# A Target Detection Method of Distributed Passive Radar without Direct-Path Signal
## 1 Introduction  
Passive radar refers to a radar detection system that does not emit electromagnetic wave signals but relies on existing electromagnetic waves in space to achieve detection, positioning, and tracking [1; 2; 3; 4; 5]. Passive radar can detect targets by receiving electromagnetic wave signals from third-party non-cooperative illuminators reflected by targets. Such third-party non-cooperative illuminators include commercial radio stations, television stations, communication base stations, and satellites. There are many types of signals emitted by external illuminators available. The passive radar system has the following advantages: high concealment, with strong anti-interference and anti-radiation missile capabilities; it has the potential of anti-stealth; the available external illuminators have abundant signals and strong networking capability; the system has low cost and no pollution to the environment; it can detect low altitude and ultra-low altitude targets. Because the passive radar system has many advantages, scholars at home and abroad have performed much research on target detection algorithms for passive radar. At present, the cross-correlation method is mainly used for target detection. The cross-correlation detection method is based on the matched filtering theory, which uses the ambiguity function of the reference signal and the target echo signal to accumulate the target energy, obtaining the target delay and Doppler information so as to achieve target detection. The passive radar detection system obtains the reference signal by purifying the direct-path signals. The purity of the reference signal has an important impact on the system detection performance. The reference signal can eliminate the clutter in the echo signal and improve the signal-to-noise ratio of the target echo through matched filtering. Therefore, it is crucial for the passive radar to obtain pure direct-path signals for targetdetection [6; 7; 8; 9; 10]. At present, scholars have performed much research on direct-path signal purification. [PERSON] proposed a new direct-path signals purification method based on sparse characteristics for the problem of direct wave purification in non-cooperative passive detection [11] [PERSON] proposed a direct wave purification method for passive radar in the scene with strong multipath signals reflected [12]. Although the target detection algorithm based on passive radar has gradually matured, there are still some limitations that limit the applicable environment of passive radar. The significance of direct-path signals target detection for passive radar is very significant. Therefore, if there is a scene where direct-path signals cannot be received, as shown in Figure 1. When each receiver cannot receive direct-path signals and can only receive the echo signals reflected from the target separately, the traditional cross-correlation method is no longer applicable, and the target detection performance will decline sharply. At present, experts have conducted some research on target detection algorithms based on the condition of without-direct-path signals and generally use the generalized likelihood ratio test (GLRT) detection method. Its core idea is to substitute the maximum likelihood estimates of all unknown parameters into the likelihood ratio to form a detector and compare them with the threshold to achieve target detection [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. For a passive radar system with one illuminator and multiple receivers, [PERSON] proposed a generalized canonical correlation (GCC) detector for passive multistate radar detection under the condition of known noise power [24] [PERSON] proposed a generalized likelihood ratio test (GLRT) detector to deal with the target detection problem of passive radar under the condition of without-direct-path signals. Although it solves the detection problem of without-direct-path signal scenes to a certain extent, the improvement of SNR by these two detection algorithms is not very obvious [13].