A Brief History

The first attempt to measure SSS from space took place 20 years ago on Skylab (Lerner and Hollinger, 1977). A 1.4 GHz microwave radiometer collected data intermittently, there was no "ground truth" other than standard surface charts, and many of the ambient corrections were not as well understood then as they are today• Nevertheless, a correlation was found between the sensor data (after correcting for other influences) and
SSS. This was an encouraging early result.

Spacecraft remote sensing of SSS, using microwave radiometry at low frequencies, was first proposed by Swift and McIntosh . At L-band the polarized brightness temperature (TB) measured by a radiometer is linked to salinity in the first centimeter of the ocean through the dielectric constant of sea water. The sensitivity to salinity increases with decreasing frequency (until around 600 MHz), as well as decreases attenuation by the atmosphere (except for heavy rain), and the 1400-1427 MHz window, reserved for passive observations, has advantages for SSS remote sensing. This requires special care because of the low sensitivity of TB to SSS: from 0.8 K to 0.2 K per psu, which depends on the ocean temperature, the radiometer incidence angle, and the polarization. It is necessary to separate out the effects on TB from other parameters such as SST, the impact of ocean roughness, Faraday rotation, etc., as is described in section "How it works". The stringent requirements pose technical challenges achieving the required radiometric accuracy and stability. Finally, the low frequency involved requires the use of very large antennas to achieve a moderate spatial resolution on ground. For these reasons, only two L-band space-borne radiometers, until present, have been flown: in 1968 aboard the Cosmos 243 and in 1973 aboard the Skylab S-194.

In 1995, at the "Soil Moisture and Ocean Salinity" Workshop organized at ESTEC (the European Space Research and Technology Centre, Noordwijk, The Netherlands), microwave radiometry at L-band was still considered as the most adequate technique to remotely measure these two geophysical variables. However, instead of the real aperture microwave radiometers that were considered until then, it was concluded that the most promising technique was aperture synthesis radiometry that had successfully been demonstrated a few years earlier. Le Vine et al. [15] created in 2000 an SSS map using the Electronically Steered Thinned Array Radiometer (ESTAR), the first 1D synthetic aperture radiometer flown on an aircraft.