The Defense Meteorological Satellite Program's (DMSP) Special Sensor Microwave Imager/Sounder (SSMIS) is a space-based conically-scanning operational microwave radiometer designed to measure atmospheric emissions from the Earth's surface into the Mesosphere (~80 km altitude. The conically-scanning SSMIS combines both the imaging and sounding capabilities of current DMSP operational instruments, the Special Sensor Microwave Imager (SSM/I), and the Special Sensor Microwave Temperature 1 and 2 (SSM/T-1 and SSM/T-2). The SSMIS Upper Atmosphere Sounding (UAS) channels also extend the current upper limits of operational microwave sounders from 45 km (~ 1 hPa) for the NOAA Advanced Microwave Sounding Unit-A (AMSU-A) to the 80 km height (~ 0.03 hPa).

The SSMIS Upper Atmospheric Sounding (UAS) channels utilize the oxygen absorption line complex near 60 GHz. In order to have sufficient absorption and emission to provide information related to atmospheric temperature above 40 km, the SSMIS UAS channel passbands are co-located with several of the strongest rotational transitions within the 60 GHz oxygen complex. In order to achieve adequate vertical resolution at the higher altitudes, very narrow channel bandwidths are needed. These characteristics pose several challenges, for example 1) the narrow-band UAS channels are extremely sensitive to frequency shifts; 2) Doppler shifts generated from the combination of conical scan and spacecraft velocity and from earth rotation, must be accounted for; and 3) the Zeeman splitting effect leads to polarization dependent UAS brightness temperatures which depend on the local magnetic field strength and orientation relative to the propagation vector.

As the upper vertical levels within Numerical Weather Predication (NWP) models approach 100 km, the SSMIS UAS measurements are expected to help provide improvements to NWP models and forecasts. This is due largely to the lack of any operational observational capability for temperatures in the upper stratosphere and mesosphere constraining the model background. However, the full utility of SSMIS UAS channel data for NWP and associated data assimilation systems may require direct assimilation of brightness temperatures. Direct assimilation requires development of a fast radiative transfer model including a method for parameterizing the Zeeman effects to the O2 absorption complex. A prototype fast model (CRTM-Z) has been developed for the SSMIS UAS channels and is under evaluation at the Naval Research Laboratory. Temperature analyses from the operational global NWP model of the European Center for Medium-range Weather Forecasting, which recently was extended to 0.01 hPa, and at present is the only NWP model with upper levels in the mesosphere are being used for the evaluation. As the need to monitor trends in global climate change becomes increasingly important, the ability to monitor the mesospheric cooling associated with lower atmospheric warming, requires us continue to measure, understand, and fully utilize the measurements provided by the SSMIS and future sensors.