  In this paper, we analyze the impact of a friendly unmanned aerial vehicle
(UAV) jammer on UAV communications in the presence of multiple eavesdroppers.
We first present channel components determined by the line-of-sight (LoS)
probability between the friendly UAV jammer and the ground device, and
introduce different channel fadings for LoS and non-line-of-sight (NLoS) links.
We then derive the secrecy transmission probability satisfying both constraints
of legitimate and wiretap channels. We also analyze the secrecy transmission
probability in the presence of randomly distributed multiple friendly UAV
jammers. Finally, we show the existence of the optimal UAV jammer location, and
the impact of the density of eavesdroppers, the transmission power of the UAV
jammer, and the density of UAV jammers on the optimal location.
