**1. Introduction**

With the development of space technologies, Global Navigation Satellite Systems (GNSSs) have been widely applied in various applications and have been playing an extremely important role in many fields [1–3]. In a GNSS, GNSS navigation satellites need to be widely distributed and used as radiation sources. Among existing GNSSs [4–7], Global Positioning System (GPS) has been broadly recognized and acknowledged as an advanced and mature technology in target detection due to its wider coverage and shorter observation time compared to other GNSSs, such as the Beidou satellite system [8] and Golbal navigation satellite system (GLONASS). Generally, target detection is conducted using a single GPS as the radiation source [9]. However, due to the widely deployed GNSSs, a GPS satellite signal receiver will inevitably receive multiple GPS signals, causing signal contamination and the failure of echo signal detection. Hence, instead of relying on only a single GPS satellite radiation

source, effective weak echo signal detection methods using multiple GPS satellite radiation sources need to be studied.

Due to the benefits of global coverage, 24-h operation, and easy-to-access signal sources, GPS navigation satellite signals will be employed as third-party illumination sources to study GPS-based external radiation source target detection methods in this paper. For weak echoes of GPS satellites, many researchers have made preliminary explorations [10–14]. These works focus on studying the detection of a single GPS satellite signal. In fact, different GPS satellites in the zenith may share the same frequency bands to send GPS signals. On the other hand, a near-Earth orbit target may be simultaneously covered by multiple satellite beams. As a result, reference signals transmitted on reference channels may be contaminated by other unexpected GPS signals [15], which makes the estimation of reference signals very difficult. In addition, interference caused by other GPS signals may degrade the performance of direct-path interference (DPI)/multi-path interference (MPI) suppression and the subsequent echo detection on an echo channel [16]. For effective echo detection, multiple GPS signals received on the reference channels and the DPI/MPI on monitoring channels should be purified and suppressed.

On the other hand, the received power of GPS signals is very low due to a long-distance propagation [17,18]. Furthermore, the signals after the target reflection, also referred to as the target echo, would be even weaker. Even under the condition that the clutter and interference suppression could be realized, the extremely weak GPS echo still needs a long coherent accumulation time to be detected. In order to improve the detection probability of the target echo, the existing methods improved the detection performance by constructing a multi-station joint detection system and merging the detection results of multiple radiation sources [19–21]. Unfortunately, the signal processing methods introduced in these works are not suitable for GPS signals. Therefore, effective use of the received multiple GPS signals to construct a joint detection system with high signal-to-noise ratio (SNR) is an important technical problem that has to be tackled.

The main contributions of this paper can be summarized as follows. A GPS weak echo signal detection method is proposed based on multi-star data fusion. To be specific, under the condition of multiple satellite sources, multiple reference signals will be mixed into a reference channel, resulting in degrading the DPI and MPI suppression effects on a monitoring channel. Therefore, firstly, the proposed method separates and reconstructs multiple GPS reference signals on the reference channel, based on which the Extensive Cancellation Algorithm (ECA) is used to monitor and suppress DPI and MPI. Then, to address the problem of the weak target reflection echo, which is very difficult to be detected, a coordinate conversion algorithm is applied to fuse detection statistics of multiple GPS satellites and obtain a final detection statistic. By this way, the peak value of weak echo detections and the probability of weak echo detections could be improved. Finally, by defining an adaptive detection threshold, the weak echo could be adaptively detected.

The reminder of this paper is organized as follows. In Section 2, the system model under multiple GPS satellite radiation sources is presented. A novel method of joint target detection is proposed and the corresponding technical details are described in the third Section 3, including the separation and reconstruction of direct wave in a reference channel, the suppression of direct and multi-path interference in a monitoring channel, and the construction of detection quantity and the design of detector. In the Section 4, the extensive simulation studies are conducted.
