# Reconfigurable Intelligent Surface Assisted Target Three-Dimensional Localization with 2-D Radar
## 1 Introduction
The RIS is a planar array consisting of many low-cost passive or active reflecting elements, which are each capable of independently controlling controllable amplitude and/or phase changes in the incident signal. As a nearly passive low-cost planar structure, the RIS can be easily deployed to realize smart radio environments [4; 5]. Compared with the dedicated altimeter radar or the construction of multi-station radar networking, the RIS has low power consumption and low cost. It can not only move at any time by Unmanned Aerial Vehicle (UAV) but also change the propagation direction of electromagnetic waves through phase shifts. Then, the radar can receive the target echo signal, and it does not require data transmission of fusion information, which will improve the real-time performance. In addition, it is easier to achieve high-precision alignment for the RIS-assisted radar system. At the earliest time, the RIS was used in the field of wireless communication. It can realize directional transmission and the reception of wireless signals by dynamically adjusting the phase and amplitude of its surface elements, improving the transmission efficiency of signals and the overall performance of the network [6]. In addition, the RIS is used to adaptively adjust the propagation path and phase of signals according to the real-time changes of wireless channels, thus effectively resisting multipath fading and interference and reducing the bit error rate [7]. In Ref. [8], the transmission rate has been improved by using the RIS to realize the real-time adjustment and optimization of wireless signals. The RIS-assisted channel estimation technology can improve the accuracy and efficiency of channel estimation by using its dynamic control ability on wireless signals [9; 10; 11]. In Ref. [12], the RIS was used as a measuring device to monitor and feedback the propagation characteristics and path loss of wireless signals in real time, which provides an important basis for the design and optimization of wireless communication systems. The RIS has also been widely used in mobile networks and intelligent transportation [13; 14], since it can dynamically adjust and control wireless signals to achieve the accurate localization of users. In addition, the RIS has been successfully applied to dual-function radar communication systems [15; 16], which can realize radar detection and communication functions at the same time. In the radar mode, the RIS is used to control the propagation direction of electromagnetic waves and improve the performance of the radar system. In the communication mode, the RIS is used to realize the transmission and reception of signals and improve the performance of the communication system [17; 18; 19; 20]. The excellent characteristics of the RIS in changing the propagation direction of electromagnetic waves should be able to provide additional receiving channels for the radar, which is a research direction with great application prospects. The RIS has been applied to radar systems, which helps the radar improve its performance in many ways. [PERSON] and [PERSON] incorporated the radar system with the RIS technique to improve the target detection performance [21; 22; 23]. Then, the RIS-assisted MIMO radar [24; 25] and the multi-target detection application [26] are considered, in which the detection performance is improved by jointly optimizing both radar waveforms and the RIS phase shifts. The author in [27] introduced the RISs to extend the radar surveillance in non-line-of-sight (N-LOS) scenarios. In addition, the RIS has been used to improve the accuracy of target measurement by controlling the propagation direction of electromagnetic waves. By analyzing the received electromagnetic wave signal, the position of the target can be determined, and its accurate tracking can be realized [28; 29]. In Ref. [30], the parameters of the target, such as size and shape, were estimated by analyzing the received electromagnetic wave signal in the RIS-assisted radar.