18.1 DME & VOR

18.1 DME & VOR

Quick Answer: DME = airborne interrogator + ground transponder; times pulse-pair round-trip in ~962–1213 MHz → slant range. Ground reply delay enables close-range operation. Most DME auto-tunes with paired VOR/LOC. VOR = 108.00–117.95 MHz; all directions from magnetic north; radials = magnetic lines from station; principle = phase difference between reference and 30 rps variable signal, in phase at magnetic north.

Topic 3-K (Aircraft) opens with the two classic enroute navaids GROL techs still find on virtually every IFR airframe: Distance Measuring Equipment (DME) and VHF Omnidirectional Range (VOR). Key topics 068 and 069 test frequencies, geometry words (slant range, radial), and the physical principle each system uses—time for DME, phase for VOR.

Distance Measuring Equipment (DME) — interrogator / transponder

DME is a secondary-radar ranging system. The aircraft unit is the interrogator; the ground station is the transponder (often co-located with a VOR or as part of a VORTAC / TACAN suite).

Operating principle (pool anchor)

The main underlying operating principle of airborne DME is that a measurable amount of time is required to send and receive a radio signal through the Earth’s atmosphere. Radio waves travel at (essentially) the speed of light. The airborne computer converts the measured round-trip interval—minus the known ground-station processing delay—into distance.

Pool wording also singles out DME as the aid that determines distance to a selected VORTAC by measuring the length of time the radio signal takes to travel to and from the station.

Frequency range

Element 3 states the DME frequency range used to indicate slant-range distance as 962 MHz to 1213 MHz (L-band). Do not confuse this with:

Slant range (not map distance)

The distance DME reports is slant range—the straight-line path through the air from aircraft to ground station—not the horizontal ground distance a chart might suggest. Directly overhead a station, DME still shows roughly the aircraft’s height in nautical miles; far away at low altitude, slant range ≈ ground range. Pool term for “distance from the aircraft to the DME ground station”: the slant range (not “DME bearing,” “glide slope angle,” or “localizer course width”).

Built-in reply delay and close-range operation

The ground station inserts a fixed delay between reception of an interrogation and transmission of the reply. That intentional hold-off allows operation at close range—without the delay, very short geometric times would be hard to process cleanly and the airborne unit could not separate its own transmit pulse from the reply. Pool distractors (“someone to answer the call,” “VOR mechanical hook-up,” “clear other traffic”) are wrong; the technical purpose is close-range operation.

Channel pairing with VOR / LOC

The majority of airborne DME systems automatically tune their transmitter and receiver frequencies to the paired VOR/LOC channel. When the pilot (or FMS) selects a VOR or localizer frequency, the DME receiver/transmitter pair selects the standardized L-band channel assigned to that VHF navaid. That is why a single NAV radio frequency selection often yields radial + distance on a VORTAC or localizer course + DME on many ILS/DME approaches.

Service notes for GROL maintainers

1. Verify antenna condition (blade/L-band) and coax integrity—high VSWR kills interrogation power and reply sensitivity.
2. Confirm channel pairing tables when a DME “won’t lock” after a NAV frequency change.
3. Expect slant-range complaints over the station; explain geometry before condemning the unit.
4. Ground-station saturation (too many aircraft interrogating one transponder) can reduce reply rate—an operational limit, not always a failed interrogator.
5. Co-located TACAN provides military-compatible ranging/azimuth; civil aircraft still extract DME distance from the TACAN pulse structure.

VHF Omnidirectional Range (VOR)

VOR is the classic VHF azimuth navaid. A ground station radiates a complex signal from which any aircraft can determine its magnetic bearing from the station—a radial.

Frequency range

Ground-based VOR stations used for aircraft navigation operate 108.00 MHz to 117.95 MHz. That band is shared allocation space with localizer channels in the lower portion (odd-tenth localizer frequencies live in 108.10–111.95 MHz); service techs must know which selected frequency is a VOR vs a LOC by chart/channel plan, not by “any number in the 108s.”

Magnetic north and radials

All directions associated with a VOR station are related to magnetic north (not true north / the geographic pole / the North Star). Lines drawn from the VOR station in a particular magnetic direction are radials. The 090° radial is the magnetic line east of the station; an aircraft on the 090° radial is east of the VOR regardless of the aircraft’s heading.

Phase comparison—the VOR operating principle

The main underlying operating principle of VOR is that a phase difference between two AC voltages may be used to determine an aircraft’s azimuth position in relation to a selected VOR station. The station radiates:

1. A reference phase signal (classically frequency-modulated onto a subcarrier).
2. A variable phase signal (amplitude-modulated radiation pattern that effectively rotates).

The rate that the transmitted VOR variable signal rotates is equivalent to 30 revolutions per second (30 Hz). The aircraft receiver compares the phase of the reference and variable components; that phase angle equals the magnetic radial.

The amplitude-modulated variable-phase signal and the frequency-modulated reference-phase signal are synchronized so that both signals are in phase with each other at 360 degrees North, magnetic bearing position of the VOR station. At magnetic north, phase difference = 0°; elsewhere the phase lag encodes the radial.

TO / FROM sense (how pilots use what you repair)

Selecting a course on the OBS does not change which radial the aircraft is physically on—it changes the CDI centering and the TO/FROM flag:

• FROM with a centered CDI: you are on the selected radial outbound from the station.
• TO with a centered CDI: you are on the reciprocal radial, tracking inbound on the selected course.
• Off-scale CDI: left/right of the selected course; reverse sensing on the back course of some setups is a pilot procedure issue, but the underlying RF is still phase comparison.

GROL techs do not fly approaches for a living, but understanding TO/FROM prevents misdiagnosing a healthy receiver when the pilot simply has the wrong OBS setting or is on the opposite side of the station.

VOR + DME = rho–theta navigation

Before GNSS became primary, enroute and terminal navigation were built on this rho–theta pair. Element 3 still expects the classical radio principles because GROL holders install, inspect, and repair the RF hardware that generates those signals.

Frequency memory table (068–069)

Exam-day checklist (3-K 068–069)

1. DME band 962–1213 MHz; distance = slant range; principle = time of flight.
2. Ground delay exists so DME can work at close range.
3. Airborne DME usually pairs with VOR/LOC channels.
4. VOR band 108.00–117.95 MHz; directions from magnetic north; lines = radials.
5. Variable signal 30 rps; signals in phase at magnetic north; principle = phase difference for azimuth (not distance).
6. Do not mix DME L-band numbers with VOR VHF or glideslope UHF distractors.

Master time-for-DME and phase-for-VOR and the 068–069 items become mechanical. Next section adds precision approach (ILS), low-frequency bearings (ADF/NDB), and ATC transponders.