DME

Question 0

How DME works

Answer 0

1. Works on the principle of secondary radar
2. Air-born transceiver sends out an interrogations signal to the ground
3. Ground station receives the signal and sends a reply signal back to the transceiver
4. Air-born transceiver then measures the response time and converts it to slant range distance in NM
   - slant range; direct distance from aircraft to the ground equipment
   - slant range error; because the horizontal distance is measured there is a slight error in distance. To minimize this error be 1000’ or less for every 1NM away
   - it can be accurate up to 3% or .5 NM which ever is greater

Question 1

DME (distance measuring equipment)

Answer 1

• DME is line of sight and ranges up to 199NM
• DME station can handle up to 100 aircraft at one time (based on who has the strongest signal)
  DME Frequencies:
• operates in UHF band (962-1213 MHz)
• 252 possible frequencies

Question 2

DME cock-pit display

Answer 2

• distance to or from the station
• ground speed; only accurate when tracking directly to the station
• ETA; only accurate when tracking directly to the station
• DME only provides a circular position line by itself you need it combined with another NAV-AID to provide a fix on your position

Question 3

DME pairing

Answer 3

VOR/DME pairing; the DME is co-located with a VOR station and frequencies are paired together. When you tune into the VOR the DME is automatically tuned in as well, this reduces pilot work load and error
- VOR ID is every 10 seconds
- DME ID is every 30 and is a higher pitch
ILS/DME pairing:
ILS localizer frequency is paired with a DME station located close to the runway threshold, it provides you with “distance to go” to the runway threshold

Question 4

DME arcs: are a circular flight path centered on a DME ground station

Answer 4

• protected airspace of a published DME arc is +/- 4NM (Jackson hole)
• remain within 1 mile when performing a DME arc
  Used for:
  -change radials
• obstacle clearance
• align aircraft with approach course, instead of using a procedure turn
  EXAMPLE:
  1. Begin turn 1/2 mile prior to distance given (depending on your speed you may need more than a 1/2 mile)
  2. At 10.5 start your turn; continue turn until you have gone 90 degrees
  3. When turn is complete turn 10 more degrees and then tune 10 degrees
  4. The last turn of the arc should be inbound on the radial
• to determine distance on a DME arc:
• take the number of radials times the distance from the NAV-AID divided by 60