In this paper a control strategy for a mobile robot enabling to track a manually driven vehicle or a moving target is proposed in the context of natural environment. In such a context, the motion does not meet classical assumptions usually proposed for mobile robots, since the terrain geometry is not necessarily flat and wheels are subject to sliding. As a result, in order to preserve the accuracy of tracking, it appears necessary to account for such phenomena in the control law. Several observer-based approaches have already been developed in the framework of path following in off-road conditions, but suffer from several limitations. In particular, the velocity should not be null, which appears to be an important drawback in the proposed application: the tracking of a non-autonomous vehicle indeed imposes possible stops. In this paper, a new observation strategy is proposed allowing to avoid non-observable situations (null velocity). This permits to achieve an accurate vehicle tracking whatever its velocity, its trajectory and the grip conditions.