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9.5 Transfer Scans

The acquisition of an object for an FGS operating in TRANSFER mode is carried out in the same sequence described above, with the exception that the FGS remains in CoarseTrack until a specified spacecraft time. Thereafter it proceeds to FineLock by slewing the IFOV to the starting point of the first scan. As described earlier for the POSITION mode WalkDown, the FGS averages and differences the PMT data for 0.4 seconds, computes the fine error signal, and compares it to a commanded threshold. However, this threshold equals zero for TRANSFER mode observations, immediately prompting the FGE to declare that FineLock has been achieved and to set the DataValid flag. Setting this flag signals the DF 224 that the FGS is ready to begin the TRANSFER mode observation.

In a TRANSFER mode observation the DF 224 computer steps the FGS's IFOV across the photocenter (determined by CoarseTrack) along a diagonal path in detector space for a distance specified in the original proposal. Each sweep across the target is referred to as a scan. After the completion of a given scan, the IFOV reverses direction and scans the object again until the total number of scans specified in the proposal have been completed. Every 25 msec the PMT data and star selector rotation angles are reported in the telemetry. The FGS samples the entire S-curve, which can be reconstructed by post observation data processing.

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