The scattering of high frequency electromagnetic radiation from deterministic targets on the ocean surface

Royle, Michael (2025) The scattering of high frequency electromagnetic radiation from deterministic targets on the ocean surface. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

An analysis of the electromagnetic scattering from deterministic targets embedded in time-varying random rough surfaces is presented. The approach combines and extends previous works addressing high frequency (HF) electromagnetic scattering from the ocean and stationary surface targets separately. The analysis begins with first- and second-order expressions for the normal component of the scattered electric field from a conducting surface that is small in height and slope and described by a time-varying Fourier series. A vertical pulsed-dipole transmitting source is assumed while the observation point of the scattered field remains general. These expressions are modified to introduce a finite, deterministic target with arbitrary motion via a Fourier transform of the surface target’s profile. The Fourier integrals in the resulting expressions are evaluated through asymptotic methods. The analysis produces two bistatic scattered field expressions involving the surface target. These are attributed to (1) first- and second-order scatters solely from the target and (2) a double scatter involving the target and nearby surrounding ocean. The two components are added to existing expressions for the first- and second-order scattered fields from the ocean surface to model the total scattered field from an ocean patch containing a surface target. It is shown that the HF Doppler cross section of the ocean patch and target may be found as the sum of the cross sections obtained in treating each field component independently. The target-only and target-ocean cross sections are formally evaluated, while the ocean-only components are obtained from existing models. The target-only component is shown to agree with existing monostatic, zero-velocity results when appropriate substitutions are made. The target-ocean ii cross section represents a new expression not seen in previous work, but its general form is seen to agree with existing bistatic ocean cross section models. Both cross section components involving target scatter contain a motion-related Fourier factor similar to one that arises in ocean cross section models for a radar installed on a floating platform. The HF Doppler cross sections are simplified for the case of a surface target moving with constant velocity. It is shown that the target-only cross section contains a Dirac delta function with an argument restricting the response to an impulse at the bistatic Doppler frequency shift predicted for uniform linear motion. The oceantarget component also contains a Dirac delta function with an argument containing the constant-velocity bistatic Doppler shift in addition to terms related to ocean dispersion and the change in target location between radar acquisitions. A system model of an HF radar suitable for predicting the received Doppler power spectral density from an ocean patch containing a surface target is presented. The system model is used to predict the received signal strength for a variety of target, environmental, and radar operating parameters. The results of the computations show, that under certain conditions, a constant velocity target whose first-order cross section is masked by ocean clutter may be detected through a secondary scatter from the ocean surface. The models derived in this work enable the establishment of suitable design specifications and operating parameters when developing new or utilizing existing HF surface wave radar systems for the purposes of monitoring targets on the ocean surface. In addition, the physical interpretation of the scattering process and simple computation provided by the models should prove relevant in developing and testing novel signal processing techniques for both target identification and clutter rejection.

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