A technique is presented for measuring ocean wave directional spectra from aircraft using microwave Dopper radar. The technique involves backscattering coherent microwave radiation from a patch of sea surface which is small compared to dominant ocean wavelengths in the antenna look direction, and large compared to these lengths in the perpendicular (azimuthal) direction. The mean Doppler shift of the return signal measured over short time intervals is proportional to the mean sea surface velocity of the illuminated patch. Variable sea surface velocities induced by wave motion therefore produce time-varying Doppler shifts in the received signal. The large azimuthal dimension of the patch implies that these variations must be produced by surface waves travelling near the horizontal antenna look direction thus allowing determination of the direction of wave travel. Linear wave theory is used to convert the measured velocities into ocean wave spectral densities. Spectra measured simultaneously with this technique and two laser profilometers, and nearly simultaneously with a surface buoy, are presented. Applications and limitations of this airborne Doppler technique are discussed.
Frequency agility is applied to dual-frequency scattering from the ocean using an L-band microwave system in order to reduce the clutter background which has previously limited signal detectability in such scattcrometers. Doppler spectra of the return show improvements of up to 13dB in signal-to-clutter ratio using this technique compared with the standard nonfrequency-agile technique. This improvement greatly enhances the ability of a dual-frequency scatterometer to measure ocean surface currents. Composite surface scattering theory is applied to explain the signal improvement. The clutter background is shown to be proportional to the ocean surface wave spectrum evaluated at a wavenumber corresponding to the wavenumber separation between the carrier frequencies of two transmitted pairs of frequencies. Thus, if the frequency separation between these carriers is equal to the frequency separation between the lines of each pair, the entire signal received by the system is proportional to the ocean surface wave spectrum evaluated at the common wavenumber difference. This three-frequency technique completely removes previous limitations on the measurement of ocean wave spectra by dualfrequency scatterometers by converting the clutter background into part of the desired signal. Signaldetectability is then limited only by thermal noise.
Investigators in several countries have been studying a twofrequency microwave radar technique for the remote-sensing of direction ocean wave spectra and surface currents. This technique is conceptually attractive because its operational physical principle involves a spatial electro-magnetic scattering resonance with a single, but selectable, long gravity wave. Multiplexing of signals having different spacing of the two transmitted frequencies allows practical measurements of the entire long wave ocean spectrum to be carried out. The scatterometer signal/background ratio is. using conventional processing, only adequate for measurements made relatively near the dominant wave wavenumber and when sufficient temporal averaging of spectra is employed.A new scatterometer has been developed and experimentally tested which is capable of making measurements having much larger signallbackground values than were previously possible. This new instrument couples in a hybrid fashion the resonance technique with coherent, frequency-agility capabilities. Frequency-agility provides a means of acquiring virtually instantaneously the signal degrees of freedom necessary to obtain accurate spectral estimates.The effect of increasing the number of coherently summed frequency-agile signals is to enhance the return from large ocean surface features such as long-wave trains which are correlated over the range of frequency changes used at the expense of signals from shorter centimetric wavelength waves which are decorrelated. It was. however, not known a priori whether or not the induced correlation of centimetric waves by the longer waves would be insignificant for the small scattering cells used in the experiments.
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