page1SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002

SCIAMACHY Tangent Height Verification

H.Bovensmann

University of BremenInstitute of Remote Sensing

http://www.sciamachy.de

page2SCIAMACHY Calibration Review – ESTEC – 9 - 13 Sept 2002

Content

Expectations from Instrument On-Ground Verification

General Approach Limb tangent height verification Occultation tangent height verification Summary/Future Work

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On-Ground Verification

IFOV 0.045° (2.8 km) Absolute pointing knowledge error 0.079° (4.6

km) Relative pointing knowledge error 0.027° (1.6

km) Cycle step size error 0.003° (170 m)

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General Approach

Use features in the backscattered or transmitted radiance with well known height dependence Absorption of O2 and CO2

Scattering and absorption characteristics at 305 nm around 45 km (so-caled „knee“)

Retrieve tangent height from this „features“ Compare retrieved tangent height to data from

L0/L1

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Limb tangent height verification

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)

UV-Knee, Sioris et al. TRUE (modified Knee), J. Kaiser et al.

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0

20

40

60

80

100

0.1 1 10 100 1000

= 260 nm = 270 nm = 280 nm = 290 nm = 300 nm

Uncalibrated limb radiance [BU]

Ta

ng

en

t he

igh

t [km

]

UV limb radiance profiles

optical thin

optical thick

„knee“

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UV-Knee (Sioris et al.)

find the knee heights for each spectral pixel around 305 nm on the original TH grid

find the knee heights simulated by a model (single scattering, O3 from model atmosphere)

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).

compare THs given by the polynomial expressions for observation and model.

Accuracy of better than 220 m is deduced for Orbit 1230

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UV-Knee (Sioris et al.)

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Tangent Height Retrieval by UV-B Exploitation (TRUE) (J. Kaiser et al.)

Generalised Knee Method (Janz et al. 1996) normalised model spectra (UV-B) for all tangent

heights are fitted simultaneously to the measured spectrum by varying: tangent height (mean and step size), O3 concentration profile pressure profile

Recommended fit interval 289 – 310 nm (ch. 1b)

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theoretical retrieval precision reaches 100 m for tangent heights 33–54 km

For 1.6 km step size measurements the following results were observed: mean tangent height error: 0.119 km tangent height step: 1.657 km tangent height step error: 0.012 km

Method is also sufficient to investigate parallelism of limb azimuth scan

Results consistent with Sioris et al. (220 m accuracy)

Report on methodology and first results is available

TRUE Results

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Occultation tangent height verification

O2 B band absorption ch. 4 (690 nm) 10 – 65 km

CO2 absorption ch. 6 (1560 nm) 10 – 20 km L0 data Solar Occultations from orbit 514, 947 L1 data Solar Occultations from orbit 2056

Systematic shift of approx. 2 km observed w.r.t. L1 data

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Occultation

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Summary

Limb tangent height accurate within 100 – 200 m Limb pointing performance seems to be an order

of magnitude better than predicted from on-ground performance values (SCIA & ENVISAT)

Occultation tangent height shows a systematic deviation of approx. 2 km

Future Work: Systematic analysis of orbit variations in tangent height Data Analysis w.r.t. parallelism azimuth scan of limb

measurements based on high data rate data with consolidated β-states (Tint = 0.375 s down to 290 nm)

Documentation of results