ATPL · illustrated course

ATPL radio navigation course: 14 illustrated chapters

The ATPL radio navigation course follows the syllabus in 14 chapters: 133 key points explained in a few paragraphs, often with a diagram, what to remember and, for many, the classic exam trap. The first chapter, « Radio waves, modulation and antennas », can be read in full on this page.

Read chapter 1 The 883 questions of this subject

Each chapter explains, point by point, what the questions of the bank ask, with a « Remember » box and often the classic exam trap. The first chapter is free, below and in the app; the next ones are for ATPL subscribers.

1.Radio waves, modulation and antennas
Free
Nature of the wave, frequency and wavelength, bands, modulations and emission classes, antennas and siting.
8 key points · 48 questions of the bank
2.Wave propagation
Subscribers
Ground, space and sky waves, ionosphere, skip zone, fading, line-of-sight range, attenuation and the Doppler effect.
9 key points · 45 questions of the bank
3.Direction finding: VDF, NDB and ADF
Subscribers
The ground direction finder (VDF), the NDB beacon, the airborne ADF, its indicators, its use and its errors.
14 key points · 127 questions of the bank
4.The VOR
Subscribers
Frequencies, principle of the conventional VOR and the Doppler VOR, radial and variation, cone of confusion, range, DOC, accuracy and identification.
9 key points · 76 questions of the bank
5.VOR indicators: CDI, HSI and RMI
Subscribers
Reading the CDI and the TO/FROM indicator, method for solving exercises, HSI, RMI and failures, EFIS presentation.
11 key points · 66 questions of the bank
6.DME
Subscribers
Secondary radar principle, frequencies and pairing, operating modes, saturation, slant range, accuracy, groundspeed and position fixing.
8 key points · 80 questions of the bank
7.ILS and MLS
Subscribers
Localizer, glide path, marker beacons, indicators, categories, rate of descent and height calculations, operations, critical areas, then MLS.
14 key points · 148 questions of the bank
8.Primary radar and ground radars
Subscribers
Pulse principle, PRF and maximum unambiguous range, resolution, frequencies, ATC radars and radar approaches.
6 key points · 60 questions of the bank
9.Airborne weather radar
Subscribers
Principle, beam and antenna, colours, tilt setting, range scale, shadow areas, ground mapping and ground precautions.
9 key points · 61 questions of the bank
10.Secondary radar (SSR) and the transponder
Subscribers
Interrogation and reply, modes A, C and S, codes, IDENT, shortcomings of classic SSR and ADS-B.
8 key points · 55 questions of the bank
11.GNSS: GPS and the other constellations
Subscribers
Segments, GPS constellation, signals and codes, pseudo-range, position, navigation message, receivers, WGS 84, errors, DOP, GLONASS and Galileo.
12 key points · 137 questions of the bank
12.GNSS Integrity and Augmentation
Subscribers
ABAS and RAIM, RAIM prediction and loss, SBAS and EGNOS, GBAS and GLS approaches, differential GPS.
6 key points · 55 questions of the bank
13.Area Navigation: RNAV and FMS
Subscribers
Definition of RNAV, sensors and hybrid systems, VOR/DME RNAV and phantom stations, FMS and databases, initialisation, position calculation, functions, colours and EFIS modes.
9 key points · 109 questions of the bank
14.Performance-Based Navigation (PBN)
Subscribers
PBN concept, RNAV and RNP, errors and TSE, specifications, oceanic operations, path terminators, turns, alerts, RNP APCH, RNP AR and A-RNP approaches.
10 key points · 45 questions of the bank

Chapter 1: Radio waves, modulation and antennas

Nature of the wave, frequency and wavelength, bands, modulations and emission classes, antennas and siting.

The electromagnetic wave

← Faites glisser le schéma →

E and H fields of a vertically polarised wave

A radio wave combines two inseparable fields: an electric field E and a magnetic field H, perpendicular to each other and to the direction of propagation, and varying in phase. In a vacuum (and practically in air) it travels at the speed of light, c = 300,000 km/s.

Polarisation is the orientation of the E field. The E field is parallel to the radiating antenna: a vertical antenna emits a vertically polarised wave (E vertical, H horizontal), a horizontal antenna a horizontally polarised wave.

A receiving antenna picks up best a wave whose E field is parallel to it: a wave whose magnetic component is horizontal is received on a vertical antenna.

Remember

Polarisation = direction of E = direction of the transmitting antenna. E ⟂ H ⟂ propagation.

Exam trap

“Horizontally polarised” means that the E field is horizontal (H is then vertical). An NDB wave is vertically polarised: its magnetic component propagates in the horizontal plane.

Frequency, period and wavelength

← Faites glisser le schéma →

Wavelength, amplitude and common conversions

The frequency F is the number of complete cycles per second (in hertz). The period T = 1/F is the duration of one cycle. The wavelength λ is the distance travelled during one cycle, from one crest to the next:

λ = c / F

With c = 3·10⁸ m/s, the practical form is λ (m) = 300 / F (MHz). Keep units consistent: 300 kHz = 0.3 MHz gives 1,000 m; 9,375 MHz gives 3.2 cm.

The amplitude is the maximum value reached by the wave; it does not depend on frequency.

Remember

λ (m) = 300 / F (MHz); F (MHz) = 300 / λ (m). 1 GHz = 1,000 MHz; 1 MHz = 1,000 kHz.

Exam trap

Common unit errors: 1 cm → 30 GHz (not 3 GHz); 2.22 cm → 13,500 MHz (band of Doppler navigation radars).

Frequency bands

← Faites glisser le schéma →

Frequency bands, wave names and associated radio aids

The radio spectrum is divided into bands by factors of 10. Each band has a name by frequency (LF, MF, VHF…) and a name by wavelength (kilometric, metric…).

LF/MF: NDBs and locators (190 to 1,750 kHz).
HF: long-distance communications by ionospheric reflection.
VHF (30–300 MHz, metric waves): markers, localizer, VOR, VHF radio, VDF.
UHF (300–3,000 MHz, decimetric waves): glide path, DME, transponder, GNSS.
SHF (3–30 GHz, centimetric waves): radio altimeter, MLS, weather radar, aerodrome radars.

Radars also use sub-bands: L (≈ 1.3 GHz, en-route radars, GNSS), C (≈ 5 GHz) and X (≈ 9.4 GHz, airborne weather radar).

Remember

VHF = 30–300 MHz = 10 m to 1 m (metric). UHF = 300–3,000 MHz (decimetric). SHF = 3–30 GHz (centimetric).

Exam trap

The ILS glide path is in UHF (decimetric) whereas the localizer and the markers are in VHF (metric). GNSS are classed in UHF (L band).

Modulation

← Faites glisser le schéma →

Carrier, amplitude, frequency and phase modulation

The carrier (high frequency) carries energy well but contains no information. The modulator superimposes the useful low-frequency signal (audio, data) on it.

Amplitude modulation (AM): the information is in the variation of amplitude. Simple, but sensitive to interference (thunderstorms, industrial). To save spectrum only one sideband is transmitted: SSB, used in HF and for VOLMET.
Frequency modulation (FM): constant amplitude, the frequency varies; insensitive to interference.
Phase modulation: phase shifts, suited to data, used by GPS.
Pulse modulation: radar, DME, transponder.

A receiver with a reduced bandwidth receives less noise.

Remember

Classes: A = amplitude, F = frequency, P = pulses. AM is sensitive to interference, FM is not.

Emission classes

← Faites glisser le schéma →

Reading an emission class and aeronautical examples

The ITU describes an emission by three symbols: the type of modulation of the carrier, the nature of the modulating signal, the type of information.

N0N: pure carrier. This is what the ADF uses to measure the bearing of an NDB.
A1A: the carrier is keyed at the rhythm of Morse code. After demodulation no sound is left: the BFO is needed to hear the identifier.
A2A: the Morse is carried by an audible tone; no BFO needed.
A3E: amplitude-modulated telephony, that of VHF COM radios.
J3E: single-sideband telephony (HF).
A8W: localizer emission (several 90/150 Hz channels plus the identifier).

Remember

Typical long-range NDB: LF, N0N A1A (e.g. 200 kHz). A3E = VHF COM.

The dipole and radiation

← Faites glisser le schéma →

Vertical half-wave dipole and its radiation patterns

An antenna converts an alternating current into a wave (and vice versa: it is reciprocal). To radiate efficiently, its length must be in a simple relationship with the wavelength.

The basic antenna is the half-wave dipole: L = λ/2. At 18 MHz (λ = 16.7 m) it measures 8.33 m; at 110 MHz (λ = 2.73 m) it measures 1.36 m.

A vertical dipole or whip is omnidirectional in the horizontal plane, but radiates nothing along its own axis. The radiated E field is parallel to the conductor.

Remember

Half-wave dipole: L = λ/2 = 150 / F (MHz) in metres.

Exam trap

Do not confuse λ and λ/2: a question asking for “the length of the half-wave dipole” expects half the wavelength.

Directional antennas and gain

← Faites glisser le schéma →

In-phase reflection on a parabolic dish and beamwidth formula

A directional antenna concentrates the energy in a beam: for the same power, it reaches further in the chosen direction and picks up weak signals better. This gain is used to compensate for path attenuation.

Parabolic dish: all paths focus → dish → aperture plane have the same length, so the waves leave in phase and add up. The beamwidth is about 70 λ / D: it depends on the frequency and the diameter. For a narrow beam, D must be large compared with λ.
Flat slotted-array antenna (modern radars): narrower beam, fewer side lobes, hence more energy in the main lobe.

A primary radar uses a single directional antenna for both transmission and reception.

Remember

Beamwidth (°) ≈ 70 × λ / D. For the same λ, an antenna twice as large = a beam twice as narrow.

Other antennas and siting

← Faites glisser le schéma →

Typical antenna locations on a transport aircraft

Loop: sensitive to the magnetic component, figure-of-eight pattern; this is the ADF antenna.
Helix: circularly polarised wave, on GPS satellites. GNSS receivers use a small flat (patch) antenna on top of the fuselage, near the centre of gravity.
Antenna-to-equipment link: coaxial cable up to 3,000 MHz, waveguide above (radars).

Location matters: shielding by the wing in a turn (small UHF antennas), multipath, influence of the landing gear on nose antennas. The standby VHF COM antenna is on top so that it remains usable after a gear-up landing or a ditching.

Remember

Coaxial ≤ 3 GHz, waveguide above. Loop = H component, figure-of-eight. Helix = circular polarisation (GPS satellites).

The course goes on with chapter 2, « Wave propagation ». It is read in the Course tab of the app with a ATPL subscription, and each chapter ends with the questions of the bank about it. See pricing

How ForAllPilots trains you

Series of 20 or 30 questions
New questions, mistakes to review or a mix of both, with the answer and its explanation straight away.
Flashcards
Question on the front, explained answer on the back, filtered by subject, to learn without a timer.
Timed mock exam
100 questions in 150 minutes, drawn from the whole ATPL bank: a training format, not a copy of an official paper.
AI instructor, optional
From €4.99 a month on top of the Private pilot or PRO offer: it quizzes you in writing and explains your mistakes. A beta tool, it does not replace an approved instructor.

ATPL access comes with the PRO offer (which also includes Private pilot): €19.99 a month, cancel anytime, or €129 once with the Exam Pass (18 months of access, nothing renews). Prices include all taxes.

See ATPL pricing Create an account

The other ATPL courses

All about the ATPL exams: format, subjects and method

Contents

Chapter 1: Radio waves, modulation and antennas

The electromagnetic wave

Frequency, period and wavelength

Frequency bands

Modulation

Emission classes

The dipole and radiation

Directional antennas and gain

Other antennas and siting

How ForAllPilots trains you

The other ATPL courses