AN EVALUATION OF A SELF-CALIBRATING INFRARED RADIOMETER FOR MEASURINGSEA-SURFACE TEMPERATURE

Satellite radiometer measurements of global sea surface temperature (S ST) with an accuracy of 0.3 K are required for climate change monitori ng. In order to validate that this accuracy can be achieved, in situ m easurements of sea surface radiance must be made during satellite over passes. In the past decade attempts have been made to design self-cali brating, infrared radiometers for measuring SST from research ships, a nd some commercially manufactured models are now available. The Britis h Antarctic Survey deployed one such radiometer on board the royal res earch ship Bransfield between October 1991 and May 1992. Its purpose w as to measure SST within the Along Track Scanning Radiometer (ATSR) sw ath when the ERS-1 satellite passed over the ship. The ship radiometer was claimed to have an accuracy of +/- 0.1 K but this had not been ve rified under realistic measurement conditions. An evaluation of the ra diometer's accuracy was therefore carried out during a voyage from the British Isles to Antarctica. At intervals throughout the voyage the t emperature of well-stirred seawater in a tank on the deck of the ship was measured using both the radiometer and thermometers, which were ac curate to +/- 0.1 K. These measurements revealed that the radiometer v alues of water temperature were more than 1.5 K warmer than the values given by the thermometers. The cause of this offset was thought to be incorrect calibration of platinum resistance thermometers within the instrument, and an empirical correction was derived. When the correcti on was applied the rms difference between the thermometer and the radi ometer measurements of the temperature of the seawater in the tank was 0.1 K using the radiometer's 11-mu m channel. The rms difference usin g the 12-mu m channel was 0.14 K, which was larger because of an unide ntified beat signal that affected this channel. These results, therefo re, showed that this radiometer was capable of making SST measurements that were accurate enough to validate the ATSR SST and also to carry out useful investigations of the ocean skin effect.