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In-orbit Mirror Performance

In-orbit Mirror Performance. B.Stewart RAL. Topics. Instrument layout Mirror mechanism operation Background Housekeeping data FIFO data Typical data Torque level Performance since launch autoSAFE events description Feb 28 th Apr 1 st Future operation of GERB 2 & 1 paliatives

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In-orbit Mirror Performance

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  1. In-orbit Mirror Performance B.Stewart RAL

  2. 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

  3. GERB calmon Earth View Spinning MSG detector filter wheel rotating mirror BodyBody Instrument layout folded telescope optics

  4. 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?

  5. 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 ~50sec (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%

  6. 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

  7. 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.

  8. FIFO data (2) • Snapshot From 17th Jan 2003 TL 4 a: Rotation b: Rotation – nominal (*1250) SOLs Scanning Inductosyn Motor pole • Earth view Yellow box a B b

  9. 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.

  10. Performance since launch • GVMPERR • Trend • Dependent on TL • NORMAL/ SUNBLOCK modes

  11. Performance since launch (2) • GVMVEL (noise) • dependent on TL • Variation • GSMCORSE • Some association with GVMVEL (not specific) • Worse in 2005 • autoSAFEs 2003 2004 2005

  12. SCANSIG/VELSIG

  13. 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.

  14. 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

  15. 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)

  16. 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

  17. 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

  18. 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

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