# Reconfigurable Intelligent Surface Assisted Target Three-Dimensional Localization with 2-D Radar
## 1 Introduction  
Two-dimensional (2-D) radar is the earliest radar system, produced with the Second World War, which can provide the slant range and azimuth measurements of air targets. After years of development, 2-D radar technology has matured. Because of its small equipment, low cost, simplicity and reliability, it is still widely used in battlefield surveillance. However, due to the wide elevation dimension beam, the 2-D radar has poor resolution in the elevation dimension. In other words, it cannot effectively measure the elevation of air targets. Without elevation measurement, it is impossible to accurately locate the target, which will affect the intelligence quality. However, the high localization accuracy of the target is significant in modern warfare. Therefore, the target three-dimensional localization for 2-D radar is one of the difficult problems to be solved urgently. Currently, three classes of methods have been proposed to achieve 2-D radar elevation estimation. The first method is that 2-D radar usually needs to be fused with other sensor data to provide accurate elevation information. Take Height Finder Radar (HFR) aided 2-D radar as an example. Due to the advanced development and maturity of 2-D radar technology, the utilization of 2-D radar for target azimuth and range measurement, along with the HFR for precise height measurement, offers significant technical advantages including high accuracy, robust anti-interference capability, and easy implementation, which has been widely used in the air defense system. However, equipping the radarwith dedicated sensors not only increases the cost of the radar but also complicates the measurement process and increases the processing of intelligence data. Therefore, the real-time performance of this method is poor, and it is not conducive to the maneuverability of the radar position. Secondly, a single 2-D radar can estimate the target height by using the Doppler history of the target over several scans, since the rate of the Doppler frequency changes depends on the target height. In Ref. [1], the changing rate of the radial velocity is estimated by Radon transform, and it is used to calculate the target height with the estimated radial velocity and the target measurements of 2-D radar. However, target echoes over multiple scans are used here, and it is assumed that the target position and velocity remain unchanged during these scanning intervals. Therefore, the method is not suitable for high-speed moving or maneuvering targets. In addition, there is a certain delay in obtaining the target height information. Thirdly, with the continuous development of radar networking technology in recent years, it has become a new way to estimate the target elevation by using the redundant information of the radar network consisting of multiple 2-D radar stations. The target elevation or altitude can be estimated by using the single point tracking initialization algorithm with the range and azimuth measurements of the target obtained in multiple 2-D radars [2; 3]. The measurements from multiple observation angles need to be combined and fused; therefore, the estimation accuracy depends on the number and the distribution of multiple 2-D radar stations. However, high-precision spatial-temporal registration is the premise of networking, and the cooperative estimation in the fusion center will also inevitably bring time delay. Moreover, in the electronic countermeasure scene, the conditions required for radar cooperation in networking are difficult to meet, and even if they are met, networking is always at risk of being destroyed. Based on the analysis of available methods, the target three-dimensional localization algorithm for 2-D radar with low cost and low delay is still worthy of further study. The Reconfigurable Intelligent Surface (RIS), which has been widely used in the field of wireless communication, provides the possibility for developing a novel three-dimensional localization method for 2-D radar based on its characteristic of reconstructing signal propagation.