GPM microwave imager instrument design and predicted performance

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) Instrument is being developed by Ball Aerospace and Technology Corporation (BATC) for the GPM program at NASA Goddard. We describe the instrument and predict the performance of the GMI instrument. The GMI instrument is a conical-scanned passive microwave radiometer used to make calibrated, radiometric measurements from space at multiple microwave frequencies and polarizations from 10 GHz to 190 GHz. The instrument has a 1.22 meter offset parabolic reflector for high spatial resolution and high beam efficiency. Receivers from 10 to 190 GHz frequency receive, amplify and detect the microwave radiation with different detection approaches used at each frequency to maximize performance while minimizing cost and risk. Mechanical features include a deployment assembly for the main reflector and slip rings to transfer power and signals to the rotating portion of the instrument. The core GPM spacecraft, including GMI, will be used to develop a retrieval transfer standard for the purpose of calibrating precipitation retrieval algorithms. For this reason calibration is critical for GMI and a number of features have been included in the GMI design to allow GMI to meet the 1.35 K calibration uncertainty requirement.