We propose a novel vector field based guidance scheme for tracking and
surveillance of a convoy, moving along a possibly nonlinear trajectory on the
ground, by an aerial agent. The scheme first computes a time varying ellipse
that encompasses all the targets in the convoy using a simple regression based
algorithm. It then ensures convergence of the agent to a trajectory that
repeatedly traverses this moving ellipse. The scheme is analyzed using
perturbation theory of nonlinear differential equations and supporting
simulations are provided. Some related implementation issues are discussed and
advantages of the scheme are highlighted