A resolution limitation on sea‐echo radar spectra inferred from point to point ionospheric Doppler broadening

Absrracr-Recent advances in propagation modeling, ionospheric diagnostics, and signal processing have helped overcome the limitations the ionosphere imposes on sea-state measurements with HF skywave radar. Wind-direction fields in tropical storms can be routinely mapped under most ionospheric conditions, but waveheight and wave-spectrum extraction is more sensitive to ionospheric distortions and requires care in signal processing and in selecting an ionospheric path. Spot measurements with a high-resolution radar have verified its ability to measure (in order of increasing dif€iculty) wind-direction fields, rms waveheight, and the scalar ocean-wave spectrum at ranges up to 3000 km using one ionospheric hop. Although such a radar can in principle map these quantities over millions of square kilometers of an ocean area, the time required to do so under various ionospheric conditions remains to be determined. A minimum objective of one map of rms waveheight per day seems attainable. S I. HISTORICAL PERSPECTIVE KYWAVE radars sense natural and man-made targets beyond the optical horizon by bouncing HF radio waves off the ionosphere. The first skywave radars were research instruments developed in the late forties and early fifties to study the structure and motions of the ionosphere itself, using echoes backscattered from the earth. Many researchers tried t o develop ground-backscatter radar into an operational diagnoseic tool for ionospheric mapping or HF frequency management, but because of problems in uniquely transforming radar signatures into ionospheric structure: such efforts have been largely abandoned. Consequently, the reviews of such applications by Hayden [ 11 and Croft [ 2 ] remain reasonably up-to-date. In the fifties, the prospect of detecting and tracking military targets (missiles, aircraft, and ships) with an over-thehorizon (OTH) radar -spurred a distinct and largely classified line of research that led to the construction in the sixties and seventies of large-aperture HF-OTH radar arrays by the major powers [31, 141, [511.The subject of this review is the third and most recent application of skywave radar-sea-state monitoring-in which information about the wind and wave fields at the sea surface is derived from the sea-clutter echo that is regarded as noise in surveillance systems.A sea-state radar need not employ an ionospheric (skywave) path. Indeed, the whole field of HF radar oceanography evolved from surface-wave experiments by Crombie [ 51 who first demonstrated HF Bragg scattering by sea waves of half the radar wavelength. Refinements in the mathematical model of HF scattering from the sea and the evolution of high-speed signal-processing systems have rapidly led to the present state of surface-wave radar development represented by

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