_
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V V SSSS OOO PPPP \__ |_| __/
V V S O O P P --____/ \____--
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V V S O O P |_|_|_| @|_|_|_|
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*** N E W S *** <)
OPERATIONS MEETING
From November 11-13, thirty or so members from the various VSOP mission
elements gathered in Vancouver to discuss the status of the mission. A wide
range of issues was discussed and below is a brief summary of some of the
issues of more relevance to readers of the VSOP News.
In-Orbit Checkout at 1.6 and 5 GHz is effectively over. Engineering tests
will continue to be made but, for example, the current schedule for December
(available from the VSOP WWW site) contains 15 General Observing Time observ-
ations, 7 VSOP Survey Program observations and one test observation. In-Orbit
Checkout at 22 GHz is likely to continue for the rest of the mission lifetime.
The VSOP observing efficiency has been limited thus far by sun-angle
limitations, HALCA maneuvering overheads, tracking station anomalies and
correlator throughput. Taking these in turn:
- The sun-angle is the angle between the sun and the target source. HALCA
cannot observe sources within 70 degrees of the sun as the main antenna shadows
the solar paddles at these angles. The allowed sun-angle range has also been
limited to date to regions around 90 degrees and 180 degrees by excessive
angular momentum build-up experienced at intermediate angles. On-board software
upgrades have improved this situation, and the allowed sun angle range is
slowly being extended.
- After maneuvering between sources, both of HALCA's star trackers need an
hour or so to calibrate and fine-tune the pointing. Until recently, observing
has had to wait until the next tracking pass over the Kagoshima Space Center
to confirm that the maneuvering and calibration has proceeded correctly. It is
now believed the on-board software is sufficiently robust that maneuvers can
be carried out `automatically', and so the limitation of six maneuvers per week
(as there are only six KSC passes per week) has now been lifted.
- Two distinct 15 GHz tracking station problems currently exist. The tracking
stations generate Time Correction Files (TCFs) which are needed at the
correlators to correct the delay and Doppler errors in the predicted satellite
orbit. Problems with the TCF generation at the Usuda tracking station
have limited its usefulness to 1.6 GHz observations thus far. The DSN tracking
stations have a different, intermittent problem with discontinuities in the
delay term occurring. Both problems are being actively investigated.
- Finally, all three correlators have now gained experience in handling
space VLBI data, and have been fine-tuning their software and procedures.
Efforts are underway to tighten the delay and delay-rate windows used in
correlation to reduce the output data files to more manageable sizes.
All in all, although some problems remain, significant progress has been
made, and data flow through the mission to PIs is increasing.
One final decision endorsed by the meeting: by the end of December an observing
schedule through to the end of the first AO period will be available. All PIs
will be contacted individually regarding the observations they had proposed.
Editors: Phil Edwards and Hirax Hirabayashi