Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

2.2 HST Two-Gyro Science Mode

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HST normally uses three gyros out of the six installed onboard the spacecraft to provide accurate pointing information. All information about STIS performance in this Handbook has been written under the assumption that three gyros are being used during observations. However, over time these gyros eventually fail, leaving the spacecraft with less redundancy, and eventually with fewer than the number required for normal pointing. The HST Project at GSFC and STScI are developing a Two-Gyro Science (TGS) mode for use when fewer than three gyros are available. The TGS mode will use only two gyros to provide slew capability and spacecraft attitude control (guiding) of sufficient accuracy to continue obtaining high-quality science data.

There is a finite probability that HST will have to switch to TGS mode during Cycle 14. At the time this Handbook is being finalized, a (rough) estimate of the time when there is a 50% probability that HST will be down to two gyros is March 2006 (i.e., during the second half of Cycle 14).

The potential impacts of the Two-Gyro Science mode to STIS observations (e.g., exposure time calculations), along with all other relevant information regarding this mode, are provided in a separate Handbook.

See the Two-Gyro Handbook for detailed information.

Up-to-date information on the TGS mode can be found at the following URL: http://www.stsci.edu/hst/HST_overview/TwoGyroMode

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