SCIAMACHY Limb pointing: Geolocation and Intensities on the Level 1b product Sander Slijkhuis,...

SCIAMACHY Limb pointing: Geolocation and Intensities on the Level 1b product

Sander Slijkhuis,

Remote Sensing Technology Institute

/ 06.12.2006 /ACVE-3/ G. Lichtenberg et al

Overview

Consistency check: expected TH vs. values on L1b product

Effect of ASM movement and detector readout time on measured Intensity profile

(Straylight characterisation)


ESM mirror position and nominal TH values

The ESM LOS angle on L1b product is:

Direct transformation of encoder values: ESM = <encoder> x (#degree/enc.step) – zero_offset

Reflects command execution of instrument

TH value on L1b product is:

Orbit propagator calculation, using LOS from ASM/ESM, satellite position (state vector), mispointing angles, Ellipsoid model (’84)

Purely geometric, no refraction

TH w.r.t Ellipsoid, not above ground

From intended TH to L1b TH:

TH(input) orbit.prop.> SM.angles(input) scan profile > SM.angles(true) SM Positioning > SM.encoder orbit.prop.>TH (L1b)

?? Can scan profile cope with real constant TH during horizontal sweep


ESM mirror position and nominal TH values(II)

ESM LOS, Orbit 21883, first state


ESM mirror position and nominal TH values(III)

TH L1b, Orbit 21883, first state


ESM mirror position and nominal TH values(IV)

TH minus 3.28 km step size, Orbit 21883, first state


ESM mirror position and nominal TH values(V)

TH minus 3.28 km step size, Orbit 21883, states 1-16


ESM mirror position and nominal TH values(VI)

TH minus 3.28 km step size, Orbit 21883, states 17-30


ASM mirror position and nominal Longitude

ASM LOS, Orbit 21883, first state


ASM mirror position and nominal Longitude(II)

There is a shift in longitude between even and odd horizontal sweeps:

Note: plotted shift is for pixel 0 (1023 in channel 2)

For pixel 1023 (0 in channel 2) shift is 1/6 pixel smaller and near-zero

For a homogeneous ground/cloud albedo, there is an intensity gradient from East to West

Due to shift, this gives alternating intensities in profile

If ‘knee’ is always at same profile step, this might lead to systematic offset in knee height

Straylight comes from the total clear FOV. If straylight influences the limb retrieval, its

relative contribution is statistically larger for West columns than for East columns


ASM mirror position and Intensity profiles

Intensity profile at 350 nm, for 4 vertical columns, Orbit 5914, states 0-3


Summary

There is a jitter in ESM / TH retrieval with amplitude of ~100m.Not understood.

ASM scan profile results in column shift between odd/even horizontal sweeps.

East-West gradient in intensity, in combination with column shift or straylight, might give rise to East-West differences in retrieved profile.

SCIAMACHY Limb pointing: Geolocation and Intensities on the Level 1b product Sander Slijkhuis,...

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