1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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?