**1. Introduction**

The high-frequency surface wave radar (HFSWR), also known as the HF surface over-the-horizon radar, operates in the 3–30 MHz frequency band at wavelengths between 100 m and 10 m, respectively. The HFSWR can provide additional information on maritime traffic because it can detect targets over the horizon, has continuous temporal coverage, and can estimate vessel velocity based on Doppler data [1,2]. Currently, most HFSWR systems operate in a monostatic mode that requires the collocation of the transmitter and the receiver. This raises practical issues in terms of the coastal space required for installation of both the transmitting and the receiving antenna arrays, as well as the problem of mutual interference between antennas. These issues can be overcome by resorting to a bistatic radar system in which the transmitter and the receiver are located some distance apart [3–5]. In a bistatic HFSWR system, the receiver has robust anti-active directional jamming and anti-destruction characteristics because of the physical separation of the transmitter and the receiver, giving it unique advantages and potential regarding anti-electronic interference. A shipborne HFSWR system has the

advantage of flexibility and it can increase the radar detection range beyond that of an onshore HFSWR. A system in which one of the transmitting or receiving stations is installed on the coast and the other is placed on a ship forms a coast–ship bistatic HFSWR system. Such systems can be classified either as a coast-transmit ship-receive (CTSR) bistatic HFSWR or as a ship-transmit coast-receive (STCR) bistatic HFSWR depending on whether the receiving station is placed on the ship or the coast, respectively.

A CTSR bistatic HFSWR has the advantages of anti-stealth, anti–interference, and no onboard electromagnetic radiation because the ship carrying the receiver can move to areas far from the coast. Compared with STCR systems, CTSR systems can exploit fully the flexibility of a shipborne platform and further expand the radar detection range by adjusting the attitude of the shipborne platform and changing the radar system configuration, e.g., adjusting the radar spindle angle. However, the azimuth resolution of such systems is reduced because shipboard platforms are limited by the size of the platform and thus the radar receiving station aperture is typically limited to ≤100 m. In an STCR bistatic HFSWR system, the shipborne equipment comprises only the transmitter, and there is no need to consider the deployment of a receiver, signal processor, or other equipment. In addition, the antenna aperture of the radar system is not limited by the size of the ship, which means that a large aperture-receiving antenna array could be installed onshore to improve azimuthal resolution. However, the fixed nature of the coast-based receiving station means that the receiving array spindle angle cannot be changed, which limits the detection range of the radar system to a certain extent. In addition, a transmitter/receiver–receiver radar system could be formed by adding a second coast-based/shipborne transmitter–receiver monostatic radar on the same basis as the coast–ship bistatic radar. Then, the detection performance and positioning accuracy of the marine target could be improved through fusion of the results of the two systems.

In their research into onshore and shipborne bistatic HFSWR systems, Gill and Walsh derived analysis relevant to onshore bistatic HFSWRs [6,7] based on the equation for the first-order radar echo spectrum derived by Barrick and by Lipa and Barrick for an onshore multistatic HFSWR [8,9]. Based on the space–time distribution of the first-order radar echo spectrum of an onshore bistatic HFSWR system, Xie et al. proposed and verified a spreading model through simulation and experiment [10]. In research of shipborne HFSWRs, both Walsh et al. and Khoury and Guinvarch derived the first-order ocean surface cross section of a monostatic HFSWR system on a moving platform [11–13]. In addition, Xie et al. studied the first-order ocean surface cross-section for a shipborne HFSWR both theoretically and through experiment [14,15]. In research of coast–ship bistatic HFSWRs, Li et al., Chen et al., and Liu et al. all presented the first-order sea clutter based on system configuration and analyzed the mechanism of broadening [16–19]. It is worth noting that the HFSWR system used in their research employed multichannel transmitting arrays mounted on the coast and only one receiving antenna mounted on a ship, whereas the HFSWR system of consideration in the present study employed one transmitting antenna and multichannel receiving arrays. Zhu et al. analyzed the influence of bistatic angle and shipborne platform motion on the characteristics of the broadened sea clutter spectrum [20]. Most previous research on coast–ship bistatic HFSWR systems focused on simulation analysis, with few reports on experimental verification.

The main characteristic of the coast–ship bistatic HFSWR is that it combines the advantages (and disadvantages) of monostatic coast-based and shipborne HFSWR systems. The radar spectrum of the coast–ship bistatic radar, including the first-order sea clutter and moving target echoes, will change with variation of the motion of the shipborne platform and the existence of the bistatic angle. A frequency shift of the first-order sea clutter spectrum will lead to expansion of the first-order sea clutter, and the blind area, due to the broadening of the first-order sea clutter spectrum, could cause difficulty in detecting targets within it, significantly reducing the overall detection performance. Therefore, it is necessary to investigate the mechanism of broadening of the first-order sea clutter spectrum and its influence on target detection. A comprehensive understanding of the bistatic spreading mechanism of the first-order sea clutter spectrum is essential for study of bistatic clutter suppression in target monitoring.

The remainder of this paper is organized as follows. In Section 2, the theoretical formulas of the first-order sea clutter scattering cross-section and the Doppler frequency shift of moving ship targets for the two types of coast–ship bistatic radar system are derived. In Section 3, simulation results of the first-order sea clutter spectrum under different operating conditions are presented and the influence on target detection is analyzed. In Section 4, the coast–ship bistatic HFSWR experiment is introduced and the derived results interpreted. Finally, brief conclusions are outlined in Section 5.
