# Target Localization Based on Bistatic T/R Pair Selection in GNSS-Based Multistatic Radar System
## 1 Introduction  
With the rapid development of science and technology, the modern electromagnetic environment has become more complex and changeable. Although the monostatic radar is powerful, it has a single detection of perspective and obtains incomplete information, so it cannot fight against the complex electromagnetic environment [1]. In response to the complex situation, a number of new concepts and new radar systems have gradually emerged. Among them, multistatic radar system will be a trend of future radar development. Currently, the widely studied multistatic radar systems [1, 2, 3] include the networked radar [4, 5], bistatic (multistatic) radar [6, 7], and distributed MIMO radar [8, 9, 10]. Compared with monostatic radar, multistatic radar has the following advantages [11, 12, 13]: (1) It can detect targets in multiple directions to facilitate target recognition and anti-stealth. (2) Each transmitter can have different frequency bands and data rates, so it is beneficial to enhance the anti-jamming capability and reliability of the system. (3) It can increase the target detection probability and improve the positioning accuracy. (4) The system's survivability is stronger, because the local transmitting or receiving abnormality cannot cause the collapse of the entire system. At present, many relevant studies have been done on themultistatic radar [14; 15; 16; 17; 18], and they have also been successfully applied in the fields of strategic indication warning, navigation system, and air traffic control. The representative research results are the Russian barrier radar [19], Japan's bistatic radar system for ground detection [6], and the passive detection and positioning system researched by the British Defense Research Agency [20]. For the multistatic radar, the use of external illuminators is also a mainstream trend. On the one hand, with the rapid development of technology, there are many types of external illuminators that can be used, such as navigation satellites, communication satellites, and various ground-based base stations. On the other hand, in the face of an increasingly complex electromagnetic environment, a passive radar has more obvious advantages than an active radar. All major countries are now developing the wide-area detection networks combining active and passive radars [21; 22; 23; 24]. However, the wide-area detection network also has a problem, in that it brings a huge amount of information and requires considerable computing power. For active networks, there is a closed-loop problem of cognitive transmission and the intelligent processing of receiving signal. For example, the literature [25] focuses on the joint processing of cognitive transmission and reception. For passive networks, the problem of selecting external illuminators is introduced. In essence, both these problems can be transformed into modeling optimization problems of T/R pairs configuration and system parameters. This paper aims at the selection of external illuminators in the multistatic radar system and uses GNSS as the external illuminator. At present, with the continuous improvement of GNSS, the GNSS-based radar has received widespread attention, and it also has the advantages of all-weather and global coverage, and no extra electromagnetic pollution from the active radar transmitter. Moreover, for an arbitrary point on Earth, there are more than 30 satellites illuminators at any time. Therefore, many related research works have been conducted on it, such as target detection [26; 27; 28], positioning [29; 30], kinematic state estimation [31], imaging [32; 33], etc. The work of this paper is based on the literature [26; 27; 29]. On the premise that the approximate location of the target is known, this paper proposes two multi-source optimization algorithms from the perspective of resolution unit and resolution capability: Covariance Matrix Fusion Method and Convex Hull Optimization Method, and then uses GNSS as an external illuminator to verify the algorithms.