Distance Measurement Equipment (DME) is a transponder-based radio navigation system that measures slant range distance by timing the propagation delay of radio signals. The DME system is composed of a transmitter/receiver (interrogator) in the aircraft and a receiver/transmitter (transponder) on the ground. Aircraft interrogate and the DME ground station responds.
Aircraft use DMEs to determine their distance from a land-based transponder (terrestrial station) by sending and receiving pulse pulses of fixed duration and separation. The ground stations are typically co-located with VORs. A low-power DME can also be co-located with an ILS glide slope antenna installation where it provides an accurate distance to touchdown. The simultaneous syntonization of two terrestrial DME stations by the aircraft allows us to locate the aircraft in two dimensions: latitude and longitude. By means of an altimeter, the aircraft is located in the 3D space. When co-located with a VOR, it also provides the direction of flight (read at the on-board ADF equipment). Therefore, the duple VOR-DME provides both position and course. It’s important to understand that DME provides the physical distance from the aircraft to the DME transponder. This distance is often referred to as slant range and depends trigonometrically upon both the altitude above the transponder and the ground distance from it. DME operation will continue and possibly expand as an alternate navigation source to space-based navigational systems such as GPS and Galileo.
Figure 11.8: GNSS systems