Rutherford Appleton Laboratory

Requirements Consolidation of the Near-Infrared Channel of the GMES-Sentinel-5 UVNS

Instrument:

Initial trade-off:Height-resolved aerosol retrieval

R.Siddans

MTR: Estec, 1 October 2013

Overview• Follows work in Eumetsat , ESA Camelot, ESA S4 studies to define

instrument requirements • Uses same optimal estimation retrieval simulation scheme employed in the

previous studies. These updated to simulate S5.• Retrieval scheme based on OE

• Aerosol optical properties (single-scat albedo, phase fn) assumed• Extinction profile retrieved (2km vertical grid)• Wavelength shift retrieved• Results integrated to various layer aerosol optical depths (AOD) for

comparison to user requirements • Requirement on BL and free-trop column AOD=0.05 (@5-50 km

spatial resolution)• A priori error on surface albedo 0.01 assumed

• Estimated standard deviation (ESD) from measurement noise derived from solution covariance

• Errors to be assessed by performing “linear mapping” of error spectra (Δy) into L2 error (Δx): Δx = G Δy

Extension of retrieval scheme

• Albedo retrieved with linear wavelength dependence (2 terms)• For Concept A, spectral range 705-754nm omitted and linear

dependence fitted in both oxygen A and B bands (4 terms)• Error in albedo variation with 2nd order spectral dependence mapped• Peak intensity of both Fluorescence Gaussians mapped or optionally fitted • Retrieval of ILS width implemented as option

• Retrieval conditions now based on SWIR study scenarios• For aerosol just defines surface albedo assumed and spatial grid of

scenarios

Instrument model• Simulations re-run with latest information from ESA on noise model and

spectral response functions.• Noise model specifies Signal:Noise (SNR) as funciton of radiance (L):

• Two instrument concepts defined:• Concept A: 0.39nm resolution, 685-773nm, O2 A+B bands, H2O• Concept B: 0.12nm resolution, 755-775m, only O2 A-band

• Concept B has assumes the “tropical-dark” noise model.• Similar to requirement of SNR 500 in continuum of tropical dark

(with b=0)• Both concepts simulated + concept B scaled to different resolutions

Simulated instruments

Geophysical scenarios• Aerosol retrieval performance very dependent on

• view / solar geometry due to variations in aerosol phase function, light path for aerosol light-path for O2 absorption etc.

• Surface albedo• Assumed aerosol type + size (asymmetry, single scatter albedo etc)

• Earlier simulations show that 0.05 requirement cannot be met for height-resolved quantities in all (most) S5 observing conditions

• Difficult to concisely summarise performance and optimise inst/L1 requirements without considering many conditions

• Aim to provide realistic along-orbit simulations of retrieval peformance• Based on S5 orbit model + linear retrieval simultions for range of conditions:

• Solar zenith angle: 30, 45, 60, 70.,75, 80o • View zenith angle: 0, 30, 50, 60, 70o • Relative azimuth angle: 0, 30, 60, 90,120,150,180o • Surface albedo: 0.01,0.1,0.2,0.3,0.5,0.7,0.9 • Aerosol profile: Camelot Mid-latitude background and “tropical dust ocean” conditions (total

AOD 0.2 and 0.67, respectively)

Instrumental errors simulated• Instrument noise: The impact of instrument noise on the estimated

precision of aerosol layer optical depth is estimated via the ESD as described above.

• Sensitivity to errors in the spectral response function or instrument line shape (ILS): This is determined by linearly mapping a 1% error in the width of the assumed ILS.

• Sensitivity to errors in radiometric gain is determined by linearly mapping the impact of a 10% gain error, i.e. multiplying the observed spectrum by a factor 1.1

• Sensitivity to an additive absolute radiometric accuracy requirement (ARA)• Mapped MTRD specification as gain (~3%)• Mapped proposed relaxation as a separate radiometric offset.• Other representations of the error possible / more realistic ?

• Intra-band co-registration• Mapping spatial variation in albedo associated with spatial shift, with 2nd

order wavelength dependence in band• First order would be handled by linear retrieval of albedo

Absolute Radiometric Accuracy (ARA) (MR-LEO-UVN-160)

• MRTD:

• At the MAG the requirement was relaxed to apply only over a given signal level (Tropical dark at 755nm);

Co-registration requirements

These identified as challenging at S4/5 MAG, proposal to relax to 0.3 inter (keep 0.1 intra for NIR)

Change in albedo at 858nm for 20% shift in Spatial response

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)

With H2O modelled and retrieved

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)

With fluorescence retrieved

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)

With fluorescence and spectral response function width retrieved

Weighting functions

Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)

S5B (with fluorescence and ILS fitted)

S5A A-band (with fluorescence and ILS fitted)

S5A A+B band (with fluorescence and ILS fitted)

S5B (with fluorescence and ILS fitted)

S5B (with fluorescence and ILS fitted)

Conclusions• Height resolved aerosol retrievals improve with increasing (finer) spectral

resolution, even considering an instrument with fixed total throughput.• Dependence on geometry large• Purely on grounds of theoretical precision both instrument concepts A and

B considered here are competitive• Option A compensates for lower resolution via

• including B-band (but this involves modelling aerosol spectral properties)

• more favourable noise model• Concept B definitely preferred if only A-band used.

• Sensitivity to spectral response function errors seems well mitigated by retrieval. Issue now is spectral dependence in ILS knowledge/stability

• Relaxation to ARA leads to significant error (when mapped as offset)• Mapping of 2nd order albedo variations generated by 20% intra-band co-

registration, gives large errors• Amplitude vs spectral dependence of intra-band co-registration needs

to be better understood to estimate realistic effect

S5B 0.06nm resolution (with fluorescence and ILS fitted)