SADDU meeting, IUP Bremen, June 16/17, 2008 SCIAMACHY limb pointing performance: 2002 – 2008 C....

SADDU meeting, IUP Bremen, June 16/17, 2008

SCIAMACHY limb pointing performance: 2002 – 2008

C. von Savigny1, M. Gottwald2, E. Krieg2, K. Bramstedt1, S. Noël1, J. W. Kaiser3, A. Rozanov1, H. Bovensmann1, John P. Burrows1, 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, 2008IUP Bremen


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


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 O3.

• 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

0

20

40

60

80

10-6

10-5

10-4

10-3

10-2

10-1

750 nm

355 nm

305 nm

280 nm

Limb radiance [a.u.]

Ta

ng

en

t h

eig

ht

[km

]

“knee”


Current version of TRUE (1.7)

Technical features of TRUE (Vs 1.7):

• IUP dynamical O3 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 O3 profiles in upper stratosphere and lower mesosphere not well established not usable for pointing retrievals

TRUE version O3 climatology P & T profiles Mean offset 2004 (L1 vs. 5.04)

1.4 UGAMP NASA ≈ 1.1 km

1.6 UGAMP ECMWF ≈ 1.7 km

1.7 IUP dymamic ECMWF ≈ 1.5 km


Sample fits

35 km

50 km

Measurements in blackModel in red

August 4, 2007


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


SCIAMACHY measurement sequence

Solar spectrum measurement in sub-solar geometry(state 53)

Sun-aquisition at TOA during solar occultation (state 47)


SF active

SF not yet active

mism

atchState 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 = 2nd last readout)


Comparison of sun acquisition (state 47) and TRUE retrievals

TRUE vs. 1.7 State 47

TRUE State 47Offset: 0.99 km 1.51 kmAmplitude: 0.92 km 0.95 kmLinear trend: 16 m / month 22 m / month

Least-sqares-fit parameters:


• Observed Anomalies (LoS angle)

- Solar occultation: azimuth = 0.1 deg2 elevation = -0.04 deg3

jumps of the ASM and ESM readings when switching to Sun Follower (State 47)2 at 17.2 km3 at 100 km

- Subsolar: azimuth = 0.05 deg4 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 deg1

‘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)


‘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: pt = pi + pp + pe (t = total misalignment, i = instrument, p = platform, e = extra)

- roll: rt = ri + rp + re

- yaw: yt = yi + yp + ye

Misalignment modeling II


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 misalignment Limb 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°


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 O3 profile validation results [Brinksma et al., 2005]

TRUE results (Level 1 data 5.04 and 6.01/02)


O3

z

West

East

1234

West – East variations of TH offsets

150 – 200 m

Indications for horizontally tilted scanning


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 O3 validation results for the 4 azimuthal segments [Lolkema et al., 2005]

Vertical shifts between adjacent O3 profiles: 150 – 200 m

for a horizontal distance of 240 km

Consistency of different results

TRUE, extra roll misalignment and O3 profile validation yield qualitatively consistent results. TRUE and roll misalignment also in good quantitative agreement.


TRUE results for Level 1 version 6.03


Long-term trends in tropical mean pointing offset ?

Apparent trend may have many reasons:

e.g., solar cycle variations of tropical, upper stratospheric O3

No final conclusions possible right now, but monitoring required


East – West difference in TRUE retrievals

East-West difference for L1 vs. 6.03 smaller due to extra roll-misalignment correction


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) O3 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) O3 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

SADDU meeting, IUP Bremen, June 16/17, 2008 SCIAMACHY limb pointing performance: 2002 – 2008 C....

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