Part 1 – Aircraft Radio Operation


To gain a basic understanding of the requirements and use of aircraft radio and its operation in the field of aviation.


At the end of this briefing you will be able to:

  • State the authorisation granted by, and operating condition associated with, the aeronautical radio operator certificate
  • Make a properly formatted broadcast appropriate to RPAS operations

Aircraft Radio

The Function of Aircraft Radio

When using in the context of aviation, it is sometimes called “aircraft radio” or “air-band radio“.

The use of a radio does not guarantee that all your communication problems will be resolved. A radio is a tool that we can choose to use, but we need to understand it’s limitations!

In electing to use a radio we assume:

  • That the other aircraft/person actually has an appropriate radio.
  • That all radios are working as expected.
  • That the other radio is turned on.
  • That the other radio is tuned to the same frequency.
  • That the other radio has been set to a volume that can be heard.
  • That the person monitoring the radio is actually listening to the radio.
  • That what you intended the message to mean is what the other person understood the message to mean!

All of these things things to all be in place at the same time!

Definite “Show-Stopper” or Combination of Errors?

If the radio is turned off, or the ‘wrong’ frequency selected – this is a show-stopper!

More subtly, although it seems like the correct action is being taken, a combination of events may lead to disaster!

“Swiss Cheese” model (source: James Reason’s “Managing the Risks of Organizational Accidents”).

Aircraft Radio as a Threat & Error Countermeasure

Radio creates an opportunity for you to:

  • Develop & maintain your situational awareness
  • Identify and (possibly) manage threats
  • Possibly communicate with potential threats and de-conflict

Aviation Task Prioritisation

Task prioritisation, also referred to as “Applied Risk Management”, consists of four primary tasks:

  • Aviate
  • Navigate
  • Communicate
  • Administrate


The Phonetic Alphabet

Annunciation of the Phonetic Alphabet.


All numbers shall be transmitted by pronouncing each digit separately (e.g. 10 is WUN ZE-RO, 236 is TOO THREE SIX), except for:

  • Whole hundreds
    • 500 is FIFE HUN-dred
  • Whole thousands
    • 7,000 is SEV-en TOU-SAND
  • Combinations of thousands and whole hundreds
    • 7,500 is SEV-en TOU-SAND FIFE  HUN-dred

Note: The meteorological way of expressing cloud cover is in eighths of the sky covered:

  • Eighths in radio transmissions is expressed as “okta’s“.
  • A little over half the sky covered, say 5/8, would be expressed as:
    • FIFE-oktas
900NIN-er HUN-dred (whole hundreds only)
3,000TREE TOU-SAND (whole thouands only)
12,700WUN TOO TOU-SAND SEV-en HUN-dred
Annunciation of the numbers.

Standard Words & Phrases

Word/Phrase  Meaning

  • ACKNOWLEDGE: Let me know that you have received and understood this message.
  • AFFIRM: Yes.
  • APPROVED: Permission for proposed action granted.
  • BREAK: I hereby indicate the separation between portions of the message.
    • To be used where there is no clear distinction between the text and other portions of the message.
  • BREAK BREAK: I hereby indicate separation between messages transmitted to different aircraft in a very busy environment.
  • CANCEL: Annul the previously transmitted clearance.
  • CHECK: Examine a system or procedure.
    • No answer is normally expected.
  • CLEARED: Authorised to proceed under the conditions specified.
  • CONFIRM: Used in the context of “Have I correctly received the following … ?” or “Did you correctly receive this message?”.
  • CONTACT: Establish radio contact with.
  • CORRECT: That is correct.
  • CORRECTION: An error has been made or the message indicated the wrong information in this transmission, followed by the correct version “[wrong information] CORRECTION [correct information…]”.
  • DISREGARD: Consider that transmission as not sent.
  • HOW DO YOU: What is the readability of my transmission?
  • I SAY AGAIN: I repeat for clarity or emphasis.
  • MONITOR: Listen out on: [frequency].
  • NEGATIVE: “No”, “Permission is not granted”, or “That is not correct”.
  • OVER: My transmission is ended and I expect a response from you
    • Not normally used in VHF communication
  • OUT: My transmission is ended and I expect no response from you
    • Not normally used in VHF communication
  • READ: The readability is:Unreadable
    • Readable now and then
    • Readable but with difficulty
    • Readable
    • Perfectly readable
  • READ BACK: Repeat all, or the specified part a’ this message back to me exactly as received
  • RE-CLEARED: A change has been made to your last clearance and this new clearance supersedes your previous clearance or part thereof.
  • REPORT: Pass ore the following information
  • REQUEST: I should like! to know or I wish to obtain
  • ROGER: I have received all of your last transmission, [Under NO circumstances to be used in reply to a question requiring read back or a direct answer in the affirmative or negative]
  • SAY AGAIN: Repeat all or the following part at your last transmission
  • SPEAK SLOWER: Reduce your rate of speech.
  • STANDBY: Wait and I will call you.
  • VERIFY: Check and confirm with originator.
  • WILCO: I understand your message and will comply with it.
  • WORDS TWICE: Can mean:
    • As a request: Communication is difficult: “Please send every word or group of words twice.”
    • As information: Since communication is difficult, every word or group of words in this message will be sent twice.

Radio Certification & Standard Calls

Aeronautical Radio Operator Certificate

Aeronautical Radio Operator Certificate (AROC) is specifically the Authorisation & Condition (CASR 64.035).

Why do we need it and what does an AROC allow us to do?

  • It will allow us to transmit on a frequency of a kind used for the purpose of ensuring the safety of air navigation.
  • You may only make a transmission if you are the holder of a current a current aviation English language proficiency assessment!

Operator Conduct

Radiocommunications Act 1992
International Radio Regulations (published by International Telecommunications Union)
Maintain secrecy:
Must preserve secrecy of communications to which the operator may become acquainted
Unauthorised transmissions:
– Interference
– False of deceptive messages
– Bad language
– Unnecessary conversations
– Harassing, alarming or affronting behaviour
– Using radio as an explosives trigger
Summary of the rosponsibilities imposed by the Radiocommunications Act 1992.

Brevity and Clarity

Remember: when you make a transmission you are saying something for someone else to hear, and hopefully understand what you meant!

Some points (and common issues):

  • Speak clearly.
  • Position the microphone (know your equipment).
  • Remember transmitter latency (press, pause-2-3, speak).
  • Keep a constant volume – no need to yell (or swallow the mic).
  • Keep an even rate of speech – in the excitement don’t speak quickly.
  • Choose your words. Send enough to communicate, but do not flood the airwaves!

Paraphrasing George Bernard Shaw: The greatest fallacy with communication is that there was!

Declaring an Emergency

There are three types of emergency, and the pilot must preface his call with the appropriate words:

  • MAYDAY: (repeated three times) for a distress call.
  • PAN-PAN: (repeated three times for an urgency call.
  • SECURITY: (repeated three times) for a safety call.

Distress Call (MAYDAY)

The MAYDAY call is derived from m’aidez” “help me”.

This is the absolute top priority call. It has priority over all others, and the word mayday should force everyone else into immediate radio silence.

Used when you require immediate assistance, and are in grave and immediate danger.

Any aircraft making an emergency radio call is to be given priority over all other aircraft/ground operations.

If a distress call is received radio operators may initiate or impose radio silence on all other stations.

This will be heard as:

  • “STOP transmitting; MAYDAY”
  • “ALL stations; STOP transmitting; MAYDAY”

This process ensures that all communications between the aircraft in distress are received by radio operators and emergency response teams clearly & accurately.

Construction of a MAYDAY Call

The distress message should contain as much of the following information as possible, and if possible in the sequence below:

  • [Aircraft callsign] [Aircraft callsign] [Aircraft callsign]
  • [Position and time]
  • [Heading]
  • [Airspeed]
  • [Altitude]
  • [Aircraft type]
  • [Nature of distress]
  • [Captain’s intentions]
  • [Any other information that may facilitate the rescue]

Urgency Call (PAN-PAN)

Panne (pronounced: “pan”) is a breakdown, such as a mechanical failure.

Used when and emergency exists but does not require immediate assistance.

Typical Situations include:

  • If you are experiencing navigational difficulties and require the urgent assistance of traffic services.
  • If you have a passenger on board who is seriously ill and required urgent attention.
  • If you see another airplane or ship the safety of which is threatened and urgent action is perhaps needed.
  • If you are making an emergency change of level in controlled airspace and you may conflict with traffic below.

PAN – A Three-Letter Acronym

As a Three-Letter Acronym (TLA):

  • “Possible assistance needed”
  • “Pay attention now”

The TLA is derived from “pan” and is used in maritime and aeronautical radio communications courses as a mnemonic to radio and communications operators.

Note: it is very important remember the difference between mayday and pan-pan emergency communications.

Construction of a PAN-PAN Call

For an urgency call, the pilot should transmit:

  • [Callsign of a specific station or ‘all stations’]
  • [Aircraft callsign]
  • [Request for bearing, course or position, if required]
  • [Position and time]
  • [Heading]
  • [Airspeed]
  • [Altitude]
  • [Aircraft type]
  • [Available flight time]
  • [Nature of emergency]
  • [Captain’s intentions]

Security Call (SECURITY)

194 specifies that a message known as a safety signal shall be transmitted by an aircraft when considered necessary to advise the existence of a hazard to air navigation or hazardous meteorological conditions.

The specific circumstances for this call:

  • The safety signal shall be transmitted when an aircraft wishes to transmit a message concerning the safety of navigation or to give important meteorological warnings.
  • The safety signal shall be sent before the call and in the case of radiotelegraphy:
    • Shall consist of 3 repetitions of the group TTT (- – -), sent with the letters of each group and the successive groups clearly separated from each other.
    • Shall consist of the word “SECURITY”, repeated 3 times.

The “SECURITY” call may be used when there has been a breach in the security of the aircraft or its crew during flight.

An example of this would be an aggressive or agitated passenger on board who is interfering with the pilot.

Construction of a SECURITY Call

There are very few occasions when it would be necessary to transmit a security call, nevertheless you are required to know of the existence of this type of message in case it should ever become necessary to send one – or should one be received.

  • Pilot:
    • Melbourne Centre
    • Alfa November Kilo
    • One two five decimal nine
    • Severe turbulence and windshear experienced in the easter-lee of Wilson’s Promontory up to three thousand feet
  • Melbourne ATS:
    • Alfa November Kilo
    • Melbourne Centre
    • Copied

Radio Silence

An aircraft in distress or the appropriate ground station may impose radio silence on all other stations in the area or on any particular station causing interference, asking them to stop transmitting.

For example:

  • All stations
  • Stop transmitting

Types of Radio Calls

There are three main types of radio transmissions:

  1. Report:
    • A report is generally made to a specific air traffic services unit.
    • A response is expected.
  2. Broadcast:
    • A broadcast is usually made in the form of a traffic advisory.
    • May be addressed to “all stations“.
    • No response is expected.
  3. Call:
    • A call is made to a specific station.
    • A response is expected.

Clearance and Read-Back

At certain times during a manned flight a clearance must be obtained from an air traffic control unit before proceeding. (e.g. takeoff, landing and taxi.)

If a manned aircraft intends to operate in controlled airspace it must obtain prior clearance.

The following clearances and instructions must be read back:

  • An ATC route clearances in its entirety, and any amendments
  • En route holding instructions.
  • Any route or holding point specified in a taxi clearance.
  • Any clearances and instructions to hold short of, enter, land on, conditional line-up.
  • On, take-off from, cross, taxi or backtrack on, any runway.
  • Any approach clearance.
  • Assigned runway, altimeter settings directed to a specific aircraft, radio and radio navigation aid frequency instructions.
    • Note: an “expectation” of the runway to be used is not to be read back.
  • SSR (secondary surveillance radar) codes, data link logon codes.
  • Level instructions, direction of turn, heading and speed instructions.

A Typical Read-Back Call

A “read back” is to:

  • Confirm receipt of the transmission.
  • Confirm that the pilot has understood the direction (no confusion or misunderstanding).
  • To keep transmissions short, sharp and to the point.

For example:

  • The call received from the tower:
    • Delta Alfa November cleared to land.
  • The pilot should respond:
    • Cleared to land Delta Alfa November.

The pilot has “read back” the direction so as to ensure no confusion. The pilot has also given his call sign to again ensure the intended recipient has received the transmission and is acting on it.

Broadcast Calls

A call we might reasonably be expected to make:

  • A broadcast is a traffic advisory passed to all interested parties on that frequency
  • No response required.
  • Unmanned aircraft must always have the word unmanned before aircraft type.
<Location> TrafficAmberley Traffic
Aircraft TypeUnmanned Aircraft
Possibly: small unmanned aircraft
<Callsign>[Unless assigned, we do not have a callsign]
Position and Intentions7 miles north-west, not above 400 AGL, for the next 20 minutes
Examples of broadcast call formats.

Part 2 – Radio Technologies


To gain a basic understanding of the types of equipment and technologies that enable aircraft radio and its operation in the field of aviation.


At the end of this briefing you will be able to:

  • Have a basic understanding of aircraft radio devices.
  • Describe the principles and limitations of VHF and HF radio wave propagation
  • Use aeronautical documents to select VHF radio frequencies for operations in Class G airspace (i.e. outside controlled airspace)

Radio Devices

The Transponder

A transponder (short for transmitter-responder, and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Secondary Surveillance Radar (SSR) is referred to as “secondary“, to distinguish it from the “primary” radar that works by passively reflecting a radio signal off the skin of the aircraft, and displays them on a screen with a simple blip.

Typical transponder control panel.

Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar. This response most often includes the aircraft’s pressure altitude and a 4-digit octal identifier.

Secondary surveillance radar (SSR) equipment detects the stronger specific transponder signal (called a “squawk“), allowing better identification of your aircraft.

Transponder Operation

Set the transponder to ON or ALT (in this case ALT is ON in Mode “C”).

Activating the transponder will make the aircraft (and altitude if Mode “C”) visible to ATC, and allow air traffic controllers to receive conflict alerts involving your aircraft. Larger aircraft will also be able to receive Airborne Collision Avoidance System (ACAS) advisories.

Make sure you know how to operate the “IDENT” function on your transponder, however do not operate it unless directed to do so by ATC.

Transponder Codes (Manned Aircraft)

If you have not been given a specific code by ATC, set code:

  • Code 1200: Civil VFR flights in class E or G airspace (Australia)
  • Code 3000: Civil flights in classes A, C and D airspace, or IFR flights in Class E airspace (Australia)

Other Allocated Codes within Australia:

  • Code 0100: Flights operating at aerodromes (in lieu of codes 1200, 2000 or 3000 when assigned by ATC or noted in the Enroute Supplement (Australia))
  • Code 2000: Civil IFR flights in Class G airspace (Australia)
  • Code 4000: Civil flights not involved in special operations or SAR, operating in Class G airspace in excess of 15NM offshore (Australia)
  • Code 5000: Aircraft flying on military operations (Australia)
  • Code 6000: Military flights in Class G airspace (Australia)

“Special” Codes:

  • Code 7700: Emergency
  • Code 7600: Radio Failure
  • Code 7500: Aircraft subject to unlawful interference (hijacking)

Emergency Locator Transmitter

Usually fixed in aircraft, ELTs are designed to be activated automatically during a crash, typically by a g-force activated switch or, less commonly, by a water-activated switch. Additionally, ELTs can also be wired into a remote switch on the instrument panel of an aircraft for manual activation by the pilot or a passenger.

An example of an Emergency Locator Transmitter

Once activated, ELTs are required to operate continuously for at least 24 hours. About the size of a 1 litre carton of milk, many ELTs are designed to be portable so that in the event of a crash they can be removed from the aircraft (or wreckage) to a safer location and manually activated if necessary.

Emergency Locator Transmitter Regulation

From 1 February 2009, all Emergency Locator Transmitters (ELTs) must operate on frequencies 406 and 121.5 MHz. ELTs which operate solely on 121.5 and 243 MHz are now obsolete.

The Civil Aviation Regulations 1988 (CAR) require the carriage of an ELT on most flights in Australian airspace.

ELTs are distress beacons which are activated following an accident either automatically by embedded electronics, or manually by a pilot or other person. An active beacon is detected by orbiting satellites which transmit a signal to search and rescue coordinators.

Radio Transmission

Radio Wave Propagation

As the pilot of an unmanned aircraft it is important to have a firm understanding of the mechanism of radio wave propagation.

This understanding will afford you a better understanding of the advantages and limitations of the various modes of transmission, and where communications may not be possible.

As an example, the VHF band is less susceptible to static interference, making them well suited for voice communications – however VHF signals are “line of sight” signals, meaning that they travel in straight lines. The transmitting and receiving antennae must be “visible” to each other, with no obstructions between them.


Mathematically, the wavelength (λ) is defined by the speed with which the wave propagates (c) divided by frequency (f) of the wave.

A depiction of a radio wave.

Wavelength Formula (λ):

λ = c / f 

Speed (c): Considering that our waves propagate in air, we can consider as the speed of light in vacuum: c ≈ 300,000,000 m/s;

Frequency (f): frequency of the signal will be using.

Example of a 900 MHz system:

λ = (300 Mm/s) / (900 MHz) = 0.33333, or 33.33 cm.

Encoding Data into Radio Waves – Amplitude Modulation (AM)

Amplitude modulation of a radio wave.

Encoding Data into Radio Waves – Frequency Modulation (FM)

Frequency modulation of a radio wave.

Encoding Data into Radio Waves – Pulse Modulation (PM)

The aim of pulse modulation methods is to transfer a narrowband analogue signal, (for example a phone call), over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.

Pulse modulation of a radio wave.

In terms of what we are looking at , pulse modulation can be something as “simple” as ON/OFF encoding!

In music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform.

Radio Wave Propagation Paths

The frequency of the radio wave governs the mode of propagation of the radio wave.

Understanding the benefits and disadvantages of the different modes of radio wave propagation allows a better choice of radio system that is better suited to your application.

Groundwaves and Skywaves

A groundwave (or surfacewave) is the part of the signal that travels along the surface of the earth, from the transmitter to the receiver. The strength of the groundwave is inversely proportional to frequency, being strongest at low frequencies. As frequency is increased, the distance covered by the groundwave decreases.

The range of a MW groundwave signal is limited to a 100 or so km during daylight, stations that are geographically separated by 200km or so can operate on the same frequency without causing interference to each other (in theory!).

The skywave is the part of the signal that is sent skywards and is reflected from the troposphere with the spacewave (not a term in common use these days) being the part that is reflected from the ionosphere.

The skywave is now taken to mean any signal other than the groundwave. As the D layer disappears at night, MW signals from the continent and further afield can be heard as they are being propagated by skywave.

Normally skywave signals on MW would not be heard during daylight as the D region would absorb the signals like an RF sponge.

Propagation Modes

Example of a groundwave mode of propagation.


  • Follow earths contour
  • Affected by natural and man-made terrain
  • Salt water forms low loss path
  • Several hundred mile range
  • Common use with 2-3 MHz signals
Example of a spacewave mode of propagation.

Skywaves and Spacewaves:

  • Line of Sight (LOS) wave
  • Ground diffraction allows for greater distance
  • Approximate Maximum Distance (D) in miles:
    • D = √(2htx +2hrx)
    • Note: antenna height in feet.
  • No strict signal frequency limitations.
  • Reflected off ionosphere (20-250 miles high)
  • Large ranges possible with single hop or multi-hop
  • Transmit angle affects distance, coverage, refracted energy
Categorisation of wave propagation categories.
Radio wave reflection off of the ionoshpere.

The Ionosphere

The ionosphere is a layer of partially ionized gasses below troposphere:

  • Ionization caused by ultra-violet radiation from the sun
  • It is affected by:
    • Available sunlight
    • Season
    • Weather
    • Terrain
  • Free ions and electrons reflect radiated energy

It consists of several ionized layers with varying ion density. Each layer has a central region of dense ionization.

LayerAltitude (Miles)Frequency RangeAvailability
D20-25Several MHzDay Only
E55-9020MHzDay, partially at night
F190-14030Mhz24 Hrs
F2200-25030Mhz24 Hrs
Ionosphere characteristics (note: F1 & F2 separate during daylight, merge at night).

VHF Radio

Primary characteristics:

  • Direct wave
  • Line-of-sight
  • Can be easily blocked or shielded by:
    • Terrain (e.g. mountains)
    • Natural obstructions (e.g. trees)
    • Unnatural obstructions (e.g. buildings)
Emaple of terrain blocking VHF radio.

What this means for an operator:

  • If you cant see the aircraft, neither can the radio.
  • No radio means “loss of comms.” protocol
  • Loss of comms. protocol means a failure to complete the mission!
  • Worst case: – Loss of comms. means loss of the aircraft!

HF Radio

Primary characteristics:

  • Used for communication over great distances.
  • Skywaves reflected by ionosphere.
Example of how different HF radio signals are reflected at different layers of the ionosphere.

Radio Class & Frequencies

Frequency Selection

Outside Controlled AirspaceFlight Information Area (Glass G) Broadcast Area (e.g. Redcliffe)FIA/FISChart
Non-controlled (non-towered) AerodromeCommon Traffic Advisory Frequency Automatic Terminal Information ServiceCTAF ATISChart ERSA
Breakdown of Class G airspace (i.e. outside controlled airspace).

Frequency Selection – Class G

Depiction of Class G controlled airspace on a typical navigation chart for Sydney, Australia (source: Airservices Australia. (2014).
Depiction of Class G controlled airspace on a typical navigation chart for Melbourne, Australia (source: Airservices Australia. (2014).
Depiction of Class G controlled airspace on a typical terminal chart for Melbourne, Australia (source: Airservices Australia. (2014).

MULTICOM Frequency

At a non-towered aerodrome, or in an area where no specific frequency has been nominated on the aeronautical charts, there is a common “multicom” frequency that may be used.

This frequency is 126.7MHz.