19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 1 GERB mirror mechanism, pointing and repeatability (Tech Note MSG-RAL-GE-TN-2011) B.C.Stewart RAL

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 2 Topics Mirror data and noise E-W Scanning and SOE-SOL MSG rotation –Effect on mirror rotation rate –Effect on scanning –Effect on SOE-SOL Conclusions Mirror timing offset Motor Pole + torque level

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 3 Mirror data INDUCTOSYN data from ‘SNAP shot’ mode 4096 samples, 343.3μsec sample time Resolution: Dn = 360º (encoder) 20Dn  4.2’  1 pixel (Earth view) Characterisation SOL pulse STEP size and rotation rate

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 4 Mirror position noise Difference between a typical scan and mean RMS  1.3Dn  0.27’  0.07 pixel

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 5 Scanning Scanning achieved by changing step size (scan rate constant) 2 values corresponding to E-W and W-E scans Difference of ~40 Dn corresponds to a scan STEP size of ~4.2’ in each direction i.e. NORMAL mode

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 6 SOE - SOL Measured SOL–SOE for a scan, shows variation from 126º-146º (apparent width of Earth at GEO orbit) Subtracting a straight line shows RMS residual of ~0.2’ (0.05 pixel) - consistent with RMS in mirror data

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 7 MSG rotation MSG spin period ~ msec 15min cycle from SEVERI scanning also long term smaller amplitude periodicities 0.01% change in period (~0.5 pixel)

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 8 Mirror Rotation (SEVIRI on) Mirror control system ensures GERB mirror rotation rate follows MSG spin period digital system, GERB mirror rate changes in discrete steps max difference between ‘ideal’ and ‘actual’ positions is ~0.1 pixel (i.e. 3.8’ rather than 4.2’ scan step) ‘noise’ at transitions

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 9 Scanning (SEVIRI on) Two slopes as before but ‘tops’ are not flat Re-plotted with average value subtracted Saw-tooth pattern corresponds to changes in rotation rate Steps are ~ 4Dn i.e. 0.2 pixel

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 10 SOE – SOL (SEVIRI on) SOL–SOE residuals for many scans ‘spikes’ correspond to 1 st line of each scan Mean of sets of 16 packets. Same behaviour as for the mirror rotation steps Step size ~0.4 arc min ~0.8 at Earth (0.2 pixel) RMS (not shown, again ~0.2’)

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 11 Conclusions Mirror control system locates scan lines to ~0.05 pixel SEVIRI scanning does not affect RMS of line positions but causes them to be non-uniform, at level of 0.2 pixel, at the mirror rotation rate transitions Non-uniformity is not repeatable from scan to scan

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 12 Mirror face effect Observation of Earth limb with step size of 0.15’ (~1/28 th NORMAL scan step) Alternate values correspond to the 2 mirror faces. Difference is ~2-3 steps

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 13 Post timing correction Observation of Earth limb with step size of 0.15’ (~1/28th NORMAL scan step) ’Optimum’ correction of ~2.7 pixels (0.4’) Residual effects probably from variations and noise in step size (0.05’ and up to 0.2’)

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 14 Motor pole Filtered to remove high frequency INDUCTOSYN effect Enlargement has slope removed

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 15 Torque level Torque levels 1, 2 and 3 Effect is to add higher frequency components

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 16 RMS scan residuals Fig Torque levels 1, 2 and 3 Effect of higher level is to reduce the RMS scan residual

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 17 Typical limb data 2 nd Aug 2003 Observation of Earth limb with step size (x-axis) of 0.15’ (~1/28th NORMAL scan step) Plots are averages over 10 detector pixels and are 150 scan steps wide Slopes are average over 4 scan steps 40 pixels * 150 scan steps

19 th GIST Meeting 27 th –29 th August 2003 GERB mirror mechanism 18 E-W limb position 1 st -4 th Aug rd Aug 1 st Aug 2 nd Aug 4 th Aug