On-orbit calibration performance of MODIS TDI bands

摘要

Bands 13 (667 run) and 14 (678 run) of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites are built with a time delay and integration (TDI) circuit and a high-low dual-gain setting intended for the monitoring of the ocean environment. An important on-orbit performance issue for the high-gain output of Terra Band 14 and Aqua Band 13 and 14 is that these three bands saturate during standard on-orbit calibrations using the onboard solar diffuser (SD). Consequently, their calibration procedure requires a derivation different from other reflective solar bands (RSB). A high-low scaling-ratio approach is employed for these bands to obtain the high-gain coefficients via the low-gain output 13L and 14L through a calculated constant ratio derived from other sources. Currently, earth view (EV) scenes over the North Atlantic Ocean are used as the primary sources. As it also has been observed that the response versus scan-angle (RVS) exhibits noticeable changes with time for RSB, a time-dependent RVS calibration algorithm has been implemented based on the SD and lunar calibration results. This paper reviews the electronic design and operation of Band 13 and 14, discusses the SD calibration and the saturation issue of the high-gain output, and the time-dependent RVS algorithms. Long-term performance of the calibration coefficient and RVS are presented. The long-term trend shows that the gain-change for Terra MODIS Band 13 and 14, for both high- and low-gain bands, is significant at 6% and 23%, but is more modest for Aqua MODIS Band 13 and 14 at 6% and 2%. The gain ratios 13H/13L and 14H/14L are very stable at the level of 0.01% for both MODIS instruments. The long-term gain difference due to RVS effect is up to 3% for Terra MODIS and about 2% and below for Aqua MODIS.