GIST 24 14 th -16 th Dec B.Stewart RAL Mirror Anomaly Review Board Update

GIST 24 14th-16th Dec

B.StewartRAL

Mirror Anomaly Review Board Update


Topics

• Resumeé• Mirror events since April 2005• Operation of GERBs 2 & 1

– Revised SAFEing software

• GERBs 3 & 4– Investigation of possible causes– DM, simulation of events– GERB 3/4 tests (dry bearing)– Lubricated bearings


Resumeé

• GERB scan mirror has been seen to stop rotating on a number of occasions - this is dangerous during eclipse seasons

• On Feb 28th 2005 (spring eclipse season) , this resulted in the loss of 2 pixels and significant changes in responsivity to 6-8 others.

• Causes unknown and events not predictable.• Frequency of events, to ~April 2005, was increasing• Current GERB AutoSAFE is not fast enough to save pixels• Increase in torque level 3 to 4 reduced frequency of events but

4 to 5 did not make any difference and at 5 GEO is poor.• Anomaly Review Board set up by ESA/EUMETSAT (KO 18th

May), independent experts to investigate problem and suggest how it might be resolved for the various GERB instruments

• Strategy during/ outside eclipse seasons– decision at last GIST to operate in SAFE mode for the Aug-Oct

2005 season


Mirror events since April 2005

• GSM Coarse events good indicator of problems• Plot shows decreasing frequency of events


On-board software changesGERB2 software patch to improve SAFEing response time

1. The descan mirror error signal and “COARSE" flag repeatedly checked during 600 ms( was once per 600 ms).

2. The QFM is set to reverse rotation direction (shortest path) and commanded to BLOCKED immediately (within <1ms)• An option is also available to increase the ‘stepping’ rate.

3. Mirror mechanism commanded to SUNBLOCK mode (immediate monitoring of mechanism recovery)

4. Telemetry changes to flag and record the ‘event’

5. Patch has been successfully tested at RAL using a combination of the DM, GERB 4 QFM and the GERB 2 software.


Response Time

• Pre patch, response time> 2*600ms, i.e. if the mirror stops in the earth view some pixels are certain to see the sun.

• Post Patch,– Software detection ~2.5 to ~59 ms– Time to blocked 195 ms (from filter), 385 ms (from

open) at nominal rotation rate

(time from earth view to viewing the sun is 0 to 600 msec, depending on time of day, from just before to just after midnight)


Possible causes

• Mechanical– After event see ball-bearing spikes (caused by

debris?) and increased noise which fades away, however no precursors

– Deceleration greater than is seen to be generated by the motor

– DM simulation– However, nothing in life tests

• Electronic– No obvious artefacts in FIFO or HK data, eg

currents/ Voltages/ flags changing– Attempts at simulation using the DM have failed


DM simulation

• Mechanical– Eric’s mirror

interrupter– Manually stopping the

mirror mechanism– (debris – yet to be

tried)

• Electronic– Attempts to change

electronic signal in an extreme way (zeroing most significant 8 bits of error signal)


Mechanical - GERB 2 v. DM

GERB 2

DM


Electrical - GERB 2 v. DM

GERB 2

DM


GERB3 /4 tests

• GERB 3 / 4– Different control system– Less smear, control precision limited by

INDUCTOSYN digitisation– Better matches changes in rotation rate introduced

by SEVIRI (correction of effects seen in GERB-2)– No torque level setting, dynamically adjusted (ie no

3-4-5)– Extended running, identified systematics resolved– Low level ball-bearing noise detectable, at 1/10th to

1/100th that seen in GERB 2, and could be monitored.

– Mirror can be stopped at any given position– TCFs could work!


Bearing lubrication

• GERB 1 / 2– Dry (lead in cage)– Sacrificial, debris build-up

• Greased bearing (GERB 3/ 4 option)– Labyrinth, low vapour pressure bearing grease, no debris – Possible contamination of GERB and SEVIRI optics - time

for mono-layer build-up on GERB optics (estimated at ~70 years)

– Other re-design (?)– Effects on science (?)– Timescale :

• For nominal launch of MSG-3 a fix would need to be implemented by the end of 2007. The decision needs to go via Eumetsat Council.

• Assuming 1 year for the change and 6 months to go via the Eumetsat decision process, initiation of activities by mid 2006 at the latest. Implies ARB recommendations, change requests and quotations should be available and assessed by this date. ARB recommendations need to be available by Dec 2005 (now looking like Jan-Feb 2006).

Documents

GIST 24 14 th -16 th Dec B.Stewart RAL Mirror Anomaly Review Board Update