1. GIST 23 27 th -29 th Apr B.Stewart RAL In-orbit Mirror Performance

2. GIST 23 27 th -29 th Apr 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 GERBs 3 & 4 –Potential hardware change

3. GIST 23 27 th -29 th Apr Instrument layout GERB Earth View Body calmon filter wheel rotating mirror detector Spinning MSG folded telescope optics

4. GIST 23 27 th -29 th 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?

5. GIST 23 27 th -29 th 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 ~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. GIST 23 27 th -29 th 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

7. GIST 23 27 th -29 th Apr 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 32 nd rotation is down-linked. Default in SUNBLOCK mode (‘page’ defines 128 samples within 4096, 1 st 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. GIST 23 27 th -29 th Apr FIFO data (2) Snapshot From 17 th Jan 2003 TL 4 a: Rotation b: Rotation – nominal (*1250) SOLs Scanning Inductosyn Motor pole Earth view Yellow box a B b

9. GIST 23 27 th -29 th 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.

10. GIST 23 27 th -29 th Apr Performance since launch GVMPERR Trend Dependent on TL NORMAL/ SUNBLOCK modes

11. GIST 23 27 th -29 th Apr Performance since launch (2) GVMVEL (noise) dependent on TL Variation GSMCORSE Some association with GVMVEL (not specific) Worse in 2005 autoSAFEs 200320042005

12. GIST 23 27 th -29 th Apr SCANSIG/VELSIG

13. GIST 23 27 th -29 th 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.

14. GIST 23 27 th -29 th 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

15. GIST 23 27 th -29 th Apr 28 th 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 30 th 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. GIST 23 27 th -29 th Apr What happens? – 1 st 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. GIST 23 27 th -29 th Apr 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. GIST 23 27 th -29 th 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