Improved Geolocation and Earth Incidence Angle Information for a Fundamental Climate Data Record of the SSM/I Sensors

The long-term data record of microwave imager data from the series of six Special Sensor Microwave/Imagers (SSM/Is) on board the Defense Meteorological Satellite Program (DMSP) spacecraft has been used to produce global multidecadal time series of a number of geophysical parameters, including precipitation, total precipitable water, ocean surface wind speed, and sea ice extent. As part of an effort to produce an intercalibrated fundamental climate data record (CDR) of the brightness temperature (Tb) data from the SSM/I, an examination of geolocation errors and the subsequent impact on the view angle [or the Earth incidence angle (EIA)] is performed. Using a combination of techniques, estimates of changes in the sensor/spacecraft attitude, including deviations in roll, pitch, and yaw, have been computed for the life of each of the SSM/I sensors. Applying these corrections results in an improved pixel geolocation, but more importantly, it provides accurate estimates of the EIA across the scan and throughout each orbit. An analysis of uncertainties in the calculation of EIA shows mean errors within 0.1 degrees, which translates to errors in the calibration of less than 0.2 K for all channels. The availability of these precise estimates of EIA is extremely important for producing CDRs since the mean EIA decreases over time due to the decay in the DMSP orbits, which will lead to an artificial climate trend if not properly accounted for by the geophysical retrieval algorithms.