SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

First verification of SCIAMACHY’s polarisation

measurements

Gijs Tilstra, Martin de Graaf, Piet Stammes

KNMI, De Bilt, The Netherlands

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

SCIAMACHY has 7 broadband polarisation monitoring devices (PMDs):

Central wavelengths:

340, 485, 650, 850, 1560, 2320 nm (Q), 850 nm (U)

Purpose:

1. Polarisation-correction: correct radiance I for instrumental sensitivity to Q and U polarisation.

2. Imaging of Earth scenes: PMDs have higher spatial resolution (30x7.5 km2) than the spectral channels; useful for cloud detection.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Verification of fractional polarisation Q/I

for orbit 2209 (2 Aug. 2002) – nadir states

Blue=West pixels

Black=East pixels

Model value data

( < 300 nm):

Conclusion: OK.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 1 (c = 340 nm):

Conclusions:

- Resembles model

values.

- Zero’s occur.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 2 (c = 485 nm):

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 3 (c = 650 nm):

Conclusions:

- A “hole” appears

without any values.

- “Mirroring” of Q/I

between 0-30 N.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 4 (c = 840 nm):

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 5 (c = 1560 nm):

Conclusions:

- Q/I is around 0.5.

- Many zero’s.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

PMD 6 (c = 2320 nm):

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Verification of U; model value

U/Q ( < 300 nm)

vs. tan 2ss

for 00 pixels

Conclusion:

U is switching sign

(due to wrong use

of -0 ?)

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Verification of U; PMD values

Measured U/Q

vs. tan 2ss

for 00 points

Conclusions:

- U is switching sign

like the model value.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Use of PMDs for imaging nadir states

- PMD2= “Blue”, PMD3=“Red”, PMD4=“Green”.

- 4x32 = 128 PMD pixels in swath of 960 km.

- Pixel size: 30 km along track x 7.5 km across track.

- Pixel stretching needed to fill gaps between consecutive forward scans.

- Geolocation for individual PMD pixels is not given, but has to be calculated by interpolation.

- Signals are normalized to maximum per state.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Global overview of nadir states on 2 Aug. 2002

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Netherlands/Germany/Belgium

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

MODIS image of 2 Aug. 2002, 10:30 UTC

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Corsica (non-stretched pixels)

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Conclusions

1. Problems with nadir polarisation Q/I and U/I from the PMDs, due to:

- probably wrong usage of (relative) azimuth

- errors in processing of PMD signals into polarisation data Q/I and U/I.

2. Problem: overlap polarisation is exactly zero everywhere.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Conclusions (cont’d)

3. The following data seem to be OK:

- date/time, lat/lon, solar zenith angle, viewing zenith angle, viewing azimuth, solar azimuth.

4. From PMD images we may conclude:

- The signals of PMD 2, 3, and 4 seem to be OK.

- The nadir lat/lon coordinates are accurate to at least a few km.

SCIAMACHY Calibration Review, ESTEC, 11 September 2002 Tilstra, De Graaf, Stammes

Corsica (stretched pixels)