DME

1
DME

• AST 241

2
DME Theory

• Most VORs in the US have DME- distance measuring
  equipment or are co-located with a military TACAN
  facility which provides the same information.
• VOR navigation is much improved with DME as it
  only provides
• Azimuth information

3
DME Theory

• DME is required for aerial navigation above?
• FL 240 or 24,000 ft.
• Especially useful in IFR operations
• VOR and DME are essentially unified systems which
  are seamless to the pilot when co-located

4
DME theory

• Each VOR frequency from 108.00 to 117.95 MHz
  (VHF) has a corresponding UHF freq. Ranging from
  962 1213 MHz
• Pilots usually have to tune only one freq. With
  newer receivers
• DMEs can be received independent of the VOR if
  the tuner is separate but the information is of
  little use for most pilots

5
DME Theory

• For civilian users VOR/DME and VORs co-located
  with TACAN (VORTACs) are essentially the same
  thing providing pilots with the same information.
• TACAN was developed by the military to
• Rectify terrain interference problems
• Rectify portability problems
• Rectify the need for a level platform- (carriers)

6
DME Theory

• Civilian users cant take advantage of TACANs
  azimuth information and military pilots cant use
  DME information

7
DME Operation

• Similar to the ATC Radio beacon system or
  Transponder system
• VORs are a Passive System giving continuous
  information
• DMEs respond only when interrogated making them
  an Active System

8
DME Operation

• Each DME receiver emits two distinctly (to that
  receiver) spaced pulses which are replied to in
  identical fashion by the ground based unit.
• The DME receiver ignores all other replies.
• This is why the DME can take up to 10 seconds to
  lock on to its unique reply
• Once locked it updates every 1/10th mile

9
DME Operation

• The time difference between interrogation and
  reply corresponds to the aircrafts straight line
  distance from the station.
• 12/1,000,000 of a second in time corresponds to 1
  NM in distance

10
DME Operation

• DME accuracy varies from 3 of total distance (6
  NM at 200 miles out) up to being within .1 NM at
  all distances for the higher end receivers- as
  good as GPS
• The most common operational mistake is to forget
  to select the correct VOR if the aircraft is
  equipped with 2 VORs or leaving it in the hold
  mode which holds the last frequency tuned rather
  than the current one.

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DME operation

• The most common units today are selectable
  between the dual VOR receivers- if not then the
  DME signal is usually taken from the 1 VOR but
  BE SURE!-
• ALWAYS VERIFY!!

12
DME Operation

• Groundspeed and time to Station- Since DME uses
  rate of response information to get distance it
  is a small step for the receiver to calculate
  groundspeed and time to the station.
• This information is usually displayed in a
  separate window labeled- KTS. MIN. OFF

13
DME Operation

• It is important to remember that this only works
  if heading directly to or from the station-
  angular motion w.r.t. the station makes all DME
  information USELESS!

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DME identification

• DME stations are identified in the same coded
  manner using the same identification as the
  corresponding VOR station.
• The DME portion is broadcast at a higher pitch
  and is less frequent than the VOR identification-
  every 30 seconds.

15
DME overload

• DME transmitters can become overloaded during
  heavy usage causing the late comers not to lock
  on the the freq.
• It is courteous to always turn off the DME
  receiver when not in use such as when on the
  ground or when DME info. Is not needed.

16
DME error

• Slant range error occurs because the DME
  calculates its straight line or slant range
  distance from the station which is not as useful
  to pilots as ground distance.
• This causes all DME indications to be inaccurate
  when greater than 1,000 ft. AGL for each NM from
  the station- otherwise it can be ignored.

17
DME Error

• For example if a pilot is at 3,000 ft AGL at 2
  miles from the station the error is out of
  tolerance.
• Remember for accuracy be at least 1NM from the
  station for each 1,000 ft AGL

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DME Arc

• Older instrument approach procedure where the DME
  information is used to fly a constant radius
  around the transmitter
• This becomes easier when VOR lead radials are
  used to prompt the turn

19
DME Operation

• DME is Rho-Rho navigation
• Rho distance
• Theta azimuth
• VOR/DME together is theta rho nav.
• 2 VORs are theta - theta nav.
• Rho rho nav. Requires 3 DME readings to
  determine position

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Questions?

• The End