Radio Navigation Systems - Complete Reference Guide

Radio Navigation Systems

Comprehensive Reference Guide for Aviation Radio Navigation

Quick Reference - Critical Values

108-118 MHz
VOR Frequency Range
108-112 MHz
ILS Frequency (Odd Tenths)
190-1750 kHz
NDB Frequency Range
960-1215 MHz
DME Frequency Range
±5°
VOR Accuracy (95%)
±5°
NDB Accuracy (Day)
0.5°
ILS Localizer (1 Dot)
0.14°
ILS Glideslope (1 Dot)
1030/1090 MHz
SSR Frequencies
24 SATs
GPS Constellation
±0.2 NM
DME/P Accuracy
300 m/µs
Speed of Light (Radar)

THEORY Basic Radio Principles

Core Radio Wave Properties

  • Frequency (f): Number of complete cycles per second, measured in Hertz (Hz)
  • Wavelength (λ): Distance traveled in one complete cycle, measured in meters
  • Time Period (T): Time taken to complete one cycle, T = 1/f
  • Phase: Fraction of one wavelength expressed in degrees from 0-360°

Fundamental Formula

c = λ × f
Where: c = 3 × 10⁸ m/s (speed of light)
  • Low Frequency = Long Wavelength
  • High Frequency = Short Wavelength

Phase Difference

Critical: Can only be measured when signals have the same frequency (or wavelength). This principle is fundamental to VOR operation.

Unit Equivalent Example Use
1 kHz 10³ Hz NDB frequencies
1 MHz 10⁶ Hz VOR, ILS frequencies
1 GHz 10⁹ Hz DME, GPS frequencies

1. Keying (NON A1A)

Method: Interrupting carrier wave to create dots and dashes (Morse code)

Characteristics:

  • Does NOT change frequency or amplitude
  • Temporarily interrupts nav aid output
  • Used for identification

2. AM (Amplitude Modulation)

Method: Varying carrier wave amplitude

Characteristics:

  • Carrier frequency kept constant
  • Oldest method apart from keying
  • Used by: NDB, ILS, VOR reference signal

Disadvantages:

  • Small amplitude areas = weak signal
  • Prone to interference (LF spectrum)
  • Requires extra modulation power

3. FM (Frequency Modulation)

Method: Varying carrier wave frequency

Characteristics:

  • Carrier amplitude kept constant
  • +ve amplitude = higher frequency
  • -ve amplitude = lower frequency
  • Used by: DVOR, Radio Altimeter

Advantages:

  • Simpler and cheaper TX than AM
  • Lower modulation power required
  • Constant amplitude = stronger signal
  • VHF operation = almost static free
  • Horizontally polarized (less weather static)

Disadvantages:

  • More complex receivers
  • Wider frequency band required
  • FM has many more sidebands than AM

4. Pulse Modulation

Method: Radio wave switched on/off at regular intervals

Applications:

  • Radar systems
  • DME ranging
  • Transmits 0's and 1's effectively

5. Phase Modulation

Applications:

  • GPS signals
  • Uses helical antennas
  • Circular polarization

AM Sidebands & Single Sideband (SSB)

When a carrier is AM modulated by a lower frequency, sidebands are created:

Carrier Wave = 500 kHz, Audio Freq = 4 kHz Output: 496 kHz / 500 kHz / 504 kHz

Single Sideband (SSB):

  • Only ONE sideband transmitted (4 kHz filtered out)
  • Sideband carries information, not carrier
  • All TX power focused on one sideband = increased range
  • Used in: HF Communications & HF VOLMET (J3E)

Ideal Antenna Length

Ideal: λ/2 (half wavelength)

If not possible: λ/4, λ/8, etc. will work

Polarization

Definition: Orientation of the plane of oscillation of the electrical component (E)

  • Electrical (E) and Magnetic (H) components travel at right angles
  • Both perpendicular to direction of propagation
  • Vertical aerial: Vertical E field, Horizontal H field
  • Horizontal aerial: Horizontal E field, Vertical H field
  • Circular propagation: Both components spin about axis of advance

Important Polarization Facts

  • Electrical field (E) same direction as aerial
  • Magnetic field (H) perpendicular to aerial
  • AC induced parallel to wire, but remote from it
  • First wire radiates energy, second wire induces AC
  • Plane of electrical component = plane of polarization

Omnidirectional Antennas

Radiates equally in all horizontal directions

Examples: NDB, VOR

Directional Antennas

Focused radiation pattern

Types:

  • Loop (ADF)
  • Parabolic (Radar)
  • Slotted Planar (Radar - less side lobes)
  • Helical (GPS - circular polarization)

Note: Flat/slotted plate generates less side lobes than parabolic

Dipole Antennas

Simplest type - Requires receiver and transmitter of electrical field on the same planes

Three-Letter Code Structure

  • 1st Letter: Type of Modulation
  • 2nd Letter: Nature of modulating Signal
  • 3rd Letter: Type of Information transmitted
Code Description Used By
NON A1A Unmodulated carrier (Keying) NDB (no audio)
NON A2A AM Modulated NDB (with audio)
J3E Single Sideband HF SSB Communications
A9W Composite VOR
A8W Composite ILS
PON Pulse DME
Band Abbr. Frequency Wavelength Aviation Use
Very Low VLF 3-30 kHz 100-10 km (Myriametric) -
Low Frequency LF 30-300 kHz 10-1 km (Kilometric) NDB/Locator
Medium Frequency MF 300-3000 kHz 1000-100 m (Hectometric) NDB/ADF
High Frequency HF 3-30 MHz 100-10 m (Decametric) HF Comms, VOLMET
Very High Frequency VHF 30-300 MHz 10-1 m (Metric) VOR, ILS Localizer, VHF Comms, VDF, Markers
Ultra High Frequency UHF 300-3000 MHz 100-10 cm (Decimetric) DME, ILS Glideslope, GPS, Terminal Radar
Super High Frequency SHF 3-30 GHz 10-1 cm (Centimetric) AWR, MLS, ASMR, Radio Altimeter
Extremely High Frequency EHF 30-300 GHz 10-1 mm (Millimetric) -

Wavelength Calculation

λ = c / f where c = 300,000,000 m/s or 300 m/µs

PROPAGATION Radio Wave Propagation

Main Propagation Types

VLF/LF/MF/HF: Surface Wave + Sky Wave

VHF/UHF/SHF: Direct Wave (Space Wave)

Surface Waves

Mechanism: Diffraction + Surface attenuation

  • Attenuation slows bottom of wave
  • Forward tilt allows following Earth's curvature
  • Lower frequencies have longer range (less attenuation)
  • Attenuation reduced over sea (waves travel twice as far)

Drawbacks: Low efficiency aerials (not λ/2), static, high TX power required

Sky Waves

Mechanism: Refraction by ionosphere

  • Signals bent sufficiently to return to Earth
  • Lower frequencies refracted more
  • Ionosphere approx. 50-500 km altitude

Critical: ONLY HF propagates via skywave for practical use

Direct Waves (Space Waves)

Essentially 'Line of Sight'

Max Range (nm) = 1.23(√H_T + √H_R) where H = height in feet

Range depends on:

  • Height of transmitter + receiver
  • Power of transmitter
  • Height of intervening high ground

Components: Direct + Reflected + Sky

Range can be reduced by lowering TX power

Three Ionospheric Layers

Higher frequencies refracted by higher layers, but VHF and above pass straight through

Layer Frequencies Refracted Day vs Night
F Layer HF Increases in height at night
E Layer LF / MF Increases in height at night
D Layer VLF Disappears at night

Night Effects on Sky Waves

  • D layer disappears at night
  • E & F layers increase in height
  • 8 MHz frequency goes higher at night due to layer height increase
  • To avoid signal going out of range: Use approx. HALF the frequency (causes more refraction)
  • At dawn/dusk: May be NO signal due to re-ionization

Sky Wave Terminology

  • Critical Angle: Minimum angle at which radio wave will refract and return to Earth
    • Anything less = no refraction
    • Anything more = increased skip distance
  • Skip Distance: Distance between TX and point where first sky wave arrives
  • Dead Space: Area between limit of surface wave and 1st sky wave
    • Mainly HF band
    • Minimized with lower frequency

Frequency Increase Effects

Higher frequency = Less refraction:

  • Critical angle increases
  • Skip distance increases
  • Higher ionospheric layer penetration

MF/LF Sky Waves - Day vs Night

  • DAY: MF/LF gets attenuated too much, NO skywaves during day
  • NIGHT: Attenuation is less, skywaves present

Night Effect Problems

Causes interference by night with surface waves (e.g., NDBs)

Solutions:

  • TX power may be reduced at night
  • Listen to BFO to check clean signal
  • Regular ident checks

Characteristics:

  • Fluctuating indications
  • Interference of required ground wave with sky wave
  • Sky wave reflected from ionosphere
  • Sky wave distortion of null
  • Maximum effect at dusk & dawn

Refraction

Definition: Change in direction/bending due to change in speed

Occurs when traveling obliquely from one density medium to another

Types:

  • Coastal: Land to sea (higher altitude or moving beacon toward coast reduces effects)
  • Atmospheric: Density change with altitude
  • Ionospheric: Skywave propagation

Rule: Low to high density = slows down and bends toward normal

Reflection

Definition: Radio waves bounce off solid surface

Effects:

  • If two signals arrive same time but out of phase: fading/temporary losses
  • Used by HF communications
  • Creates multipath errors

Diffraction

Definition: When radio wave passes solid object, radio energy is scattered

Benefit: Allows radio waves to be received behind a mountain

Application: Surface wave propagation

Interference

Definition: Superimposition of two radio waves of same frequency

Results in: Fading, signal enhancement, or cancellation

Surface Attenuation

Definition: As radio wave passes over surface, it loses energy

  • Higher frequencies more susceptible (hit surface more often)
  • Lower frequency = Greater attenuation = Affects RANGE
  • Reduced over sea vs land

Ionospheric Attenuation

Ionosphere and atmospheric particles can absorb and block radio waves

Most significant error for GNSS systems

Atmospheric/Radar Attenuation

When radar energy strikes water droplets:

  • Some energy absorbed (attenuated)
  • Some energy reflected

Note: Short wavelength (high frequency) subject to greater attenuation (suited for weather radar)

Long wavelength (low frequency) subject to less attenuation (suited for ATC radar)

Absorption

Definition: Energy taken up by atmosphere

Weakening of radio wave

Duct Propagation

Created by:

  • Temperature inversion AND/OR
  • Rapid decrease in humidity with height

Effects:

  • Causes super-refraction
  • VHF and above can have unexpected ranges
  • Layer normally no more than 1,000 ft

Can increase VDF range

Can cause interference with other stations

Sub-refraction

Caused by: Temperature and humidity conditions

Effect: Decreased range (opposite of duct propagation)

Doppler Shift Principles

Important: Actual wavelength stays the same

+VE Doppler Shift

Condition: Distance between source and receiver is reducing

Effect: Received frequency appears greater than transmitted

Reason: More waves detected than if stationary

-VE Doppler Shift

Condition: Distance increasing

Effect: Frequency appears lower

Applications in Aviation

  • Doppler VOR: Uses Doppler shift to calculate phase difference
  • Weather Radar Turbulence Detection: WX/TURB mode uses Doppler to detect wind shear
  • DME Groundspeed: Rate of change of distance (Doppler effect on ranging)

SNR Definition

High SNR: Amplitude of wanted signal is greater than that of unwanted signal

Critical for: All navigation systems, especially VHF/UHF systems

VDF VHF Direction Finding (Ground Based)

Primary Characteristics

  • Provides HOMING
  • Measures RELATIVE TO/AT STATION
  • Do NOT BANK when using VDF for navigation
  • Uses METRIC wavelengths
  • Frequency: 118-136 MHz (VHF band)
  • ATC must have at least 2 VDFs
  • Military also uses UHF

VDF Q-Codes

Code Meaning Mnemonic Reference
QDM Magnetic TO station "To your Mom" Magnetic
QDR Magnetic FROM station "Radials go outwards" Magnetic
QUJ True TO station "To Juliet" True
QTE True FROM station "Cutey from ATC" True

Important: True bearing is always given FROM VDF station (QTE)

Adcock Aerial

Type: Traditional

Characteristics:

  • Multiple vertical elements
  • Less common in modern installations

Doppler Aerial

Type: Most common modern system

Method: Direction calculated by phase of Doppler shift

Polarization: Vertically polarized

Channel Spacing

  • 25 kHz = "Frequencies"
  • 8.33 kHz = "Channels"

QDL Procedure

Method: Series of QDMs are given

Interpretation: Pilot interpreted

Pilot calculates and flies headings based on QDMs provided

QGH Procedure

Method: Heading and heights issued to aircraft

Pattern: Maintains published pattern

Interpretation: ATC interpreted

Controller provides specific headings to fly

Class Accuracy Usage
Class A ±2° Not normally used
Class B ±5° Standard
Class C ±10° Limited accuracy
Class D Worse than 10° Poor conditions

Range Formula

Range (nm) = 1.23 × (√H_TX + √H_RX) where H = height in feet

Range Factors

  • Line of sight limitations - TX, RX and terrain height
  • Power of transmitter (aircraft)
  • Sensitivity/quality of receiver (ground station)
  • Range may be increased by duct propagation
  • Range may be decreased by sub-refraction (temperature & humidity)

Accuracy Factors (Errors)

  • Equipment errors: Calibration, maintenance
  • Propagation errors: Reflections, refraction, duct propagation
  • Site errors: Reflections from objects near receiver
  • Multipath errors: Reflections from objects between aircraft and ATC (results in bearing error)
  • Crossed transmission: Multiple aircraft on same frequency

NDB/ADF Non-Directional Beacon / Automatic Direction Finder

Primary Characteristics

  • Omnidirectional transmission
  • Vertically Polarized
  • Frequency: 190-1750 kHz (LF & MF Bands)
  • Wavelength: Hectometric/Kilometric
  • Propagation: Surface Wave (Sky waves could interfere at night)
  • Range: 10-500 nm
  • Power Range: 25 Watts - 10 Kilowatts
  • Ident: 2/3 Morse Code Letters

Emission Types

  • NON A1A - Unmodulated (no audio tone)
  • NON A2A - AM Modulated (with audio tone)

NON A2A uses power for modulation, so has lower range than NON A1A

NDB Range Formulas

Land Range (nm) = 2 × √Power(watts) Sea Range (nm) = 3 × √Power(watts)

Power vs Range: To double range, power must increase by 4 times (Power²)

(Range_new / Range_old)² = Power_new / Power_old

Range Factors

  • Determined by power & surface (land/water)
  • NOT affected by aircraft height
  • Increase TX power = Increase range
  • Increase frequency = Decrease range (more attenuation)

Relative Bearing Formula

MH + RB = MB (Magnetic Bearing TO station / QDM) "My Hairy Red Balls Make Babies"

RB: Angle between where aircraft is heading (nose) and where ADF needle is pointing

Maintaining constant RB = Maintaining constant TRACK

Fixed RBI

Relative Bearing Indicator

Shows Relative Bearing only

Requires mental math to obtain QDM

RBI with Moving Card

Shows QDM when current MH is set on display

Manual compass card rotation

RMI

Radio Magnetic Indicator

Always shows QDM (linked to compass)

Needle tip: QDM

Needle tail: QDR

Difference: Homing vs Tracking

  • Homing: Keeping needle on nose (RB = 0°) - results in curved path in wind
  • Tracking: Maintaining desired track by allowing drift correction - straight path

Loop Aerial

Principle: Different orientations give different voltage differentials

  • Maximum differential: When parallel to signal
  • Minimum differential: When perpendicular to signal
  • Loop null error: When plane of loop is at right angles to direction of transmitter

Sense Aerial

Purpose: Required to resolve 180° ambiguity

Two positions can give same voltage differential

Result: Loop (Figure-8) + Sense (Circular) = Cardioid pattern

Cardioid Polar Diagram

Direction of zero signal strength points toward NDB

Important: Due to 0V, ident must be performed regularly to check NDB is still active (lack of failure warning)

Fixed Loop Theory (Modern Systems)

Rotating loop replaced with pair of fixed loops 90° apart

Electromagnetic field set up for direction finding

ANT Switch (REC / OMNI / SENSE)

Function: Sense aerial only is used

Expected: Needle should point to 90°

After deselection: Needle should point to beacon

BFO Selector (Beat Frequency Oscillator)

Primary Use: Ident NON A1A (unmodulated) beacons

NON A2A beacons are AM modulated with audio, so no BFO required for ident

How it works:

  • NDB frequencies outside audio range
  • Appropriate sideband created to hear ident
  • Example: 299 kHz mixed with 300 kHz NDB = 1 kHz beat frequency

Manual Tuning with BFO

Another use: Manual tuning of ADF

Required for both NON A1A and NON A2A

Signal Quality: Even if not required for ident (NON A2A), BFO will always produce higher quality signal as loop aerial is removed - can be used to check interference

Standard Selection Procedure

  1. Check aircraft within NDB stated range
  2. Increase gain
  3. Select frequency
  4. Select ANT to test
  5. Select BFO as required **
  6. Check Ident
  7. Select ADF

** BFO Note: Even if not required for ident (NON A2A), the BFO will always produce higher quality signal. It can be used to check interference.

1. Mutual Interference

NDBs transmitting on same or similar frequency

Solution:

  • Cannot use them inside overlapping area
  • Stick to published range (applicable to day only)

2. Night Effect / Fading / Fluctuations

Most common error for NDB/ADF

Cause: Skywaves from other NDBs by night cause interference

Characteristics:

  • Interference of REQUIRED ground wave with sky wave
  • Sky wave distortion of null
  • Maximum at dusk & dawn
  • Sky wave reflected from ionosphere
  • Fluctuating indications

Minimize by:

  • Listening to BFO (check clean signal)
  • Regular identing

3. Static Interference

Two Types:

A) Precipitation Static

  • Cause: Dust and water droplets rub against aerial creating static
  • Causes PD disruptions
  • Must make physical contact with aerial

B) Thunderstorm Static (Most Significant)

  • Cause: Nearby thunderstorms cause ADF to point toward lightning strikes
  • Just vicinity sufficient (not direct contact)
  • Most significant error

4. Mountain / Multi-Path Effect

Cause: Reflection/refraction of signal in mountainous areas

Results in erroneous bearings

5. Coastal Refraction

Effect: Aircraft RBI points at 270° rather than 220° (example)

When plotted on map, aircraft appears 'closer to coast' than in reality

Minimize by:

  • Flying higher, OR
  • Moving NDB closer to coast

Rule: Higher altitude = Lesser error

6. Quadrantal Effect

Cause: Signal arriving at 45° to aircraft structure bent by metal framework

Modern aircraft: Normally fixed internally, no longer an issue

7. Dip Error

Cause: When in banked condition, PD is distorted

Effect: Needle dips toward lower wing

Magnitude: Approx. 10° in light aircraft (varies by aircraft type)

8. Lack of Failure Warning

Problem: 0V (as used in cardioid) is also present when:

  • NDB is off, OR
  • Aircraft in cone of silence (above NDB)

Cannot distinguish between these conditions and normal operation

Accuracy

  • NDB Accuracy: ±5° (Day)
  • ADF Accuracy: ±6.9°
  • Best accuracy: Flying directly TO/FROM beacon
  • Worst accuracy: Flying abeam beacon

NDB Locators

Definition: Low powered NDB

Usage: Usually installed as supplement to ILS at sites of outer and middle markers

Range: 10-25 nm

Type: En-route type with long range, LF A1A

Different from NDB in: Operations and power

Finding Distance from NDB

Distance (nm) = (Elapsed Time(mins) × Groundspeed) / Change in Radial

Variations Reference

NDB/ADF: Use variation at AIRCRAFT

VOR: Use variation at VOR beacon

VOR VHF Omnidirectional Range

Frequency Allocation

  • 108-112 MHz: VOR & ILS
    • 40 Channels each
    • ILS if first decimal digit is ODD (108.10, 108.15)
  • 112-118 MHz: VOR only
    • 120 Channels
  • Total VOR Channels: 160
  • Channel spacing: 50 kHz

Emission Type & Identification

  • Emission: A9W
  • Polarization: Horizontally Polarized (less noise - atmosphere vertically polarized)
  • Ident Methods:
    • Keyed AM morse code every 10 seconds
    • Voice identification

Terminal VOR (108-112 MHz)

  • Power: Up to 50 watts
  • Range: 25-100 nm
  • Channels: 40
  • Frequencies: EVEN tenths (108.25, 108.45, etc.)

En-route VOR (112-118 MHz)

  • Power: Up to 200 watts
  • Range: 200 nm (Max 300 nm)
  • Channels: 120

Special VOR Types

  • Broadcast VOR:
    • Normally terminal VOR
    • Transmits radial & ATIS
  • Test VOR (VOT):
    • Transmits just 360° radial
    • ±4° requires servicing

CVOR (Conventional VOR)

Signal 1: Reference Signal

  • Omni-directional
  • Transmitted on sub-carrier
  • FM modulated at 30 Hz

Signal 2: Variable/Variphase Signal

  • Directional
  • Transmitted on main carrier wave
  • Appears AM modulated at 30 Hz
  • Rotates at 1800 rpm (30 times/second)

Result: Reference + Variphase = Rotating Limacon

Phase Difference Measurement:

  • Measured when voltage drops on rotating limacon
  • Phase difference = Radial (QDR) from VOR
  • Zero phase difference on 360° radial
  • Does NOT drop to zero like cardioid

DVOR (Doppler VOR)

Signal 1: Reference Signal

  • AM Modulated
  • Transmitted on main carrier

Signal 2: Variable Signal

  • FM Modulated
  • Transmitted on sub-carrier
  • Transmitted on 50 different aerials in turn around reference signal

Doppler shift used to calculate phase difference

DVOR vs CVOR Comparison

  • CVOR: Large aerial required to reduce site error
    • Impractical to rotate large aerial
  • DVOR: Much more accurate, very little site error
  • Uses frequency modulation (FM)

Key Concept: RADIAL

RADIAL: Magnetic bearing of aircraft FROM station (QDR)

Phase difference = Radial from VOR

RMI Display

  • Needle TAIL: QDR (Radial FROM)
  • Needle HEAD: Points to QDM (TO station)
  • Always shows QDM (linked to compass)

RMI Note: Shows bearing, not displacement

CDI/OBI (Course Deviation Indicator)

Components:

  • OBS: Omni Bearing Selector (rotatable)
  • CDI: Course Deviation Indicator (needle)
  • TO/FROM indicator: Ambiguity indicator

CDI Note: Shows displacement, not bearing

Important Display Differences

  • Questions on CDI/OBS: Disregard magnetic heading
  • Questions on HSI: Take into account magnetic heading
  • RMI: Compass stuck & RMI working = RB unavailable, radial available
  • RMI: RMI stuck & ADF working = RB still available

TO/FROM Indicator

Tells whether track equal to selected bearing will bring you TO or AWAY FROM VOR

Not a simple indication of current position

Scale Deflection - CDI

  • Each dot: 2° displacement
  • Full scale deflection: 10°
  • Outside of centre circle (if present) counts as one dot

1 in 60 Rule: Angle = (Distance off track × 60) / Distance along track

Radial & Distance Combinations

  • Same radial, different distance: Reference & variable both unequal
  • Different radial, same distance: Reference equal, variable unequal

Overall VOR Accuracy

  • ±5° 95% of the time
  • Worst case: ±7.5°
  • Quoted accuracy applied at all times

VOR Errors

  • Site Error: Obstacles near transmitter cause reflections
    • Limacon pattern distorted
    • Amplitude doesn't rise/fall in predicted manner
    • Reduced by Doppler VOR
  • Propagation Error:
    • Irregular terrain causing oscillations
    • Slow oscillations = Bends
    • Rapid oscillations = Scalloping
    • Scallops cannot be followed
    • Uneven propagation over irregular ground surfaces
  • Airborne Equipment Error
  • Interference Error: DOC / Below LOS
    • Irregular terrain causing oscillations

Cone of Confusion

Location: Overhead VOR

Effects:

  • Flickering of ambiguity indicator (TO/FROM flag)
  • Possible failure flag (often prevented in modern systems)

Using VOR Outside Published Range

May cause interference from other transmitters

VOR Monitor & Shutdown

VOR switches off when:

  • Measured error greater than 1°
  • Signal strength drops by 15% or more
  • VOR monitor fails

Failure Indications

  • VOR Failure: NAV flag appears
  • VOR failure on display: CRS, deviation bar & pointer removed from display

Distance Between VORs

Distance required to ensure no conflict = Range × 2

VOR Distance - Airways Formula

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

Example: If VOR accuracy ±7.5°, what's max distance VORs can be apart assuming airway 10 nm wide?

7.5 = (5 / Distance Gone) × 60 Distance Gone = 40 nm (Midway Point) ⇒ Max Distance = 80 nm

Great Circle Tracking

Flying along a VOR radial, you will be following a great circle track

VOR vs DME Accuracy

DME is more accurate than VOR except when directly overhead the beacon

Variation Reference

  • VOR: Use variation at VOR beacon
  • NDB/ADF: Use variation at AIRCRAFT

DME Distance Measuring Equipment

Primary Characteristics

  • Polarization: Vertically Polarized
  • Modulation: PON (Pulse modulation)
  • Frequency: 962-1213 MHz (UHF)
  • Wavelength: Decimetric
  • Total Channels: 256
    • 126 X Channels (12 µs pulse spacing)
    • 126 Y Channels (36 µs pulse spacing)
  • Typical Range: 200 nm
  • Terminal VOR/DME: 108-112 MHz (paired frequency)

Frequencies - Pairing

  • Interrogator (in aircraft): Transmits on DME frequency (962-1213 MHz)
  • Ground station: Replies with frequency ±63 MHz different
  • Transmit & receive on two different frequencies to prevent self-triggering

Ghost Frequency

Civil pilots cannot tune UHF directly

Solution: Ghost VHF frequency displayed on charts

This is frequency paired to correct UHF frequency

1. Jittered PRF (Pulse Repetition Frequency)

Interrogator transmits: Series of pulses in pairs

  • Interval between two pulses in pair: Kept constant
  • Interval between pairs: Randomly generated
  • Purpose: Prevents fruiting (picking up wrong replies)

Irregular transmission sequence determines from which aircraft pulse pairs are received

Time between pulse pairs is at random

2. Ground Station Reply

On ±63 MHz frequency, ground station sends back any pulses received after delay of 50 µs

EPC (Equipment Protection Coding): 50 µs delay to protect receiver from reflected pulses, prevent self-triggering

3. Receiver Processing

Receiver on aircraft tuned to ±63 MHz frequency

Process:

  • Picks up all replies from ground station
  • Searches for its unique interval pattern
  • Once match found, computes range

Range Calculation Formula

Range (nm) = (Time Elapsed - Delay) / 12.36 (Radar Mile) where: 12.36 µs = Radar Mile Time for transmission to travel 1 nm out and 1 nm back

Principle of Operation

Based on: Time measurement between transmission & reception of radio pulses

Transponder: In aircraft

Interrogator: On ground

Search Mode

When: Before 'lock-on' has occurred

PRF: 150 PPS (pulse pairs per second)

Process: Searches out to max range to identify presence of DME ground station

Timeout: If 15,000 pulses sent and no lock-on, reduces to 60 PPS

Note: No lock-on during search mode

Tracking Mode

When: After lock-on has occurred

PRF: 24-30 PPS

Purpose: Reduce load on ground station

Memory Mode

When: Signal drops out / interrupted

Duration: 10-15 seconds

Function: Range calculated based on last trend info

After timeout: Re-enters search mode

More PPS During Search Than Tracking

This loading consideration affects ground station capacity

Beacon Saturation Limits

DME ground equipment normally limited to 2700 pulse pairs per second (PPS)

Mode PPS per Aircraft Max Aircraft
All on Search 150 18 aircraft (2700/150)
All on Tracking ~24 120 aircraft
Mixed (Typical) Varies ~100 aircraft average

Oversaturation Response

When more than 100 aircraft interrogate:

Receiver gain is reduced

Result: DME adjusts to strongest signals (closer aircraft have higher priority)

Ground station only listens to closer aircraft

Co-Located VOR/DME Idents

Co-Located when within:

  • 2000 ft (En-Route)
  • 100 ft (Terminal)

Ident characteristics:

  • VOR & DME idents will be same code
  • VOR: Idents every 10 seconds
  • DME: Idents every 30-40 seconds
  • DME ident frequency: 1350 Hz (higher pitch) = 2700/2
  • In 40s, DME ident sounds once at higher pitch

Same Location (Not Co-Located) Idents

When in same location but too far apart to be 'co-located'

Last letter of DME ident changed to Z

DME Accuracy

  • DME/P (Precision): ±0.2 NM
  • DME/N (Standard): 0.25 NM + (1.25% × Distance)
Old Beacon Accuracy = ±[0.25 nm + (1.25/100) × Distance]

Comparison: DME is more accurate than VOR except when directly overhead beacon

Accuracy vs Range Relationship

DME accuracy DECREASES with increase in range

BUT: Groundspeed accuracy INCREASES with increase in range

Reason: Difference between ground range and slant range gets bigger at longer ranges

Groundspeed LESS accurate near DME

Best/Worst Accuracy Conditions

  • Best accuracy: Flying directly TO/FROM beacon
  • Worst accuracy: Flying abeam beacon

Slant Range vs Plan Range

At long ranges: Slant range ≈ Plan range

When closer than: 3 × Height, significant difference

LOP (Line of Position): Ground distance on chart with DME at centre

Groundspeed & Time Behavior

Decrease in groundspeed readout can be expected when nearing beacon at constant height

Reason: Change in slant distance decreases closer to beacon

On ILS: Following constant slant, so this is NOT a factor