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

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, 1997. 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/I’s 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/I’s 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. These differences were dramatically reduced when analyzed on the basis of the temperature data record (TDR), i.e., prior to application of the APC, suggesting that studies requiring high intersensor accuracy should use a single set of APC coefficients. The statistical analyses have obvious inherent limitations themselves. The results of this study indicate that the “noise floor” to which we can justifiably compare individual SSM/I sensors is approximately 0.3 K, depending on the channel, and is a combination of actual sensor calibration differences and the comparison methodology. Manuscript received April 22, 1997; revised February 17, 1998. This work was supported by the Navy Space and Warfare Command Contract [Nooo3997WXDF524, CDR L. Burgess (retired) and CDR T. Sheridan] and DMSP System Program Office (N000CI73700173, Capt. M. Murphy). M. C. Colton was with the Satellite Division, Fleet Numerical Meteorology and Oceanography Center, Monterey, CA, 93943 USA. She is now with the Office of Naval Research, Arlington, VA 22217 USA (e-mail: [email protected]). G. A. Poe is with the Naval Research Laboratory, Washington, DC 20375 USA. Publisher Item Identifier S 0196-2892(99)00038-8.