  In recent years, satellite communication (SatCom) systems have been widely
used for navigation, broadcasting application, disaster recovery, weather
sensing, and even spying on the Earth. As the number of satellites is highly
increasing and with the radical revolution in wireless technology,
eavesdropping on SatCom will be possible in next-generation networks. In this
context, we introduce the satellite eavesdropping approach, where an
eavesdropping spacecraft can intercept optical communications established
between a low Earth orbit satellite and a high altitude platform station
(HAPS). Specifically, we propose two practical eavesdropping scenarios for
satellite-to-HAPS (downlink) and HAPS-to-satellite (uplink) optical
communications, where the attacker spacecraft can eavesdrop on the transmitted
signal or the received signal. To quantify the secrecy performance of the
scenarios, the average secrecy capacity and secrecy outage probability
expressions are derived and validated with Monte Carlo simulations. Moreover,
secrecy throughput of the proposed models is investigated. We observe that
turbulence-induced fading significantly impacts the secrecy performance of
free-space optical communication.
