Consider a satellite in orbit about a spherically symmetric planet. If the orbit is circular, the sub-satellite point travels uniformly along its circular track. If the orbit is not circular, however, the sub-satellite track is still circular, but the sub-satellite point moves along it at a variable rate. Show that the maximum along-track error in calculating the position of the sub-satellite point on the assumption that it moves uniformly is about eD, where e is the eccentricity of the orbit and D is the planet’s diameter.