ADS-B and ADS-C

Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance technique in which each aircraft works out its own position, normally from GNSS, and broadcasts it with its identity, altitude and velocity about once a second. Anyone with a suitable receiver can use the message: ground stations feeding an air traffic control display, other aircraft, and commercial flight-tracking networks. Nothing interrogates the aircraft and the ground measures nothing; it only listens.

The name describes the method. The broadcast is automatic, needing neither a crew action nor an interrogation; it is dependent, because the position is only as good as the aircraft's navigation source; and it is broadcast to all. Its counterpart, Automatic Dependent Surveillance - Contract (ADS-C), sends reports by contract to particular ground systems and is the main surveillance tool over the oceans. This article builds on secondary surveillance radar and transponders, which describes the Mode S transponder that most ADS-B installations use.

Dependent surveillance explained

Radar, primary or secondary, is independent of the aircraft's navigation: the ground station measures range from timing and bearing from the direction of its antenna. ADS-B reverses that. The position is computed on board, so the ground station needs no rotating antenna and no timing measurement, only a receiver. That is why coverage can be extended cheaply to places where a radar head would be impractical.

An ADS-B Out broadcast carries:

• the ICAO 24-bit aircraft address and the flight identification entered by the crew;
• the Mode A code;
• latitude and longitude from the position source;
• two altitudes: pressure altitude, the same value Mode C and Mode S report, and geometric altitude from GNSS;
• horizontal and vertical velocity;
• indicators of accuracy and integrity, described below.

Dependence cuts both ways. A wrong flight identification reaches every user of the data, because a Mode S transponder that transmits ADS-B takes it from the same entry as its replies. A GNSS fault, jamming or spoofing corrupts or removes the position while the aircraft is otherwise healthy, which is why the message has to say how far it can be trusted, and why an air navigation service provider may keep an independent layer, radar or multilateration, where the traffic justifies it.

ADS-B Out and ADS-B In

ADS-B Out is the transmitting function, the part that makes the aircraft visible. It is what mandates require, and it benefits everyone else's picture rather than the crew's own. ADS-B In is the receiving function: a receiver and a display that show the crew other ADS-B traffic and, in the United States, the broadcast services described below. An aircraft with Out only is seen but sees nothing. The US rule requires Out; In remains optional, although the FAA has developed applications that use it, such as cockpit traffic displays and guidance for spacing behind another aircraft.

A traffic picture from ADS-B In supports situational awareness and the visual search. It is not collision avoidance. ACAS II still interrogates the transponders around it and issues resolution advisories from those replies; in hybrid surveillance it uses ADS-B squitters only to track distant, non-threatening aircraft passively. Neither system shows an aircraft that carries no transponder and no ADS-B Out.

1090 MHz extended squitter

1090 MHz Extended Squitter (1090ES) is ADS-B transmitted on 1090 MHz, normally by a Mode S transponder. A squitter is a transmission the transponder makes on its own initiative, without being asked; the extended squitter is the long, 112-bit Mode S format, transmitted on 1090 MHz alongside ordinary SSR replies. The transponder already holds the 24-bit address, flight identification and pressure altitude, so an extended-squitter transponder connected to a suitable GNSS position source becomes an ADS-B Out transmitter.

1090ES is the international standard. It is the only link used in Europe, the only one accepted in US Class A airspace, and the only one that space-based receivers pick up. US rules call for equipment meeting TSO-C166b (RTCA DO-260B) or its successor TSO-C166c. The frequency is shared with SSR replies and with TCAS, and the FAA cites that congestion as the reason it sends no weather on 1090 MHz.

Universal access transceiver

The Universal Access Transceiver (UAT) is a dedicated ADS-B data link on 978 MHz, used only in the United States. It is a separate box, not a transponder function, so an aircraft with UAT still needs its Mode C or Mode S transponder wherever 14 CFR 91.215 requires one. Under 14 CFR 91.225, UAT equipment (TSO-C154c, RTCA DO-282B, or its successor TSO-C154d) satisfies the ADS-B Out rule only outside Class A, which in the United States begins at 18,000 ft MSL. UAT therefore suits aircraft that stay below Class A and inside the United States; anything that flies higher or abroad needs 1090ES.

With two links in use, aircraft on one would not see aircraft on the other. The FAA ground network fills that gap with Automatic Dependent Surveillance-Rebroadcast (ADS-R): it receives a message on one link and retransmits it on the other when an aircraft using the other link is nearby.

TIS-B and FIS-B

Traffic Information Service–Broadcast (TIS-B) uplinks radar-tracked traffic from FAA ground stations to ADS-B In aircraft, so that aircraft with only a transponder appear on the display. The service is built for each aircraft that is itself transmitting ADS-B Out, covering a cylinder of airspace around it, and like ADS-R it is available at or below 24,000 ft. Three limitations follow. An aircraft with In but no Out sees TIS-B targets only when it happens to be near an aircraft that has Out. Traffic that radar cannot see, including aircraft without a transponder, never appears. Radar-derived positions are less precise and update less often than an ADS-B report.

Flight Information Service–Broadcast (FIS-B) uplinks free weather and aeronautical information, including radar mosaics, reports and forecasts, temporary flight restrictions and NOTAMs. It is broadcast only on 978 MHz, so a receiver that listens only to 1090 MHz gets none of it. The age shown for a radar mosaic refers to when the mosaic was assembled, and the radar data inside it can be older still, so it supports strategic avoidance of weather, never tactical manoeuvring between thunderstorm cells.

Accuracy and integrity parameters

A position is useful for separation only if the receiving system knows how good it is. Each message therefore carries figures that describe the position source, and 14 CFR 91.227 sets the minimum values for ADS-B Out in the United States:

Changes in NACp, NACv, SDA and SIL must be broadcast within 10 seconds, and a change in NIC within 12 seconds. In practice these values come from the GNSS receiver's own integrity monitoring (see GNSS), so a receiver that loses integrity reports it at once and ground systems and other users can judge how far to trust each report. In the United States an operator can request a Public ADS-B Performance Report after a flight to check that the installation meets the rule.

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ADS-B mandates: FAA and EASA

In the United States, 14 CFR 91.225 has required ADS-B Out since 1 January 2020 in Class A, B and C airspace, within the 30 NM Mode C veil, above Class B and C up to 10,000 ft MSL, at and above 10,000 ft MSL except within 2,500 ft of the surface, and in Class E over the Gulf of Mexico at and above 3,000 ft MSL within 12 NM of the coast. The airspace is essentially that of the transponder rule, 91.215; airspace classification covers the areas and the ADAPT tool through which an operator without ADS-B can request access.

In the European Union, Regulation (EU) No 1207/2011, as amended by Regulation (EU) 2020/587, requires ADS-B Out, together with Mode S elementary and enhanced surveillance, on fixed-wing aircraft flying IFR as general air traffic with a maximum certificated take-off mass above 5,700 kg or a maximum cruising true airspeed above 250 kt, whose individual certificate of airworthiness was first issued on or after 7 June 1995. The requirement has applied since 7 December 2020; operators with an established retrofit programme were allowed until 7 June 2023.

Because ADS-B is public, anyone can track a flight. The FAA offers two programmes for operators who want less visibility: Limiting Aircraft Data Displayed (LADD) restricts the data the FAA passes to third parties, and the Privacy ICAO Address (PIA) programme assigns a temporary 24-bit address not linked to the civil aircraft registry. PIA is open to US-registered aircraft with 1090ES that use a third-party call sign in US domestic airspace.

Space-based ADS-B

Space-Based ADS-B places the receivers in orbit. The Aireon system carries ADS-B receivers on the Iridium NEXT satellites in low Earth orbit and relays what they hear to air traffic service providers, so aircraft transmitting 1090ES can be tracked over oceans and remote areas where no ground station could be built. NATS and NAV CANADA began using it operationally in the Shanwick and Gander oceanic regions on 27 March 2019, applying reduced separation minima under ICAO's advanced surveillance-enhanced procedural separation standards, which allowed in-trail spacing of 40 NM to be reduced to as little as 14 NM.

For the crew nothing changes beyond keeping a serviceable 1090ES installation, but surveillance from space does not give the controller a way to talk to the aircraft. Oceanic communication still relies on data link, HF or satellite voice.

ADS-C in oceanic airspace

ADS-C sends position reports from the flight management system to a particular ATS ground system, under contracts that the ground system sets up after the crew logs on. The main contract types are:

• periodic, a report at an interval set by ATC and changed when needed;
• event, a report when a defined event occurs, such as a waypoint change, a lateral deviation beyond a set value, leaving an altitude range or a change in vertical rate;
• demand, a single report requested now.

A ground system can hold one periodic and one event contract with an aircraft at the same time and add demand contracts as needed. An emergency mode, which only the crew can start and cancel, alerts the controller. Reports carry position, level and time, and can include the predicted route, so the controller sees both where the aircraft is and where the system will fly it next. Because the reports travel by satellite or VHF data link, ADS-C needs no ground stations along the route.

ADS-C is packaged with controller-pilot data link communications (CPDLC) in FANS 1/A, the avionics standard used over the oceans. On the North Atlantic, the data link mandate requires FANS 1/A CPDLC and ADS-C, or equivalent, for flights between FL290 and FL410 inclusive throughout the region, with limited exceptions. With the navigation and communication performance these systems support, ICAO allows much smaller oceanic minima than position reports by voice, described in separation standards.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.