1. SADDU meeting, IUP Bremen, June 16/17, 2008 SCIAMACHY limb pointing performance: 2002 – 2008 C. von Savigny 1, M. Gottwald 2, E. Krieg 2, K. Bramstedt 1, S. Noël 1, J. W. Kaiser 3, A. Rozanov 1, H. Bovensmann 1, John P. Burrows 1, and the SCIAMACHY Quality Working Group 1 Institute of Environmental Physics, University of Bremen, Bremen, Germany 2 Deutsches Zentrum für Luft- und Raumfahrt (DLR), Wessling, Germany 3 European Centre for Medium-Range Weather Forecasts (ECMWF), Reading, UK SADDU meeting, June 16/17, 2008 IUP Bremen

2. SADDU meeting, IUP Bremen, June 16/17, 2008 Outline Information on SCIAMACHY pointing errors / Envisat attitude errors „Knee“ retrievals using TRUE Results on Level 1 version 5.04 data - Elevation/azimuth discontinuities occuring at sun aquisition during solar occultation (State 47 measurement) - Sun position measurements in „sub-solar mode“ (State 53 measurements) Results on Level 1 version 6.03 data

3. SADDU meeting, IUP Bremen, June 16/17, 2008 TRUE = Tangent height Retrieval by UV-B Exploitation TRUE [Kaiser et al., 2004] exploits the maximum (”knee”) in the UV limb radiance profiles caused by the absorption in the Huggins- and Hartley-bands of O 3. An optimal estimation (OE) scheme drives the RT model SCIARAYS [Kaiser, 2001] Tangent height weighting functions are calculated with the perturbation technique Field of view integration is considered (Gaussian FOV, FWHM = 2.8 km) Measured Limb spectra interpolated on 1 nm wavelength grid between 295 nm and 305 nm SCIAMACHY TH steps 11 – 15 used, i.e., tangent heights between ≈ 35 km and ≈ 50 km “knee”

4. SADDU meeting, IUP Bremen, June 16/17, 2008 Current version of TRUE (1.7) Technical features of TRUE (Vs 1.7): IUP dynamical O 3 climatology [Lamsal et al., 2003] together with EP-TOMS and OMI Total ozone column data for each day and location Use of ECMWF background atmosphere data rather than the NASA climatology (NASA report 1292) Quality of ECMWF O 3 profiles in upper stratosphere and lower mesosphere not well established  not usable for pointing retrievals TRUE versionO 3 climatologyP & T profilesMean offset 2004 (L1 vs. 5.04) 1.4UGAMPNASA≈ 1.1 km 1.6UGAMPECMWF≈ 1.7 km 1.7IUP dymamicECMWF≈ 1.5 km

5. SADDU meeting, IUP Bremen, June 16/17, 2008 Sample fits 35 km 50 km Measurements in black Model in red August 4, 2007

6. SADDU meeting, IUP Bremen, June 16/17, 2008 Monthly and tropical (20°S – 20°N) mean TH offsets Mean value 06/2006 – 06/2007: 0.27 ± 0.07 km Level 1 vs. 6.02/6.02 Level 1 vs. 5.04

7. SADDU meeting, IUP Bremen, June 16/17, 2008 SCIAMACHY measurement sequence Solar spectrum measure- ment in sub-solar geometry (state 53) Sun-aquisition at TOA during solar occultation (state 47)

8. SADDU meeting, IUP Bremen, June 16/17, 2008 SF active SF not yet active mismatch State 47 elevation discontinuities (Stefan Noël, IUP) Analysis is based on measurements during pointing phase above atmosphere within solar occultation (State ID 47) Lv0 data have been processed with CFI s/w Mispointing = LOS – PredictedSun (@BCPS=1055 = 2 nd last readout)

9. SADDU meeting, IUP Bremen, June 16/17, 2008 Comparison of sun acquisition (state 47) and TRUE retrievals TRUE vs. 1.7 State 47 TRUEState 47 Offset:0.99 km1.51 km Amplitude:0.92 km0.95 km Linear trend:16 m / month22 m / month Least-sqares-fit parameters:

10. SADDU meeting, IUP Bremen, June 16/17, 2008 Observed Anomalies (LoS angle) -Solar occultation: azimuth = 0.1 deg 2 elevation = -0.04 deg 3 jumps of the ASM and ESM readings when switching to Sun Follower (State 47) 2 at 17.2 km 3 at 100 km -Subsolar:azimuth = 0.05 deg 4 elevation = -0.02 deg jump of the ESM readings when switching to Sun Follower and temporal shift of maximum signal (State 53) -Limb: elevation = -0.016 deg 1 ‘SCIAMACHY is pointing too low  tangent heights are too large’ 1 possibly larger (not considered for determination of misalignment correction) Misalignment modeling I (Manfred Gottwald, DLR)

11. SADDU meeting, IUP Bremen, June 16/17, 2008 ‘Modelling the Discontinuities’ Assumption -jumps, temporal shift in subsolar signal and limb BIAS are caused by extra misalignment (known misalignment LoS: pitch = 0.00065°, roll = 0.00167°, yaw = - 0.22746°) Misalignment budget -pitch: p t = p i + p p + p e (t = total misalignment, i = instrument, p = platform, e = extra) -roll: r t = r i + r p + r e -yaw: y t = y i + y p + y e Misalignment modeling II Which Jump results from which Misalignment? subsolar elevation  roll subsolar azimuth = time of maximum signal  yaw and pitch Sun occultation azimuth: not analysed Sun occultation elevation  pitch and roll

12. SADDU meeting, IUP Bremen, June 16/17, 2008 Derived extra misalignments Total misalignment correction: AOCS = [pitch, roll, yaw] Level 1 data vs 6.03: init.file version 4.05 with AOCS = [-0.0260°, -0.020°, -0.21846°] Level 1 data vs 6.02: init.file version 4.04 with AOCS = [-0.0196°,0.000°,-0.22746°] Pitch misalignmentLimb tangent height difference 19.6 mdeg≈ 1.1 km 26 mdeg≈ 1.45 km Extra misalignment correction: pitch: -0.0260° roll: -0.020° yaw:+0.009°

13. SADDU meeting, IUP Bremen, June 16/17, 2008 Mean TH offset (01/2004 – 04/2006): approximately 1.5 km This is in very good agreement with: the extra pitch misalignment derived from solar occultation measurements by Gottwald et al. [2007] leading to limb TH error of about 1.45 km. stratospheric O 3 profile validation results [Brinksma et al., 2005] TRUE results (Level 1 data 5.04 and 6.01/02)

14. SADDU meeting, IUP Bremen, June 16/17, 2008 O3O3 z West East 1 2 3 4 West – East variations of TH offsets 150 – 200 m Indications for horizontally tilted scanning

15. SADDU meeting, IUP Bremen, June 16/17, 2008 East-West difference in TRUE TH offsets: ≈ 200 m no apparent change with 12/2003 orbit model update no pronounced seasonal variation increase since mid 2006 ? This is in qualitative and good quantitative agreement with the roll misalignment derived from solar elevation jumps in sub-solar measurements: Extra roll misalignment: -0.020° ± 0.001°  vertical shift for 480 km horiz. distance: ≈ 160 m Also: qualitative agreement with O 3 validation results for the 4 azimuthal segments [Lolkema et al., 2005] Vertical shifts between adjacent O 3 profiles: 150 – 200 m for a horizontal distance of 240 km Consistency of different results  TRUE, extra roll misalignment and O 3 profile validation yield qualitatively consistent results. TRUE and roll misalignment also in good quantitative agreement.

16. SADDU meeting, IUP Bremen, June 16/17, 2008 TRUE results for Level 1 version 6.03

17. SADDU meeting, IUP Bremen, June 16/17, 2008 Long-term trends in tropical mean pointing offset ? Apparent trend may have many reasons: e.g., solar cycle variations of tropical, upper stratospheric O 3  No final conclusions possible right now, but monitoring required

18. SADDU meeting, IUP Bremen, June 16/17, 2008 East – West difference in TRUE retrievals  East-West difference for L1 vs. 6.03 smaller due to extra roll-misalignment correction

19. SADDU meeting, IUP Bremen, June 16/17, 2008 Conclusions Absolute tangent height offset – pitch misalignment (L1 vs. 5.04): 1) Modelling of elevation/azimuth discontinuities [Gottwald et al.] about 1.45 km in limb 2) TRUE vs. 1.7 about 1.5 km (01/2004 – 04/2006) 3) O 3 profile validation at low latitudes about 1.5 – 2 km offset at low latitudes At high latitudes inconsistent picture, particularly in NH Horizontally tilted scanning – roll misalignment (L1 vs. 5.04): 3 independent techniques show qualitatively (i.e. in terms of sign) consistent results: 1) O 3 profile validation [Lolkema et al.]: about 300 – 400 m East-West difference * 2) TRUE: about 200 m East-West difference 3) Modelling of Sub-solar elevation jumps [Gottwald et al.]: about 160 m East-West difference TRUE TH offsets for L1 vs. 6.03 much smaller than for vs. 5.04. Remaining variations (seasonal, long-term) need to be investigated further * Vertical East-West difference for 480 km horizontal distance

20. SADDU meeting, IUP Bremen, June 16/17, 2008 Grey bars: Australian updates Red bars: Caribbean updates Before December 2003 orbit model improvement After December 2003 orbit model improvement Effect of December 2003 orbit model improvement

21. SADDU meeting, IUP Bremen, June 16/17, 2008 Longitudinal variation of TH offset Before December 2003 orbit model improvement After December 2003 orbit model improvement

22. SADDU meeting, IUP Bremen, June 16/17, 2008 Validation of Stratozone (vs. 1.63) O 3 profiles with TRUE correction Borchi and Pommerau, ACP [2007] SAOZ – SCIAMACHY comparison Dupuy et al., ACPD [2007] ACE-FTS – SCIAMACHY comparison  New validation results indicate that TRUE vs. 1.7 (i.e., a TH offset of about 1.5 km for 2004 and 2005) provide a good TH correction for the trace gas retrievals