  In this paper, we consider a problem motivated by search-and-rescue
applications, where an unmanned aerial vehicle (UAV) seeks to approach the
vicinity of a distant quasi-stationary radio frequency (RF) emitter surrounded
by local scatterers. The UAV employs only measurements of the Doppler frequency
of the received RF signal, along with its own bearing, to continuously adapt
its trajectory. We propose and evaluate a trajectory planning approach that
addresses technical difficulties such as the unknown carrier frequency offset
between the emitter and the UAV's receiver, the frequency drifts of the local
oscillators over time, the direction ambiguity in Doppler, and the noise in the
observations. For the initial trajectory, the UAV estimates the direction of
the emitter using a circular motion, which resolves direction ambiguity. The
trajectory is then continuously adapted using feedback from frequency
measurements obtained by perturbing the bearing around the current trajectory.
We show that the proposed algorithm converges to the vicinity of the emitter,
and illustrate its efficacy using simulations.
