Vertically polarised MF/HF groundwave radar has been used over the frequency range 2-30 MHz for sea-state sensing, surface-current measurement and ship tracking. The paper reviews the propagation factors and noise environment which control the performance of such systems. The various definitions of antenna gain, propagation loss and sea backscattering coefficient are discussed and related to physical concepts. The various first-order and higher-order mechanisms of scattering are reviewed and related to the relevant electromagnetic and hydrodynamic processes involved. These processes, linked with the velocities of the water waves responsible, result in a Doppler spectrum of the radar echoes which may be used to infer the wave height and wave directional spectra on the sea surface. Particular attention is directed towards remaining problems in understanding these processes. In evolving radars for ship tracking, detectability depends on the knowledge of the Doppler spectra of the sea clutter and, in particular, on limitations in directional determination of ship echoes with practical antenna systems located on available coastal sites. In conclusion, experimental measurements of sea echo, ship tracks and antenna-array characterisations are compared with theoretical considerations. List of principal symbols A(d)h F,F r ,F r ,F R = one-way attenuation over lossy ground relative to perfectly conducting ground = effective absorbing aperture of receiving antenna = propagation distance =; EMF in transmitting antenna = electric field incident on target = reradiated electric field from target as received at transmitter = wave frequency, Doppler frequency, Bragg frequency = incident power flux (field intensity) on target = reradiated power flux from target as received at transmitter = Norton attenuation coefficients 9 9, 9T > 0 L Pi > P R » PRR = acceleration due to gravity = gain factors of transmitting/receiving antennas relative to isotropic = modified gains in presence of ground G' r , G' R = gains expressed in dB = directional pattern of wind-waves as a function of angle, 6, relative to mean wind = heights of transmitting and receiving antennas above ground plane = lengths of transmitting, receiving whip antennas = radio wave number 2n/X = sea wave number 2n/L = effective length of antenna = basic transmission loss factor between isotropics in free space = transmission loss factor between antennas in free space = transmission loss factor between antennas in reflective space = transmission loss factor between whips = sea wave length = transmitted, received, reradiated power A A T , A R = radiation resistance of antenna in free space -2s is exponent of cos 9 in spreading function for wind waves = reactance of transmitting, receiving antenna -(1 + A) is factor by which radiation resistance of antenna is increased in proximity of ground plane = values of A for transmitting, receiving antennas = proportion of sea-wave energy travelling in opposite direction to wind = characteristic impedance of free space, 377 ohm/square = an...

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The three main radio techniques for remote sensing of the ocean surface, microwave radar, microwave radiometry, and dekametric or HF radar, are surveyed. The generation and characterization of wind waves and swell on the ocean surface, and the first‐order mechanism of Bragg resonant scattering of radio waves by such surfaces are outlined. Doppler HF radar techniques, ground wave and sky wave, developed for utilization of this effect to sense remotely wave, wind direction and surface current velocity are reviewed. Second‐order scattering effects, hydrodynamic and electromagnetic, are also present, and their signatures on the Doppler echo spectrum of an HF radar permit measurement of wave height, wavelength spectrum and directional spectrum. Finally, the scattering mechanisms relevant in satellite‐borne microwave radar are outlined. Bragg resonance occurs with short waves, these being modulated by the longer gravity waves of practical interest. The potential of the radar altimeter, scatterometer and synthetic aperture radar for global ocean surveillance is illustrated by examples from the satellite.

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