Abstract
The Global Precipitation Measurement (GPM) Microwave Imager (GMI) program delivered the GMI Instrument to Goddard Space Flight Center early 2012. Currently, the GMI instrument is undergoing observatory-level integration and test. Goddard Space Flight Center plans to launch the GMI in 2014 aboard the GPM spacecraft with the Dual-frequency Precipitation Radar, affording better correlation of active and passive measurement techniques. The GMI will fly in a 65 degree inclination orbit and is intended to become the calibration standard for radiometer precipitation measurements. The GMI employs a unique dual calibration system. As other scanning microwave imagers, the GMI provides primary calibration using a hot load blackbody and cold sky view. Unlike other microwave imagers, the GMI uses a secondary calibration system with noise diodes on the lower frequency channels. The noise diodes provide four calibration points, rather than just 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. Using GMI, noise diode performance analysis can be done directly using the hot load and cold sky views. This paper presents the pre-flight calibration performance of the noise diodes based on thermal vacuum measurements taken during GMI testing. We address stability of the noise diodes, and draw conclusions of on-orbit noise diode performance based on the ground measurements.
Manuscript
Global Precipitation Measurement (GPM) Microwave Imager (GMI) Pre-flight Noise Diode Calibration
The Global Precipitation Measurement (GPM) Microwave Imager (GMI) program delivered the GMI Instrument to Goddard Space Flight Center early 2012. Currently, the GMI instrument is undergoing observatory-level integration and test. Goddard Space Flight Center plans to launch the GMI in 2014 aboard the GPM spacecraft with the Dual-frequency Precipitation Radar, affording better correlation of active and passive measurement techniques. The GMI will fly in a 65 degree inclination orbit and is intended to become the calibration standard for radiometer precipitation measurements. The GMI employs a unique dual calibration system. As other scanning microwave imagers, the GMI provides primary calibration using a hot load blackbody and cold sky view. Unlike other microwave imagers, the GMI uses a secondary calibration system with noise diodes on the lower frequency channels. The noise diodes provide four calibration points, rather than just 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. Using GMI, noise diode performance analysis can be done directly using the hot load and cold sky views. This paper presents the pre-flight calibration performance of the noise diodes based on thermal vacuum measurements taken during GMI testing. We address stability of the noise diodes, and draw conclusions of on-orbit noise diode performance based on the ground measurements.