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So the slant range distance = v  t/2
v = propagation velocity of the pulse pair
t = elapsed time
[2] The DME interrogator is a transmitter/receiver. It transmits a pulse-pair
interrogation signal, receives the reply signal, computes the slant range to the
tuned station, and provides distance output data. Transmission frequency is
Fig.25-1 DME system
between 1025 to 1150 MHz, receive frequency is between 962 to 1213 MHz.
The slant range computing capability of the interrogator is from 0 to 200
nautical miles, however, a program pin will allow expanded range capability
up to 389.99 nautical miles(NM). The interrogator contains monitor circuitry
that continuously monitors system performance. If a fault is detected, the
interrogator will stop sending the DME valid signal. The interrogator is
enclosed in a 1/2 ATR case and weighs 16 lbs (7.3kg). It is secured by two hold
-down devices. Interface with the airplane wiring is through a rear-mounted dual
connector.
[3]The DME antenna
provides the means of
radiating and receiving RF
energy for the DME system
operation. The DME antenna
is a blade-type antenna, that is
5.25 1.75 inches at the base
and extends 2.20 inches down.
The DME antenna is designed
for L-band use. It has a
nominal impedance of 50
ohms and a maximum VSWR
of 2:1 .The antenna has an
o-ring moisture seal and is
attached to the airplane by
four screws. The coaxial cable
connector should be
connected to the airplane. The
antennas for the DME and
ATC systems are identical
and interchangeable.
[4] Figure 25-3 shows the
VHF NAV/DME control
panel. This panel provides
manual frequency selection
for the VOR/ILS NAV and
DME system, also it provides
the selection of manual or
automatic tuning mode. The
Fig. 25-2 DME antenna
Fig. 25-3 VHF NAV/DME control panel
AUTO/MAN selector switch
is a push-on/push-off
illuminated switch that is used
to select either automatic or
manual frequency control of
the VOR/ILS NAV and DME
system. The selected mode is
indicated on the switch.
[5] Manual frequency selection is provided by dual concentric knobs. The outer
knob controls tens and units, and the inner knob controls tenths and hundredths.
[6] When the system is in the AUTO tune mode a flag is raised over the manual
display window, and the VOR/ILS NAV frequency selected by the flight
management computer is displayed in the AUTO window. If the system is in
agility tune mode, the AUTO window will display dashes. When the system is
in the manual tune mode, the manually selected frequency is displayed in the
MAN window, the flag is retracted, and the AUTO window is blank.
[7] On the VHF NAV/DME control panel, two switches are provided for test of
the VHF NAV and DME systems. Both switches are three-position switches and
are spring-loaded to the center position.
[8] The DME distance can be displayed on either RDDMI (Radio Direction
Distance Magnetic Indicator) or EHSI (Electronic Horizontal Situation Indicator)
The RDDMI displays the distance computed by the DME interrogators. The
DME indicators are located on the upper portion of the RDDMI. Each indicator
consists of four magnetic wheel
displaying hundreds, tens, units
and tenths of miles. Each indicator
has a red and white flag that is
retracted out of view during
normal DME operation. The
indicators receive binary coded
inputs from the DME interrogators,
decode the data and display slant
range distance. When there is no
computed data, the indicators will
display dashes. A loss of valid
Fig. 25-4 RDDMI DME distance displays input data, or a faulty power
supply, will result in the red and white striped flag covering the display. When
auto tune is selected, the display shows the distance to the ground station
selected by the FMC. If the agility tune mode is selected by the FMC. The
RDDMI display will be dashes.
[9] The EHSI displays the
distance computed by the DME
interrogators. The distance is
used to aid navigation. The
distance is displayed in the upper
left corner of EHSI, 3 digit
display shows miles above 100
nautical miles and tenths of miles
below 100 nautical miles. If there
is no computed data, dashes
replace the distance. If there is a
DME failure, the complete DME
display blanks. With AUTO
selected on the VHF/NAV
control panel, the DME distance
will not be displayed on the EHSI.
The frequency display in the
lower right corner will be
replaced with the word AUTO.
　
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