Combination Approaches GSICS Annual Meeting 29th Feb.- 4th March 2016, Tsukuba, Japan Combination Approaches Bertrand Fougnie for CNES Calibration Center (CNES-DCT/SI/MO + support from CNES-DCT/ME)

Introduction From an initial question addressed on previous GSICS Webmeeting /Workshop GSICS intends to implement various reference methods various results will be available Synergy : How to merge results from various methods ? Is it possible to derive a unique approach ? Which answers could be derived ? GSICS must define/construct a clear strategy

Observation Indicative behavior of targets Several calibration methods could be available Cloud-DCC, Moon, PICS-Desert, Rayleigh, Sunglint, PICS-Antarctica, SNO, Ray-matching, Land… Each target has its own behavior : Magnitude: from very dark to very bright Spectral shape : from white to very pronounced Angular signature : from nearly uniform to large BRDF Polarized properties : from non-polarized to nearly fully polarized Short-term stability : from variable to fully stable Long-term stability : from seasonal variable to fully stable So efficiency range … Indicative behavior of targets May sensitively vary with various parameters

Observation Several calibration methods could be available Cloud-DCC, Moon, PICS-Desert, Rayleigh, Sunglint, PICS-Antarctica, SNO, Ray-matching… Implement several methods will provide various results which will differ (in general) sometimes consistant, sometimes not at all What’s the definition of « consistant » for GSICS needs ? It is often called “calibration error” a radiometric artifact which is not a calibration error  Straylight, spectral rejection, non-linearity, variation with scan, polarization… They are radiometric errors, but not calibration errors They are radiometric biases, but varying with every different situation They are not full instrumental biases  What do we mean by “calibration” ? GSICS has to face the way to provide to users the results : alternatives One single , the best One blended Multiple sets  What’s the best alternative for GSICS needs ?

Example The single The multiple set The Blend

To be evaluated for each band The One-method matrix Sensor to calibrate To be evaluated for each band Uncertainty from implemented method (depending on data sampling) Uncertainty from sensor Characterization to be addressed Spectral response knowledge Straylight Linearity Polarization Radiometric noise … Trending Absolute Interband Cross-calibration … DCC

To be evaluated for each band The Synergy matrix Sensor to calibrate To be evaluated for each band Uncertainty from implemented method (depending on data sampling) Uncertainty from sensor Characterization to be addressed DCC Moon PICS-desert PICS-snow SNO Rayleigh Sunglint Spectral response knowledge Straylight Linearity Polarization Radiometric noise … Trending Absolute Interband Cross-calibration …

Example : combination of Desert, DCC and Rayleigh The Synergy matrix Example : combination of Desert, DCC and Rayleigh for MERIS and PARASOL Sensor to calibrate To be evaluated for each band Uncertainty from implemented method (depending on data sampling) Uncertainty from sensor Characterization to be addressed DCC Moon PICS-desert PICS-snow SNO Rayleigh Sunglint Spectral response knowledge Straylight Linearity Polarization Radiometric noise … Trending Absolute Interband Cross-calibration …

443nm band as a function of VZA MERIS PARASOL DCC Desert Rayleigh

Indicative map for a synergy Generale baseline

Example Application to PARASOL (According Fougnie, IEEE, 2016) Strategy used for the PARASOL revision trending and variation within FOV for band 765 using desert and snow trending for all other interbands based on a compromise between methods Compromise = find for each band the adjustment that minimises all stdev with a high weight for methods with stdev < 0.6% variation within FOV for all other interbands using clouds validation with all other methods checking of the consistency of absolute radiometric calibration between all methods (According Fougnie, IEEE, 2016)

Ex: PARASOL - Monitoring Best compromise B490 = 0.16 D=0.018 B670 = 0.062 Synergy : calibration of the temporal monitoring Calibration versus month B565 = 0.11 B865 = 0.024 B1020 = 0.018 B765 = 0.01

Combination Combination for Trending Synergy for Absolute Calibration Absolute adjusment (Fougnie et al., 2007): mean of 490/565/670 over Rayleigh NIR bands over mean of clouds/sunglint Other methods for validation

Discussion – Questions Derive results from various methods How to define the mean value, or weighted value ? How to define corresponding weights ? Consider theoretical accuracy of methods, as well as sensor behavior Is the mean (or weighted) result the most optimal result for GSICS needs ? How discrepancies will be interpreted ? Ignored ? Will GSICS try to understand what radiometric artifacts could be ? This is the job of every agencies or GPRC Not only a question of maturity for the method, but also maturity for the result itself If results are consistent between methods Do we keep the best theoretical method ? Do we derive a mean/blend in order to reduce residual bias ? If results differ between methods : Do we stay at the level where agency has to investigate the radiometric artifact but try to select the “most relevant” result ? Do we propose all results and clarifying the “applicability” range of each ?

Best method or weighted mean Radiometric explanation The Synergy matrix Sensor to calibrate To be evaluated for each band Uncertainty from implemented method (depending on data sampling) Uncertainty from sensor Characterization to be addressed DCC Moon PICS-desert PICS-snow SNO Rayleigh Sunglint Spectral response knowledge Straylight Linearity Polarization Radiometric noise … Trending Absolute Interband Cross-calibration … 1 One calibration coefficient per band per time … Best method or weighted mean GSICS Output Radiometric explanation

Sorry, more questions than answers… Wrap-up In general, derive results from various (accurate) methods = provide different information from the radiometry In general, results will slightly/largely differ Probably, no universal approach can be defined in advance for combining At least at the beginning, this would be a case-by-case analysis and conclusion GSICS must define how the various calibration sets will be used (see previous list of questions) Unclear information toward users may endanger the future of calibration correction proposed by GSICS Sorry, more questions than answers… « This is not Science, this is Art… » (Dave Doelling)

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