DME

Question 1

intro

Answer 1

● The DME provides the distance to station in nautical miles, and in some systems a groundspeed will also be shown
● DME receivers can obtain distance information from a VORTAC, TACAN or DME installation
● DME operates in the UHF frequency band.
○ However, this frequency can be “paired” with a VOR or ILS or localizer (LOC) frequency.
● The receiving equipment in most aircraft provides for automatic DME selection through a coupled VOR or lLS receiver.
○ Selection of the appropriate VOR or ILS frequency automatically tunes the DME

Question 2

DME Frequency

Answer 2

● The DME operates in the ultra-high frequency (UHF) band and therefore is restricted to line-of-sight transmission.
● With adequate altitude the pilot can receive en route DME signals at distances over 200 NM.
● Like all navigation aids, DME has inherent error.
○ This error is certified to be within ±0.5 nautical miles (NM) or 3% of the distance, whichever is greater

Question 3

Basic Principles

Answer 3

● The DME operates by transmitting and then receiving paired pulses from the ground station
● These signals are received at the ground station and trigger a second transmission on a different frequency.
● These reply pulses are sensed by timing circuits in the aircraft’s receiver that measure the elapsed time between transmission and reception

Question 4

DME Components

Answer 4

● The antenna, used for both transmission and reception, is a very small “shark’s fin” normally mounted on the underside of the aircraft
● Modern DME controls incorporate digital readouts of frequency, DME, and ground speed information

Question 5

Slant Range

Answer 5

● The distance to the station is a slant range, expressed in nautical miles.
● Slant range error is greatest under the following circumstances
○ at high altitudes, and
○ when closest to the station
● If an aircraft is far away and low there is little slant range error.

Question 6

Ground Speed

Answer 6

● The DME receiver can express ground speed in knots.
● This speed value used as ground speed is accurate only if the aircraft is flying directly to or from the DME since the DME system is measuring the speed by comparing the time lapse between a series of pulses.
● DME indicates the speed in relation to a DME and the aircraft.
➢ If flying directly to or from the DME it will indicate a ground speed.
➢ But if not flying directly to or from the DME it will not be indicating ground speed, just the speed of closure to or departure from the DME

● The DME must be on the ground speed function long enough to compare the time lapse between several pulse signals.
● Not all DME displays are equipped with a ground speed feature

Question 7

Frequency Hold

Answer 7

● Certain DME radios have a frequency-hold function.
● The DME hold function retains the frequency of the original VORTAC station on the DME while the pilot switches the VOR receiver to another frequency

Question 8

Finding Position

Answer 8

● A pilot with a DME may pinpoint the aircraft’s position using the radial of a VORTAC and the distance information from the same station.
● A pilot without DME must use the radials from two stations to get a position fix
● The pilot can also use the DME to establish intersections and holding patterns.
● When so equipped and cleared by ATC, pilots can establish holding patterns by reference to radials and DME
● Many airports have instrument approach procedures based on use of VOR and DME equipment.
● Normally, an aircraft making this type of approach has lower minimums than when only the VOR is used.