From AC Wed Jan 11 07:01:02 1995
From: AC (AC)
Date: Wed, 11 Jan 1995 7:01:02 PST
Subject: [IGSMAIL-0832] Wk0730-764 IGS Final Orb Update
Message-ID:
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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IGS Rapid orbits (prior wk 742) were not recomputed as they were superseded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 ]