VOR & DME

062 Radio NavigationChapter 2

Schematic

Fly TO the needle when FROM flag shown; fly FROM when TO flag shown.

VOR frequencies

Key fact

108–112 MHz: VOR and ILS — 40 channels each. ILS frequency if first decimal digit is odd (e.g. 108.10 and 108.15 MHz). 108–118 MHz: VOR only — 120 channels. 50 kHz spacing. Total VOR channels = 160. Emission type A9W, horizontally polarised (less atmospheric noise). Ident: keyed AM Morse every 10 s or voice.

VOR type	Frequency	Power	Range	Channels
Terminal (TVOR)	108–112 MHz	up to 50 W	25–100 nm	40
En-route	112–118 MHz	up to 200 W	200 nm (max 300)	120

Key fact

Broadcast VOR — normally terminal; transmits radial and ATIS. TEST VOR (VOT) — transmits just the 360° radial; ± 4° requires servicing.

CVOR (conventional VOR)

Key fact

Signal 1 — Reference: omnidirectional, FM modulated at 30 Hz on a sub-carrier. Signal 2 — Variphase: directional, on main carrier, appears AM modulated at 30 Hz, rotates at 1,800 rpm. Reference + variphase = rotating limacon. Phase difference measured when voltage drops on the limacon = radial (QDR). Zero phase difference on the 360° radial; does not drop to zero like a cardioid.

DVOR (Doppler VOR)

Key fact

Reference: AM modulated on main carrier. Variphase: FM modulated on sub-carrier, transmitted on 50 different aerials in turn. Doppler shift calculates phase difference. More accurate with very little site error — impractical to rotate a large CVOR aerial.

Displays and terminology

Term	Meaning
CDI	Course Deviation Indicator
OBI	Omni Bearing Indicator
OBS	Omni Bearing Selector

Key fact

RMI: tail of needle = QDR; head = QDM. CDI shows displacement, not bearing. Each dot = 2° displacement; full-scale deflection = 10°. Outside of centre circle counts as one dot.

VOR errors and accuracy

Error	Cause
Site error	Obstacles near transmitter reflect waves; limacon distorted
Cone of confusion	Overhead VOR — flickering to/from flag
Bends	Slow oscillations from irregular terrain
Scalloping	Rapid oscillations — cannot be followed

Key fact

Overall VOR accuracy: ± 5° 95% of the time; worst case ± 7.5°. VOR monitor powers off VOR or removes ident when bearing exceeds ± 1°, signal strength drops 15% or more, or monitor fails. Distance between VORs on airways: range × 2 to avoid conflict.

Formula — Track error from VOR (1-in-60)

Track Error = (Distance Off × 60) ÷ Distance Gone

Exam trap

Variation at the VOR beacon for VOR; variation at the aircraft for ADF. Flying along a VOR radial follows a great circle track. Using VOR outside published range may cause interference from other transmitters. Compass stuck but RMI working: RB unavailable, radial still available.

DME characteristics

Key fact

Vertically polarised, PON, UHF 962–1213 MHz, 256 channels (126 X at 12 μs, 126 Y at 36 μs). Typical range 200 nm (line of sight). Aircraft interrogator transmits on DME frequency; ground station replies ±63 MHz different.

DME operation sequence

1. Jittered PRF — interrogator sends pulse pairs; interval between pairs in a pair is constant; interval between pairs is random (prevents fruiting).
2. Ground reply — on ±63 MHz frequency, echoes pulses after 50 μs delay.
3. Receiver — tuned to ±63 MHz, searches for its own interval pattern, computes range.

Formula — DME range

Range = (Time Elapsed − Delay) ÷ 12.36   (radar mile)

DME modes and saturation

Mode	Pulse rate	Notes
Search	150 pps	Before lock-on; reduces to 60 pps after 15,000 pulses with no lock
Tracking	24–30 pps	After lock-on — reduces ground-station load
Memory	—	Signal dropout — 10 s trend-based range, then re-enters search

Key fact

Ground equipment limited to 2,700 pulse pairs per second. Average capacity ≈ 100 aircraft. Oversaturation: receiver gain reduced — station listens only to closer aircraft. Co-located VOR/DME: within 2,000 ft (en-route) or 100 ft (terminal); same ident. DME ident every 30–40 s at 1,350 Hz (higher pitch than VOR’s 10 s ident).

Exam trap

Slant range ≈ plan range at long distances. When closer than 3 × height, slant range error increases. DME groundspeed decreases when nearing the beacon at constant height (slant distance change). DME is more accurate than VOR except directly overhead. Old beacon accuracy: ±(0.25 nm + 1.25% of distance); new: ± 0.2 nm.

RNAV

Key fact

B-RNAV: ± 5 nm 95% of time. P-RNAV: ± 1 nm 95% of time. With VOR, error is a radial error; with RNAV, error is cross-track error. DME–DME is most accurate; ADF not used in RNAV. In AUTO mode, navaids selected automatically — best geometry when two navaids intersect at 90°.

Display	Mode	Full scale	1 dot
5-dot	VOR	10°	2°
5-dot	ENR	5 nm	1 nm
5-dot	APP	1.25 nm	0.25 nm
2-dot	ENR	4 nm	2 nm
2-dot	APP	1 nm	0.5 nm