Global Precipitation Measurement (GPM) Microwave Imager (GMI) pre-flight calibration performance

The GMI calibration design incorporates unique features to limit calibration errors that have been observed with previous radiometers. The design geometrically blocks solar intrusion into the hot load, encloses the hot load in a tray with known temperature, utilizes a dual calibration system using hot load/ cold-sky views plus noise diodes, and has a proven main reflector coating technology to ensure high reflectance. Calibration performance is based on ground calibration data taken in thermal vacuum tests together with component tests and analyses. The main measurements taken to demonstrate calibration performance include thermal vacuum radiometric measurements using an external calibration system, beam pattern measurements including modeled backlobe estimates for the main beam and cold sky beams, validated thermal model predictions of hot load thermal gradient performance, and reflector coating conductance. The calibration design meets the GMI calibration uncertainty requirements.