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The Tropical Rainfall Measuring Mission (TRMM), the joint U.S./Japan space mission to measure rainfall over the global tropics, has been formally approved in both nations, with launch scheduled for late 1996 or 1997.

As described in the Bulletin of the American Meteorological Society (vol. 69, pp. 278–295), TRMM, in its early planning stages, will be the first space mission to fly a rain radar. The “rain package” also includes a multichannel passive microwave radiometer and a high-resolution visible and infrared instrument, similar to the AVHRR (advanced very high-resolution radiometer). Orbiting at 350-km elevation for high resolution, TRMM is inclined at 35° in order to overfly tropical locations at a different local time each day, and to document the diurnal variability in precipitation. An important TRMM measurement will monitor monthly rainfall over 5° by 5° boxes over oceans to an accuracy of approximately 10–15%, with the largest error arising from sampling by the selected orbit. Rainfall over individual swaths will also be valuable and can become useful data sets, particularly when combined with products from other satellites.