  Being a key technology for beyond fifth-generation wireless systems, joint
communication and radar sensing (JCAS) utilizes the reflections of
communication signals to detect foreign objects and deliver situational
awareness. A cellular-connected unmanned aerial vehicle (UAV) is uniquely
suited to form a mobile bistatic synthetic aperture radar (SAR) with its
serving base station (BS) to sense over large areas with superb sensing
resolutions at no additional requirement of spectrum. This paper designs this
novel BS-UAV bistatic SAR platform, and optimizes the flight path of the UAV to
minimize its propulsion energy and guarantee the required sensing resolutions
on a series of interesting landmarks. A new trajectory planning algorithm is
developed to convexify the propulsion energy and resolution requirements by
using successive convex approximation and block coordinate descent. Effective
trajectories are obtained with a polynomial complexity. Extensive simulations
reveal that the proposed trajectory planning algorithm outperforms
significantly its alternative that minimizes the flight distance of
cellular-aided sensing missions in terms of energy efficiency and effective
consumption fluctuation. The energy saving offered by the proposed algorithm
can be as significant as 55\%.
