  This work investigates the secrecy outage performance of the uplink
transmission of a radio-frequency (RF)-free-space optical (FSO) hybrid
cooperative satellite-aerial-terrestrial network (SATN). Specifically, in the
considered cooperative SATN, a terrestrial source (S) transmits its information
to a satellite receiver (D) via the help of a cache-enabled aerial relay (R)
terminal with the most popular content caching scheme, while a group of
eavesdropping aerial terminals (Eves) trying to overhear the transmitted
confidential information. Moreover, RF and FSO transmissions are employed over
S-R and R-D links, respectively. Considering the randomness of R, D, and Eves,
and employing a stochastic geometry framework, the secrecy outage performance
of the cooperative uplink transmission in the considered SATN is investigated
and a closed-form analytical expression for the end-to-end secrecy outage
probability is derived. Finally, Monte-Carlo simulations are shown to verify
the accuracy of our analysis.
