CNS/ATM resource kit – Chapter 4: Surveillance and ADS-B

The surveillance component of CNS/ATM, GNSS-based automatic dependent surveillance-broadcast (ADS-B) has evolved from radar.

ADS-B offers a substantial increase in the areas covered by surveillance.

This chapter will introduce ADS-B, discuss airspace changes and cover installation and operation of equipment. Human factors in ADS-B is covered in Chapter 8.

What is ADS-B?

ADS-B is a broadcast surveillance system in which an aircraft automatically transmits to a ground station and other air traffic its identity, precise location, altitude, velocity and other information. The system requires an aircraft to be fitted with systems such as a barometric encoder and global navigation satellite system (GNSS) equipment. While ADS-B is required only for aircraft operating under instrument flight rules (IFR), it also offers substantial benefits for visual flight rules (VFR) pilots.

ADS-B ground stations comprise a receiver unit, an antenna and a site monitor. Ground stations across Australia are connected to the Airservices Australia digital communication infrastructure and, combined with radar, provide continent-wide, line-of-sight surveillance coverage above 30,000 ft, as well as significant coverage at lower levels.

ADS-B uses the same transponder as, but operates independently of, the aircraft radar and traffic collision alerting and avoidance (TCAS) systems. Most modern Mode S secondary surveillance radar (SSR) transponders are capable of transmitting SSR and ADS-B (also termed extended squitter) data. However Mode A/C and some older Mode S transponders do not support ADS-B.

Australia, Europe, the US, and the rest of the world have implemented ADS-B on the Mode S frequency band of 1090 MHz—most commonly called ten-ninety ES (extended squitter). In the USA, the FAA has deployed a redundant ADS-B system on 978 MHz called universal access transponder (UAT) for aircraft that operate below 18,000 feet. This system is not deployed in Australia and the avionics will not work—when importing an aircraft from the US, ensure it has the correct ADS-B equipment on board.

Combined radar and ADS-B coverage with initial ADS-B rollout
Combined radar and ADS-B coverage with initial ADS-B rollout

ADS-B transmission and display

On-board ADS-B equipment can consist solely of a transmission system to send ADS-B information (ADS-B OUT). Aircraft can also be equipped with ADS-B IN—a cockpit display of traffic information (CDTI).


An ADS-B transmitter enables the identity, position and altitude of an aircraft to be determined and displayed to an air traffic controller. The signal is broadcast from the aircraft approximately every half second and, provided the aircraft is within the coverage volume of an ADS-B ground station, the data can be fed to the ATC facility and used to provide air traffic services.

ADS-B OUT transmission
ADS-B OUT transmission


Aircraft may also be equipped with a cockpit display of traffic information (CDTI) and associated receiver to display the broadcast positions of ADS-B OUT aircraft. CDTI may be combined with other systems, such as moving map navigation displays.

ADS-B IN transmission
ADS-B IN transmission

Benefits of ADS-B

Australia’s adoption of ADS-B for air traffic surveillance across the continent outside conventional radar coverage offers a range of benefits to commercial and general aviation pilots flying using IFR. These are:

  • Position reports by voice no longer required for identified ADS-B aircraft.
  • Ability to approve continuous rather than stepped climbs and descents to and from cruising level.
  • Greater flexibility in allocating appropriate flight levels at the request of pilots. (That is, to climb to optimum flight level, as aircraft weight decreases with fuel burn.)
  • Airspace which previously had no radar, and only procedural separation services, can now have an ATC surveillance service.
  • Greater ability for ATC to grant clearances to fly requested routes or levels.
  • Aircraft are easier to locate for search and rescue (SAR).
  • Giving priority to ADS-B equipped aircraft is Airservices’ policy (when doing so lowers the workload of ATC, thereby improving safety).
  • Ability to replace radar.

What the air traffic controller sees

The ongoing rollout of ADS-B ground stations significantly improves surveillance coverage of aircraft across the continent, including at lower altitudes. By allowing controllers to more precisely ‘see’ aircraft, rather than relying on estimated position reports, ADS-B increases the level of service ATC can provide while increasing safety for everyone in the sky.

The control panel provides controllers with an integrated air situation display, showing positions from ADS-B and other sources on a single screen.

Assistance in-flight

Aircraft fitted with ADS-B experiencing any kind of in-flight incident or emergency will have their location more accurately pinpointed, meaning search and rescue operators will be able to respond more rapidly, particularly in remote areas.

Controllers routinely provide pilots with navigation assistance if they lose their position due to instrument failure, weather, or if they become incapacitated. This availability is enhanced by the precise accuracy of ADS-B.

There are many reasons to request altitude or flight level changes during flight, including avoiding poor weather, and reducing fuel burn and carbon dioxide emissions by improving the efficiency of your engine. Fitting ADS-B in your aircraft will raise the likelihood of ATC granting altitude or flight level changes during your flight, as they will more accurately ‘see’ your position in the air in relation to other aircraft nearby.

Using ADS-B

Before you fly with ADS-B, find out what you have to do. In most cases, this will be very little other than entering the correct fields in the flight plan, as the ADS-B broadcast is automatic. However, safe flight relies on you understanding what ATC might require of you, how to handle emergencies, and how to use the cockpit interface effectively.

ADS-B systems typically broadcast two means of identifying the transmitting aircraft:

  • the aircraft address (also known as the 24-bit code), and
  • the flight identification (FLTID)—the visual equivalent of a call sign—used to identify targets on a display and link them to their flight plans.

Aircraft address

Each aircraft has a unique aircraft address, which consists of a 24-bit code allocated by CASA. This code is usually entered into the unit by a licensed aviation maintenance engineer (LAME) at installation and may be expressed in either binary or hexadecimal format. The code is on the aircraft registration letter sent to aircraft owners by CASA. If your aircraft is not registered by CASA, you can get a code from the aircraft registry. See the CASA website for more information.

The 24-bit aircraft address is safety critical information, so ensure it is correct before flying. An incorrect address could lead to traffic collision and avoidance systems (TCAS) on your or other aircraft not functioning correctly, to ATC confusing your aircraft with another, or not being able to ‘see’ it at all.

Flight identification

The FLTID is used in both ADS-B and Mode S secondary surveillance radar (SSR) technology. Up to seven characters long, it is usually set in airline aircraft by the flight crew via a cockpit interface. It enables air traffic controllers to identify an aircraft on a display and to correlate a radar or ADS-B track with the flight plan data.

Aircraft identification is critical information, so enter it carefully; punching in the wrong characters could lead to ATC confusing your aircraft with another. It is important that the identification exactly matches the aircraft identification (ACID) entered in the flight notification.

Air traffic control might ask you to change your FLTID if possible, so you must know if you can do so and how (see below). They might also ask you to stop transmitting an ADS-B signal because they have detected an error, such as altimeter failure, in your equipment. If you cannot do this, notify ATC immediately.

If the ADS-B transmitter and SSR transponder are combined, switching ADS-B off may also make the aircraft invisible to SSR and TCAS.

Intuitive correlation between an aircraft’s flight identification and radio call sign enhances situational awareness and communication. Airline aircraft will use the three-letter ICAO airline code used in flight plans, not the two-letter IATA codes.

Check the manual

Procedures for use of ADS-B transponders are likely to be different from those for older Mode C types. It is therefore important that pilots of ADS-B equipped aircraft ensure that their transponders are switched to the correct mode.

Case study—Which button would you push?

The VFR pilot of an aircraft which had recently been fitted with an ADS-B transponder selected the unit’s ON button (see illustration) before undertaking some circuits in class C airspace. Once airborne, the pilot was advised by air traffic control that the aircraft was not returning altitude information.

On the unit in question, a Garmin 335, the ON button powers the transponder, but disables altitude reporting. The pilot’s guide for the transponder says that the unit should always be in ALT mode (altitude reporting enabled) on the ground and in the air, unless instructed otherwise by ATC.

Aircraft control and functions buttons
CNS/ATM aircraft control and functions buttons

Setting the FLTID

Your call sign normally dictates the applicable option:

  1. the flight number using the ICAO three-letter designator for the aircraft operator if a flight number call sign is being used (e.g. QFA1 for Qantas 1, VOZ702 for Virgin 702)
  2. the nationality and registration mark (without a hyphen) of the aircraft if the call sign is the full version of the registration (e.g. VHABC for international operations)
  3. the registration mark alone of the aircraft if the call sign is the abbreviated version of the registration (e.g. ABC for domestic operations)
  4. the designator corresponding to a particular call sign approved by Airservices Australia or the Australian Defence Force (e.g. SPTR3 for Firespotter 3, ROLR45 for Roller 45)
  5. the designator corresponding to a particular call sign in accordance with the operations manual of the relevant recreational aircraft administrative organisation (e.g. G123 for Gyroplane 123). Don’t add any leading zeros, hyphens, dashes or spaces to the FLTID.

Sometimes an aircraft may need to use an aircraft identification and call sign other than that corresponding to the FLTID. Air traffic control may approve or direct the use of an alternative FLTID.

Correction of FLTID in flight

  • If FLTID has been entered incorrectly, ATC will instruct you to re-enter ADS-B aircraft identification.
  • If you are able to, you must then re-set the FLTID to exactly match the aircraft identification in the ATS flight notification.
  • If you are unable to re-set the FLTID in flight (for example if the FlightID was configured by LAME) advise ATC that you are unable to comply.

Failure to enter your FLTID correctly means information from your ATS flight plan is not automatically linked to your ADS-B information. This causes:

  • screen clutter on air traffic control displays—two different labels are displayed instead of one
  • distraction
  • increased controller and pilot workload to resolve FLTID error.

Incorrect FLTID entry by an air transport operator is a reportable event, and ATC is required to raise a safety incident report.

Flight planning and ATC displays

The flight planning format requires pilots to indicate the surveillance equipment carried and operated on the aircraft. Field 10b of the flight plan provides the following options for ADS-B equipped aircraft:

  • E: Mode S transponder with Flight ID, pressure altitude and extended squitter (ADS-B) capability
  • L: Mode S transponder with Flight ID, pressure altitude, extended squitter (ADS-B), and enhanced surveillance capability.

And for ADS-B fitment one of:

  • B1: ADS-B OUT using 1090MHz, or
  • B2: ADS-B IN and OUT using 1090MHz.

There are also other codes for ADS-B installations complying with other international standards, i.e. UAT.

Complete guidance on notifying surveillance equipment and capabilities in flight plans can be found in AIP ENR 1.10.

A typical field 10b flight plan entry for an ADS-B equipped aircraft will be: (Field 10a content)/EB1.

An aircraft address code is not usually needed on flight plans. However, if ATC has approved the use of a FLTID different from the ACID, the aircraft address will be needed to correlate the flight plan to the aircraft. Enter the aircraft address in item 18 of the flight notification as hexadecimal code (e.g. CODE/7C81CB).

Hexadecimal code is complex and non-intuitive, and easy to enter incorrectly, so use it only when necessary and check it carefully. If you lodge flight notification by radio, tell air traffic services the aircraft FLTID if it differs from the call sign.

Common errors in FLTID entries

This table contains common errors in FLTID entries
ABC_123 ✗
Added spaces designator omitted
ABC123 ✓ 123 ✗
ICAO airline
ABC123 ✓
AB123 ✗
IATA airline designator used
ABC123 ✓ ABC0123 ✗
Additional zero inserted
ABC123 ✓
AB0123 ✗
Zero added and one letter of ICAO airline designator dropped
DEP/DEST/Alternate details used instead of Flight ID
ABC123 ✓
How the FLTID must exactly match aircraft in your ATS flight plan
How the FLTID must exactly match aircraft in your ATS flight plan

Flight plan procedures

Operators who meet the Australian requirements for ADS-B operations must indicate ADS-B capability in the flight notification (ATS flight plan) of all approved ADS-B equipped aircraft when planning to operate in Australian airspace.


-M077F370 DCT ENTRA Y245 BANDA H185 CG/NO452F360 Q69 ITIDE DCT

If you enter ACID or FLTID codes incorrectly, ATC might not be able to see your aircraft, or might confuse it with another. You could also affect other systems, such as TCAS. The codes are flight critical information, so enter them carefully.

ADS-B transmitters must not send spurious information. If ATC instructs you to stop transmitting, you must turn off your ADS-B transmitter* and not turn it on again before it has been checked, and if necessary, repaired or reset.

* Many ADS-B installations share controls with the SSR transponder, so you cannot operate the two systems independently. If you cannot comply with a particular instruction, advise ATC and ask for alternative instructions.

ADS-B performance requirements for ATC

Airservices uses a hierarchy system on The Australian advanced air traffic system (TAAATS) displays. Radar is displayed first, followed by ADS-B, followed by ADS-C and then the flight plan track of the aircraft if there is no surveillance coverage.

In the future, however, ADS-B will be displayed first, then radar.

Loss or degradation of GNSS integrity in the airborne receiver is transmitted to air traffic control in the ADS-B messages. ADS-B services may be terminated if GNSS integrity is inadequate.

ADS-B phraseology

Specific and generic radio phraseology is used for ADS-B and radar services. You should use specific phraseology when it is necessary to differentiate between radar and ADS-B. The ADS-B equivalent of ‘squawk’ is ‘transmit’ and ADS-B is pronounced ‘ay-dee-ess-bee’ over the radio.

Otherwise, you can use generic phraseology when it is not necessary to differentiate between a service provided by radar and one provided by ADS-B. In many of these cases, no change to existing phraseology is required. For example, ‘identified’ and the various vectoring instructions apply to either technology.

The following table lists some examples, but users should check the AIP for the current terminology.

This table contains ADS-B phraseology
Circumstance Radar phraseology ADS-B phraseology
Termination of radar
and/or ADS-B service
Identification terminated (due [reason]) (instructions)
Radar or ADS-B ground equipment unserviceability Secondary radar out of service or Primary radar out of service (appropriate information as necessary) ADS-B out of service (appropriate information as necessary)
To request the aircraft’s SSR or ADS-B capability Advise transponder capability Advise ADS-B capability
To advise the aircraft’s SSR or ADS-B capability Transponder (Alpha, Charlie or Sierra as shown in the flight plan) or Negative transponder ADS-B transmitter/ receiver (Ten Ninety data link) or Negative ADS-B
To request reselection of FLTID* Re-enter Mode S aircraft identification Re-enter ADS-B aircraft identification
To request the operation of the IDENT feature* Squawk ([code]) (and) Ident Transmit ADS-B Ident
To request termination of SSR transponder or ADS-B transmitter operation* Stop squawk (transmit ADS-B only) Stop ADS-B transmission (squawk [code] only)
To request transmission of pressure altitude* Squawk Charlie Transmit ADS-B altitude
To request termination of pressure altitude transmission due to faulty operation* Stop squawk Charlie
Wrong indication
Stop ADS-B transmission ([wrong indication or reason])

* Some older ADS-B installations may not provide for entry of FLTID, transmission of Ident, or isolation of pressure altitude by the pilot. Such systems are no longer compliant with CAOs. Some ADS-B installations may share controls with the SSR transponder, so that you cannot operate the two systems independently. If you cannot comply with a particular instruction, advise ATC and ask for other instructions.


Emergency codes

The method for notifying ATC of an emergency depends on the type of equipment carried and the surveillance coverage available, so make sure you know which equipment is on board. Does the aircraft have an ADS-B emergency function? Or an on/off switch only? Is it linked to the transponder, so that squawking 7600 also sends an ADS-B communications failure message?

Selection of an emergency transponder code, e.g. 7600, automatically generates an emergency indication in the ADS-B message. However, many transponders transmit only a generic ADS-B emergency indication. That means the specific type of emergency, such as communications failure, may not be conveyed to controllers in an ADS-B environment. Some general aviation installations may not broadcast any form of ADS-B emergency indication.

Required emergency procedures

If an emergency indication is received from an aircraft in ADS-B airspace and the flight crew does not verbally communicate the nature of the emergency, the controller will initiate procedures for suspected unlawful interference.


If no response from the pilot is received within a reasonable time, the controller will assume the possibility of unlawful interference.

Note: Some transponders cannot transmit an ADS-B ‘IDENT’ (SPI) while an emergency transponder code is selected.

Does your transponder transmit discrete emergency codes, or does it transmit a generic emergency code only?

In an emergency, use all available means to signal your status, regardless of expected surveillance and communications coverage.

Traffic awareness with cockpit displays

Before flying, you must understand the capabilities of your cockpit display of traffic information (CDTI) and how to best use it for traffic awareness.

CDTIs will help you spot other ADS-B traffic more easily by showing you where to look. However:

  • depending on the unit’s filtering capability, your CDTI might not show all ADS-B traffic
  • CDTIs will not display non-ADS-B traffic
  • don’t try to second-guess ATC instructions with CDTI information
  • do not attempt to take evasive action, or to separate your aircraft from other traffic, using a CDTI. It is there to enhance situational awareness, not to replace separation procedures.

Cockpit display of traffic information does not replace see-and-avoid. You still have to look out the window for other traffic.

Radar or ADS-B?

You may not always know which surveillance system is being used and how you are being controlled. You may be told only that you have been ‘identified’, but it may not be clear whether you have been identified with radar, ADS-B or both. Unless ATC uses specific phraseology, use both ADS-B and transponder equipment to give the controller the best surveillance picture.

Surveillance coverage and controlled airspace often have different boundaries.

Aircraft equipment

In Australia, ADS-B is transmitted on the 1090 MHz extended squitter datalink, also known as ‘Ten Ninety’. Standards for the extended squitter avionics are defined in Civil Aviation Order 20:18. CASA has also published Australian TSOs C1004 and C1005.

What are the rules?

  • Non-compliant ADS-B transmissions must be disabled.
  • Operation at/above FL290 requires ADS-B.
  • All aircraft flying IFR must be equipped for GNSS navigation and be fitted with ADS-B.

GNSS provides the positioning information for ADS-B, so if you turn the GNSS receiver off, your aircraft will become invisible to ADS-B surveillance.

ADS-B equipment can have various pilot interfaces, ranging from a simple on/off switch for the transmitter to a pilot control interface with advanced features, such as a cockpit display of traffic information.

It may also be combined with other systems, such as a secondary surveillance radar (SSR) transponder, traffic collision avoidance system (TCAS) or multifunction display (MFD). In most aircraft installations, the SSR transponder control module in the cockpit also controls the ADS-B transmitter; operating the SSR system will also operate the ADS-B system.

International variations

A number of states around the world are implementing ADS-B. While harmonised as much as possible, there are some differences in equipment requirements and fitment. For example, the USA requires either DO260B (also accepted by Australia) or universal access transceiver (UAT) technology, which is not used in Australia.

Anyone importing a GA category aircraft from the United States should make sure the ADS-B fitted is the 1090MHz system, not the UAT system. Anyone who has imported an IFR aircraft since February 2014 should make sure it is ADS-B and Mode S capable.

What to do when something goes wrong?

Chapter 5 explores installation and recovery procedures for aircraft equipment.

Key points

  • ADS-B avionics broadcast identification, position, altitude, velocity and other data automatically about every half second, with air-to-ground and air-to-air applications.
  • Aircraft fitted with ADS-B experiencing any kind of in-flight incident or emergency will have their location more accurately pinpointed, meaning search and rescue operators will be able to respond more rapidly, particularly in remote areas.
  • Operators who meet the Australian requirements for ADS-B operations must indicate ADS-B capability in the flight notification (ATS flight plan) of all approved ADS-B equipped aircraft when planning to operate in Australian airspace.
  • If the ADS-B transmitter and SSR transponder are combined, switching ADS-B off may also make the aircraft invisible to SSR and TCAS.
  • GNSS provides the positioning information for ADS-B, so if you turn the GNSS receiver off, your aircraft will become invisible to ADS-B surveillance.
  • In most aircraft installations, the SSR transponder control module in the cockpit also controls the ADS-B transmitter; operating the SSR system will also operate the ADS-B system.


Further reading


Published date: 23 June 2021
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