Chapter 10: Extra: Radio Communication, ELT and ADS-B

Radio waves

An alternating current in an antenna sets up changing electric and magnetic fields that detach and travel outwards as an electromagnetic wave. The electric (E) and magnetic (H) fields are at right angles to each other and to the direction of travel, and the wave moves at the speed of light, c ≈ 300 000 km/s. Frequency and wavelength are linked by λ = c ÷ f. For example, 121.5 MHz has a wavelength of 2.47 m, 406 MHz 0.74 m and 1 090 MHz about 0.28 m.

Types of radio waves — how they travel

• Ground (surface) waves follow the curvature of the Earth. They suit LF and MF — non-directional beacons and ADF — and reach hundreds of kilometres, further over sea than land.
• Sky waves are refracted back to Earth by the ionosphere (the D, E and F layers), mainly at HF, giving ranges of thousands of kilometres. Between the point where the ground wave dies out and the point where the first sky wave returns lies the skip zone, where nothing is received. The best frequency changes with time of day and solar activity, so HF operators use higher frequencies by day and lower by night.
• Space (direct) waves — VHF and above pass through the ionosphere, so they work only by line of sight, and range depends on height: roughly 1.23 × √(height in feet) nautical miles to a ground station at sea level. An aircraft at 10 000 ft can work a station about 120 nm away. Satellites use these frequencies precisely because they get through the ionosphere.

Loading information onto a radio wave

A high-frequency carrier travels well, but carries nothing until it is modulated.

• Amplitude modulation (AM) varies the carrier's amplitude with the audio. It produces a carrier plus upper and lower sidebands, and the bandwidth is twice the highest audio frequency. It is simple, but noise and static add directly to the signal. Aviation VHF voice uses AM (118–137 MHz) — one reason is that two stations transmitting at once are both heard rather than one silently blocking the other.
• Frequency modulation (FM) varies the carrier's frequency with the audio and keeps the amplitude constant. It is far less affected by noise, but needs more bandwidth.
• Single sideband (SSB) is AM with the carrier and one sideband removed. It uses half the bandwidth and puts all the transmitter's power into information, giving more range for the same power. Aircraft HF radios use SSB, normally the upper sideband (USB). The receiver re-inserts a carrier to recover the audio.
• Data links key the carrier digitally — for example ACARS and controller-pilot data link messages, and the pulse-position modulation of 1090 MHz ADS-B.

Transmitters, receivers and transceivers

• A transmitter has an oscillator (crystal-controlled or a frequency synthesiser) to generate a stable carrier, buffer and multiplier stages to reach the working frequency, a modulator fed by the microphone amplifier, and a power amplifier feeding the antenna through a matching network. It transmits only while the push-to-talk is pressed.
• Almost every receiver is a superheterodyne. An RF amplifier selects and boosts the wanted signal. A mixer combines it with a local oscillator to produce a fixed intermediate frequency (IF), where most of the gain and selectivity are provided. A detector recovers the audio, and an AF amplifier drives the headsets. Automatic gain control (AGC) holds the output steady for weak and strong signals, and squelch mutes the hiss when no signal is present. A receiver is judged by its sensitivity (the weakest usable signal) and selectivity (rejection of neighbouring channels).
• A transceiver combines transmitter and receiver in one unit sharing the synthesiser, power supply and antenna. A transmit/receive switch connects the antenna to the transmitter only while transmitting.

Antennas

• Length — an antenna radiates efficiently when it is resonant, with its length set by the wavelength. A half-wave dipole is two quarter-wave elements fed at the centre (about 1.2 m at 125 MHz). A Marconi (monopole) antenna is a quarter-wave vertical element; the conducting ground plane — on an aircraft, the skin — supplies the other half as an "image". That is why aircraft VHF blade antennas are only about 0.6 m long and must be well bonded to the structure.
• Polarisation is the orientation of the electric field, which follows the element. VHF communication, DME and transponder antennas are vertically polarised; VOR and localiser antennas are horizontally polarised.
• Directivity and field pattern — a radiation pattern shows where an antenna sends (or hears) best. A vertical monopole is omnidirectional in the horizontal plane. A loop has a figure-of-eight pattern with two sharp nulls, which the ADF uses for direction finding (a sense antenna removes the 180° ambiguity). Dishes and arrays are highly directional.
• Types on aircraft include blades (VHF, transponder, DME), whips, HF wire, probe or notch antennas, flat patch antennas (GNSS, satellite communications), loops (ADF), horns and slotted arrays (radar).
• Installation: the location the manufacturer specifies, clear of shadowing by other antennas; a doubler on the structure; a low-resistance bond to the skin; and sealing against moisture.

Transmission lines

• Radio frequency travels between radio and antenna in coaxial cable: a centre conductor, a dielectric (such as PTFE or foam), a braided shield and an outer jacket. The shield keeps the field inside and interference out.
• Each cable has a characteristic impedance (Z₀), set by its dimensions and dielectric — 50 Ω for most aircraft RF (75 Ω for video). Radio, cable and antenna must be matched. Otherwise power reflects back, forming standing waves measured as VSWR: 1:1 is perfect, and readings much above about 2:1 point to a fault.
• Kinks, crushing, a too-tight bend radius, a badly assembled connector (BNC, TNC, N, SMA) or water in a connector all change the impedance. VSWR meters and time-domain reflectometers find such faults.

ADS-B

Automatic Dependent Surveillance–Broadcast is automatic (no crew or controller action), dependent on the aircraft's own navigation source — normally GNSS — and a broadcast to anyone listening, about twice a second.

• ADS-B Out broadcasts the aircraft's 24-bit ICAO address, flight identification, GNSS position, pressure altitude and velocity, with accuracy and integrity indicators and any emergency status.
• It uses one of two links. 1090 MHz extended squitter (1090 ES), sent by the Mode S transponder, is the worldwide standard. UAT on 978 MHz is used only in the USA, below 18 000 ft.
• ADS-B In receives other aircraft's broadcasts for cockpit traffic displays and, in the USA, ground-sent traffic and weather services.
• Ground stations — and satellite receivers covering the oceans — feed ATC with positions that are more accurate and more frequent than radar. ADS-B Out is mandatory in much of the world's airspace, for example in the USA since 1 January 2020 and in Europe for aircraft above 5 700 kg or faster than 250 kt.
• Maintenance: the transponder must hold the correct 24-bit address, flight ID and configuration, and be fed by the correct position source. Ramp test sets check the broadcast, and authorities monitor ADS-B performance and report faulty installations.

Emergency locator transmitters (ELT)

• 406 MHz: a digital burst of about 5 W roughly every 50 seconds, coded with the beacon's unique identity (and its GNSS position if fitted). It is detected and located by the Cospas-Sarsat satellites — low-earth-orbit, geostationary and medium-earth-orbit systems — and relayed through local user terminals and mission control centres to a rescue co-ordination centre. Beacons must be registered so the alert can be traced to the aircraft and owner.
• 121.5 MHz: a low-power swept tone that search-and-rescue teams home on. Satellites stopped processing 121.5/243 MHz signals on 1 February 2009, so a 406 MHz ELT is needed for satellite alerting.
• Types: ELT(AF) automatic fixed; ELT(AP) automatic portable (removable for survivors' use); ELT(AD) automatically deployable (ejected on impact, and it floats); ELT(S) survival, manually activated and carried in rafts. The newest large aircraft also carry distress-tracking ELTs, which activate in flight when a distress condition is detected.
• Activation: automatically by an inertia (g) switch on impact, or manually by the switch on the unit or the cockpit remote panel (ON / ARM / RESET-TEST).
• Installation: fixed ELTs are mounted to structure as far aft as practical, with the unit's arrow pointing forward so the g-switch senses a crash along the flight path. They have a short coaxial run to an external antenna.
• Battery and testing (FAA 14 CFR 91.207 as an example): replace the battery after more than 1 cumulative hour of use or when 50% of its useful life has passed (the expiry date is marked on the unit), and inspect the ELT every 12 calendar months. Use the built-in self-test. FAA guidance limits live 121.5 MHz tests to the first 5 minutes after the hour, no more than three sweeps. After a hard landing, listen on 121.5 MHz for an accidental activation, and report any false alert to ATC at once.