Foreword to the Special Issue on the DMSP SSMIS

THE JOINT Air Force and Navy Defense Meteorological Satellite Program (DMSP) has a rich heritage of flying new state-of-the art remote sensing instruments to provide timely and critical global weather intelligence to the military users and stable global radiometric observations to the earth science community. In particular, DMSP has had a string of successes flying passive microwave sensors with new capability. The Special Sensor Microwave Temperature (SSM/T) was developed in the 1970s as an “all-weather” cross-track scanning microwave temperature profiler and still serves the users today. The Special Sensor Microwave Imager (SSM/I) followed in the 1980s with a revolutionary conically scanning imager to measure surface parameters, such as ocean surface wind speed, sea-ice concentration, land surface temperature, and soil moisture, and atmospheric parameters, such as rainfall rate, cloud liquid water, and integrated water vapor. The SSM/I was soon followed by the SSM/T-2 microwave water vapor profiler with cross-track scan geometry synchronized to the SSM/T. This instrument pushed millimeter-wave technology into higher frequencies (150–183 GHz). This suite of instruments is still flying today and provide an excellent source of independent collaborative data to help verify the newest instrument program for DMSP: the Special Sensor Microwave Imager/Sounder (SSMIS). The SSMIS program began in the spring of 1989 at Aerojet Corp. (now Northrop-Grumman Electronic Systems), in Azusa, CA. The SSMIS was considered to be a major step forward for the user communities, because this sensor combined the functionality of the heritage DMSP sounders (SSM/T and SSM/T-2) and imager (SSM/I) into a single integrated conically scanning instrument with additional channels to profile the mesosphere. For the first time, atmospheric soundings would be derived by an instrument with a constant viewing geometry in lieu of the more traditional cross-track, dwell, and step-stare geometry, such as the heritage DMSP and National Oceanic and Atmospheric Administration Advanced Microwave Sounding Unit sensors. The users anticipated several benefits from the common conical scan geometry: constant pixel resolution across the swath, constant polarization, and common fields of view of the surface and atmosphere for both sounding and imaging channels. The SSMIS development process was challenging. Many components in the complex sensor system initially did not pass rigorous tests, resulting in a protracted development cycle. Lessons learned were captured along the way and are being