How Flight Tracking Works: ADS-B, Radar, and Satellite Surveillance

Radar Basics: Primary and Secondary Surveillance

Aviation radar falls into two fundamental categories with distinct operating principles and different information content. Understanding the distinction is essential to understanding why modern flight tracking uses multiple complementary technologies rather than relying on any single system.

Primary surveillance radar (PSR) works by transmitting a pulse of radio energy and detecting the reflection from any object in the beam's path. The radar does not require any cooperation from the aircraft: it simply detects the reflection of its own transmitted energy. PSR provides range (from the time delay between transmission and reception) and bearing (from the antenna's pointing direction), giving a two-dimensional position fix. It provides no identification, no altitude, and no information that the aircraft itself is transmitting — it simply shows a "blip" on a display indicating that something is in a particular location. PSR is essential for detecting aircraft with transponder failure, military aircraft operating in "squawk standby" mode, and any object that returns a radar reflection, including weather phenomena and ground clutter.

Secondary surveillance radar (SSR) works on a cooperative basis. The ground station transmits an interrogation signal; the aircraft's transponder receives the interrogation and transmits a reply containing encoded information. In Mode A operation, the reply contains a four-digit octal squawk code (0–7 in each digit, giving 4,096 possible codes) that identifies the aircraft to the controller. In Mode C operation, the reply additionally contains the aircraft's pressure altitude derived from an encoding altimeter, giving the controller a three-dimensional position. In Mode S operation — the basis for all modern secondary radar — the reply contains the aircraft's 24-bit ICAO address (unique to each aircraft registration) and can carry a range of data messages in addition to basic identification and altitude.

SSR provides vastly more information than PSR but depends on the aircraft having a functioning, compliant transponder. ICAO regulations require transponders on all aircraft operating in controlled airspace above certain altitudes, but compliance varies in uncontrolled airspace and in airspace with limited radar coverage. The 2014 disappearance of Malaysia Airlines MH370 demonstrated the limitations of transponder-dependent tracking: once MH370's transponder stopped transmitting (due to either malfunction or deliberate deactivation), the aircraft became invisible to SSR across most of its subsequent flight path.

Radar coverage is inherently limited by geography and line-of-sight physics. A radar antenna on the ground cannot see aircraft at low altitude beyond approximately 200–300 km due to Earth's curvature. Over oceanic routes — the North Atlantic, the Pacific, polar routes — there is no radar coverage at all. For decades, oceanic flights operated under a procedural separation system where aircraft transmitted position reports by HF radio at defined waypoints, with controllers maintaining separation based on these periodic reports rather than continuous radar tracking. This procedural system allowed large uncertainties in aircraft position and required conservative separation minima of 50–100 nautical miles.

ADS-B Technology: Automatic, Dependent, Broadcast

Automatic Dependent Surveillance-Broadcast (ADS-B) is the technology that has transformed flight tracking from a radar-dependent, ground-station-limited system to a global, near-real-time tracking capability accessible to anyone with an inexpensive receiver. The three adjectives in the name capture its operating principle: it is automatic (transmitting without pilot action or ground interrogation), dependent (on the aircraft's own navigation system for position data), and broadcast (transmitting to any receiver within range, not just responding to a specific interrogation).

An ADS-B-equipped aircraft continuously broadcasts a data packet on 1090 MHz that contains the aircraft's ICAO 24-bit address, GPS-derived position (latitude and longitude), barometric altitude, GPS altitude, track, ground speed, and a timestamp. This broadcast occurs approximately twice per second and can be received by any antenna and receiver tuned to 1090 MHz — including the small, inexpensive software-defined radio (SDR) dongles that aviation enthusiasts use to build personal ADS-B ground stations for a cost of around $25. The combination of dense coverage from thousands of volunteer-operated ground stations and the aggregation of their data by services like Flightradar24, FlightAware, and ADS-B Exchange has created a comprehensive picture of global aviation traffic that would have been technically and financially impossible with radar alone.

ADS-B mandate timelines have driven rapid adoption. The FAA required ADS-B Out (transmitting) equipment on all aircraft operating in most US controlled airspace from January 1, 2020. EASA extended a similar mandate to European airspace. Australia, Japan, and Singapore have implemented their own ADS-B mandates. As a result, the vast majority of commercial aviation now operates ADS-B-equipped aircraft, and the global flight tracking picture is substantially complete for commercial operations over areas with any ground-based receiver coverage.

The limitation of ground-based ADS-B reception is identical to the radar line-of-sight problem: receivers on the ground cannot hear aircraft at low altitude at great distances, and oceanic routes remain outside ground station range. This gap has been addressed through space-based ADS-B, in which low Earth orbit (LEO) satellite constellations receive ADS-B broadcasts directly from aircraft over oceanic and polar regions where no ground stations exist. Aireon, a joint venture between Iridium Communications and a consortium of air navigation service providers, deployed space-based ADS-B receivers on all 66 Iridium NEXT satellites, achieving global coverage including polar and oceanic regions in 2019. Airlines operating North Atlantic and Pacific crossings are now tracked continuously rather than through periodic HF position reports, enabling a reduction in oceanic separation minima from 50 to 15 nautical miles and allowing airlines to fly more fuel-efficient routes through previously unavoidable separation buffers.

Satellite Tracking: ACARS and Space-Based Systems

Satellite communication and tracking technologies complement ADS-B to provide additional layers of aircraft position data and two-way communication capability. The Aircraft Communications Addressing and Reporting System (ACARS), developed by ARINC in 1978 and now managed by SITA and ARINC (now Collins Aerospace), provides a digital datalink between aircraft and their airline ground systems. ACARS transmits position reports, engine data, maintenance alerts, weather requests, and operational messages — originally over VHF radio, then via SATCOM on long-haul routes — allowing airlines and maintenance organizations to monitor aircraft performance in real time.

ACARS position reports are generated at programmed intervals (typically every 15–30 minutes) and transmitted to the airline's operational control center. These reports are used for flight following — monitoring that the aircraft is on course and that its systems are operating normally — and they form the primary real-time data feed for many airlines' operational control systems. The ACARS data from MH370 provided the basis for Inmarsat's analysis of the aircraft's final trajectory: by analyzing the frequency shifts (Doppler effect) in the regular ACARS "handshakes" between MH370 and the Inmarsat-3 F1 satellite, investigators were able to narrow the search area to a southern arc of the Indian Ocean, demonstrating the forensic value of satellite communication records even when conventional tracking fails.

The Global Aeronautical Distress and Safety System (GADSS), developed by ICAO in response to MH370, mandates that all new large aircraft manufactured after November 2023 transmit position at least every 15 minutes throughout the flight, with an automatic distress tracking capability that increases transmission frequency to every minute when an emergency is detected. The 15-minute position reporting standard, already adopted voluntarily by most major airlines after MH370, means that a missing aircraft can be located within approximately 200 nautical miles (15 minutes at 500 knots) of its last known position — dramatically narrowing the search area compared to the MH370 scenario where the last radar contact was hours before the aircraft's likely terminal point.

Flightradar24 and FlightAware: The Consumer Data Layer

The consumer-accessible flight tracking services that have made aviation tracking familiar to the general public — Flightradar24 (founded in Sweden in 2009), FlightAware (founded in the US in 2005), and Plane Finder — are data aggregation and visualization platforms built on top of the raw ADS-B and multilateration data collected by networks of volunteer and commercial ground stations.

Flightradar24 operates the largest global network of ADS-B receivers, with over 35,000 ground stations operated by volunteers who receive free premium subscriptions in return for contributing data. The company aggregates data from its own station network, from satellite-based ADS-B (Aireon and others), from MLAT (multilateration — a technique that uses timing differences between multiple receivers to locate aircraft without ADS-B transponders), and from airline and airport data feeds to construct the near-real-time global traffic picture displayed on its website and app. Flightradar24 processes approximately 180,000 flights daily and serves over 4 million active users per day in normal times, with spikes during major aviation incidents when public interest drives traffic multiples higher.

FlightAware, acquired by Collins Aerospace in 2021, has historically focused more on data services for aviation professionals and airlines rather than consumer visualization. Its Firehose data feed, which provides raw ADS-B and airline operational data, is used by airlines, airports, and technology companies for operational monitoring. FlightAware's predictive analytics, which use machine learning on historical flight data to forecast delays and gate arrival times, are integrated into American Airlines' customer-facing app and various airport systems.

The commercial model of flight tracking services has evolved from advertising-supported consumer apps toward B2B data licensing. Airlines, airports, ground handlers, and corporate travel tools pay significant fees for real-time and historical flight data APIs. The insurance and financial derivatives market — aviation delay derivatives, weather disruption insurance — also purchases flight tracking data for underwriting and claims verification. The data that volunteers generate by running $25 SDR receivers in their windows has significant commercial value when aggregated at scale.

Coverage Gaps and MH370: The Case for Global Tracking

The disappearance of Malaysia Airlines flight MH370 on March 8, 2014, exposed the most serious gap in global aviation tracking: the inability to continuously track aircraft over oceanic regions. MH370 departed Kuala Lumpur for Beijing at 00:41 local time carrying 239 passengers and crew. At 01:21, the aircraft's transponder stopped transmitting. Malaysian military primary radar tracked it for approximately 70 minutes as it turned back across the Malay Peninsula and flew northwest into the Strait of Malacca, but this radar data was not shared with civilian air traffic control in real time. After the military radar contact ended, MH370 was untracked — a commercial widebody aircraft carrying 239 people, somewhere over the Indian Ocean, with no position information whatsoever except the Inmarsat satellite handshakes that provided only an arc of possible positions.

The search for MH370 became the most expensive in aviation history, ultimately costing over $200 million across the primary Australian-led underwater search and subsequent private contract searches. As of 2025, the wreckage of MH370's main body has not been located. The debris that has washed ashore on Indian Ocean islands and the East African coast over the years confirms the aircraft ended in the southern Indian Ocean, consistent with the Inmarsat arc analysis, but the exact location remains unknown. Without the black boxes (flight data recorder and cockpit voice recorder), the cause of the transponder deactivation and the flight's subsequent path remain subject to competing hypotheses.

The regulatory response — GADSS 15-minute position reporting, enhanced transponder tamper detection requirements — addresses the monitoring gap but not the root cause of the disappearance. ICAO has also worked to improve the sharing of military radar data with civil aviation search and rescue, one of the key breakdowns in the MH370 response. The Aireon space-based ADS-B system, completed in 2019, provides the continuous global tracking capability that would have allowed MH370 to be tracked even over the Indian Ocean — had it been operating in 2014. The lesson of MH370 has been applied, but at the cost of the most profound unsolved mystery in modern aviation.