Level 1.5 Processing Updates

Martin Bates, RALGIST 23

28th April, 2005

Contents

• Gain calculation• Geolocation• ARG Duplication

GainsLF

VSpace view

Black body view

1 / GTOT

TOTAL 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

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

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

TOTAL Response (All pixels)

Stray light bar

Stray light in blackbody

Lunar Data

Lunar Data

without sd cut

with sd cut

Typical response ~ -2200 to -2600

0.5 % => need to be sensitive to change of ~ 10 counts

Centring Problem

• 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

E-W Centring Spin Period

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

Running Averages• 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.

Gain = ra_factor * Inst_Gain

+ (1- ra_factor) * Ave_Gain

ra_factor = 0.008

TOT Gain 05-06 Feb 2005 Pix 200

TOT Gain 05-06 Feb 2005 Pix 200

TOT Gain 05-06 Feb 2005 Ave Pix

TOT Gain 01 Jan 2005 Pix 24

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.

Stray Light in Black Body

• 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

Sep 27

Jul 05

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.

E/W Shift (Spin rate)

Spin rate change associated with 08 Feb manoeuvre

Effect of spin rate changes

MSG SLOWS DOWN

MSG SPEEDS UP

A known consequence of

de-spin mechanism

But predicted magnitude out by

~1 pixel in 7.

N/S Shift (??)

N/S Shift commencing 15 Jan 2005

N/S Shift Jan 2005

Coincident with increase in spin axis tilt

Also follows SEVIRI

decontamination

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 ?

Effects of Axis Misalignments

Effect of axis misalignments on determination of optical model

Getting knowledge of axis misalignments now a high priority.

No misalignment

0.3 degrees (radial

rotation)

0.3 degrees (transverse rotation)

L15G 90 min fits

20050208 20050227

20050115 20050207

ARG Duplication

Renaming

Duplication

G2_SEV1_L20S_20050101_145013_V002.hdf

G2_SEV1_L20S_20050101_150711_V002.hdf

G2_SEV1_L20S_20050101_144500_V002.hdf

G2_SEV1_L20S_20050101_151500_V002.hdf

G2_SEV1_L20S_20050101_150000_V002.hdf

15

17

SEVIRI

GERB

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 ?