MICROWAVE BACKSCATTER FROM THE SEA - MODULATION OF RECEIVED POWER AND DOPPLER BANDWIDTH BY LONG WAVES

Two continuous wave microwave systems were mounted on an elevator on the German Forschungsplattform Nordsee (FPN) during the Synthetic Aperture and X Band Ocean Nonlinearities (SAXON)-FPN experiment. Here we report on measurements of long-wave effects in the power received by these systems and in the Doppler bandwidths recorded from the systems. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH- and VV-polarized backscatter signals simultaneously. The elevator system allowed us to vary the altitude of the microwave antennas above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45-degrees incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We have attempted to characterize the modulation of both received power and Doppler bandwidth in terms of the standard modulation transfer function (MTF) concept. We find that our measurements of the MTF for received power agree well with previous measurements at X band but that the phase of the MTF for VV polarization is different from that previously reported for Ka band at a 60-degrees incidence angle. We show how the amplitude and phase of the MTF as well as the coherence function vary with wind speed, long-wave frequency, long-wave propagation direction, and antenna height. In the case of the first three parameters our results agree with previous work except as noted above for Ka band. The dependence of the MTF on antenna height has not been investigated previously, Our results show that this dependence is small and well explained by the height variations induced as long waves propagate through the footprint on the surface. When we apply the MTF concept to the Doppler bandwidth, we find that the MTF of the bandwidth is small except at very low wind speeds. In view of the association of this bandwidth with velocities of subresolution-scale, or intermediate-scale, waves which are themselves modulated by the long waves, we find these small MTFs associated with Doppler bandwidths to be somewhat surprising. We present a simple model to attempt to draw inferences about the amplitude modulation of intermediate-scale waves from our results. We also conclude from the small MTFs for Doppler bandwidths that the assumption of a constant correlation time in theories of synthetic aperture radar imagery of the ocean is well justified.