[IGSMAIL-0832] Wk0730-764 IGS Final Orb Update

[AC]

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IGS Electronic Mail      Wed Jan 11  7:01:02 PST 1995      Message Number 0832
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Author: AC Coordinator/NRCan GSDivision
Subject: Wk0730-764 IGS Final Orb Update

    
    
    Dear colleagues,
    

    During a routine check we have discovered a small bug in the application of
    the ITRF-IERS reference frame misalignments for the IGS final orbits during
    the GPS wks 734-764 (Jan. 30 - Sep 3, 1994). The IGS Final orbits prior to 
    wk 734 did not include any misalignment corrections and the IGS final
    orbits after wk 764 (Sep 4, 1994) employ the correct misalignment
    corrections. Consequently to correct this problem and to make the IGS orbit
    consistent throughout the year we have corrected the IGS Final orbits Wk
    730-764 for the correct misalignment corrections. This change resulted in
    small net x,y-pole rotations of less than 1.6mas for wk 730-733 and less
    than 1mas for Wk 734-764. The clock combination during these wks is
    unchanged. All the corrected orbits (wks 730-764) were uploaded into CDDIS.

    The IGS Rapid orbits, which employ the misalignment corrections since wk
    742 (March 27, 1994), are correct and were not affected by this bug. Before
    wk 742  the IGS Rapid orbits did not include this alignment correction. The
    IGS Rapid orbits (prior wk 742) were not recomputed as they were superseded
    by the IGS Final orbits.

    More specifically, the IERS-ITRF misalignment given in the IERS 1992 Annual
    Report, p. II-17 (Table II-3), for the epoch of 1988.0:
    
             x             y            UT1 
    A       -0.1mas      0.7mas       -0.06ms
    A'       0.08mas/y   0.15mas/y     0.004ms/y
    
    were originally computed by using only the constant term (A) for the IGS
    Final orbits during wk 734-764.

    There are three possible ways for IGS product users to correct results
    based on IGS products, namely:

    1. Ignore the change as the (orientation) effect is less than 8 ppb for wk
    730-733 and less than 5 ppb for wk 734-764, which makes 8 or 5 mm
    orientation error for a 1000km baseline and may likely be negligible in
    most applications;

    2. rotate the results by the x-pole (R2) and y-pole (R1) corrections, using
    the values A, A' above and the respective epoch difference (in years),
    since 1988.0;

    3. redo the analysis using the corrected IGS orbits, which may be rather
    time and labour intensive. 
    
    Both 2. and 3 . are rigorous and equivalent but option 2. is clearly more
    efficient and likely preferable (in fact this is the way the IGS Final
    orbits above were corrected).

    
    Finally we would like to apologize for any inconvenience this mistake of
    ours might have caused.


                              J. Kouba


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