Recently an algorithm was developed by Lefas et at. (1988), for estimating the absolute altitude of civil aircraft using measurements from two sets of distance measuring equipment (DME). An interesting conclusion was that if the aeroplane deviates from the airlane marked by the two DMEs, secondary surveillance radar (SSR) measurements are also necessary, and this fact causes the degradation of the filter's performance. The present paper investigates the use of three DMEs for the estimation of the aircraft's absolute altitude, in which case no SSR is needed. The DME measurements are processed on-board using a first-order Kalman filter and an accuracy of 30 ft is achieved with normal DMEs and a sampling interval of 01 s.