**Time period 2**: STCR bistatic HFSWR experimental data analysis

Measured data of the STCR bistatic HFSRW system with different platform speeds were obtained in the second period. The STCR bistatic HFSWR spectrum and the synchronous shipborne monostatic HFSWR data at the velocity of 0 km/h, 7.96 km/h (4.3 knots), and 18.5 km/h (10.0 knots) are shown in Figures 20–22, respectively. The heading at the three moments was 54◦, 141◦, and 115◦, respectively (relative to the baseline). For the receiving station, the angle of the main axis was fixed at 310◦.

**Figure 20.** Doppler spectra of the STCR bistatic HFSWR and ship-based monostatic HFSWR systems at the velocity of 0 knots. (**a**) RD spectrum of the STCR bistatic HFSWR; (**b**) RD spectrum of the ship-based monostatic HFSWR; (**c**) Doppler spectrum of the STCR bistatic HFSWR at the range of 32.5 km; (**d**) Doppler spectrum of the ship-based monostatic HFSWR at the range of 22.5 km.

**Figure 21.** Doppler spectra of the STCR bistatic HFSWR and ship-based monostatic HFSWR systems at the velocity of approximately 4.3 knots. (**a**) RD spectrum of the STCR bistatic HFSWR; (**b**) RD spectrum of the ship-based monostatic HFSWR; (**c**) Doppler spectrum of the STCR bistatic HFSWR at the range of 45 km; (**d**) Doppler spectrum of the ship-based monostatic HFSWR at the range of 35 km.

**Figure 22.** Doppler spectra of the STCR bistatic HFSWR and ship-based monostatic HFSWR systems at the velocity of approximately 10.0 knots. (**a**) RD spectrum of the STCR bistatic HFSWR; (**b**) RD spectrum of the ship-based monostatic HFSWR; (**c**) Doppler spectrum of the STCR bistatic HFSWR at the range of 52.5 km; (**d**) Doppler spectrum of the ship-based monostatic HFSWR at the range of 45 km.

As can be seen from Figures 20–22, the bending characteristics of the first-order Doppler spectrum caused by the bistatic angle can be observed, although they are not obvious. When the platform was anchored, the velocity of the platform was zero (Figure 20). In this case, the radar can be regarded as having a fixed transmitting station and a fixed receiving station. Therefore, the first-order sea clutter of the STCR bistatic HFSWR was not broadened, which was the same outcome as that of the shipborne monostatic HFSWR. The target echo in the RD spectrum of each of the two radar systems was very clear.

When the platform velocity was approximately 4.3 knots, the broadening of the first-order sea clutter in the RD spectrum (Figure 21) of the bistatic HFSWR was not obvious and most targets were easily detected. The width of the left first-order sea clutter was 6.2 km/h (−20.6 to −26.8 km/h), and that of the right first-order sea clutter was 6.1 km/h (24.1 to 30.2 km/h). According to the simulation results (Figure 10a in Section 3), under the same conditions, the theoretical width of the left first-order sea clutter should be 6.1 km/h (−20.6 to −26.7 km/h), and that of the right first-order sea clutter should be 6.5 km/h (23.7 to 30.2 km/h). Thus, the broadening range of the measured sea clutter spectrum is consistent with the theoretical value. In Figure 22c, the SNR value of the target of concern was approximately 17 dB for the STCR bistatic HFSWR. In contrast, the first-order sea clutter in the RD spectrum of the monostatic HFSWR was obviously broadened. Although the range of the first-order spectrum was wide, the obvious outer boundary can be seen coupled with the influence of land clutter, i.e., the moving target marked in the figure can still be detected, but the signal-to-clutter ratio was reduced to approximately 10 dB.

When the platform velocity reached 10 knots (Figure 22), the scene was very different from the previous two examples. For the STCR bistatic HFSWR, as the coast-based receiving station was obscured by surrounding land, the received signal came only from a limited range of the incoming direction. Therefore, the range of broadening of the left first-order sea clutter spectrum and the Doppler shift of the land clutter were obviously smaller than the theoretical values. For the shipborne monostatic HFSWR, as the shipborne platform was on the way back, the main axis angle of the radar receiving station was 205◦, and the detection range of the shipborne receiving station was mainly facing land. Consequently, there was no obvious boundary to be found because of the wide range of the first-order sea clutter. However, the presence of land clutter was obvious, varying continuously with range and velocity in the radar echo.

Similar to the above, Figure 23 shows the D-T distributions and AIS results of the STCR bistatic HFSWR and ship-based monostatic HFSWR at the platform velocity of 4.3 knots (*vT* = 7.96 km/h). At this time, ϕ = 37.56◦, <sup>β</sup> <sup>2</sup> = 2.9◦, ϕ*<sup>T</sup>* = 141◦, and θ*<sup>T</sup>* = 92◦. The moving target (*v* = 18.71 km/h ) was detected at the range of 44.75 km with an elliptical velocity of approximately 17.46 km/h by the STCR bistatic HFSWR based on Equation (6), whereas it was detected at the range of 35 km with a radial velocity of approximately 18.87 km/h by the ship-based monostatic HFSWR. The Doppler shift of the target echo in the measured radar data is largely consistent with the AIS track results.

**Figure 23.** D-T distribution and AIS results for the STCR bistatic HFSWR and ship-based monostatic HFSWR. (**a**) D-T distribution of the STCR bistatic HFSWR; (**b**) D-T distribution of the ship-based monostatic HFSWR.
