Distance Measuring Equipment (DME)
This
system uses basic radio telemetry to provide information on the distance
between the aircraft and the ground station. It manipulates both the radio
signal received from an on-board interrogator and the reply transmitted from
the ground transponder. The principle is based on distance = time x speed where
speed is the velocity of electromagnetic wave. The
distance is determined by measuring the propagation delay of a radio frequency
(RF) pulse that is emitted by the aircraft transmitter and returned at a
different frequency by the ground station.
DME equipped (interrogator) aircraft
transmit encoded interrogating RF pulse pairs to the ground beacon
(transponder). The transponder replies with encoded pulse pairs to the interrogator.
DME transponders transmit on a channel in the 962 to 1213 MHz
range and receive on a corresponding channel between 1025 to 1150 MHz,
where the two channels are is 63 MHz apart. This principle of measurement of
distance is illustrated in figure below:
Measurement of distance based on radio frequency signals assumes
free space propagation. However due to natural and artificial obstacles around
airports, RF signals interact with this structures. This interaction is likely
to result into attenuation and more particularly reflection. Reflection arising from the roof of a building introduces errors in DME measurements.
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