This book attempts to bring together those aspects of maritime radar relating to scattering from the sea surface, and their exploitation in radar systems. The presentation aims to emphasise the unity and simplicity of the underlying principles and so should facilitate their application in these changing circumstances.

This chapter discusses radar scattering and sea clutter.

In this chapter we consider simulation techniques that enable us to study various aspects of radar performance, in circumstances where an analytic attack may not be possible, or particularly informative. To complement the analytic clutter modelling discussed in Chapter 4, we will develop methods for the numerical simulation of unwanted radar returns. The clutter models in Chapter 4 are exclusively statistical; this prejudice is still evident in our choice of simulation methods. In essence, we address the problem of generating correlated random numbers with prescribed one and two point statistics (i.e. pdf and correlation function) that incorporate the physical insights developed in Chapters 2-4.

In the previous chapter we considered how we might best detect small, localised targets in a background of sea clutter. The identification of the likelihood ratio as an optimum discriminant, and of its more practically useful approximations, pro vided us with a unifying framework for this discussion. However, small target returns are not the only features of interest in maritime radar imagery. Large-scale correlated structures arising from surface currents, ship wakes, the presence of sur factants and other sources can frequently be discerned and are a valuable source of information in many circumstances. In this chapter we will discuss how such ocean surface features might best be enhanced and detected. Once again the like lihood ratio concept is a very useful guiding principle, which leads us to methods that enable us to both enhance these features and exploit our prior knowledge of their structure to detect them more effectively. So, paradoxically, a discussion of the processing of images that are frequently interpreted and assessed in qualitative terms, will involve us in a fair amount of detailed formal analysis. Much of this will be based on the multivariate Gaussian distribution.

A radar is often required to detect targets against a changing background of clutter and thermal noise, the clutter reflectivity and statistics varying with range and look direc tion, dependent on the chosen radar parameters, operating height and the prevailing weather conditions. For a scanning radar over the sea, this change is continuous and usually unpredictable.

In this appendix we give an informal review of probability theory and other matters relevant to the modelling of clutter and its impact on radar systems. Our selection of topics is rather eclectic and our treatment pragmatic; the principal aim is to remove those obstacles to the evaluation of radar performance presented by any unfamiliarity with the concepts and practice involved in calculating probabilities.

In this appendix the authors have set up the integral equation formulation o fthe scalar scattering problem. Complete solutions, such as those describing the reflection and transmission of radiation by a planar interface, can be extracted from this only in the most idealised of circumstances. Nonetheless, the integral equation formulation provides us with a framework within which many aspects of scattering by a rough surface can be developed systematically. Approximate descriptions of scattering by a perfectly conducting surface can be derived, both through iterative solution of the integral equation (physical optics and small height perturbation theory) and progressive refinement of the Green's function (half space and Lorentz reciprocity based calculations). The latter Lorentz reciprocity based approach also allows us to derive the small height perturbation theory result for an imperfectly conducting rough surface with relatively little trouble; this in turn can be modified to take account of the large scale swell structure of the sea surface through its incorporation into the composite model. Methods for the numerical solution of the scattering integral equations have been developed. Sufficient detail has been given for explicit expressions to be identified and coded up for both perfectly and imperfectly conducting surfaces. The modelling of low grazing angle scattering invariably encounters difficulties that arise from the finite size of scattering surface that can be accommodated within the computer; these has been ameliorated by introducing semi-infinite adjunct planes. A different and improved method for the calculation of the terms in the integral equations that arise from these planes has been described, which also allows us to treat the imperfectly conducting surface through the use of the impedance boundary condition that was derived.