Characterization of CRCS Dunhuang test site and vicarious calibration utilization for Fengyun (FY) series sensors

The Dunhuang test site, one of the China Radiometric Calibration Sites (CRCS) for the vicarious calibration (VC) of Chinese spaceborne sensors, was selected in 2008 by the Working Group on Calibration and Validation (WGCV) of the Committee on Earth Observation Satellites (CEOS) as one of the instrumented reference sites. The site is spatially uniform with a coefficient of variation (CV) (standard deviation / mean) less than 2% of spectral reflectance over the 10 km × 10 km central region, based on the imagery of the Medium Resolution Spectral Imager (MERSI) onboard the Fengyun 3A (FY-3A) satellite. This result is also proven by several ground truth measurements. The surface spectral reflectance is temporally stable with a CV of about 3% from 7-year Moderate Resolution Imaging Spectroradiometer (MODIS) surface albedo product and has been validated using the ground-based measurement eight times in 10 years. Several calibration/validation (Cal/Val) field campaigns for the site evaluation and VC have been conducted since 1999, and a database including atmospheric and surface characteristics has been established. The aerosol optical depth (AOD) at the Dunhuang site is low (τ550 is about 0.2), except for the dusty spring season from March to May. The surface reflectance of this site is about 15%–30% from the visible to near-infrared spectral region. The site surface is not Lambertian, with clearly direction-dependent reflectance variation especially at the large solar or observation zenith angles. The CRCS VC was the baseline operational calibration approach from 2002 for the Multichannel Visible and Infrared Scanning Radiometer (MVISR) onboard the FY-1C and FY-1D satellites. The approach is inherited by the optical imaging sensors (Visible and Infrared Radiometer (VIRR) and MERSI) of the Chinese second-generation polar-orbiting satellite FY-3A. The CRCS VC is used not only to correct the significant bias of preflight calibration in some bands of these sensors but also to monitor the sensor radiometric degradation. The Dunhuang site can also be used as the earth target in the cross-calibration for the FY satellite sensors.