  This paper presents two control algorithms enabling a UAV to circumnavigate
an unknown target using range and range rate (i.e., the derivative of range)
measurements. Given a prescribed orbit radius, both control algorithms (i) tend
to drive the UAV toward the tangent of prescribed orbit when the UAV is outside
or on the orbit, and (ii) apply zero control input if the UAV is inside the
desired orbit. The algorithms differ in that, the first algorithm is smooth and
unsaturated while the second algorithm is non-smooth and saturated. By
analyzing properties associated with the bearing angle of the UAV relative to
the target and through proper design of Lyapunov functions, it is shown that
both algorithms produce the desired orbit for an arbitrary initial state. Three
examples are provided as a proof of concept.
