Tracking long-term stability of MODIS thermal emissive bands response versus scan-angle using Dome C observations

Abstract

Since their launch in December 1999 and May 2002, Terra and Aqua MODIS (MODerate Resolution Imaging Spectroradiometer) have successfully operated for over 19 and 17 years, respectively. MODIS is a scanning radiometer that uses a two-sided scan mirror rotating at 20.3 rpm. MODIS data are collected in 36 spectral bands: 20 reflective solar bands (RSBs) and 16 thermal emissive bands (TEBs). Earth observations are made each scan over a wide scan-angle range of ±55° from the instrument nadir. MODIS TEBs are calibrated on-orbit on a scan-by-scan basis using a blackbody at a fixed scan-angle. For Terra MODIS, the current TEB Response Versus Scan-angle (RVS) of its scan mirror was derived using observations made during a deep-space pitch maneuver in early 2003, while the Aqua MODIS TEB RVS was characterized pre-launch. In this study, the RVS on-orbit stability for the MODIS TEBs (over mission lifetime) is evaluated using multiple daily Dome C observations over the entire range of angles of incidence (AOIs) using a 20 x 20 km region of interest (ROI) centered at 75.12° S, 123.39° E. In this study, a total of approximately 3000 individual granules per year per instrument are analyzed. Except for band 29, the estimated brightness temperature drift for every AOI and band is small for both MODIS instruments over their mission lifetimes. The large variability and noise in the Dome C data sets make it difficult to determine any small RVS changes that may have occurred, but the stability of the results does give confidence that the current RVS used in the calibration is sufficient.