On-orbit monitoring of MODIS thermal emissive bands response versus scan angle

MODIS is a scanning radiometer that uses a two-sided scan mirror, making the Earth view (EV) observations over an angular range of 550 relative to instrument nadir. The 16 thermal emissive bands (TEB) are calibrated on a scan-by-scan basis using a quadratic algorithm that includes corrections for the sensor response versus scan angle (RVS). Currently, there are two MODIS instruments operated on-orbit. For Terra MODIS (launched in December 1999), the TEB RVS was characterized on-orbit using deep space observations made during spacecraft maneuvers. For Aqua MODIS (launched in May 2002), the TEB RVS was measured pre-launch. In this paper, we describe an approach used to monitor the on-orbit stability of MODIS TEB RVS using observations made during its data sector rotation (SR). The SR is implemented via flight software commands, during which the EV data collection is purposely shifted such that the EV data is taken when the sensor actually views the instrument scan cavity. We will show that this approach can accurately determine the TEB RVS differences between the mirror sides and track the TEB RVS changes on-orbit.