On March 15, 2005 NASA Headquarters announced the selection of Ball Aerospace and Technologies Corporation (BATC) to design and fabricate the GPM Microwave Imager (GMI). GMI, like the TRMM Microwave Imager (TMI), will be a passive, nine-channel, conical-scan microwave radiometer whose measurement frequencies are selected to optimize its capabilities for the measurement of rainfall. The conical-scan configuration of the instrument will provide a large measurement swath (approximately 850 km from edge-to-edge), with a constant-size ground measurement footprint, regardless of scan position.

Figure 1: GPM Microwave Imager

The most obvious difference between GMI and TMI is the size of the main reflector; GMI will have an effective aperture of 1.2 meters, which will reduce the size of the measurement footprints to approximately one-half of the size of the TMI footprints. This improved measurement capability is of particular value to the scientists, and will enable them to understand changes of a small spatial scale in clouds and rainfall events, like those that may occur during thunderstorms. In addition to this improved image resolution, GMI will have a carefully designed on-orbit calibration system that will ensure the collection of well calibrated measurements.

The GPM Core Spacecraft will carry two instruments—the GMI and the Dual-frequency Precipitation Radar (DPR). The DPR, currently under development by the Japanese Aerospace and Exploration Agency (JAXA), will have the capability to make detailed, three-dimensional measurements of cloud structure, rainfall, and rain rates. These detailed measurements will provide further understanding of the microwave radiometric measurements made by the GMI. The well calibrated GMI measurements and the well characterized scene obtained from the DPR will combine to provide an excellent reference against which to calibrate other microwave radiometers in the GPM constellation when overlapping measurements of same Earth scene are made.

The GMI Contract will initially provide for the development of one GMI to be placed on the GPM Core Spacecraft. An option to procure a second instrument is available and will be exercised when a GPM Constellation Spacecraft is selected.

The procurement of GMI is a milestone for the GPM Project in that it is the first major procurement action undertaken by the Project.

Shown below are some of the key GMI parameters.

Table 1: GMI Channel Set

Figure 2: GMI Scan Geometry

Figure 3: GMI Field-of-View Projections

By Mark Flaming
GMI Instrument Manager
Gilbert.M.Flaming@nasa.gov
+1 301 286-7374

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