| Abstract: |
The Global Precipitation Measurement (GPM) Mission Microwave Imager (GMI), launched on-board the GPM core spacecraft in March of 2014, incorporates a number of enhancements to the calibration subsystem in order to address issues with past space-borne radiometers. These include additional additional thermal shielding for the warm target, additional temperature monitoring for the warm target and main reflector, shaping of the cold sky reflector, and the use of noise diodes in the 10 GHz through 36 GHz receiver chains for both on-orbit estimates of receiver non-linearity and as back-up for the warm and cold targets. The thermal shielding for the warm target is intended to avoid thermal gradients seen in past radiometers, while additional temperature sensors are for monitoring and correcting for any residual gradients. Shaping of the cold sky reflector is intended to avoid interference in the cold sky view from the spacecraft. Space-borne radiometers prior to GMI principally used two-point calibration between the cold sky reflector and warm target for a linear fit to the gain, antenna temperature relationship for calibration of observed brightness temperatures. With the addition of noise diodes, GMI can do both two-point and four point calibration. The additional degrees of freedom in four-point calibration relative to two-point permit estimates of receiver non-linearity with every calibration cycle. In addition, two-point calibration can be performed in three different ways: cold sky reflector and warm target, cold sky reflector and noise diode, and warm target and noise diode. This provides redundancy if any issues arise with any element of the calibration, as well as providing an independent check on the stability of any of the three.
The early on-orbit performance of the calibration subsystem was presented in Draper et al. (JSTARS, 2015). Based on the first 6 months of data, the GMI warm target, cold sky reflector view, and noise diodes were shown to be very stable. This study for MicroRad 2018 provides an update, trending performance of the calibration subsystem based on approximately 4 years of on-orbit data to look at long-term stability. The stability of the warm load, cold sky view, and each of the 6 noise diodes will be quantified using the built-in calibration redundancies discussed above. In addition, receiver stability including non-linearity, gain, and noise equivalent delta temperature (NEDT). |
