Terra and Aqua MODIS inter-comparison using LEO-GEO double difference method

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites have successfully operated since their launch in 1999 and in 2002, providing more than 18 and 16 years of continuous global observations, respectively. The inter-comparison between the two MODIS instruments can be very supportive for the instrument calibration and uncertainty assessment. Aqua and Terra MODIS have almost identical relative spectral response, spatial resolution, and dynamic range for each band. Therefore, a site dependent correction for a sensor spectral band pair is not necessary for their comparison. However, Terra is in the morning orbit with an equator crossing time of 10:30 am, and Aqua is in the afternoon orbit with equator crossing time of 1:30 pm. Consequently, there is a dearth of simultaneous nadir overpasses (SNOs) between the two satellites. Major challenges in cross-sensor comparison of instruments on different satellites include differences in observation time, solar angle, and view angle over selected pseudo-invariant sites. In this work, the inter-comparisons of thermal emissive bands are performed over a pseudo-invariant target, using the observations from a sensor onboard a geostationary satellite as a bridge. Himawari8 was launched on October 7, 2014. The Advanced Himawari Imager (AHI) onboard Himawari8 can be used as a reference to bridge the comparison between Terra and Aqua MODIS. AHI has 16 channels; with spatial resolutions from 0.5 km to 2 km at nadir and produces a full disk observations every 10 minutes. The band spectral coverage matchup, comparable spatial resolution and near-simultaneous observation between MODIS and AHI provide feasibility to implement a double difference method. This comparison method minimizes the impact of the difference in observation time and solar angle. The comparison results will be used as an assessment for MODIS instrument calibration and will be helpful for future enhancement of the L1B product.