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

GIST th -16 th Dec 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

GIST th -16 th Dec Resumeé GERB scan mirror has been seen to stop rotating on a number of occasions - this is dangerous during eclipse seasons On Feb 28 th 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 18 th 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

GIST th -16 th Dec Mirror events since April 2005 GSM Coarse events good indicator of problems Plot shows decreasing frequency of events

GIST th -16 th Dec On-board software changes GERB2 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.

GIST th -16 th Dec 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)

GIST th -16 th Dec 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

GIST th -16 th Dec 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)

GIST th -16 th Dec Mechanical - GERB 2 v. DM GERB 2 DM

GIST th -16 th Dec Electrical - GERB 2 v. DM GERB 2 DM

GIST th -16 th Dec 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 1/10 th to 1/100 th that seen in GERB 2, and could be monitored. –Mirror can be stopped at any given position –TCFs could work!

GIST th -16 th Dec 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 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).