How ADS-B Tracking Works: The 1090MHz Radio Signal Behind Real-Time Flight Trackers

1. Introduction to ADS-B Surveillance

Automatic Dependent Surveillance-Broadcast (ADS-B) is the backbone of modern global flight tracking networks. Unlike legacy secondary surveillance radar (SSR) which relied solely on rotating ground station antennas sending radio interrogations, ADS-B allows aircraft to autonomously determine their position via satellite navigation systems (GPS, GLONASS, Galileo) and broadcast that telemetry unencrypted twice per second to any ground receiver or satellite constellation within line-of-sight.

The term ADS-B breaks down into four fundamental aviation engineering concepts:

• Automatic: Requires no pilot or air traffic controller interaction to transmit position reports.
• Dependent: Relies on accurate position data derived from the aircraft's onboard Global Navigation Satellite System (GNSS) and Inertial Reference System (IRS).
• Surveillance: Provides altitude, ground speed, heading, squawk codes, and callsign information to air traffic control (ATC).
• Broadcast: Transmits radio signals openly across 1090 MHz to all nearby receivers (ADS-B In and ground stations).

2. Radio Frequency Physics & Modulation (1090 MHz Extended Squitter)

Commercial aircraft transmit ADS-B Out messages using 1090 MHz Extended Squitter (1090ES) transponders. The signal employs Pulse Position Modulation (PPM) at a data rate of 1 Megabit per second (1 Mbps). Each ADS-B Extended Squitter message consists of a 112-bit binary frame transmitted over a duration of exactly 120 microseconds (µs).

3. Multilateration (MLAT) for Non-ADS-B Aircraft

When an aircraft is equipped only with older Mode A/C or Mode S transponders without GPS position encoding, flight tracking platforms use Multilateration (MLAT). MLAT calculates the aircraft's 3D position by measuring the Time Difference of Arrival (TDOA) of transponder signals at four or more ground-based SDR (Software Defined Radio) receiver nodes situated at known geographic locations.

Because radio waves travel at the speed of light (c ≈ 299,792,458 m/s), even nanosecond differences in signal arrival times enable high-precision position calculations using hyperbolic equations.

4. Space-Based ADS-B Tracking & Satellite Constellations

Over remote oceanic airspaces like the North Atlantic Organised Track System (NAT-OTS) or Pacific corridors, ground receivers cannot capture 1090MHz signals due to Earth's curvature (line-of-sight limit of ~250 nautical miles at 37,000 feet). To solve this, low-Earth orbit (LEO) satellite constellations equipped with sensitive ADS-B receivers pick up broadcasts from above, streaming real-time satellite telemetry directly into the global flight radar engine.

See the runways and live weather on our JFK International Airport and London Heathrow Airport pages, or how flight numbers and radio callsigns look for American Airlines and British Airways.