Pier-based observations of polarized sea surface emission

A Ka band polarimetric radiometer operating at 37 GHz was deployed from the seaward end of a small pier colocated with an X band Doppler scatterometer used to measure the instantaneous wave field. Brightness temperature time series from a small spot on the surface for four linear polarizations were measured: T
V
, T
H
, T
P
, and T
M
, corresponding to vertical, horizontal, +45°, and –45°, respectively. Expected behavior of the mean values of T
V
and T
H
was observed as a function of incidence and azimuth angles. Varying sensitivities to the wave field in T
P
and T
M
were observed as a function of azimuth angle with respect to the upwind direction. To the left of upwind, wave-induced modulations of T
P
exceeded those of T
M
, while the opposite behavior was observed to the right of upwind. Since U is the mean value of the difference of these signals, it is relatively easy to interpret the sine variation of U with wind-relative azimuth angle. Finally, modulations of brightness temperatures by the wave field were found to be consistent with the “smoothed” slope distribution provided by the radar.