Global Precipitation Measurement (GPM) Microwave Imager (GMI) calibration features and predicted performance

The GMI, scheduled to launch in 2013, includes innovative calibration features designed to mitigate or eliminate sources of calibration error that have plagued past microwave radiometers. Solar radiation illuminating the surface of the flight calibration hot load is a common calibration problem. Tests at BATC show that hot load solar intrusion can cause a calibration error on the order of twice GMFs required uncertainty. To reduce the likelihood of hot load solar intrusion, the GMI includes stray light shrouding features around the hot load. The GMI contains a dual-calibration system with noise diodes to provide a backup calibration method in the unlikely case the hot load experiences solar intrusion. Also, the GMI program is working with coating vendors to avoid reflector emissivity problems on-orbit. GMI includes temperature sensors on the main and cold-sky reflectors to correct possible nonzero emissivity. The GMI instrument design meets the required calibration uncertainty.