Maintaining the Long-Term Calibration of the Jason-2/OSTM Advanced Microwave Radiometer Through Intersatellite Calibration

A method is applied to maintain the long-term calibration of a microwave radiometer through intersatellite calibration and is used to mitigate an observed calibration drift of the Advanced Microwave Radiometer (AMR) on Jason-2/Ocean Surface Topography Mission. The AMR provides a correction for the wet tropospheric path delay (PD) of the radar altimeter signal, and it is critical that any drift in the radiometer be estimated and removed to enable studies of global mean sea-level variability. The intersatellite calibration method transfers the long-term calibration from other satellite microwave radiometers using a transfer function to map the other sensor's brightness temperature (TB) observations to those of the AMR. Intersensor mapping functions are derived separately for ocean observations and observations over the Amazon rainforest. This provides a warm and cold TB calibration reference to enable the distinction between long-term gain and offset drifts. A database of co-incident observations is generated between the AMR and conically scanning microwave sensors, namely, AMSR-E, TMI, and SSMIS. Monthly averaged differences are found between the AMR and the AMR equivalent TBs computed from the reference sensors. The apparent change in the AMR calibration determined from the three reference sensors is intercompared between the sensors and compared to that determined using natural on-Earth references. It is found that apparent trends in the AMR TBs between the reference sensors and the natural on-Earth references agree within a month to better than 0.4 K. The AMR 18.7- and 23.8-GHz channels are found to be stable to 0.5 K over the first three years of the mission, and the calibration 34.0-GHz channel is found to drift downward by approximately 6 K. In all channels, the calibration change is determined to be a series of offset jumps (independent of TB). These calibration changes in each AMR channel are estimated and removed using the comparisons to the reference sensors. The uncertainty in the PD long-term stability after recalibration is estimated to be less than 0.5 mm/year from July 2008 to August 2011.