The Sun as a Source of Error for Sea Surface Salinity Retrieval by Means of Passive Microwave Remote Sensing at L-band

The Aquarius/SAC-D mission is dedicated to the remote sensing of Sea Surface Salinity(SSS) and is to be launched in 2009 by NASA. A three-beam L-band (1.41 GHz) radiometer is the core instrument for retrieving SSS. The required accuracy on the radiometric measurements for retrieving SSS within 0.2 psu is 0.1 Kelvin (K), with an ocean brightness temperature being of the order of 100 K. One potential source of noise is the Sun, because of its very large brightness temperature at this frequency, on the order of 150,000 K. Its radiation affects the measurements directly through the antenna sidelobes, and after reflection at the ocean surface. The latter influence is being minimized by adopting an orbit with 6 AM/PM equator passing times and by orienting the three antenna beams across the satellite track towards the dark side of the Earth surface. However, as the 98 degrees polar orbit is not always aligned with the day/night terminator, the satellite ground track will be on the illuminated side of the Earth for half of the time. Since the satellite altitude is relatively low (~660 km), beams pointing at the angles selected for Aquarius, between 26 and 40 degrees, will not always be in the dark side due to their limited distance from the satellite ground track. In fact, because the sea surface roughness causes scattering even from non-specular directions, radiation from the Sun will enter the antenna beams approximately one fourth of the time. The different contributions of the Sun to the antenna temperature and their potential influence on the SSS retrieval as a function of time of the year and latitude are investigated. Aquarius orbit simulations are used to calculate the Sun position with respect to the antenna during one year. In addition, the Sun specular image position is identified on the Earth surface, and the bistatic angles of the illuminated part of the antenna field of view are computed. A two-scale and a Kirchhoff electromagnetic model for the scattering from the sea surface are then used to evaluate the amount of sun radiation reflected and scattered toward the instruments. Finally, the Aquarius antenna patterns are used to weight and add together the direct, reflected and scattered contributions. The final results are discussed in terms of potential seasonal or regional biases on the retrieved SSS.