page1 SCIAMACHY Calibration Review – ESTEC – Sept 2002 SCIAMACHY Tangent Height Verification H.Bovensmann University of Bremen Institute of Remote Sensing

page2 SCIAMACHY Calibration Review – ESTEC – 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

page3 SCIAMACHY Calibration Review – ESTEC – 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)

page4 SCIAMACHY Calibration Review – ESTEC – 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

page5 SCIAMACHY Calibration Review – ESTEC – 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.

page6 SCIAMACHY Calibration Review – ESTEC – Sept 2002 UV limb radiance profiles optical thin optical thick „knee“

page7 SCIAMACHY Calibration Review – ESTEC – 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

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

page9 SCIAMACHY Calibration Review – ESTEC – 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)

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

page11 SCIAMACHY Calibration Review – ESTEC – 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

page12 SCIAMACHY Calibration Review – ESTEC – 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: km u tangent height step: km u tangent height step error: 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

page13 SCIAMACHY Calibration Review – ESTEC – Sept 2002 First Results

page14 SCIAMACHY Calibration Review – ESTEC – 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

page15 SCIAMACHY Calibration Review – ESTEC – Sept 2002 Occultation

page16 SCIAMACHY Calibration Review – ESTEC – 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 = s down to 290 nm) u Documentation of results