**1. Introduction**

High-frequency surface wave radar (HFSWR) operated in the 3–30 MHz frequency band has been recognized as an important maritime surveillance tool [1] for both sea state monitoring [2] and hard target detection [3] due to its superiority of over-the-horizon coverage, all-weather and continuous surveillance, high time resolution, low-cost, etc. Existing HFSWRs for hard target detection are monostatic with the transmitter and receiver being co-located. Some HFSWRs employ a linear receiving array with a large aperture size and high transmitting power to achieve high azimuth resolution and long detection range. e.g., the SWR-503 system developed in Canada uses a receiving array with an aperture size of 660 m and a transmitting power of 30 kW. Such a radar system usually requires a large coastline area, which makes it difficult for site selection, system deployment, and maintenance, and thus limits its operational applications. Therefore, system miniaturization has become a new development trend [4]. So far, two kinds of compact HFSWR systems with small aperture size have been developed. One utilizes crossed-loop/monopole antennas, such as the SeaSonde system developed by CODAR [5,6], the OSMAR system developed by Wuhan University [7], etc., while the other still uses phased array antenna but with less antenna array elements (e.g., 3–8 elements), e.g., the WERA-S system developed by Helzel MessTechnik [8], the Compact Over-the-horizon Radar for Maritime Surveillance (CORMS) system developed by our team [9–11], etc.

Compared with an HFSWR system that has large aperture size, a small-aperture compact system has the advantages of flexible deployment and maintenance. It can be installed on a small island, or even on a large ship [12], thus increasing the detection flexibility and extending coverage area. However, it should be noted that the direction of arrival estimation performance degrades due to the reduced number of antenna elements. The resultant poor azimuth estimation accuracy may lead to large target positioning errors. In addition, the lower transmitting power and clutter interference, such as the sea clutter, ionospheric clutter, etc., make target detection more challenging.

To improve the target detection performance of compact HFSWRs, on one hand, super-resolution direction-finding methods, such as MUSIC [13], were proposed to improve the azimuth estimation accuracy. On the other hand, distributed multi-radar systems, such as bistatic and multi-static radar [14], MIMO radar [15], etc., were designed to simultaneously monitor the targets in a common area of interest from different perspectives. Also, bistatic or multi-static configuration makes the influence of sea clutter, ionosphere clutter, etc., diverse from different sites. The complementary information obtained from different radars can be associated and fused to produce more precise results. However, the distributed radar systems with multiple radars are inherently more complex and hence tend to be more expensive. Multi-static radar offers a way to obtain good target detection performance but requires extensive research, especially advanced signal processing techniques. Among distributed radar systems, the combination of one monostatic radar and one bistatic radar with a common transmitting station and two separate receiving stations, i.e., T/R-R radar, is a typical configuration that offers a tradeoff between system performance and complexity. In this paper, the target detection and tracking performance of a T/R-R bistatic compact HFSWR system is investigated. It is worth noting that although the T/R-R configuration brings several advantages in target detection, the problems caused by the compact HFSWR system itself, such as low detection precision for both range and azimuth, are inherent.

Although bistatic system is not a new concept, very few bistatic HFSWRs have been developed and limited experiments have been reported. Theoretical studies on bistatic HFSWR have been carried out extensively. For example, various issues related to bistatic HFSWR, such as system configuration, site selection, spectral characteristics, detection performance, etc., were analyzed in [16–18]. Clutter models were established in [19] and analyzed in [20], and interference suppression methods were proposed in [21]. Different ocean surface radar cross section models of bistatic HFSWR were derived and analyzed in [22–28]. However, experimental work is relatively limited. So far, bistatic HFSWRs have been utilized for surface current mapping [29–31], directional ocean spectrum measurement [32], and wind direction measurement [33] over a very short period. Existing research related to bistatic HFSWR mainly focuses on system design and performance analysis, scattering mechanism investigation, clutter suppression, and sea state mapping applications. Most of the research work lies in theoretical analysis and numerical simulation. Compared with the numerous multi-target tracking (MTT) algorithms developed for monostatic HFSWRs [34–36], target detection and tracking using bistatic HFSWR have been much less explored [37], especially with field data.

The primary objective of this paper was to investigate the characteristics of target detection with a bistatic HFSWR and develop an applicable target tracking method accordingly. The field data collected by a newly developed T/R-R compact HFSWR were used to validate the correctness of the derived results and verify the performance of the proposed tracking method. Besides, comparisons were made between the tracking results of monostatic and bistatic HFSWR. The remainder of this paper is organized as follows. Section 2 describes the characteristics of target representation for a bistatic HFSWR, followed by a detailed description of an applicable target tracking method. In Section 3, the experimental results are presented and analyzed. Discussions are provided in Section 4 and conclusions are drawn in Section 5.

#### **2. Target Detection and Tracking with a Bistatic Compact HFSWR**

The combined monostatic and bistatic compact HFSWR system, abbreviated as a T/R-R compact HFSWR system in the following description, employs a common transmitter and two receivers. The co-located transmitter and receiver constitute a monostatic radar, while the bistatic radar is

composed of the shared transmitter and the other receiver deployed at a considerable distance away from the transmitter. As target detection is the basis of target tracking, the target representation method for a bistatic HFSWR is discussed first.
