DME (Distance Measurement Equipment)
Overview
1.10 Rohde & Schwarz Test of DME/TACAN Transponders 7
Figure 2 shows the time characteristic of the envelope of the DME interrogation pulses
from the aircraft (interrogation signal) and the reply pulses from the ground station
(transponder reply signal). The table below gives the pulse spacing and delay times for
the two channels X and Y.
DME/N pulse spacing and delay times
Channel Pulse spacing
of
interrogation
pulses from
aircraft
Pulse spacing
of reply
pulses
from
transponder
Delay of
1st pulse
Delay of
2
n
d
pulse
X
12 Js 12 Js 50 Js 50 Js
Y
36 Js 30 Js 56 Js 50 Js
Table 1: Pulse spacing and delay times
The table below gives the other DME pulse parameters.
DME/N pulse parameters
Pulse width (50% amplitude) 3.5 Js ± 0.5 Js
Rise time (10% 90% of amplitude) 0.8 Js to 3 Js
Fall time (90% 10% of amplitude) <3.5 Js
ON/OFF ratio 80 dB
Table 2: DME/N pulse parameters
In order to limit the bandwidth of the DME signal to the channel width of 1 MHz, the
envelope of the pulses is shaped taking the specified rise and fall times into
consideration.
There are two different DME standards (DME/N and DME/P) that mainly differ with
respect to the rise time of the pulse edge, whereby DME/N is the standard that is
almost always used at present. DME/P was defined in the 1980s with the objective of
increasing the accuracy of the system. It was intended to be used in combination with
MLS (Microwave Landing System = successor system for ILS) and to replace DME/N
in the long term. However, owing to the advent of satellite navigation (GPS), DME/P
and MLS never gained acceptance and are now no longer being pursued. Therefore,
only DME/N will be referred to below.
The overall accuracy of the DME/N system is approx. ±0.1 NM (±185 m)
(1 NM = 1 nautical mile = 1852.02 m).
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