INTRODUCTION

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All meteorologists are intimately familiar with the visual and infra-red data from such conventional satellites as the US NOAA TIROS, GOES series, European METEOSAT, and Japanese GMS. The GOES, METEOSAT, and GMS are all GEOSTATIONARY satellites: as they orbit the earth, they maintain same location over the earth (or nadir ) at all times. They orbit directly over the equator, and have a velocity that allows them to maintain the same nadir continuously. This is a great advantage, since a person can readily put images from this type of spacecraft together and form an animation (using a system like McIdas), since it always has the same field of view (FOV).
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Air Force Weather Agency SSM/I Training
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By contrast, the NOAA-TIROS satellite roughly orbits the earth from the North pole to the South pole about 14 times or so a day. The satellite orbit is referred to as SUN-SYNCHRONOUS- meaning that the plane of the satellite orbit maintains a constant angle with respect to a plane containing the earth's rotational axis and a line drawn from the center of the earth to the sun. While maintaining this orbit, the field of view continuously changes. While an image animation is more difficult with this type of satellite, much higher resolution imagery is obtained.
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This discussion will focus on the Polar orbiting, Sun Synchronous satellites, and in particular , a type of spacecraft of the US Department of Defense, the Defense Meteorological Satellite Program (DMSP). Aboard DMSP spacecraft, there are three microwave sensors: the Passive Microwave Temperature Sounder (SSM/T or SSM/T-1), the Passive Microwave Temperature Sounder [Water Vapor] 2 (SSM/T-2), and the Special Sensor Microwave Imager (SSM/I or SSMI). The SSM/T-1 and SSM/T-2 are primarily focused on the retrieval of temperatures in the upper troposphere / lower stratosphere. The SSMI is focused on the retrieval of microwave energy emitted from the surface of earth. The main thrust of this paper is to discuss the SSMI and explain its functionality, design, and applications. A brief overview of the DMSP satellite will be provided, then a description of the SSMI will follow. Each of the sensor attributes will be explained in some detail, followed by a look at the environmental products that can be derived from the SSMI. 
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