Unmanned aerial vehicles (UAVs) communications have been widely exploited in our daily life, which leads to rising concerns about the security issue. This work investigates the secrecy performance of a UAV-to-vehicle (UAV-2-V) communication system, where the information delivered over both downlink and uplink between a UAV ($S$) acting as a temporary aerial base-station and a legitimate vehicle ($D$) moving along a road which is overheard by an eavesdropping vehicle ($E$) on the same road. The location of$S$is assumed to be uniformly distributed in the sky, while the locations of$D$and$E$are uniformly distributed on the highway. The statistical characteristics, including the cumulative distribution function and probability density function of the received signal-to-noise ratio over both downlink and uplink, are characterized respectively. Closed-form expressions for the approximate and asymptotic secrecy outage probability (SOP) of the downlink experiencing Rician fading channels have been derived accordingly. Moreover, the secrecy outage performance of the uplink is investigated by deriving the closed-form expression of the exact and asymptotic SOP in two cases: the eavesdropping channel suffers Rician and Weibull fading, respectively. Finally, Monte-Carlo simulations are shown to verify our proposed analytical models.

The number of array elements significantly influences the computational complexity of space-borne phased array antenna systems. When the satellite is on-orbit, the performances of space-borne phased array antennas will be easily influenced by the environment and devices aging. Therefore, the space-borne antennas need to be regularly calibrated after satellite is launched. However, the complexity of calibration system increases in direct ratio to the radio frequency (RF) chain number of phased array antenna system. To solve the problem, a low complexity calibration method is proposed, whose system complexity is irrelevant to the number of array elements and calibration time is short. This method is especially suitable for large-scale space-borne phased array antennas. Simulation and experimental results of relative amplitude/phase inaccuracy probability under different SNR illustrate that when the value of SNR is higher than 16dB, the measurement accuracy of relative power (amplitude) and phase can reach ±0.1dB and ±1°, respectively. The simulation and experiment results also provide a significant basis to the link budget of the space-borne phased array antenna systems.