3.4.2. Scattering from Ships and Aircraft

Although target detection was once the province of HF radars deployed only for national defense, many low-power remote-sensing radars are now addressing this mission, primarily focusing on ship detection. Modelling HF radiowave scattering from ships and aircraft has been carried out for many decades by means of computational electromagnetic codes. Early work was almost exclusively carried out using the method-of-moments code NEC, but nowadays, other software packages such as FEKO, CST Studio, and HFSS are widely used, along with more advanced in-house codes. Here, we illustrate the general characteristics of bistatic HF radar cross sections (RCS) of ships and aircraft with results computed for the Fremantle-Class patrol boat 42 m in length, and the Aermacchi MB 326H aircraft, wingspan 10.6 m. These platforms are pictured in Figure 13.

**Figure 13.** The Fremantle Class patrol boat and the Aermacchi MB 326H trainer aircraft.

A format developed to present bistatic HF radar cross section is shown in Figure 14. The figure shows the predicted bistatic RCS of the Fremantle Class patrol boat, evaluated at four different radar frequencies. More precisely, it shows the squared magnitude of the VV component of the polarization scattering matrix, but we will use the term RCS where no confusion is likely. This format uses columns to index the azimuthal angle at which the radar signal is incident on the target and rows to index the azimuthal angle of departure or scattered angle. In this example, the elevation angle is set to 0◦ (vertical angle of incidence=90◦), which closely approximates the field structure for HFSWR. Below each panel showing the bistatic RCS is the monostatic RCS, i.e., the trailing diagonal of the matrix.

**Figure 14.** The bistatic RCS of the Fremantle Class patrol boat, evaluated at HF radar frequencies 5, 10, 15, and 20 MHz for HFSWR configurations. The lower panel in each case shows the monostatic RCS which is just the cut along the trailing diagonal.

For detection, we require that a ship echo exceed the clutter and noise power in the same Doppler bin by some margin ε; that is, there exists ω ∈ [−Ω, Ω] such that

$$s(\omega) > c(\omega) + n(\omega) + \varepsilon \tag{12}$$

where *s*(ω), *c*(ω), and *n*(ω) are the target, clutter, and noise power spectral densities. This situation is illustrated in Figure 15, where a nominal target echo is superimposed on Doppler spectra evaluated for four different sea states.

**Figure 15.** Ship detection in clutter and noise, showing the importance of thresholding.

The rich structure of the RCS patterns reveals the potential for exploitation by both the radar designer selecting sites for his transmit and receive systems, and the vessel seeking to minimize its detectability, in concert with other strategies that a clandestine mission might exploit. As we have seen in the previous section, the option of utilizing a bistatic configuration gives us an extra degree of freedom for 'controlling' the clutter spectrum.

While aircraft detection has seldom been a priority for HFSWR, it is certainly an established capability. Figure 16 uses a different format to show the VV RCS of the Aermacchi MB 326H as a function of azimuth (i.e., aspect) and radar frequency. The five panels correspond to different bistatic angles, namely, 0◦ (i.e., monostatic), 20◦, 40◦, 60◦, and 80◦. As aircraft move at speeds that carry them rapidly across typical HFSWR coverage, presenting a changing aspect, a distinctive temporal variation of echo strength can be observed, potentially useful for target classification.

**Figure 16.** The HFSWR RCS of the Aermacchi MB 326H aircraft as a function of aspect and radar frequency. The five panels correspond to different bistatic angles as indicated.
