Infrared Sea surface temperature Autonomous Radiometer (ISAR) is an instrument developed by the University of Southampton since 1999 to provide accurate and reliable in situ skin sea surface temperature measurements to an accuracy of +/- 0.1 K. The skin sea surface temperature is obtained by measuring upwelling infrared emission from the sea within the spectral waveband 9.8-11 microns. It is able to protect itself from sea-water spray and rain which can introduce large error or cause damage to the instrument, although this prevents the measurement. It can also self-calibrate with its two internal reference blackbodies. The instrument was designed to be mounted on ships. The data produced by the instrument is used to validate remote sensed data.