Intersensor calibration of DMSP SSM/I's: F-8 to F-14, 1987-1997

Abstract

The Defense Meteorological Satellite Program (DMSP) operational special sensor microwave imager (SSM/I) marked its ten-year anniversary on the launch date of the first SSM/I (F-8), June 19, 1987. After F-8, the DMSP has launched five more SSM/I's, F-10 (December 1990), F-11 (November 1991), F-12 (August 1994), F-13 (March 1995), and F-14 (April 1997), leaving the last SSM/I for a candidate launch in 1999. Built by Hughes Aircraft Co., these instruments have proven to be the most reliable and well-calibrated, space-based, passive microwave imaging radiometers to date, allowing the data to be used quantitatively for both operational and climatological applications. The remarkable stability of the SSM/I sensors also provides the opportunity to quantify the incremental brightness temperature differences to which the SSM/Is can be intercalibrated, thus establishing the noise floor for intercomparisons. This paper summarizes the prelaunch and postlaunch performances of each new sensor determined during calibration and validation (cal/val), starting with the formal, multiyear cal/val effort conducted by both government and public institutions under the direction of the Naval Research Laboratory (NRL) and sponsored by the joint Air Force/Navy DMSP. Sensor-specific components, orbital configuration, and systematic relative errors are examined that contribute to the total system calibration. In particular, a large (1-3 K) but correctable left-right scan asymmetry of SSM/I brightness temperatures was observed in the data and traced to an antenna field-of-view (FOV) intrusion by the spacecraft (start of scan) and a glare suppression sensor (end of scan). These effects were found to be correctable to first order using a pixel-dependent spillover correction. Empirical statistical distribution functions for rain-free ocean pixels were constructed for the entire set of SSM/Is and formed the basis for assessing intersensor calibration. Manufacturer-derived sensor-specific antenna pattern correction (APC) coefficients were found to be the source of large intersensor differences for several channels, e.g., 1-2 K for the 22-V channel.