  By observing the fact that moving in a straight line is a common flying
behavior of unmanned aerial vehicles (UAVs) in normal applications, e.g., power
line inspections, and air patrols along with highway/streets/borders, in this
paper we investigate the secrecy outage performance of a UAV system with linear
trajectory, where a UAV ($S$) flies in a straight line and transmits its
information over the downlink to a legitimate receiver ($D$) on the ground
while an eavesdropping UAV ($E$) trying to overhear the information delivery
between $S$ and $D$. Meanwhile, some information is delivered to $S$ over the
uplink from $D$, such as commanding messages to control $S$'s detecting
operations, which can also be eavesdropped by $E$. The locations of $S$, $D$,
and $E$ are randomly distributed. We first characterize the statistical
characteristics (including cumulative distribution functions and probability
density function) of the received signal-to-noise ratio over both downlink and
uplink, and then the closed-form analytical expressions for the lower boundary
of the secrecy outage probability of both downlink and uplink have also been
derived accordingly. Finally, Monte-Carlo simulations are given to testify our
proposed analytical models.
