ABSTRACT
This chapter looks at a system that provides the crew with the distance to a navigation aid. Distance measuring equipment (DME) is a short/medium- range navigation system, often used in conjunction with other systems, e.g. VOR and ILS, to provide accurate navigation fixes. DME is based on secondary radar principles and operates in the L- band of radar. The DME navigation aid contains a transponder (receiver and transmitter) contained within a single navigation aid. The aircraft equipment radiates energy pulses to the DME navigation aid; secondary signals are then transmitted back to the aircraft. The signals transmitted by the interrogator are a pair of pulses, each of 3.5 µs duration, modulated on the DME navigation aid frequency. The interrogator generates a pulse-pair repetition rate between 5 and 150 pulse-pairs per second. The TACAN navigation aid is essentially a DME transponder to which directional information has been added; both operate in the same UHF band.
