AVHRR/MODIS relative calibration bias estimates deduced from the SNO method: Uncertainties due to earth-scene inhomogeneities

The Simultaneous Nadir Overpass (SNO) method estimates the magnitude of relative calibration biases between two similar radiometers flown on different polar-orbiting satellites. For this purpose, the SNO method uses radiometer measurements taken from satellite overpasses of a given earth location that occur within 30 seconds of each other. This reduces uncertainties of intersatellite radiometer calibration bias estimates that can result from change of the atmosphere, land, and/or ocean within the time period between satellite overpasses. On the other hand, several uncertainties remain in the SNO method that are mainly related to a combination of instrument differences and earth-scene spatial/temporal inhomogeneities. Since these uncertainties need to be understood and quantified in order to increase confidence in the SNO method, we explore in this study earth-scene inhomogeneity related uncertainties of intersatellite calibration biases estimated from 22 August to 22 October 2005 SNOs occurring between Polar-orbiting Operational Environmental Satellite (POES) Advanced Very High Resolution Radiometer (AVHRR) and the Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS).