Effects of the Antenna Aperture on Remote Sensing of Sea Surface Salinity at L-band.

Remote sensing of sea surface salinity, to meet the needs of global oceanography, requires accuracy on the order of 0.1 K. But since spatial and temporal scales are very large, it is possible to use antennas with large footprints and averaging to meet this goal. However, antennas with large footprints introduce other problems such as variations over the footprint of the incidence angle and of the polarization vector directions. Examples of these effects are illustrated for antennas representative of those that will be flown on the Aquarius mission being developed for remote sensing of salinity from space. It is shown that the antenna temperature integrated over the field of view is biased relative to the value at boresight because of changes across the field of view. The bias can be as much as 4 K and depends on polarization. Polarization mixing, because of the variations of the local plane of incidence across the footprint, also induces biased polarimetric measurements (peculiarly for the third Stokes parameter). The antenna effects vary with geophysical conditions, peculiarly with surface roughness. Finally, land in the field of view contributes noticeably to the signal up to several hundred kilometers from the coast line