Flight Tracking: How It Works

What Is ADS-B? The Technology Behind Modern Flight Tracking

Automatic Dependent Surveillance–Broadcast (ADS-B) is the foundational technology behind modern consumer flight tracking. An ADS-B transponder aboard an aircraft automatically broadcasts the aircraft's GPS-derived position, altitude, speed, heading, and an aircraft identifier (ICAO 24-bit address) every half-second on the 1090 MHz radio frequency. Any receiver within radio range — whether an official air traffic control radar installation or a cheap software-defined radio (SDR) receiver attached to a Raspberry Pi — can decode these broadcasts and display the aircraft's position in real time.

ADS-B stands for: Automatic (no pilot input required), Dependent (depends on GPS/GNSS for position), Surveillance (used for surveillance of aircraft), Broadcast (information broadcast to any receiver rather than interrogated by radar). The "1090ES" variant (1090 MHz Extended Squitter) is the globally standardised format used by commercial airliners. A lower-power variant called UAT (Universal Access Transceiver, operating at 978 MHz) is used in the United States for general aviation aircraft below 18,000 feet.

The ICAO 24-bit address (also called the Mode S address or ICAO hex code) is a unique identifier assigned to each aircraft by the registry country. It is analogous to a MAC address on a computer — globally unique, assigned at registration, and embedded in the aircraft's transponder. This address allows tracking systems to link individual position reports to a specific aircraft tail number (registration) and from there to an airline, aircraft type, and flight number when the aircraft is operating a known commercial service.

ADS-B Out — the outbound broadcast described above — has been mandated for flight in most controlled airspace worldwide since 2020 (FAA) and 2020–2021 (EASA). Most modern commercial transport aircraft have been equipped since the 2010s. ADS-B In, which allows aircraft to receive ADS-B broadcasts from other aircraft for traffic awareness, is available but not yet mandatory in most jurisdictions. The combination enables aircraft to see each other without relying solely on ATC radar, improving situational awareness and enabling future airspace efficiencies.

Traditional Radar vs. ADS-B: Complementary Systems

Traditional radar comes in two types. Primary radar works by emitting a radio pulse and listening for its reflection from the metal skin of an aircraft — no cooperation from the aircraft is required, but range and altitude accuracy are limited, and the system cannot identify which specific aircraft is being tracked. Secondary Surveillance Radar (SSR) transmits an interrogation signal, to which a cooperative transponder aboard the aircraft responds with a code (Mode A for identification, Mode C for altitude, Mode S for discrete identifier and other data). Most commercial aircraft have been carrying Mode S transponders since the 1990s.

SSR radar gives ATC a position, altitude, and identity — but requires aircraft to be within the radar's line of sight and range, typically 200–250 nautical miles for en-route radar at altitudes above 10,000 feet. Mountainous terrain creates radar shadows. Ocean gaps — the North Atlantic, Pacific, Southern Ocean — were entirely beyond radar coverage until satellite-based tracking systems were developed. In those oceanic gaps, pre-ADS-B aircraft reported their position by radio voice to oceanic control centres at defined waypoints (typically every ten degrees of longitude), creating a data picture that was hours out of date by the time it was received.

ADS-B complements radar in three important ways. First, ADS-B provides much higher position update rates (every 0.5 seconds) compared to radar sweeps (typically every 4–12 seconds). Second, ADS-B position accuracy from GPS is typically better than radar position accuracy, especially at extreme ranges. Third, ADS-B works anywhere a receiver can be placed — including on the ground at small regional airports that cannot justify a full radar installation, on mountainsides above radar shadow zones, aboard ships providing maritime ADS-B coverage of ocean areas, and on low-earth orbit satellites.

ATC systems in modern facilities integrate radar and ADS-B data into a fused picture in which the best available source for each aircraft is automatically selected. In radar-covered airspace, both sources typically agree closely; the redundancy provides resilience against either system failure. In oceanic airspace, ADS-B (via satellite relay) has become the primary surveillance tool, transforming oceanic separation standards from 80 nautical miles to as little as 14 nautical miles on the busiest North Atlantic tracks.

Satellite-Based ADS-B: Tracking Across Oceans

The gap between ground-based ADS-B receiver coverage and global aircraft tracking was filled by two competing satellite-based ADS-B systems deployed between 2017 and 2021: Aireon (a partnership between Iridium Communications and NAV CANADA) and Spire Global. Both use low-earth orbit (LEO) satellite constellations equipped with ADS-B receivers to capture aircraft broadcasts from space.

Aireon operates payloads hosted on Iridium NEXT satellites in polar orbit at approximately 780 km altitude. The constellation of 66 active satellites provides global coverage with a maximum gap time — the period between successive satellite passes over any point on the earth — of roughly 6 seconds. This near-real-time oceanic coverage was made available to NAV CANADA, NATS (UK), ENAV (Italy), IAA (Ireland), and other air navigation service providers in 2019 under commercial data service agreements. The system enabled NAV CANADA to introduce Reduced Vertical Separation Minima (RVSM) and reduced lateral separation in North Atlantic airspace, allowing more efficient routing and altitude assignments.

Spire Global operates a different constellation architecture using CubeSats (small, low-cost satellites) in various orbital inclinations, collecting ADS-B data which it sells to commercial customers including airlines (for flight tracking and fuel optimisation), weather services, and government agencies. Spire's data latency is higher than Aireon's due to store-and-forward architecture on many satellites, but coverage is global and continuously improving as the constellation expands.

FlightAware's Global plans and Flightradar24's enhanced tracking both incorporate satellite ADS-B data to fill coverage gaps over oceans and remote continental areas. A user tracking a transatlantic flight on either service now sees position updates approximately every 8–15 minutes over the North Atlantic (limited by satellite pass intervals for specific coverage plans) rather than the radio waypoint reports from the pre-satellite era that could be hours apart.

How Flight Tracking Websites Work

Flightradar24, FlightAware, FlightStats, and RadarBox are the four largest publicly accessible flight tracking platforms. All combine data from multiple sources to produce the live maps and flight status information that passengers use to track their flights.

Flightradar24 operates a global network of more than 35,000 ground-based ADS-B receiver stations contributed by volunteers (anyone can host a receiver in exchange for a premium subscription). This ground network covers most of Europe, North America, parts of Asia and Australia, and urban areas in Latin America and Africa comprehensively. The platform supplements ground receiver data with MLAT (multilateration — using time-difference-of-arrival calculations from multiple receivers to position aircraft without ADS-B), satellite ADS-B data for oceanic areas, and airline schedule data for aircraft not transmitting ADS-B.

FlightAware similarly operates a receiver network plus commercial data feeds from airlines, airports, and air traffic control systems. FlightAware has deeper integrations with US airline operational data systems, making it often more accurate for domestic US flight tracking and better for departure gate information, actual off-block times, and inbound aircraft tracking (the feature showing where an aircraft is coming from before your outbound flight). FlightAware also powers the in-flight tracking displays on many US airline seatback screens.

Schedule data — what flight numbers are planned, what aircraft types are assigned, what departure times are expected — comes from SSIM (Standard Schedules Information Manual) data that airlines file through airline data providers like OAG, Cirium, or Innovata. This data is what populates a tracking site even before an aircraft takes off, and it allows the site to show you a flight's expected departure and arrival even when the aircraft is not yet transmitting.

Position accuracy varies. Over Europe and North America with dense ADS-B receiver networks, positions are accurate to within tens of metres with 0.5-second updates. Over oceans using satellite ADS-B, positions are accurate to within hundreds of metres but update every 8–15 minutes. Over truly remote areas with neither ground nor satellite receiver coverage, tracking sites fall back to interpolated positions between last known points — essentially an educated guess along the scheduled route.

Blocked and Hidden Aircraft: Why Some Flights Don't Show

Not all aircraft visible on ADS-B-capable tracking sites are actually displayed. Aircraft operators can request that their aircraft's tail number be blocked from public display on commercial tracking sites under the FAA's LADD (Limiting Aircraft Data Displayed) programme (formerly the FAA's Privacy ICAO Address programme) in the United States. Other countries have equivalent provisions.

The LADD programme allows operators to request that their aircraft's registration-to-ICAO-address mapping not be shared with commercial data aggregators. The aircraft continues to transmit ADS-B normally (it must do so for ATC purposes) and ATC systems continue to see it; the blocking applies only to commercial tracking platforms. On Flightradar24, a blocked aircraft may appear as an anonymous dot moving along a route without a tail number, or may not appear at all depending on the tracking site's policy.

Common users of tail-number blocking include: corporate flight departments protecting executive travel privacy; government aircraft (including some military and law enforcement); private individuals with high-net-worth security concerns; and occasionally celebrities or sports teams whose movements attract unwanted public attention. The system has drawn criticism from aviation journalists and transparency advocates who argue that publicly-funded airspace infrastructure should not enable private travel concealment.

Some aircraft types — including many military aircraft, intelligence agency aircraft, and law enforcement platforms — use Mode 5 military transponders or deliberately unpredictable ICAO address assignments that make continuous tracking difficult. Additionally, some states permit "temporary ICAO address reassignment" — a practice that has raised concerns about the reliability of civil-military airspace deconfliction in dense airspace environments.

It is worth noting that aircraft operated by airlines on scheduled commercial services are not typically blocked. If you are tracking an airline flight and it disappears from a tracking site, the most common explanations are: loss of ADS-B signal (entering an area with poor receiver coverage), aircraft landed and transmissions ended, or the flight is operating under an alternate callsign due to a flight plan amendment. True disappearances of commercial aircraft in controlled airspace are extremely rare and would trigger immediate ATC response.

Oceanic Tracking Before Satellite ADS-B

Before satellite ADS-B became operational in 2019–2021, tracking aircraft over the North Atlantic, Pacific, Southern Ocean, and Indian Ocean relied on a patchwork of degraded methods. Understanding the pre-satellite system illuminates both how aviation managed oceanic safety for decades and why satellite tracking was such a significant advance.

The North Atlantic Track System (NATS) organises transatlantic traffic into structured tracks — a set of routes optimised daily for the jet stream — managed cooperatively by Shanwick Oceanic Control (UK/Ireland, covering the eastern portion) and Gander Oceanic Control (Canada, covering the western portion). Before satellite ADS-B, aircraft on these tracks reported their position by HF radio voice communication to oceanic controllers at defined waypoints. Controllers maintained the aircraft's position on paper strips or early computer displays based on these reports.

The position updates were 10 degrees of longitude apart on the North Atlantic — roughly every 45–60 minutes of flight time depending on speed. Between reports, controllers had no real-time knowledge of where an aircraft was. Separation was maintained using time-based procedural separation: aircraft on the same track were required to be at least 10 minutes apart in time (corresponding to roughly 80–100 nautical miles at typical airliner speeds). Aircraft on adjacent tracks at the same altitude were required to maintain 60 nautical miles lateral separation — a margin sized to account for the navigational uncertainty in the pre-GPS era.

The Pacific posed even greater challenges. The width of the Pacific between North America and Asia required many hours of HF radio communication, and HF propagation reliability varies significantly with ionospheric conditions. SELCAL (Selective Calling) systems allowed controllers to alert specific aircraft without requiring continuous radio monitoring, but communication delays and reliability issues were constant operational challenges. Position reports could be 30–60 minutes old by the time they were relayed through position reporting systems to the controlling facility.

MH370 and the Transformation of Oceanic Surveillance

The disappearance of Malaysia Airlines Flight MH370 on 8 March 2014 — a Boeing 777 carrying 239 people that vanished over the southern Indian Ocean — exposed the catastrophic consequences of the pre-satellite tracking gap with unprecedented public visibility. The aircraft ceased communicating normally over the Gulf of Thailand, but continued flying for approximately seven hours, far beyond any available radar or ADS-B coverage, before crashing in one of the most remote and inhospitable areas of ocean on earth.

Investigators pieced together the aircraft's probable path using satellite communication (SATCOM) system handshakes — the periodic "handshake" signals exchanged between the aircraft's Inmarsat SATCOM unit and the satellite, logged for billing and network management purposes even when no voice or data communications were being made. The timing and Doppler shift of these handshakes allowed analysts to constrain the aircraft's possible positions to an arc in the southern Indian Ocean, but the uncertainty radius remained hundreds of kilometres — enough to encompass a search area of millions of square kilometres.

MH370 generated immediate and sustained industry action on global aircraft tracking. ICAO adopted the Global Aeronautical Distress Safety System (GADSS) concept, which includes requirements for airlines to track their aircraft at 15-minute position reporting intervals globally (implemented from 2018), reducing to 1-minute intervals if the aircraft appears to be in distress. This "normal tracking" requirement effectively made mandatory the use of ACARS (Aircraft Communications Addressing and Reporting System) position reporting via satellite for flights in oceanic airspace — previously optional or used on a carrier-by-carrier basis.

The GADSS also includes provisions for "autonomous distress tracking" — the requirement that aircraft in an apparent distress situation (such as rapid altitude deviation or sudden loss of normal communications) automatically begin transmitting position reports at 1-minute intervals via a separate, independent tracking system that cannot be disabled from the cockpit. The Automatic Deployable Flight Recorder (ADFR) — a flotation flight recorder that automatically ejects and deploys a locator beacon upon water impact — was mandated for new aircraft designs from 2021 and is being considered for retrofit on existing long-haul aircraft.

The deployment of Aireon's satellite ADS-B system in 2019 effectively addressed the fundamental gap MH370 exposed: an aircraft transmitting normally cannot fly for seven hours in oceanic airspace without being tracked, because the ADS-B broadcasts are now received by satellites providing near-real-time position updates regardless of where on Earth the aircraft is. MH370's disappearance, for all its tragedy, accelerated aviation's transition to global real-time surveillance by at least a decade.