Vehicle Transfer Function Test Contingency

Details Vehicle Transfer Function Test Contingency Vehicle Transfer Function Test Contingency

Jul 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001hst..prop.8971c&link_type=abstract

HST Proposal ID #8971

Computer Science

Performance

Hst Proposal Id #8971

Scientific paper

The TFT is a forced response test that measures HST system modal parameters {modal gains, modal damping ratios and frequencies} by applying a RWA forcing function and measuring RGA gyro response. This test will be performed if PCS analysis of post-SM3B HST flight data {such as the VDT} suggests that HST system modal parameters significantly differ from pre-SM3B analytical models. On-orbit modal parameters that differ from pre-SM3B models can cause degraded performance of the HST attitude control system because of reductions in stability margins and/or increased vehicle jitter. The test occurs after release and at least 24-hours after the Vehicle Disturbance Test is complete. The NCC will be operating at steady state temperature {out of surge}. The overall duration of the TFT is approximately 18 orbits of spacecraft time in gyro hold that includes nine orbits in the +V3 sunpoint orientation and nine orbits at V1 sunpoint. While in each orientation, three orbits will be allocated for applying forcing functions about each of the three vehicle axes. Forced response of the HST is required at two different vehicle attitudes because HST modal parameters are a function of solar array angle. At the beginning of the test, Stored Program Command macros are activated in real time to establish modified PN Format telemetry, to switch the attitude control law to the maneuver gain set, to command the gyros to low mode, and to disable velocity aberration and parallax {VAP} processing in the flight computer. Each TFT forcing function will be prescribed by flight software table loads that will be uplinked in real time prior to each test. As many as 14500 SPC words of flight computer memory will be required to store each forcing function, and this will require the STScI to use 8000 SPC words less than the normal allocation used for the SMS. It is anticipated that each forcing function will take approximately 12.5 minutes to uplink. After uplink, the forcing function is activated via real time command and it runs for at least one orbit. The nominal telemetry format and attitude control law configuration will be restored at the end of the test via real time command.

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