1. Page 1 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric performance ESTEC – SEPTEMBER 10, 2002 Instrument performance session Gilbert BARROT

2. Page 2 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance 1.Non-linearity 2.Non-uniformity 3.Radiometric calibration

3. Page 3 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Non-linearity determination Use of a dedicated tool (CALEX in mode NL) to compute the non-linearity factors for each gain setting of the GOMOS instrument Observation of the same star outside the atmosphere with different integration times (use of the GOMOS linearity monitoring mode)

4. Page 4 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Non-linearity Star 18 mv=1.17 T=9700K Gain setting SPA 4/4 SPB 0/0 Nominal gain setting for the star observation (except Sirius) Instrument set in Linearity monitoring mode

5. Page 5 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Non-linearity for column 250 (SPA1) Electronic chain Offset Deviation wrt Linear regression Example: Star 18 mv=1.17 T=9700K Gain setting SPA 4/4

6. Page 6 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Electronic chain offset for SPA1 gain 4 Example: Mean offset 122.4 ADU St. dev. 1.2 ADU Star 18 mv=1.17 T=9700K Very stable < 0.1 ADU

7. Page 7 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Non-linearity for SPA1 gain 4 Example: f nlin <0.1% on full range

8. Page 8 GOMOS Calibration Review - ESTEC - 10 September 2002 GOMOS radiometric performance Non-linearity calibration Specification: 0.1% per decade Measured: <0.1% on full dynamic range Conclusions Determination of the electronic chain offset during the linearity calibration

9. Page 9 GOMOS Calibration Review - ESTEC - 10 September 2002 Non-uniformity GOMOS radiometric performance Use of a dedicated tool (CALEX in modes QP/QB) to compute the Pixel Response Non-Uniformity An accurate PRNU map is needed when mixing information from different spatial bands (e.g. background correction) Correction of PRNU LUT stored in the GOMOS calibration database Analysis of GOMOS observations in bright limb conditions GOMOS is set in Uniformity Monitoring Mode (no star)

10. Page 10 GOMOS Calibration Review - ESTEC - 10 September 2002 Non-uniformity results Non-uniformity has been measured at pixel and at band level. Measurements are made in bright limb. ‘Bad pixel’ in DMSA2 (-26%) already seen on ground has not moved Interferences in IR present the same shape and amplitude than on ground (anyway the CCD temperature affects the location of the inteferences) Slit profile measurement is consistent with ground Performance is consistent with expected GOMOS radiometric performance

11. Page 11 GOMOS Calibration Review - ESTEC - 10 September 2002 Non-uniformity map of IR2 Interferences GOMOS radiometric performance

12. Page 12 GOMOS Calibration Review - ESTEC - 10 September 2002 Impact of a ‘bad’ PRNU map in bright limb GOMOS radiometric performance Using on-ground PRNU map SPB2 Using updated PRNU map Star 5 / Orbit 2130 Bright limb TGP alt = 42 km

13. Page 13 GOMOS Calibration Review - ESTEC - 10 September 2002 Non uniformity Conclusions GOMOS radiometric performance Non-uniformity shall be monitored every month, especially in IR, where interferences position is influenced by temperature. No variation is expected if temperature is stable. No trend has been noticed yet : no pixel response degradation even for ‘’hot pixels’’.

14. Page 14 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric calibration Use of a dedicated tool (CALEX in mode RS) to compute the deviation between the two spectra Comparison of GOMOS measurements (star spectra) with existing calibrated star spectra GOMOS radiometric performance Correction of the radiometric sensitivity LUT stored in the GOMOS calibration database

15. Page 15 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric calibration GOMOS radiometric performance SPA2 Observed relative variations are used to correct the LUT Mesured star flux has been converted from electrons to physical units using the Radiometric Sensitivity LUT

16. Page 16 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric calibration GOMOS radiometric performance SPA1

17. Page 17 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric sensitivity in the invalid spectral range Radiometric calibration GOMOS radiometric performance 375-405 nm

18. Page 18 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric stability GOMOS radiometric performance Sirius spectrum measured between orbit 2154 and 2186 (3 days)

19. Page 19 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric stability GOMOS radiometric performance Sirius spectrum measured between orbit 2154 and 2186 (3 days) Variation level between orbits is compatible with the theoretical noise

20. Page 20 GOMOS Calibration Review - ESTEC - 10 September 2002 Radiometric calibration Very stable radiometry. No trend observed yet Conclusions Absolute radiometric calibration has no impact on the level 2 products (species density profiles) GOMOS radiometric performance Transmission and associated error are computed from relative variations of the star spectra