Global Precipitation Measurement (GPM) microwave imager (GMI) pre-flight noise diode calibration

The GMI employs a unique dual calibration system. Unlike other microwave imagers, the GMI noise diodes on the lower frequency channels as well as black-body hot and cold sky views providing four calibration points rather than two. The dual calibration system enables on-board trending of non-linearity, as well independent cross-checking of each calibration element for stability and anomalous behavior. One important benefit of the dual calibration system is the direct evaluation of noise diode behavior on-orbit. For systems that depend solely on noise diodes for calibration, validation of the noise diode performance has to be done vicariously, using the earth as the reference. This paper presents the pre-flight and early on-orbit calibration performance of the noise diodes. We address stability of the noise diodes, and draw conclusions of continued on-orbit noise diode performance based on the ground and early flight measurements.