Radiometric characterisation of the UK Met. Office Microwave Airborne Radiometer Scanning System (MARSS)

Since 1990, the Meteorological Office has operated a scanning microwave radiometer aboard its C-130 research aircraft. This instrument, known as MARSS (Microwave Airboine Radiometer Scanning System) originally operated on two channels. In early 1999 it was upgraded (in conjunction with Rutherford Appleton Laboratories) to incorporate three additional channels. The five channels now available correspond closely to those of AMSU-B (channels 16 to 20), at frequencies of 89 GHz, 157 GHz and three channels at 183 GHz. This allows intercomparison with the satellite-borne instrument for ground truthing, as well as validation of radiative transfer models. Stronger atmospheric absorption in the new channels means zenith views at high altitude can no longer be relied on as a calibration reference. It was therefore necessary to accurately characterise of the instrument on the ground. In September 1999 a thorough radiometric characterisation of the MARSS system was undertaken using the thermal vacuum test facility at DERA Farnborough.