Level 1.5 Processing Updates

1. Level 1.5 Processing Updates Martin Bates, RAL GIST 23 28th April, 2005

2. Contents • Gain calculation • Geolocation • ARG Duplication

3. Black body view LF TOTAL 1 / GTOT V Space view Gains Instrument continually calibrated in-flight using views of (warm) internal Black Body and (cold) space Calibration can go wrong if bright objects encroach on space view Stray light and the moon are two sources of contamination

4. Gain Calculations • Currently flag affected data • stray light flagging implemented as part of new product confidence flags (11-Jan-2005) • but moon not flagged • Original suggestion was offline gains • gains are stable • but problem of keeping up with changes • Two other ideas • split gains • running averages

5. Split Gains • What are split gains ? • Calculate average response (DS-BB) per pixel in western and eastern halves of image • if either half exceeds the other by more than a set amount, remove that half from gain calculation (DS – BB)west > (DS – BB)east Use only eastern pixels for gain calculation

6. TOTAL Response (All pixels) Stray light bar Stray light in blackbody

7. Lunar Data

8. Lunar Data Typical response ~ -2200 to -2600 0.5 % => need to be sensitive to change of ~ 10 counts with sd cut without sd cut

9. Centring Problem Spin Period E-W Centring • Change in MSG rotation rate Jan 2004. • GERB looks further west • Not enough space pixels to east at equator (for split gains) • Corrected itself following 08 Feb 2005 manoeuvre 26 Oct 2004

10. Split Gains Summary • Cannot be used on 2004 data • Monitor centring more tightly to ensure feasability of use for future data • Need to cut at ~ 10 counts • level of background this will produce would need to be measured

11. Running Averages Gain = ra_factor * Inst_Gain + (1- ra_factor) * Ave_Gain ra_factor = 0.008 • Maintain a running average gain for each pixel. • Update running average with latest measurement • Weight of any given measurement drops away over 24-48 hours • No updates ~ midnight (check stray light flags) • Lunar effects washed out • Similar for BB SW Meas Radiance.

12. TOT Gain 05-06 Feb 2005 Pix 200

13. TOT Gain 05-06 Feb 2005 Pix 200

14. TOT Gain 05-06 Feb 2005 Ave Pix

15. TOT Gain 01 Jan 2005 Pix 24

16. Summary • Advantages of running gains: • successfully handles stray light and lunar data • uses more measurements • takes advantage of stability of gains • gain measurement less noisy (particularly central pixels) • (split gains would actually use fewer measurements) • still tracks longer term (> few hours) drifts in gains • should work on 2004 data and nrt data • Disadvantages ? • Time dependence to processing ? • Stability of gains means order of processing not critical • Start-up conditions ? • Write gains to file after each scan processed – file can be read if processing stopped and started • Revised weighting scheme for startup until a stable running average is derived • Should only ever come into play once • Proposal: • Switch on running averages for current level 1.5 processing. • Switching to split gains remains an option if suitable cuts can be devised.

17. Stray Light in Black Body Sep 27 • Occurs ~ midday near equinoxes • Problem because each EV pixel reading needs corresp. BB reading subtracted: (VEV – VBB) • Could use a known gain and deep space reading: • LBB = (VDS – VBB) / Gain • but would need VBB to be stable over a scan (is it ?) • Use VIS ? • Otherwise, just flag as now Jul 05

18. Geolocation • Last time: • Comparisons between GGSPS L1.5 GEO and RGP level 2 GEO (nrt and 90 min) • time dependent, unphysical corrections to optical model, variations at 0.1 – 0.2 level • noise in level 1.5 column spacings (SOL_SOE_Delay) • Plan: • devise a “good” optical model correction • implement level 1.5 GEO in level 2 processing for a pre-defined test period • still to be done • however, new problems have arisen in Jan/Feb 2005.

19. E/W Shift (Spin rate) Spin rate change associated with 08 Feb manoeuvre

20. Effect of spin rate changes MSG SLOWS DOWN A known consequence of de-spin mechanism But predicted magnitude out by ~1 pixel in 7. MSG SPEEDS UP

21. N/S Shift (??) N/S Shift commencing 15 Jan 2005

22. N/S Shift Jan 2005 Also follows SEVIRI decontamination Coincident with increase in spin axis tilt

23. Geolocation • Instantaneously, we have had a 1 pixel shift E/W in level 1.5 geolocation (manoeuvre). • Over a 2 week period, we have had a 1 pixel shift N/S in level 1.5 geolocation • not associated with any manoeuvre • coincident, if not connected, with increase in spin axis tilt (and end of SEVIRI decontamination period) • may be connected with changing axis misalignments ?

24. Effects of Axis Misalignments 0.3 degrees (radial rotation) No misalignment Effect of axis misalignments on determination of optical model 0.3 degrees (transverse rotation) Getting knowledge of axis misalignments now a high priority.

25. L15G 90 min fits 20050115 20050207 20050208 20050227

26. ARG Duplication 15 SEVIRI 17 GERB Renaming Duplication G2_SEV1_L20S_20050101_144500_V002.hdf G2_SEV1_L20S_20050101_145013_V002.hdf G2_SEV1_L20S_20050101_150000_V002.hdf G2_SEV1_L20S_20050101_150711_V002.hdf G2_SEV1_L20S_20050101_151500_V002.hdf

27. ARG Duplication • Having second thoughts about this scheme: • Current status (renaming but no duplication) • looks as though there is missing data • filename misleading as to data contents • ARG Duplication • seemingly independent files actually contain the same data • filename still misleading as to data contents • How feasible/desirable is it to abandon scheme and name ARGs according to data content ? • remember designed for level 1.5 ARG, but main product affected is level 2 ARG. • (technical): can RGP avoid renaming to a precise 15 minute slot ?