[IGSMAIL-1559] IGS orbit/clock/EOP Product Update

[AC]

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IGS Electronic Mail      Wed Mar  5  7:37:23 PST 1997      Message Number 1559
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Author: AC Coordinator/NRCan GSDivision
Subject: IGS orbit/clock/EOP Product Update


Dear IGS colleagues,

On March 2/97 (Wk 0895/day0; MJD 50509; DOY 061/97), the following changes
and enhancements have been implemented for IGS combined orbit/clock/eop
products:

1. NEW IGS COMBINED ORBIT PREDICTION (IGP) PRODUCT:
Since March 2/97 (day 0/Wk 0895) the IGS combined predictions have been
made available to IGS users at the IGS DCs (currently available at CDDIS
and hopefully, shortly at other IGS DCs, and the IGSCB Archives). Each day,
shortly after 23:00, but not later than 23:30 three combined prediction
files (WWW-GPS week; D-day No.),

 IGP0WWWD.SP3   (24h orbit/clock file in SP3 format)
 IGP0WWWD.ERP   (Bull A predicted EOP to be used with the IGP orbit file)
 IGP0WWWD.SUM   (A combination summary and statistics),

are generated from the contributed AC orbit predictions so that  the IGS
predictions for the current UT day is available at least 0.5 h before the
beginning of the current day. It is hoped that CDDIS, IGSCB and the other
DCs could make these IGP orbits/clocks/erp also available before the
beginning of the current UT day.

The IGP orbit prediction combinations have been running for almost half a
year and currently they are based on timely contributions of five AC
predictions (CODE, EMR, GFZ, JPL, SIO); 24-48h orbit predictions are, in most
cases, based on Long Arc (LA) fitting to 3-4 IGR's plus 1 day of AC orbits.
For completeness and ease of use, the CODE extrapolated (broadcast) clocks
(rms ~ 80 ns) are included in the IGP sp3 orbit files. The corresponding
erp files contain two days of Bull A predictions to allow effective
interpolation (as there are no erp rates in the IGP erp files). The sum
file contains the regular statistics, the same as for IGR with the
exception of the navigation tables. To aid IGS users with precision
evaluation, the new IGP orbits (along with the broadcast (BRD) orbits) are
also included (for statistics only) in the daily IGS Rapid orbit/clock/EOP
combinations. Typically IGP orbits are at or below 1m orbit rms.


2. IGS LOD/UT1 COMBINATIONS:
Since Day 0/Wk 0895 (March 2, 97, MJD 50509) in both the IGS Final (IGS)
and Rapid (IGR) orbits/clock/EOP combinations the Bull A UT1's have
been replaced with new IGS/IGR  UT1/LOD combinations. In the new IGS/IGR erp
files both combined LOD's and the integrated UT1's (from the nearest Bull
A UT1 value based on 24h VLBI observations), as well as the corresponding
sigmas, are coded. Prior each combination, the individual AC LOD's are
corrected for AC LOD biases, estimated by weighted averages over the most
recent 21-d period of the 24h-VLBI based Bull A LOD values. The
improvements should be most significant for IGR UT1 (EOP(IGS)96 P 01).
Based on off-line tests over more than 30 weeks an IGR UT1 rms of about
150us is expected, down from the original  UT1 rms of about 600 us which
was based on Bulletin A 6-14d predictions. In the case of the IGS (Final)
erp (EOP(IGS)96 P 02), the improvements are less pronounced, hardly
significant as the most recent 24h-VLBI Bull A values are within 0.5-3days.
In fact during most part of the week the IGS UT1's are within 0.5 day  and
thus are practically the same as the 24h VLBI based portion of the
Bull A. The main addition here is the new combined (calibrated) LOD's and
the associated sigmas. The UT1/LOD combinations complete the IGS EOP
combinations, as all the EOP values, rates and sigmas given in IGS and IGR
erp files are now obtained as combinations of input AC (GPS only)
solutions.

ACKNOWLEDGEMENTS:
A significant additional effort of all ACs, and in particular of all the
five ACs (CODE, EMR, GFZ, JPL & SIO) contributing orbit predictions is
acknowledged. Jim Ray's assistance with UT1/LOD combinations is also
acknowledged.

Suggestions/comments/questions on the above improvements/changes or on
the IGS orbit/clock/erp products should be directed to:


  Jan Kouba (kouba at geod.emr.ca)  and/or  Yves Mireault (mireault at geod.emr.ca)


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