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MADGE (Microwave Aircraft Digital Guidance Equipment) is a portable equipment which assists the landing of all types of aircraft at a variety of sites. It contains a digital data link operating at 5 GHz. Distance is derived in the air by measuring go-and-return time; angles are measured on the ground by passive interferometer receivers. Flight results relating to coverage and accuracy are presented. 1. INTRODUCTION MADGE (Microwave Aircraft Digital Guidance Equipment' 1 is a portable guidance facility intended to assist the landing of all types of aircraft in both civil and military applications. It provides angular guidance along an approach path and distance to the landing site. It is suitable for both mobile tactical sites and established airfields. MADGE is being developed for the UK government and is currently being flight tested at the Royal Aircraft Establishment, Bedford. Although the system is initially intended for military use, the design also takes account of civil applications. SYSTEM OUTLINEThe system (Fig. 1) operates in the 5 -5.25 GHz band and consists of an air-ground-air data-link and passive angle measuring interferometers on the ground. The airborne data link interrogator transmits a pulsed message to the ground transponder and interferometer receivers. The receivers measure the azimuth and elevation angles of the aircraft's transmission relative to a fixed datum and relay digital angle data to the transponder. The data is transmitted to the aircraft in the transponder reply message. Messages are protected by identity codes and parity. The reply is decoded in the aircraft and the angles are compared with desired flight path angles selected by the pilot on his control unit. The resulting deviations are presented in analog form to instruments or autopilot. Distance is derived in the air from the message go-and-return time; it is available in digital and analog form. Fig. 2 is a simplified diagram showing the wide angular coverage of the system. The data link and distance functions are omnidirectional. Position information, available over a wide volume of space, simplifies the pilot's task of initially locating the approach path. The pilot may select an approach direction (relative to the fixed datum) and a glidepath angle to suit the particular landing operation. THE INTERFEROMETERSThe direction of the aircraft's transmitter is obtained by measuring the variation of signal phase across a receiving aperture. The phase difference q between the signals received in two antennas forming a simple interferometer is related to the direction of transmission by the wellknown expression = TrD sine .

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