In this paper, we aim at estimating the state of an ensemble of mobile agents. Specifically, each agent is an oscillator which moves along a circle at a given time-varying angular velocity. We assume that an agent can measure the distance from the other oscillators only when they are in its proximity. We propose a recursive algorithm which, based on these intermittent and uncertain measurements, is capable of estimating in real-time the relative angular position of the agents. The algorithm combines the information coming from the collected measures with the information on the agents' dynamics, and its convergence is proved by means of interval analysis. The theoretical analysis is complemented with extensive numerical simulations. (C) 2014 Elsevier Ltd. All rights reserved.