The Defense Meteorological Satellite Program (DMSP) satellites collect visible and infrared cloud imagery as well as monitoring the atmospheric, oceanographic, hydrologic, cryospheric and near-Earth space environments. The DMSP program maintains a constellation of sun-synchronous, near-polar orbiting satellites. The orbital period is 101 minutes and inclination is 99 degrees. The atmospheric and oceanographic sensors record radiances at visible, infrared and microwave wavelengths. The solar geophysical sensors measure ionospheric plasma fluxes, densities, temperatures and velocities. DMSP visible and infrared imagery of clouds covers a 3,000 km swath, thus each satellite provides global coverage of both day night time conditions each day. The field view of the microwave imagers and sounders is only 1,500 km thus approximately 3 days data are required for one instrument to provide global coverage at equatorial latitudes. The solar geophysical instruments make in-situ measurements of ionospheric parameters, some of which vary very rapidly. The NOAA National Centers for Environmental Information (formerly National Geophysical Data Center) receive the complete DMSP data stream from the Air Force Weather Agency (AFWA), Offutt Air Force Base, Omaha, Nebraska. Data are currently transmitted in near realtime from AFWA directly to the archive via a designated T1 line. Archive processing prepares orbital data sets of calibrated, quality assessed data organized as a time-series, restores data lost during transmission,and accurately computes satellite positions. NCEI maintains an archive of all data recorded on DMSP satellites as relayed to The NOAA National Centers for Environmental Information (formerly National Geophysical Data Center) by the Air Force Weather Agency. Data from March 1992 to March 1994, are considered to be experimental. After March 1994, the system was fully operational. NCEI archives contain data that are post process reconstructed, positioned and geolocated using the same software.

The SSM/I is a seven-channel, four frequency, linearly-polarized, passive microwave radiometric system which measures atmospheric, ocean and terrain microwave brightness temperatures at 19.35, 22.235, 37.0 and 85.5 GHz. The data are used to obtain synoptic maps of critical atmospheric, oceanographic and selected land parameters on a global scale. The SSM/I archive data set consists of antenna temperatures recorded across a 1,400 km conical scan, satellite ephemeris, earth surface positions for each pixel and instrument calibration. Electromagnetic radiation is polarized by the ambient electric field, scattered by the atmosphere and the Earth's surface and scattered and absorbed by atmospheric water vapor, oxygen, liquid water and ice. The SSM/I instrument consists of an offset parabolic reflector that is 24 x 26 inches fed by a seven- port horn antenna. The reflector and feed are mounted on a drum which contains the radiometers, digital data subsystem, mechanical scanning subsystem and power subsystem. The drum assembly rotates about the axis of the drum. A small mirror and a hot reference absorber are mounted on the assembly. The instrument sweeps a 450 cone around the satellite velocity vector so that the Earth incidence angle is always 540. Data are recorded during the 102.40 of the cone when the antenna beam intercepts the Earth's surface. The channel footprint varies with channel energy, position in the scan, along scan or along track direction and altitude of the satellite. The 85 GHz footprint is the smallest with a 13 x 15 km and the 19 GHz footprint is the largest at 43 x 69 km. Because the 85 GHz footprint is so small, it is sampled twice as often, i.e., 128 times a scan. One data cycle consists of 4 85 GHz scans and 2 scans of the 19, 22 and 37 GHz channels. The complete cycle takes 28 seconds and it must be complete to process the data. The SSM/I processor is queried once a second by onboard computer and the data are placed into the "TS SSP" data field. Data is ingested as sent from AFWA on a T-1 line. At the NOAA National Centers for Environmental Information (formerly National Geophysical Data Center), the "TS SSP" data are decommutated, deinterleaved, bit flipped, reordered and restructured into orbits beginning with the equatorial crossing as the satellite travels from south to north. Satellite ephemeris are computed using a physically-based, orbital mechanics model. SSM/I pixels are geolocated using the satellite ephemeris and satellite attitude corrections. Antenna temperatures are computed from instrumental counts by a linear equation, i.e., the conversion is reversible. In the decommutation step, we encountered bit reversals that occurred 1.8 - 3.4% of the time and are probably caused by ionospheric scintillation. These are identified through careful checking procedures and corrected. Archive files contain metadata by orbit and geolocated antenna temperatures.

The NOAA/NASA Pathfinder climate data CD-ROM contains seven data sets: Advanced Very High Resolution Radiometer (AVHRR)Land and Ocean, TIROS Operational Vertical Sounder (TOVS) Paths A, C1, C2, Special Sensor Microwave/Imager (SSM/I) Precipitation and Atmospheric Moisture for the Benchmark Period of April 1987 to December 1988. These data sets can be viewed with a variety of applications including GeoVu, the NCEI multi-platform data browse and visualization software application, National Center for Supercomputing Applications (NCSA) Collage, IMDISP, Spyglass, and Idrisi.