1. Requirements Consolidation of the Near-Infrared Channel of the GMES-Sentinel-5 UVNS Instrument: FP, 25 April 2014, ESTEC Height-resolved aerosol R.Siddans (RAL)

2. 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 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

3. Extension of retrieval scheme
Scheme applied to both oxygen A and B bands (for various instrument concepts), over much wider range of conditions than before, leading to global maps of retrieval performance.
Albedo retrieved with linear wavelength dependence (2 terms)
For Concept A, spectral range nm omitted and linear dependence fitted in both oxygen A and B bands (4 terms)
Fluorescence included by mapping as error or including in retrieval
Retrieval of ILS width implemented as option

4. 0.39nm resolution; 2x better throughput
Concept B
0.12nm resolution
O2-A Band
Concept A
0.39nm resolution; 2x better throughput
O2-A Band
O2-B Band
H2O

5. Simulated instruments

6. 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 retrievalsfor 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)

7. 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

8. Co-registration requirements
These identified as challenging at S4/5 MAG, proposal to relax to 0.3 inter (keep 0.1 intra for NIR)

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

10. Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)
With H2O modelled and retrieved

11. Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)
With fluorescence retrieved

12. Retrieval errors for favourable geometry (LZA=60,SZA=60,RAZ=90)
With fluorescence and spectral response function width retrieved

13. Weighting functions

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

15. Retrieval Simulation results
Estimated Standard Deviation (ESD) = retrieval precision (random noise) for Free-tropospheric column
Figure shows an error spectrum (the worst case for scene 03) compared to the derivative of the measured radiance wrt wavelength.
Concept B
Concept A (A Band)
Concept A (A+B Bands)

16. Retrieval Simulation results
1% error in instrument spectral line-shape (ILS) = spectral response function
If not fitted in retrieval
Figure shows an error spectrum (the worst case for scene 03) compared to the derivative of the measured radiance wrt wavelength.
Concept B
Concept A (A Band)
Concept A (A+B Bands)

17. Retrieval Simulation results
ARA relaxation, considered as radiometric offset error (assuming High-Lat Dark limit)
Figure shows an error spectrum (the worst case for scene 03) compared to the derivative of the measured radiance wrt wavelength.
Concept B
Concept A (A Band)
Concept A (A+B Bands)

18. Retrieval Simulation results
Error due to 2% shift in spatial response with 2nd order wavelength dependence
Real magnitude not clear:
Result highlights potential issue if spatial co-registration or spectral dependence not known.
Figure shows an error spectrum (the worst case for scene 03) compared to the derivative of the measured radiance wrt wavelength.
Concept B
Concept A (A Band)
Concept A (A+B Bands)

19. Conclusions (Aerosol)
Height resolved aerosol retrievals improve with increasing (finer) spectral resolution, even considering an instrument with fixed total throughput.
Dependence on geometry large – no concept compliant over whole swath
Concept B clearly preferred but retrieval remains a challenge
Better performance over more of swath
Option A only competitive if both O2 A and B bands used
Even then sensitivity to instrumental errors larger than concept B
Introduces need to model spectral dependence of aerosol optical properties (only limited demonstrations for GOME-2 exist)
For option B relaxation of ARA requirement (to HL-dark level) seems acceptable (if mapped as offset); This is not the case for concept A.
Now that concept A selected, height-resolved aerosol in terms of layer optical depths meeting requirements very unlikely for S5 and would unrealistically drive requirements for the band (remains possible for S4)
With concept A, could aim for less challenging aerosol layer height under high aerosol load (no quantitative user requirement or this but could be useful) – implications of this should be taken into account in future