Abstract
Suomi NPP VIIRS has been going through an extensive on-orbit calibration and validation since launch to improve and maintain the radiometric accuracy well within specification. Different techniques have been used to monitor VIIRS on-orbit radiometric performance such as quantifying radiometric stability using vicarious calibration sites (using deserts, Antarctica Dome C sites etc), sensor inter-comparison with other well calibrated radiometers such as MODIS to quantify the absolute radiometric calibration accuracy and instrument stability. A number of studies in the past have shown how simultaneous nadir overpass (SNO) in high latitude polar region can be used to analyze the inter-satellite bias. SNOs between S-NPP VIIRS and AQUA MODIS occur both at high latitude polar region and low latitude tropical areas. This study demonstrates the sensor inter-comparison using SNOs extended to low latitude region over radiometrically stable and homogenous sites such as deserts. Extension of SNOs in the low latitudes (SNO-x) for sensor inter-comparison is a recently introduced approach (inherited from traditional SNO approach) that has been successfully implemented to continuously monitor and track the on-orbit radiometric stability and accuracy of VIIRS. The radiometric biases of VIIRS relative to MODIS have been estimated and analyzed using SNO-x methodology. The study suggests that VIIRS radiometric resolution bands M-1 through M-7 agrees with MODIS within 2% except band M-8 which indicates nearly 3% bias. The uncertainties in radiometric bias estimation are mainly attributed by the spectral differences, calibration uncertainties, bidirectional reflectance distribution function (BRDF) and cloud movements.
Tracking On-orbit Radiometric Stability and Accuracy of Suomi NPP VIIRS Using Extended Low Latitude SNOs
Suomi NPP VIIRS has been going through an extensive on-orbit calibration and validation since launch to improve and maintain the radiometric accuracy well within specification. Different techniques have been used to monitor VIIRS on-orbit radiometric performance such as quantifying radiometric stability using vicarious calibration sites (using deserts, Antarctica Dome C sites etc), sensor inter-comparison with other well calibrated radiometers such as MODIS to quantify the absolute radiometric calibration accuracy and instrument stability. A number of studies in the past have shown how simultaneous nadir overpass (SNO) in high latitude polar region can be used to analyze the inter-satellite bias. SNOs between S-NPP VIIRS and AQUA MODIS occur both at high latitude polar region and low latitude tropical areas. This study demonstrates the sensor inter-comparison using SNOs extended to low latitude region over radiometrically stable and homogenous sites such as deserts. Extension of SNOs in the low latitudes (SNO-x) for sensor inter-comparison is a recently introduced approach (inherited from traditional SNO approach) that has been successfully implemented to continuously monitor and track the on-orbit radiometric stability and accuracy of VIIRS. The radiometric biases of VIIRS relative to MODIS have been estimated and analyzed using SNO-x methodology. The study suggests that VIIRS radiometric resolution bands M-1 through M-7 agrees with MODIS within 2% except band M-8 which indicates nearly 3% bias. The uncertainties in radiometric bias estimation are mainly attributed by the spectral differences, calibration uncertainties, bidirectional reflectance distribution function (BRDF) and cloud movements.