**4. Detection Statistics Construction**

Due to the characteristics of the external radiation source detection system and the weak power of the GPS itself, the target echo power is relatively low. The traditional CAF often needs to increase the coherence time of the direct wave and the echo to accumulate the energy of the weak echo [33]. However, long-term coherent accumulation leads to an increase in computational complexity, and, since the range of detection is limited and the target moves faster, the accumulation time is greatly limited. In order to effectively enhance the signal-to-noise ratio of the detected quantity under the condition of equivalent coherence time, in this section, the anti-jamming properties of cycle cross ambiguity function (CCAF) are used to construct the detection statistics of multiple GPS weak echoes, and the coordinate detection algorithm is used to fuse multiple detection statistics to obtain the final detection statistics. This process enhances the signal-to-noise ratio of the detection statistic from the two aspects of detection structure and multi-star data fusion, and improves the detection probability of weak echo. In order to judge the performance of CCAF in the multi-satellite system compared with the traditional CAF detection structure, in this paper, the CAF-based detection statistic and the CCAF-based detection metric are constructed respectively, and the theoretical analysis and simulation performance verification are carried out, respectively.
