Lingling Ma, Yongguang Zhao, Na Xu, Xiuqing Hu, Chuanrong Li GRWG Web Meeting Cross-calibration through spaceborne SI-traceable reference instruments Lingling Ma, Yongguang Zhao, Na Xu, Xiuqing Hu, Chuanrong Li Key Laboratory of Quantitative Remote Sensing Information Technology, Chinese Academy of Sciences, Beijing, China Department of Earth Observation Technology Application, Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing, China National Satellite Meteorological Center, CMA, Beijing, China September 2018

SNO method based on strict element matching General cross-calibration method - flow chart SNO method based on strict element matching Observation time difference check | tFY2 − tsounder | < dtmax Satellite zenith angle difference check | cos( SZAsounder ) / cos( SZAFY2) − 1| < MaxRate Pixel Center distance 1. Subsetting Orbit + observation area matching 2. Collocating Temporal + spatial + viewing matching 3. Transforming Radiometric + spectral trans. Pixel scale trans. 4. Filtering Uniformity check; Abnormal pixel filtering SBAF Environment uniformity check STDV(FY2 Rads in ENV_BOX) < MaxSTDV 5x5 FY-2 pixels (5km grid at SSP) define Environment Diagram of generic data flow for inter-calibration of monitored (MON) instrument with respect to reference (REF) instrument

技术挑战 General cross-calibration method - limits For SNO method based on strict element matching (temporal/spectral/spatial/angular), there are some limitations, Cross-points obtained by strict SNO mainly locate at polar region, which landcover types are rather simplex. Hard to describe full-dynamic range characteristics. Especially to the response of low-radiation energy, strict SNO cannot get enough satisfied cross-points to make the calibration curve fitting to be acceptably stable. Simplex landcover types of cross-points Sensor’s response nonlinearity High resolution satellite has long revisit period and narrow swath. To obtain more cross-calibration opportunities, it is needed to enlarge the temporal difference tolerance and the viewing angle difference tolerance to find more acceptable cross-points, which will inevitably add uncertainty, sometimes even wipe out all the efforts on improving accuracy of the spaceborne reference instrument. GF-1 v.s. L8 Uncertainty associated with angular difference BRDF of Baotou desert site

技术挑战 cross-calibration method – General ideas The key step in cross-calibration is to find the proper reference TOA radiance /reflectance, corresponding to observation value of the monitored satellite. More types of reference targets are needed to better perform cross-calibration, through which wider dynamic range of the sensor can be considered. When more different reference targets are considered, and temporal/angular constraints are relaxed, it is then need to develop new models and methods to eliminate influences due to the differences (temporal/spatial/spectral/angular) between the monitored sensor and the reference sensor. PICS sites: Develop high accuracy PICS TOA reflectance model RadCalNet sites: Improve RadCalNet TOA reflectance product and assure the consistency of different sites Moon: Develop high accuracy lunar irradiance model

技术挑战 PICSs as calibration transfer targets Lots of studies have been done by both GSICS and WGCV: PICSs selection; calibration site characterization; PICS TOA reflectance model built on long time-series satellite observations (e.g. Hyperion), which can help angular correction in cross-calibration. Mishra, Nischal, et al. (2014). Radiometric calibration accuracy and spectral-coverage configuration of most onboard sensors confined accuracy of the TOA reflectance model retrieved by them. So we need PICS TOA reflectance model with higher accuracy and finer spectral resolution. Spaceborne SI-traceable reference instruments, which is hyperspectral and perfectly calibrated, could provide better input data to build advanced PICS TOA reflectance model. Long time-series observation data PICS TOA reflec. model Spaceborne SI-traceable reference instruments Corrections： Temporal difference Angular difference Spectral difference

技术挑战 RadCalNet sites as calibration transfer targets The RadCalNet provides satellite operators with SI-traceable Top-of-Atmosphere (TOA) spectrally-resolved reflectance derived over a network of sites, with associated uncertainties, at a 10nm spectral sampling interval, in the spectral range from 380nm to 2500nm and at 30 minute intervals. The RadCalNet sites may be proper to act as calibration transferrers for high-medium resolution satellite. The spaceborne SI-traceable reference instruments could be used to calibrate the RadCalNet TOA reflectance, so as to assure radiometric consistency of different sensors overpassing RadCalNet sites. RadCalNet products: L1: BOA reflectance, Measured atmospheric parameters L2: TOA reflectance atmospheric parameter TOA reflectance BOA reflectance

技术挑战 RadCalNet sites as calibration transfer targets To be better applied in the spaceborne SI-traceable reference transfer calibration, more work could be done related to RadCalNet sites: Add more calibration sites into RadCalNet: Proper site number; spatial distribution requirement of those sites; landcover requirements Improve spectral resolution of RadCalNet products: Most ideally, they should be consistent with spectral range & spectral resolution of the reference instrument. But to realize this object, calculation codes in the data processing center should be modified, and influence by extrapolation of the in-situ measurement data should also be considered. Provide angular conversion model for RadCalNet TOA reflectance product：Now different site owners already have their own BRDF models. To improve angular consistency, a standard angular conversion method is needed. All the Radcalnet sites are considered as a unified system, so the radiometric consistency of ground measurements in different sites is very important: Unified L1 product processing methods. Precisely estimate uncertainties in each step of the L1 product processing chain, make sure the ground measurements in different sites can all be traced to SI, and guarantee their comparability. Calculate the TOA reflectance RadCalNet Data Center Calculate the BOA reflectance Site owners Gobabeb Baotou La Crau Railroad Valley Playa AOE CNES NASA ESA/NPL

Radiance/reflectance Other considerations about reference targets Moon Hyperspectral and high accuracy lunar irradiance model Combination of satellite observation and ground-based observation DCC  DCC reflectance model Combination of wide-dynamic stable targets Reference satellite orbit design, to assure high frequency cross-calibration for multiple targets Weighting determination method Sea <5% Moon 5-10% Desert 20-30% Ice sheet 50-80% DCC >90% Radiance/reflectance

Spatial point spread function 技术方案 Other considerations about spatial/spectral matching and compensation Spatial Fine collocation (Wang L K) Solution + Spatial Fine Collocation Spatial point spread function 7×7km 24×12km Point 2: Spatial Non-uniformity Point 1: Spatial deformation, different IFOV Spectral Point 1: Resolution and coverage mismatching Spectral sampling reconstruction for fine resolution Typical IASI, AIRS and CRIS spectra Solution Gap filling (Xu H) Point 2: Low spectral resolution

Introduction of the “Spaceborne SI-traceable reference transfer calibration and field validation” project funded by National Key R&D Plan of China, MOST Research Contents China spaceborne SI-traceable reference system Prototype system for transfer calibration Spaceborne SI-traceable reference transfer calibration for land satellite Spaceborne SI-traceable reference transfer calibration for meteorological and ocean satellites Reflective solar bands SI① Calibration Database Thermal emissive bands SI② Models and software SI② International metrology reference Comprehensive validation site network Multi-approach reference transfer calibration and field validation system In-situ measurement to pixel scale Comprehensive validation based on multi-sites with consistent data quality SI② NPL Validation NIM System integration testing and application demonstration Standard & specification Aerostat platform at stratosphere Application Multi-series of optical satellites Land satellites Meteorological satellites Ocean satellites

技术挑战 Discussions How to compromise between the frequency and accuracy of cross-calibration when PICSs or Radcalnet site are used as reference transfers? Related research groups under WGCV/IVOS and WMO/GSICS, dedicated to different topics(RadCalNet, PICS, Lunar modelling, etc). How to involve in their researches, facilitate cooperation amongst these groups, and promote research work of the spaceborne SI-traceable reference transfer calibration. There are different programs on “spaceborne SI-traceable reference instruments” around the world. This means that there will be several spaceborne-SI. How to carry out comparisons among them？How to reach the consensus and promote international cooperation?