GIST 23 27th-29th Apr

B.StewartRAL

In-orbit Mirror Performance

GIST 23 27th-29th Apr

Topics

• Instrument layout• Mirror mechanism operation• Background

– Housekeeping data– FIFO data– Typical data– Torque level

• Performance since launch• autoSAFE events

– description– Feb 28th – Apr 1st

• Future operation of GERB 2 & 1– paliatives

• GERBs 3 & 4– Potential hardware change

GIST 23 27th-29th Apr

Instrument layout

GERB

Earth View

Body Body

calmon

filter wheel

rotating mirror

dete

ctor

Spinning MSG

folded

telescope

optics

GIST 23 27th-29th Apr

Instrument Layout

• Rotation – from perspective of GERB, the Earth appears in its FOV once every 0.6 sec.

• FOV – ~24° *20° (E-W*N-S)• Earth is ~central to N-S FOV, visible for 40msec each

0.6 sec. Rotating mirror fixes Earth’s apparent position when it is in the FOV to give 40msec integration time.

• Sun also appears within E-W FOV limits every 0.6 sec. However, is only within N-S FOV near the equinoxes for ~2 months.

• What happens if mirror should stop rotating?– AutoSAFE

• How likely is it? Can we anticipate/ what can we monitor? Are there any palliatives?

GIST 23 27th-29th Apr

Mirror mechanism operation

• 2 sided flat mirror mounted on an axle• 22 pole motor direct drive to axle• INDUCTOSYN encoder to measure rotation

phase– 204800 steps per 360° (1 step = 0.105’)

• Electronic control system– Measures time between successive SOLs (MSG

rotation rate).– After each SOL calculates a linear ‘demand’ for

rotation phase as a function of time (at exactly half MSG rotation rate) for the next mirror half rotation.

– Every ~50sec (12,800/ rotn.) calculates difference between ‘demanded’ and measured phase (error signal) and uses this to modify power to the motor to reduce the error signal.

• Science requirements imply control to ~0.002%

GIST 23 27th-29th Apr

Housekeeping data

• Telemetry – 1 ‘packet’ every MSG rotation (0.6 sec) containing science &

HK.

• Mirror Error signal (GVMPERR) – Lower 12 bits of 16 giving range of ± 7.2° (greater values

wrap)– Single value for each rotation (average of 16 1 msec

samples)

• Coarse mode flag– Set to 1 if ‘error’ exceeds 14 bits (~28 °)

• Mirror Velocity (GVMVEL)• Time of start of Earth Window (SOE-SOL)

– basis of GEO

• FIFO data• others

– Mirror face, velocity feedback, fine mode, lags-leeds

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FIFO data

• FIFO– 4096 words which can store INDUCTOSYN rotation

phase data– Downlinked as a set of 128 samples every telemetry

packet

• Modes– Snapshot

• 4096 words used to store ~1.1 complete rotations. Data transmitted to ground over 32 telemetry packets. i.e data from every 32nd rotation is down-linked. Default in SUNBLOCK mode (‘page’ defines 128 samples within 4096, 1st packet contains simultaneous recoded samples).

– Earth view• 128 samples beginning at the start of Earth view down-

linked for every mirror half rotation. Default in NORMAL mode.

GIST 23 27th-29th Apr

FIFO data (2)

• SnapshotFrom 17th Jan 2003TL 4a: Rotationb: Rotation – nominal

(*1250)SOLsScanningInductosynMotor pole

• Earth viewYellow box

a

B

b

GIST 23 27th-29th Apr

Torque level

• GERBs 1& 2 torque commandable (0-7), level fixed– For GERB on MSG-1 has been chosen to minimise

noise in SOE-SOL (minimum at TLs 3,4)– Can be increased if any effects of increased bearing

friction are seen

• GERBs 3&4 torque level dynamic• Torque level range greater than torque spikes

measured in life test.

GIST 23 27th-29th Apr

Performance since launch

•GVMPERR

•Trend

•Dependent on TL

•NORMAL/ SUNBLOCK modes

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Performance since launch (2)

• GVMVEL (noise)

•dependent on TL

• Variation

•GSMCORSE

•Some association with GVMVEL (not specific)

•Worse in 2005

•autoSAFEs 2003 2004 2005

GIST 23 27th-29th Apr

SCANSIG/VELSIG

GIST 23 27th-29th Apr

AutoSAFE

• GERB self protection system– Controlled by on-board-limits on 32 housekeeping

parameters (one of which is GVMPERR)– Operates in NORMAL mode (not in SUNBLOCK)

• What happens in an autoSAFE– GERB on-board software detects a limit violation and

issues a SAFE command.– SAFE command stops mirror rotation and causes it to

move to the zero position.– The Quartz FILTER wheel is moved to the blocked

position (200 steps {full rot}/ sec) to protect the detector• Switch back to NORMAL mode

– Circumstances reviewed with EUMETSAT before switch on agreed and implemented

• Causes– All, so far, have been provoked by GVMPERR spikes.

GIST 23 27th-29th Apr

AutoSAFE (2)

• Data loss• Limitations

– If event caused by a GVMPERR spike, mechanism recovery behaviour lost.

– GVMPERR only sampled once per S/C revolution.– QFM can take ~0.8 sec to move to BLOCKED position

• Trade-offs– Size of limits– Number of excursions

• Strategy– Non sun avoidance– Sun avoidance

• Engineering view– Keep mechanism running

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28th Feb event

• Lead up– AutoSAFE’s had become more frequent since ~start of 2005– From Jan no. GVMPERRs set to 7 to maintain data

continuity– Mid Feb Preparations for Sun avoidance

• TL tests• Tighter limits• 1 GVMPERR• Higher TL• Shorter observing time

• AutoSAFE event on 28th– Back to NORMAL on 30th

• Lost pixels (2 open circuit, evidence for damage to further 6/8 – size of Solar image)

– Decision to stay in SUNBLOCK mode (has autoSAFE disabled mechanism running to get more data)

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What happens? – 1st April

•Why even SUNBLOCK events don’t give us much -Also effects of timing•1st April first example of what happens - Sharpness of transition

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Palliatives

• GERBs 1 & 2• Precursors

– Nothing significant found so far

• RAL(Dave Parker) proposal to reduce response time– More than 1 check on GVMPERR per MSG rotation– Faster QFM move to BLOCKED (~0.2/~0.4sec)– Possibility of more FIFO data

• Imperial (Jenny Hanafin)– Time limits for latitude bands

• Alternative is not to run– Compromises daily/ annual time sampling

GIST 23 27th-29th Apr

Future

• GERBs 3 & 4 have a different control system, dynamic torque level

• ESA study– Revisit possibility of greased bearing

• Higher average torque but smaller spikes• Possible GERB contamination• Possible SEVIRI contamination• Complications of life test

– Other, electronic modifications