There is currently some interest in supersonic sea-skimming as a possible terminal phase for anti-ship guided weapons. The flight of a supersonic vehicle, close to a density interface, will produce an additional series of shock waves from the reflection of the shock waves generated by the vehicle from the interface. The angle of reflection of these shock waves will depend, interalia,on the slope of the surface upon which they are incident. Given a surface with a periodic positional displacement, such as would be produced by a train of surface waves, the variation in position of the reflected shocks, in relation to the generating vehicle, will also be periodic in nature. This coherent motion could produce periodically fluctuating forces and moments on the vehicle generating the incident shocks and these, in turn, could cause stability or control problems for the missile autopilot.