Abstract: A multi-beam rangefinder system for estimating a range-derived value of an airborne platform relative to the ground. A planar model approximates the ground. The system includes a transmitter arrangement to produce a number of pulses of electromagnetic radiation directed in three or more non-coplanar directions and a sensor arrangement for sensing the pulses of electromagnetic radiation reflected from the ground. The systems also includes a processor arrangement for processing independent measurement of time of flight of the pulses in each of the three or more non-coplanar directions, in order to estimate the range derived value of the airborne platform relative to the ground.