (Bull. B aligned to ITRF)

IGS07827.SUM    Complete summary/statistics report
IGS07837.SUM    Complete summary/statistics report
IGS07847.SUM    Complete summary/statistics report
IGS07857.SUM    Complete summary/statistics report

The old Rapid Orbit files have been renamed (on CDDIS) to:
(Note this is temporary, based on discussions with AC and GB and may be
changed )

IGR07820-6.SP3   Rapid service orbits/clocks for days 0-6, week 0782
IGR07830-6.SP3   Rapid service orbits/clocks for days 0-6, week 0783
IGR07840-6.SP3   Rapid service orbits/clocks for days 0-6, week 0784
IGR07850-6.SP3   Rapid service orbits/clocks for days 0-6, week 0785

IGR07827.ERP     Rapid service EOP associated with the above orbits (Bull A)
IGR07837.ERP     Rapid service EOP associated with the above orbits (Bull A)
IGR07847.ERP     Rapid service EOP associated with the above orbits (Bull A)
IGR07857.ERP     Rapid service EOP associated with the above orbits (Bull A)

IGR07827.SUM     Rapid service complete summary/statistic report 
IGR07837.SUM     Rapid service complete summary/statistic report 
IGR07847.SUM     Rapid service complete summary/statistic report 
IGR07857.SUM     Rapid service complete summary/statistic report 

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REMARKS/CORRECTIONS:

1. Starting on January 1, 1993 all the IGS products, including the IGS 
   combined orbits, are based on ITRF93. This reference frame change 
   introduces a small discontinuity, mainly in orientation.  More 
   specifically the following changes took effect on Jan 1/95 (day 0, 
   GPS week 782):  (IGS(ITRF93)-IGS(ITRF92); epoch 1995.0; transformation 
   parameters consistent with the IERS conventions)
    
   IGS Orbits:  T1(cm) T2(cm) T3(cm) D(ppb)  R1(mas) R2(mas) R3(mas)
                 -.9   -1.2    .3      0     1.66    .68     .4   
       rates     -.02   -.11   .15     0      .12    .15    -.13
    
   The corresponding erp file changes: (IGSERP(ITRF93)-IGSERP(ITRF92)):
     X(ITRF93)-X(ITRF92)   = -0.68mas
     Y(ITRF93)-Y(ITRF92)   = -1.66mas
     UT1(ITRF93)-UT1(ITRF92)=  0.034ms
    
   These parameters which are consistent with the IERS values, can be used
   to precisely relate the 1994 and 1995 IGS combined products. For more
   details please refer to IGSMAIL#824.

2. Starting with week 0782 the IERS Bulletin B pole used as reference is first
   corrected with the parameters published in the IERS 1993 Annual Report,
   page II-19 (Table II-3), which aligns the pole series with the ITRF93


[Mailed From: Jan Kouba <kouba at geod.emr.ca>]