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Date: Sat, 24 Dec 2011 13:35:44 -0500
From: Bill Workman
Subject: Notice of an on-board anomaly with MA Transponder 2; No STScI response required over the holiday weekend.


HST experienced an anomaly with MA transponder #2 last night. The transponder began having problems acquiring lock in coherent mode
(COHO) on the TDRS-S support that began at 358/00:57:01z. A subsequent attempt on a TDRS-W support also failed. The MA channel
itself is functional in non-COHO mode which allows for commanding and r/t engineering telemetry. However, the problem prevents
acquisition of tracking data for use by the Flight Dynamics Facility (FDF) in generating predictive HST ephemeris. An anomaly
meeting was held at noon today to review the problem and determine a forward plan. A brief summary follows:

The FOT responded immediately to restore MA communications in non-COHO mode. SA communications remain unaffected, so there will be
no loss of recorded science or engineering data and the timeline will continue to proceed nominally otherwise. The problem appears
frequent but still intermittent, and the root cause is not yet understood. Possible causes include an intermittent failure in the
transponder itself or possible RF interference from ground radiating sources.

Since the transponder is still functional in non-COHO mode, the forward plan is to have the FOT reschedule TDRS supports through
Tuesday morning to non-COHO for commanding and r/t engineering data return. The Flight Plan will continue to control the mission
timeline through the holiday weekend. Alerts will be generated if it is unable to acquire communications in that mode and the FOT
will respond as necessary. The lack of tracking data through the weekend is not expected to impact HST ephemeris accuracy for at
least a week or more. Options for alternate means of acquiring tracking data have been identified and will be considered when the
anomaly investigation begins in full force on Tuesday after the holiday weekend. A worst case response to regain coherence and 2-way
tracking capability would be to perform a full side-switch of the spacecraft to the Side-A electronics. That outcome would only come
after a full investigation. A follow-up anomaly meeting is planned for 1pm on Tuesday. I will forward any additional information as
it becomes available.

Finally, no STScI Ops or Engineering response is currently required this weekend, so enjoy your holiday!

--Bill

HSTAR 13152: Failed to lock on TDS support

HST failed to acquire on coherent (COH) TDS support scheduled from 358/00:57:01 - 01:27:01z. ME (mission engineer?) received
response required alert - "No acquisition after contingency procedures complete." ME called White Sands (WS) and CHAMP Mission
Operations Manager (CMOM). WS said they were seeing good radio frequency signal (RF) from HST. They completed several contingencies
- sent several Single Service Access Forward (SSAF) reacq, spun up several Multiple Access Forward (MAR) chains. They completed
everything they thought they could. WS thought HST was in the wrong config.

HST acquired nominally next TDW support. ME advised TDS we were locked on solid data. ME didn't see any issues on our end.

32 kb eng data loss non-recoverable 27:30 358/00:58:31-01:26:01z

CDS pending.

(From Doug Grimes 12/24/11 12:45 AM) TDS support 358/04:03:15 - 04:54:30z HST did not acquire on MAR. ACE completed all contingency
procedures and we failed to lock. WS notifed. WS completed a loop test on the chain we failed on, the chain looked good from their
end.

(From Larry Stake 12/24/11 08:33 AM)
32K Engineering data loss associated with this anomaly:
358/00:58:31z to 358/01:26:01z
358/04:04:45z to 358/04:53:30z
358/05:49:45z to 358/06:07:36z
358/08:41:22z to 358/08:44:42z
358/08:52:46z to 358/08:53:40z

Approximately 100 min non-recoverable