Signal-to-Noise Ratio Analysis in Low-Earth-Orbit-Satellite-Based Passive Backscatter Bistatic Radar

Low Earth Orbit (LEO) satellites, with their high ground-received signal power, have emerged as a promising candidate for space-based external illuminators in passive radar systems. The signal-to-noise ratio (SNR) is a critical factor determining the target detection performance of such space-based passive bistatic radar (PBR). While existing studies have investigated the SNR in LEO satellite-based PBR systems, most focus on single-satellite signal characteristics, power analysis, and detection performance. This paper systematically examines the key factors influencing SNR from both the system and user equipment perspectives. Theoretical derivations and simulation results demonstrate that: For received signal power, the Equivalent Isotropic Radiated Power (EIRP) of the transmitting antenna, the gain of the receiving antenna, and the satellite orbital altitude are the primary determinants of SNR. The EIRP and receiving antenna gain exhibit a proportional relationship with SNR. Regarding orbital altitude, when the satellite operates below 800 km, each 100 km reduction in altitude yields an SNR improvement of at least $\mathbf{1. 1 6 ~ d B}$. For SNR after processing at the receiver, the coherent integration time is the dominant factor, constrained by the target’s detectable duration. Calculations reveal that when the satellite passes through the zenith of the receiver, the target detectable time satisfies the coherent integration requirement for achieving sufficient SNR. Based on the SNR requirements of a predefined detection scenario, feasible system parameters are proposed. When utilizing Iridium LEO satellites as external illuminators, a receiving antenna with 40 dB gain and 1 s coherent integration time can achieve an SNR capable of supporting small-target detection at long ranges under ideal conditions. These findings provide a viable optimization pathway for future LEO satellite-based PBR systems.