SYNTHETIC-APERTURE RADAR INTERFEROMETRY APPLIED TO SHIP-GENERATED INTERNAL WAVES IN THE 1989 LOCH-LINNHE EXPERIMENT

Interferometer synthetic aperture radar images collected during the 19 89 Loch Linnhe experiment showed mean Doppler variations across the ph ase of ship-generated internal waves that corresponded to ''velocity'' variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave fea tures of the order of 5 to 10 cm/s and virtually ruled out the existen ce of surface currents as large as the interferometer-inferred values. In this paper we show how the pixel-to-pixel phase difference measure d by the Jet Propulsion Laboratory interferometer is related to the me an Doppler frequency of the backscattered field. Model calculations ar e used to show how this frequency can sometimes change by a large amou nt, even when rather small surface currents are present. In particular , for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations base d on our wave-current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to larg e velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity esti mates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thu s for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave p ropagation direction, the mean Doppler shifts (and the phase differenc es) are dominated mostly by advection, and interferometer current esti mates are more accurate. C band computations predict that if the anten na spacing is small enough so that the fields from the two antennas re main correlated, then the C band interferometer current estimates will be better than those at L band.