1. History of sea surface temperature sensors


The first measurements of sea surface temperature (SST) from space were largely delivered as a by-product of early efforts to remotely monitor, and forecast, weather. The nominal goal of early missions such as the Nimbus series (Nimbus-1; launched in 1964) was atmospheric and cloud characterisation, however, it soon became apparent to researchers that their radiometry data could also provide information on surface temperatures in clear scenes. Since the early 1970s, data from the ever improving Nimbus and ITOS satellites began to provide a global picture of SST - albeit one that was sparsely populated (due to low coverage and short instrument life), coarse in resolution, subject to substantial Root Mean Square (rms) errors, and subject to multiple sources of contamination. However, by the mid 1970s, the concept of infrared measurement of SST had been sufficiently proven.

In 1978, a step change in capability was achieved with the launch of the first advanced very high resolution radiometer (AVHRR), then a 4 channel imager, aboard NOAA’s TIROS-N satellite. Since then, AVHRR has formed one of the mainstays of  satellite-based SST measurement, later upgrading to 5 (AVHRR/2) and 6 channel (AVHRR/3) instruments by 1998, and still in orbit on NOAA-18, NOAA-19, MetOp-A and MetOp-B (both operated by EUMETSAT) today.

Multiple other developments occurred in parallel, bring increased coverage in the IR spectrum and adding microwave capabilities. The 1998 and 2002 launches of the NASA Terra and Aqua satellites added not only the infrared imaging capability of the Moderate Resolution Imaging Spectroradiometer (MODIS, also an ocean colour sensor), but also the Advanced Microwave Scanning Radiometer (AMSRE). The latter provided a new way to derive SST that was immune to cloud contamination, with the caveat of lower spatial resolution. The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi-PP weather satellite has also delivered SST data since 2011.

Life for the Along Track Scanning Radiometer (ATSR-1) radiometer, the historical precursor series to SLSTR (which is on Sentinel-3), began with the 1991 ESA launch of ERS-1. This was quickly followed followed by ATSR-2 on ERS-2 in 1995 and AATSR (of which SLSTR is the direct antecedent) on ENVISAT in 2002.

Incremental improvements in sensor technology and a synergy between microwave and infrared approaches have resulted in huge gains in measurement precision and thermal stability, ever increasing coverage and spatial resolutions, substantial improvements in correcting for atmospheric conditions and, at lower resolution, the ability to measure irrespective of cloud-cover.