Distance Measuring Equipment (DME) Flashcards
VOR’s, NDB’s, and localizers guide the aircraft along a course; However they do not give what?
do not give the flight crew a fix of their actual position because the aids do not provide a distance to the station
What does DME stand for?
Distance Measuring Equipment
DME lowers pilot workload in having to:
plot and reckon position
Federal regulations require DME for aircraft operating at or above:
24,000ft (FL240)
What does a DME send to a ground station?
an interrogating pulse
What happens once the DME has interrogated the ground station and received a reply?
The aircraft DME equipment measures the elapsed time to compute the distance to the station
How long does it take for a DME signal to travel one nautical mile?
just over 6 micro seconds
The ground station can be one of 3 different station:
VOR station with DME
a TACAN station
VORTAC
What is TACAN?
Tactical Navigation developer by the military
What are known as VORTAC sights?
VOR sites that also have TACAN capability
DME can also be a part of what system?
ILS
and is listed as an ILS/DME
VOR with DME is known as:
VORDME
DME operates in what band?
UHF
When a VORDME or TACAN station is tuned onto the VHF nav, it is paired with?
the UHF frequency
What are the two modes of DME frequency band use?
X Mode and Y Mode
What is X Mode?
the old system and divided the bandwidth into channels with 0 in the 50kHz
What is Y Mode?
newer system and divides the bandwidth into channels with a 5 in the 50 kHz position
The X/Y system allowed for how many available channels?
200
Unlike most radar systems, DME is an active system. This means that what?
The system in the aircraft transmits
What is the calculated time it takes a radio wave to travel one nautical mile?
6.18 micro seconds
What is RRTNM?
Radar Round Trip Nautical Mile
12.36 micro seconds
(the time it takes for a radio wave to travel one NM and back)
What does the DME transponder transmit?
random Gaussian pairs of pulses in its assigned DME channel
All DME pulses are how wide at the 50% point?
3.5 microseconds wide
Have a rise time of 1 microsecond
How far apart are X Mode station transmission pulses?
12 micro seconds apart
How far apart are Y Mode station transmission pulses?
30 micro seconds apart
DME transponders receive at a frequency different from what?
the frequency at which they transmit
What kind of system is DME?
(In terms of components, not distance measuring)
Duplex System
The ground station continuously sends random pulses known as:
Squitter Pulses or the Squitter
At regular intervals, the transponder stops sending the squitter and instead sends out pulse pairs at a constant rate resembling dots and dashes.
The aircraft DME uses the pulse pairs to create what?
a 1,350Hz identification tone
Even though the time between pulse pairs is constant, the time between pairs of pulses varies at random. This is known as:
Jitter
Jitter allows:
a DME to recognize transponder replies directed to it
The transponder receives the pulse pairs, delays them for either 50 or 56 micro seconds, depending on what?
If it is X or Y Mode
It then retransmits the pulse pair
The time delay allows the electronics equipment to operate and for the aircraft to:
lock on to the station when close by
When the DME receives the pulses back, it will lock on and:
remove any fault flags from the display and provide displayed distance to the station
Once locked on, the DME starts to track what?
the changes in timing as the pulses return to the aircraft
The DME calculates the rate at which the distance is changing and then begins to display:
other info such as speed and estimated time enroute
The order of operation for the DME is always:
listen, search, lock, and track
There are 2 additional operational modes for the DME:
Memory Mode and DME hold
What is Memory Mode?
allows the DME to continue to display distance, speed, and ETE for a short period after a loss of signal
Memory Mode prevents what?
a restart when the signal is temporarily lost due to signal blockage during aircraft maneuvers or landing gear operation
What is DME hold?
locks the DME on the current frequency
What is the heart of the DME system?
the processor
What does the processor do?
detects and validates incoming pulses and controls the timing of the transmitter and receiver sections
DME measures what range?
Slant Range
With DME, the modulator works how?
like a trigger to fire off the Gaussian shaped pulses
Light aircraft will typically use a configuration in which:
the control head, transceiver, and indicator are all in one panel mounted unit
What type of antenna is the DME antenna and where is it mounted?
Marconi and mounted on the lower fuselage of the aircraft
What are the two types of DME antenna designs available:
A small whip antenna with a ball on the end
and
a rigid blade style that is about two inches long
Most of the generators used require the technician to:
set the specific antenna distance and height
Checking the sensitivity verifies what?
that it locks on at low signal level
The test for minimum lock on should be performed on:
more than one frequency in both X and Y Modes
Where can the transmitter power can be read?
The peak transmitter power can be read from the test unit
What does PRR stand for?
Pulse Repetition Rate
May sometimes also be called Pulse Repetition Frequency (PRF)
The PRR should be checked in both:
search and track modes
The distance, speed, and time to station values should agree with:
the test box within the limits of the maintenance manual
Frequency is read from the test set and again should be within the limits of:
the maintenance manual
