ADS-B is the technology that makes live flight tracking possible. Here is everything you need to know about how it works and why some aircraft do not appear.
ADS-B stands for Automatic Dependent Surveillance-Broadcast. It is a surveillance technology where aircraft automatically broadcast their position, altitude, speed, heading, and identification to anyone with a receiver — including air traffic control, other aircraft, and hobbyists on the ground.
Unlike traditional radar which requires ground stations to actively ping aircraft, ADS-B is passive — the aircraft continuously transmits its own data using GPS to determine its position. This makes it significantly more accurate and requires no interrogation from the ground.
The data is broadcast on 1090 MHz and can be received by anyone with a simple antenna and receiver. This is why free flight tracking services like ADSB.lol and others can provide real-time data at no cost — the data is literally being broadcast publicly by every equipped aircraft.
ADS-B is significantly more accurate than traditional radar. Radar typically has position accuracy of around 100-200 metres, while ADS-B using GPS can achieve accuracy of 7.5 metres or better.
The altitude data is also more precise. ADS-B reports barometric altitude (pressure altitude) which is what air traffic control uses, as well as geometric altitude (GPS altitude) on newer implementations. The positions you see on flight trackers are updated every 0.5-2 seconds depending on the aircraft equipment.
Several reasons explain why aircraft may not appear:
Not all aircraft are required to carry ADS-B equipment. In the UK and Europe, ADS-B Out has been mandatory for most commercial aircraft since 2020, but smaller general aviation aircraft, older military aircraft, and some helicopters may not be equipped.
Military aircraft can disable their transponders or use Mode S without ADS-B, making them invisible to civilian receivers. Some military operations are conducted with transponders off entirely.
Private aircraft and business jets sometimes disable or restrict their transponders for privacy — particularly in the US where the FAA offers a privacy programme for registered aircraft owners.
Ground coverage has gaps — if there is no ADS-B receiver within range of a low-flying aircraft, its transmissions will not be picked up. Coverage is excellent over densely populated areas but can be patchy in remote regions.
Yes — ADS-B data is as close to real time as practically possible. The aircraft broadcasts its position every 0.5 to 2 seconds. Ground receivers pick this up immediately and forward it to aggregators like ADSB.lol within 1-2 seconds. By the time it appears on your screen the data is typically 2-5 seconds old.
This is far more current than traditional radar which updates every 4-12 seconds depending on the rotation speed of the antenna.
ADS-B Out — which broadcasts the aircraft position — requires a GPS receiver, a transponder capable of 1090ES (Extended Squitter) transmissions, and ideally a WAAS-enabled GPS for precision accuracy.
ADS-B In — which allows an aircraft to receive ADS-B data from other aircraft and display it in the cockpit — requires additional display equipment but is not mandated by most regulators.
In Europe, EASA mandated ADS-B Out for most commercial aircraft operating in controlled airspace from June 2020. In the US, the FAA mandated it from January 2020 for aircraft in most controlled airspace.