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

Two continuous wave microwave systems were mounted on an elevator on t he German Forschungsplattform Nordsee (FPN) during the Synthetic Apert ure and X Band Ocean Nonlinearities (SAXON)-FPN experiment. Here we re port on measurements of long-wave effects in the power received by the se systems and in the Doppler bandwidths recorded from the systems. Th e 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 abov e the sea surface from 7.5 to 27 m, always in the far field of the ant ennas. 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 Do ppler bandwidth in terms of the standard modulation transfer function (MTF) concept. We find that our measurements of the MTF for received p ower agree well with previous measurements at X band but that the phas e of the MTF for VV polarization is different from that previously rep orted for Ka band at a 60-degrees incidence angle. We show how the amp litude and phase of the MTF as well as the coherence function vary wit h wind speed, long-wave frequency, long-wave propagation direction, an d antenna height. In the case of the first three parameters our result s agree with previous work except as noted above for Ka band. The depe ndence of the MTF on antenna height has not been investigated previous ly, Our results show that this dependence is small and well explained by the height variations induced as long waves propagate through the f ootprint 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 ve ry 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 a ssociated with Doppler bandwidths to be somewhat surprising. We presen t a simple model to attempt to draw inferences about the amplitude mod ulation of intermediate-scale waves from our results. We also conclude from the small MTFs for Doppler bandwidths that the assumption of a c onstant correlation time in theories of synthetic aperture radar image ry of the ocean is well justified.