GERB mirror mechanism, pointing and repeatability

(Tech Note MSG-RAL-GE-TN-2011)B.C.StewartRAL

TopicsMirror data and noiseE-W Scanning and SOE-SOLMSG rotationEffect on mirror rotation rateEffect on scanningEffect on SOE-SOLConclusionsMirror timing offsetMotor Pole + torque level

Mirror data INDUCTOSYN data from SNAP shot mode4096 samples, 343.3sec sample time Resolution:204800Dn = 360 (encoder)20Dn 4.2 1 pixel (Earth view)

CharacterisationSOL pulseSTEP size and rotation rate

Mirror position noiseDifference between a typical scan and mean

RMS 1.3Dn 0.27 0.07 pixel

ScanningScanning 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

SOE - SOL

Measured SOLSOE for a scan, shows variation from 126-146 (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

MSG rotationMSG spin period ~601.617 msec

15min cycle from SEVERI scanning

also long term smaller amplitude periodicities

0.01% change in period (~0.5 pixel)

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

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

SOE SOL (SEVIRI on)SOLSOE residuals for many scans

spikes correspond to 1st 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)

ConclusionsMirror 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

Mirror face effectObservation of Earth limb with step size of 0.15 (~1/28th NORMAL scan step)

Alternate values correspond to the 2 mirror faces.

Difference is ~2-3 steps

Post timing correctionObservation 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)

Motor poleFiltered to remove high frequency INDUCTOSYN effect

Enlargement has slope removed

Torque levelTorque levels 1, 2 and 3

Effect is to add higher frequency components

RMS scan residuals Fig 10.1.11Torque levels 1, 2 and 3

Effect of higher level is to reduce the RMS scan residual

Typical limb data 2nd Aug 2003Observation 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 steps40 pixels * 150 scan steps

E-W limb position 1st-4th Aug 20033rd Aug1st Aug2nd Aug4th Aug