3.4.4. Scattering from Ship Wakes

The possibility of detecting ship wakes was recognized in the 1970s, but early studies focused on first-order scattering, which is critically dependent on radar geometry and frequency and hence not a practical solution. The development of a second-order theory [38–40] has opened the way to a meaningful capability, but here too, careful consideration of the radar geometry and frequency is required to approach optimum sensitivity.

According to this theory, the wake signature depends not only on the vessel, it is also a function of the ambient sea state, so no simple assertion of optimality of one or other configuration can be made. Nevertheless, extensive modelling has shown that bistatic geometries routinely emerge as optimal solutions.

One indication of the possible advantage of bistatic geometry can be seen from Figure 19, showing a measured omni-directional wave spectrum from the open sea, on which is superimposed the corresponding spectrum for a ship wake. As the annotations reveal, the wake components are concentrated at low frequencies (and hence wavenumbers) for which monostatic first-order Bragg scatter at HF is not available, but at a bistatic angle ϕ the resonant wavenumber is decreased by a factor sin(ϕ/2).

**Figure 19.** The omnidirectional spectrum of an ambient sea, and, superimposed (in red), the wake of a merchant ship at normal sailing speed; note that the abscissa is frequency, not wavenumber, connected here by the familiar deep-water dispersion relation. The radar frequencies corresponding to monostatic Bragg scatter are marked by the dashed lines.
