*4.2. Interpretation of Experiment Results*

**Time period 1**: CTSR bistatic HFSWR experimental data analysis

A typical two-dimensional range-Doppler (RD) spectrum of the CTSR bistatic HFSWR and that of the coast-based monostatic HFSWR at a specific time is shown in Figure 18a,b, respectively. At the sample time, the baseline distance between the transmitting station and the receiving station was 12.8 km. The shipborne platform was sailing in a straight line with velocity of 11.48 km/h (6.2 knots), the ship heading was 115◦ (relative to the baseline), and the main axis angle of the shipborne radar receiving station was 205◦ (relative to the baseline). The broadening range of the simulated sea clutter spectrum with the same parameter configuration as in the experiment is superimposed on the measured radar spectrum, and it is marked with a red box.

**Figure 18.** Doppler spectrum of the CTSR bistatic HFSWR and of the coast-based monostatic HFSWR at the velocity of 11.7 km/h (6.2 knots); (**a**) RD spectrum of the CTSR bistatic HFSWR; (**b**) RD spectrum of the coast-based monostatic HFSWR; (**c**) Doppler data of the CTSR bistatic HFSWR at the range of 50 km; (**d**) Doppler data of the coast-based monostatic HFSWR at the range of 55 km.

As can be seen from Figure 18, in comparison with the non-broadened first-order sea clutter spectrum of the coast-based monostatic HFSWR, both the left and right first-order sea clutter RD spectra of the CTSR bistatic HFSWR were broadened at the platform velocity *vR* = 11.48 km/h (6.2 knots), and the broadening of the right first-order sea clutter spectrum was more obvious than that of the left. The width of the right first-order sea clutter was approximately 11.1 km/h (20.8 to 31.9 km/h), and that of the left first-order sea clutter was 10.3 km/h (−18.5 to −28.8 km/h). According to the simulation results (Figure 8a) discussed in Section 3, the theoretical width of the right first-order sea clutter should be 11.3 km/h (20.2 to 31.5 km/h), and that of the left first-order sea clutter should be 10.5 km/h (−18.9 to −29.4 km/h). It can be seen that the measured width values of both the left and the right first-order sea clutter are consistent with their theoretical values. The small deviation of approximately 0.6 km/h (0.17 m/s) of the location of the sea clutter spectrum might have been caused by the existence of ocean currents. In addition, it can be discerned from Figure 18a that the bending characteristics of the first-order Doppler spectrum caused by the bistatic angle were not obvious. This is mainly attributable to two reasons. The first is that the baseline was small relative to the detection distance. The second is that the shipborne receiving station could only receive on the right side of the ship, limiting the detection range of the radar to a certain extent. Both of these factors could lead to smaller eccentricity and a smaller bistatic angle, resulting in the determined bending characteristics of the first-order sea clutter spectrum.

Compared with the broadened first-order sea clutter spectrum, the echo of a moving target (Maritime Mobile Service Identity No: 413551440) was detected simultaneously by the bistatic HFSWR and the coast-based monostatic HFSWR systems. The positions of the target in the two-dimensional RD spectra and in the one-dimensional Doppler spectra are marked with red circles and straight lines, respectively. Its velocity and heading were 19.96 km/h and 289.4◦ relative to the north, respectively, i.e., *<sup>v</sup>* <sup>=</sup> 17.96 km/h, <sup>ϕ</sup> <sup>=</sup> <sup>−</sup>167.8529◦, <sup>β</sup> <sup>2</sup> = 6.15◦, ϕ*<sup>R</sup>* = 115.5◦ relative to the baseline, and θ*<sup>R</sup>* = 116.1◦. Based on Equation (5), the theoretical radial velocity was −11.7516 km/h. Its echo appeared in the 20th range unit, i.e., the 74th Doppler unit of the RD spectrum of the CTSR bistatic HFSWR with a signal-to-noise ratio (SNR) value of approximately 30 dB, which means that the half of its range sum was 50.73 km, and the projected elliptical velocity was −11.8127 km/h. It can be seen that the measured velocity of the target is consistent with the theoretical value (−11.7516 km/h). The target's range relative to the monostatic HFSWR was 54.2269 km and the radial velocity was −17.0557 km/h, with an SNR value of approximately 25 dB. The Doppler-time (D-T) distributions over a 10-min period, onto which the elliptical velocity results were projected using Automatic-Identification-System (AIS) real information were superimposed, are shown in Figure 19. It can be seen that the track of the moving target can be detected clearly, and that the Doppler shift of the target echo within the 10-min period is consistent with the AIS track results.

**Figure 19.** Doppler-time (D-T) distribution and Automatic-Identification-System (AIS) results for the CTST bistatic HFSWR and coast-based monostatic HFSWR; (**a**) D-T distribution of the CTSR bistatic HFSWR; (**b**) D-T distribution of the coast-based monostatic HFSWR.
