Monitoring the Jason-2/AMR Stability Using SNO Observations from AMSU on MetOp-A

The microwave radiometers aboard the Jason satellites for altimetry missions are a key component of the Jason system, providing important measurements for water vapor correction, which is the most variable path delay correction in the altimeter measurement system. Unfortunately, these radiometers have exhibited on-orbit calibration drifts, step jumps, and other types of irregularities that can result in height rate errors comparable to 1 mm/yr, the stability requirement for measuring global mean sea level rise. Therefore, the calibration accuracy and short-and long-term stability of these radiometers are critical for the Jason mission to meet the stringent requirements. This study investigates the stability of the Jason-2 Advanced Microwave Radiometer (AMR) relative to the Advanced Microwave Sounding Unit (AMSU) on the MetOp-A satellite using the Simultaneous Nadir Overpass (SNO) time series method. Preliminary results show that using the AMSU as a stable reference, which has a demonstrated stability of better than 0.5 K per decade in previous studies, the Jason radiometer stability can be reliably monitored at 0.1 K per year level. The method has successfully detected a downward trend in the 23.8 GHz AMR channel (the primary one for detecting atmospheric water vapor) relative to the same AMSU channel for about -0.48 K per year since its launch on June 20, 2008, until February 2010, which is likely caused by AMR calibration problems during the period.