On-board Calibration Noise Sources for the Global Precipitation Measurement (gpm) Microwave Imager (gmi)

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) will fly on the GPM core spacecraft to be launched in 2013 and the low-inclination spacecraft to be launched in 2014 [1,2]. The GMI radiometer is a conically scanning microwave radiometer covering several bands from 10 to 183 GHz. The two primary on-board calibration targets are a warm load black-body and cold sky reflector. Additionally, the lower-frequency channels (10, 18, 23 and 36 GHz) include coupled noise sources [3]. This is the first time waveguide coupled noise sources and free space black bodies will be incorporated together in a space. In normal operation they will be turned on and off during the calibration portion of alternating antenna azimuth scans. Thus, every two scans, four calibration points are observed by the radiometers: warm load, cold sky, warm load + noise diode, and cold sky + noise diode. This unique ability will provide the potential to study several calibration problems while on-orbit: Receiver non-linearity can be tracked using a threeor four-point calibration Warm load thermal transients and their frequency dependence can be monitored using the noise sources Long-term noise source stability can be monitored using the warm and cold targets This paper summarizes the design and testing of the noise sources prior to integration into GMI.