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Telescopes Branch
HST in Safemode Due to Gyro 3 Anomaly

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On Tuesday, April 29, 2003 (119/14:21 GMT) HST entered Zero Gyro Sun Point Safemode due to system momentum failing in response to a stalled gyro 3. The failure signature of the gyro 3 motor current is similar to the event on April 13, 2003.

The current theory of this malfunction is that a gathering of lubrication material results in transient load torques causing the motor to draw more current before it finally stalls. The anomally is sensed by the flight software, which commands the vehicle into the zero gyro safemode in which the SI C&DH and some Instruments are safed and the aperture door is closed resulting in an interrupt of science operations.

Once the situation was understood, recover from the safemode was initiated by real-time commanding followed by a Health and Saftey SMS with a final intercept of the normal Science SMS on Wednesday, April 30, 2003 around 8 PM local time.

It is standard procedure to restart the stalled gyro by real time command procedures. Thre successive attempts were made, but the gyro did not restart on any of them.

After the unsuccessful attempts to get gyro 3 back operational, the spare gyro 1 was turned on and is now in the control loop.

Report written by Manfred Miebach

Instrument Operations

At the time of zero gyro entry NICMOS was in Operarate mode and WFPC2 in Decontamination Mode. ACS and STIS were configured into safemode. Since NICMOS is susceptible to TPG resets in Operate mode the instrument was configured into the SAA Operate mode by real time procedures.

ACS and STIS were recovered out of the SMS.

As for the WFPC2, engineering teams were facing a special challenge since this was the very first time that a vehicle safing coccured while the WFPC2 was in a decontamination phase. No standard, pre-tested procedures were available covering this situation. Ops Request #16948 was prepared to configure the Instrument from the decontamination to standby mode. The driver for this configuration change was the concern of the falling temperature in Bay5 (WFPC) since the duration of the decontamination phase was unknown. Fig 3 shows a typical decontamination temperature profile for the temperatures for the cold junction and the Bay 5.

Report written by Manfred Miebach.