Abstract
This work describes our findings about an evaluation of the stability and the consistency of twenty primary PICSs (Pseudo-Invariant Calibration Sites). We present an analysis of 13 years of 8-daily MODIS products of BRDF parameters and white-sky-albedos (WSA) over the shortwave band. This time series of WSA and BRDFs shows the variation of the “stability” varies significantly from site to site. Using a 10x10 km window size over all the sites, the change in of WSA stability is around 4% but the isotropicity, which is an important element in inter-satellite calibration, can vary from 75% to 98%. Moreover, some PICS, especially, Libya-4 which is one of the PICS which is most employed, has significant and relatively fast changes in wintertime. PICS observations of BRDF/albedo shows that the Libya-4 PICS has the best performance but it is not too far from some sites such as Libya-1 and Mali. This study also reveals that Niger-3 PICS has the longest continuous period of high stability per year, and Sudan has the most isotropic surface. These observations have important implications for the use of these sites.
Highlights
Pseudo-Invariant Calibration Sites (PICS) are being employed by the space agencies to calibrate and validate satellite-derived products, starting with Top of Atmosphere (ToA) radiance
The main PICS locations were identified in the 1990s using the geostationary weather satellite Meteosat-4 (MVIRI) data over the European/African disk area (Cosnefroy et al, 1996 and 1997)
In 2008, the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration/Validation (CEOS-WGCV) recommended that the space agencies should focus on six PICS: Libya-1, Libya-4, Algeria-3, Algeria5, Mauritania1 and Mauritania2
Summary
Pseudo-Invariant Calibration Sites (PICS) are being employed by the space agencies to calibrate and validate satellite-derived products, starting with Top of Atmosphere (ToA) radiance. The selection of PICS is based on many criteria such as temporal stability, flat terrain, spatial uniformity, no vegetation cover, intermittent snow, aerosol level, clear sky probability, and the uniformity of reflectance over all view angles (isotropic character). Many efforts have been made to assess the suitability of the PICS locations Most of these are based on level-1 products (radiance or top of atmosphere reflectance). These inter-comparisons do not employ atmospherically corrected reflectance and Wu et al, (Wu, 2013), have shown that this choice could contaminate the results. It is crucial that such anisotropic effects can be studied and the departure from isotropy characterised
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