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10.1 Engineering Telemetry Data

The FGS is unique among HST's science instruments in that its data downlink is carried on the engineering data stream rather than the science data stream. FGS data are transmitted with the engineering data because the FGSs are controlled by the spacecraft's housekeeping computer, consistent with their primary role as part of the pointing control system. While the contents of the downlink for the other science instruments depend upon the specifics of that instrument, the contents and rates of the FGS data are determined by the engineering telemetry format.

The fundamental FGS data of interest for astrometry and guiding are:

The photon counts from the four photomultiplier tubes.
The instantaneous values of the two Star Selector servo angles.
The twelve status flag bits. The first two items are sampled every 25 msec, while the status flag bits are reported every 150 msec for the FN format (100 msec for HN). Additional FGS mnemonics present in the telemetry either can be reconstructed from those listed above or are of interest only to the -system engineering staff.
When an FGS is operated as an astrometer, the Astrometry and Engineering Data Processing (AEDP) system, part of HST's ground support system at Goddard Space Flight Center (GFSC), extracts specific FGS mnemonics from the engineering telemetry and builds the astrometry data products. These are similar to the GEIS format files produced for the other science instruments (see Chapter 2 for more on GEIS file format). This system also accesses the Mission Support Schedule (MSS) and inserts additional information into various header keywords necessary for proper identification and interpretation of the associated data files.
As of late November 1997, generation of the astrometry data products will become the task of OMS's AST subsystem at STScI. The Astrometry and Engineering Data Processing system will then be phased out.

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