Distance Measure Equipment, DME determines for accurate determination of the inclined distance from the aircradt to the ground station (Fig. 75). Measurement range is based on measuring the transit time of the signal from the aircraft to the surface of the equipment and in the opposite direction.
Fig. 75. Distance Measure Equipment DME
DME systems consist from on-board and ground equipment. On-board equipment DME emits query signals at a certain frequencies.
Ground station receiver accepts questions signals and after some time delay initiating signal response that radiated through the antenna system transmitter to an another frequency. The signal response is taken onboard equipment DME with fixing moment of acceptance (Fig. 76).
Fig. 76. The principle of operation DME
DME evaluates the time for wich the radio signals returned with the time delay in the ground part of the equipment. By the time signal passage measured distance from aircraft to ground equipment system: Where t – time of passage radio signals from board to ground equipment DME; τ – time delay, needed to respond of parts ground equipment DME; с – speed of lights.
DME (distance measurement equipment) uses frequencies from 960 to 1215 MHz. Range of action depends on the respondent's power. Standard range on routes is 365 km, at areas of the airports – 95 km. The onboard equipment DME consists of the computing block, the control panel and the antenna (fig. 77). Standardly two sets of the equipment are established onboard of aircraft for reservation and reliability of work.
Fig. 77. DME-42 equipment (Rocwell Colins)
Two independent sets of DME equipment are allowing to calculate distances to two different radio beacons. If distances are known and including knowing coordinates it is possible to determine aircraft position.
DME transponders transmit on a channel in the 962 to 1213 MHz range and receive on a corresponding channel between 1025 to 1150 MHz. The band is divided into 126 channels for interrogation and 252 channels for reply. The interrogation and reply frequencies always differ by 63 MHz.
The ground stations are typically co-located with VORs. It allows pilot to identify the position of aircraft relative to VOR. Also the attuning for work with DME ground station taking place along with attuning of the VOR equipment.
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. So, the ground distance may be calculated from the equation: Where D – slant range; h – height of flight.
Fig. 78. Slant range error of DME
Slant range error is most pronounced at high altitudes when close to the DME station.