Modulation Transfer Functions at Ka Band

Abstract

Short dace ripples and, hence, the rem to a radar looking at the surface are modulated by the long ocean waves. These short ripples are the predominant microwave scatterers on the ocean surface. The Modulation Transfer Function (MTF) is often used to describe this modulation of the radar signal by the long waves. We measured the MTFs at 35 GHz (Ka band) with a switched-beam vector slope gauge/scatterometer on the research platform NORD- SEE as part of the SAXON-FPN experiment in November 1990. Three independent measurements of the scattering were available for each height measurement. This provided the opportunity to average the time series to reduce the effects of fading noise and sea spikes, or, alternatively, to append the time series to achieve more degrees of freedom in our spectral estimates. For upwind measurements, the phase of the W-polarized Ka-band MTF was always positive, which implies that the maximum of the radar return originates from the forward face of the long-scale waves. This phase increases with increasing wind speed. The magnitude of the MTF decreases with increasing wind speed. Intmduction The brightness of high-resolution real- and synthetic-aperture radar images of the ocean surface comlates highly with the ocean surface profile. Cross-section variation is the primary source of brightness modulation for a real-aperture radar. The radar images are often assumed to be linearly modulated about the mean brightness by changes in backscatter associated with the large-scale waves. For synthetic-aperture radar systems, in addition to cross-section modulation, the velocities of the water in the large-scale waves are important in the image formation. The usual model for scattering from the ocean surface is the two-scale Bragg-resonance model. In this, the strength of the signal depends on the amplitudes of the part of the ocean-wave spectrum that is resonant to the radar wavelength. The cross-section modula- tion apparently results from a combination of straining of the Bragg ripples by the long-wave orbital motions (hydrodynamic modulation) and the local angle of incidence caused by changing slopes of the long waves (tilt modulation).