Application of Radar and Microwave Scattering to Ocean Wave Research

The high data rate and advanced technology of modern radar systems make them attractive instruments for measuring the properties of random wave fields. When the objective is wave research, it is particularly important that the relation between wave field properties and radar output be clear and unequivocal. This is generally the case in the measurement of radar range (altimetry), the measurement of line-of-sight speeds (Doppler shift) and under those conditions where the scattered electromagnetic field is proportional to the surface displacement. The latter situation is that of first order Bragg scattering, in which case the influence of all waves outside a narrow window at the Bragg resonance condition is strongly filtered out. In this case, it has proved possible to measure such wave properties as temporal growth and approach to equilibrium in wind-wave tanks and spectral energy transport in wave tanks or at sea. Furthermore, the Doppler shift is accurately the frequency of the Bragg wave in first order scattering. Thus phase speeds may be determined not only for their intrinsic interest but as a means of measuring surface currents and probing the profile of the mean flow on both sides of the air-water interface. Results of all these techniques are now available in the case of wind-wave tanks and numerous examples are presented. The techniques have so far been less fully exploited at sea, but some examples from HF groundwave scatter and dual frequency microwave radar returns from wind-generated seas are available. Finally, opportunities for exploiting second order Bragg and two-scale scattering results for wave research are also discussed. INTRODUCTION Most of the instruments used to measure ocean surface waves sample the surface over a limited spatial region. They are essentially point probes. In consequence, it is necessary to use large, often cumbersome, arrays of probes or to record over an impractically long time In order to obtain reliable, detailed information about the random wave field, In contrast, a radar, in addition to being a remote sensor, is inherently an area-extensive probe which can automatically, and nearly instantaneously, average over a large surface region. It is worth pointing out that, in what is probably the earliest application of radar to ocean wave research, the airborne radar wave profiler of Barnett and Wilkerson (1), the radar was used as & point probe. It appears to have been largely superseded in profiling application by the laser. Two modes of radar operation are discussed here. The first takes advantage of the close match between ocean wavelengths and available radio and microwavelengths. The scattering is like that of x-rays from periodic crystal lattices--Bragg scattering. The surface displacement spectrum is obtained directly, but only at a single ocean wavelength--the Bragg wavelength which is uniquely related to the radar or radio wavelength. It is thus necessary to use a range of radio wavelengths or vary the scattering geometry, in order to cover the wave spectrum. In the second mode the precise nature of the scattering mechanism is less important. It is only necessary that the scatterers, short Bragg waves, specular points, or whatever, be modulated by the larger ocean waves. It is the modulation which is detected and utilized although this may be only indirectly related to wave amplitude. However in this case only a narrow band of radar wavelengths is required, which is a great practical convenience. BRAGG SCATTERING Suppose a transmitting antenna is oriented (Fig. 1) at depression angle 61 and a receiving antenna at angle 83 to the plane in which a wave system is propagating. If the wave is not too large in amplitude compared to the radar wavelength the scattered powet is proportional to the surface displacement spectral amplitude of the wave component at the Bragg wavevector kg where: k = [ k (cos6i + cos0a ) , 0 ] (1) ~D O and k0 is the microwavenumber. Note that this is a vector equation. In first order Bragg scattering the radar strongly filters out all other wavevectors. Since the wave is moving the scattered energy is Doppler shifted. The Doppler shift is just that associated with the line-of-sight components of the phase speed of the wave, and this proves to be identically the frequency of the Bragg wave. Let us denote the ordinary power spectrum of the scattered and linearly detected microwave field as the Doppler spectrum. In first order Bragg scatter the Doppler spectrum is proportional to f (kg, 0 1 cu), the complete surface displacement spectrum. There have been numerous measurements of such Doppler spectra of wind-'generated waves beginning with Crombie. The character of these spectra Is easily illustrated with wavetank measure*