We propose a novel scheme for surveillance of a dynamic ground convoy moving along a non-linear trajectory, by aerial agents that maintain a uniformly spaced formation on a time-varying elliptical orbit encompassing the convoy. Elliptical orbits are used as they are more economical than circular orbits for circumnavigating the group of targets in the moving convoy. The proposed scheme includes an algorithm for computing feasible elliptical orbits, a vector guidance law for agent motion along the desired orbit, and a cooperative strategy to control the speeds of the aerial agents in order to quickly achieve and maintain the desired formation. It achieves mission objectives while accounting for linear and angular speed constraints on the aerial agents. The scheme is validated through simulations and actual experiments with a convoy of ground robots and a team of quadrotors as the aerial agents, in a motion capture environment.