19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 1

GERB mirror mechanism, pointing and repeatability

(Tech Note MSG-RAL-GE-TN-2011)

B.C.StewartRAL

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 3

Mirror data

INDUCTOSYN data from ‘SNAP shot’ mode4096 samples, 343.3μsec sample

time Resolution:

204800Dn = 360º (encoder)20Dn 4.2’ 1 pixel (Earth view)

CharacterisationSOL pulseSTEP size and rotation rate

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 4

Mirror position noise

Difference between a typical scan and mean

RMS 1.3Dn 0.27’ 0.07 pixel

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 7

MSG rotation

MSG spin period ~601.617 msec

15min cycle from SEVERI scanning

also long term smaller amplitude periodicities

0.01% change in period (~0.5 pixel)

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 10

SOE – SOL (SEVIRI on)

SOL–SOE 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’)

19th GIST Meeting27th–29th 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

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 12

Mirror face effect

Observation 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

19th GIST Meeting27th–29th 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’)

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 14

Motor pole

Filtered to remove high frequency INDUCTOSYN effect

Enlargement has slope removed

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 15

Torque level

Torque levels 1, 2 and 3

Effect is to add higher frequency components

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 16

RMS scan residuals

Fig 10.1.11

Torque levels 1, 2 and 3

Effect of higher level is to reduce the RMS scan residual

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 17

Typical limb data 2nd 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

19th GIST Meeting27th–29th August 2003

GERB mirror mechanism 18

E-W limb position 1st-4th Aug 2003

3rd Aug

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