1. page1 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 SCIAMACHY Tangent Height Verification H.Bovensmann University of Bremen Institute of Remote Sensing http://www.sciamachy.de

2. page2 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Content F Expectations from Instrument On-Ground Verification F General Approach F Limb tangent height verification F Occultation tangent height verification F Summary/Future Work

3. page3 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 On-Ground Verification F IFOV 0.045° (2.8 km) F Absolute pointing knowledge error 0.079° (4.6 km) F Relative pointing knowledge error 0.027° (1.6 km) F Cycle step size error 0.003° (170 m)

4. page4 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 General Approach F Use features in the backscattered or transmitted radiance with well known height dependence u Absorption of O 2 and CO 2 u Scattering and absorption characteristics at 305 nm around 45 km (so-caled „knee“) F Retrieve tangent height from this „features“ F Compare retrieved tangent height to data from L0/L1

5. page5 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Limb tangent height verification F Idea: In the UV the backscattered radiance has a maximum („Knee“) in the upper stratosphere determined by dominating Rayleigh scattering (number density) above and (optical thick) Ozone absorption below the maximum (Janz et al. 1996) F UV-Knee, Sioris et al. F TRUE (modified Knee), J. Kaiser et al.

6. page6 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 UV limb radiance profiles optical thin optical thick „knee“

7. page7 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 UV-Knee (Sioris et al.) F find the knee heights for each spectral pixel around 305 nm on the original TH grid F find the knee heights simulated by a model (single scattering, O 3 from model atmosphere) F fit a curve to the wavelength (x-axis) vs knee z (y- axis) for the simulation and especially for the vertically undersampled observations (results have also been obtained for knee height vs pointing angle). F compare THs given by the polynomial expressions for observation and model. F Accuracy of better than 220 m is deduced for Orbit 1230

8. page8 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 UV-Knee (Sioris et al.)

9. page9 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Tangent Height Retrieval by UV-B Exploitation (TRUE) (J. Kaiser et al.) F Generalised Knee Method (Janz et al. 1996) F normalised model spectra (UV-B) for all tangent heights are fitted simultaneously to the measured spectrum by varying: u tangent height (mean and step size), u O 3 concentration profile u pressure profile F Recommended fit interval 289 – 310 nm (ch. 1b)

10. page10 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Normalised (TH 14, 32 km) Limb Spectra Channel 1 and 2

11. page11 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 TRUE Data Analysis F Up to now data with 1.6 km step size (26. May 2002) were analysed F raw data [counts] are divided by the integration time [s] to yield a flux-like quantity [counts/s] F preliminary dark current correction is performed by subtracting a measurement with 150 km tangent height F normalisation of all spectra to approx. 32 km tangent height (TH 14) eliminates most of the effects of the solar Fraunhofer structure and multiplicative instrumental artefacts

12. page12 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 F theoretical retrieval precision reaches 100 m for tangent heights 33–54 km F For 1.6 km step size measurements the following results were observed: u mean tangent height error: 0.119 km u tangent height step: 1.657 km u tangent height step error: 0.012 km F Method is also sufficient to investigate parallelism of limb azimuth scan F Results consistent with Sioris et al. (220 m accuracy) F Report on methodology and first results is available TRUE Results

13. page13 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 First Results

14. page14 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Occultation tangent height verification F O 2 B band absorption ch. 4 (690 nm) 10 – 65 km F CO 2 absorption ch. 6 (1560 nm) 10 – 20 km F L0 data Solar Occultations from orbit 514, 947 F L1 data Solar Occultations from orbit 2056 F Systematic shift of approx. 2 km observed w.r.t. L1 data

15. page15 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Occultation

16. page16 SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002 Summary F Limb tangent height accurate within 100 – 200 m F Limb pointing performance seems to be an order of magnitude better than predicted from on-ground performance values (SCIA & ENVISAT) F Occultation tangent height shows a systematic deviation of approx. 2 km F Future Work: u Systematic analysis of orbit variations in tangent height u Data Analysis w.r.t. parallelism azimuth scan of limb measurements based on high data rate data with consolidated β-states (T int = 0.375 s down to 290 nm) u Documentation of results