  Unmanned aerial vehicles (UAVs) have been undergoing fast development for
providing broader signal coverage and more extensive surveillance capabilities
in military and civilian applications. Due to the broadcast nature of the
wireless signal and the openness of the space, UAV eavesdroppers (UEDs) pose a
potential threat to ground communications. In this paper, we consider the
communications of a legitimate ground link in the presence of friendly jamming
and UEDs within a finite area of space. The spatial distribution of the UEDs
obeying a uniform binomial point process (BPP) is used to characterize the
randomness of the UEDs. The ground link is assumed to experience log-distance
path loss and Rayleigh fading, while free space path loss with/without the
averaged excess path loss due to the environment is used for the
air-to-ground/air-to-air links. A piecewise function is proposed to approximate
the line-of-sight (LoS) probability for the air-to-ground links, which provides
a better approximation than using the existing sigmoid-based fitting. The
analytical expression for the secure connection probability (SCP) of the
legitimate ground link in the presence of non-colluding UEDs is derived. The
analysis reveals some useful trends in the SCP as a function of the transmit
signal to jamming power ratio, the location of the UAV jammer, and the height
of UAVs.
