ADS-B explained

What is ADS-B?

ADS-B: Automatic Dependent Surveillance-Broadcast is a modern surveillance method in which an aircraft determines its own position (typically from GNSS (Global Navigation Satellite System)) and periodically broadcasts that information along with its identity and velocity. Unlike radar, ADS-B is dependent on the aircraft's navigation sources and broadcast because messages are openly transmitted for any compatible receiver to hear.

A short history

ADS-B evolved from the wider Mode S/SSR (secondary surveillance radar) surveillance family. Its use was driven by the need for more accurate, lower cost surveillance than secondary radar, especially in remote or oceanic regions. Mandates and infrastructure rollouts accelerated through the 2010s for example the US required ADS-B Out equipage in controlled airspace from January 1st 2020.

How it works (technical overview)

An ADS-B transmitter regularly emits short digital messages called extended squitters. A typical exchange sequence is:

1. The aircraft reads position/time from GNSS and computes ground speed/track.
2. An onboard avionics unit formats a message containing the ICAO 24-bit address (HEX code), callsign/flight ID, latitude, longitude, altitude, vertical rate, and velocity vector.
3. The message is broadcast on a dedicated frequency and may be received by nearby aircraft (ADS-B In), ground stations, or hobbyist receivers (any radio receiver listening to the 1090MHz frequency).

Message formats and basics

• Mode S Extended Squitter (1090ES) uses the Mode S physical layer on 1090 MHz. The squitter is 112 bits long (after preamble) and carries aircraft identification and state vectors.
• UAT (978 MHz) used mainly by general aviation in the USA and carries similar traffic data plus additional services such as FIS-B (weather).
• Common fields: ICAO address (unique hex code), callsign (flight number or registration), position (lat/lon), altitude (pressure or GNSS), velocity, and integrity flags.
• Update rate varies with aircraft phase: typically 0.5–2 seconds for position, and more frequent for high-speed changes.

Variants: 1090ES vs UAT

Two practical variants are in common use:

• 1090ES the global commercial standard; used by airliners, business jets and most commercial traffic. It operates on 1090 MHz and is backwards-compatible with Mode S systems.
• UAT (Universal Access Transceiver) primarily used by US general aviation on 978 MHz. UAT supports FIS-B (weather/flight information) and is intended for light aircraft.

Where ADS-B is used

• Air Traffic Surveillance replaces or supplements radar in many regions, especially where radar coverage is poor.
• Traffic Awareness ADS-B In gives pilots nearby traffic information that can improve situational awareness and reduce collision risk.
• Flight tracking hobbyist and commercial sites (e.g., FlightAware, Flightradar24) aggregate ground receivers to plot live flights.
• Weather & services UAT carries FIS-B; ground networks can forward weather and TIS-B traffic to aircraft.

Equipment and receiving ADS-B at home

You can receive ADS-B messages with modest hardware and free software:

• Hardware: a cheap RTL-SDR dongle or a dedicated 1090 MHz USB receiver (e.g., FlightAware Pro Stick) connected to a small antenna gives reliable reception nearby.
• Software: popular decoders include dump1090, runway, and aggregator software like Virtual Radar Server or the FlightAware PiAware client.
• Tips: place the antenna outdoors or near a window, high and clear of obstructions for best line-of-sight reception.

Regulation and equipage

Regulatory bodies have mandated ADS-B Out in various airspaces: the US required equipage for most aircraft in controlled airspace from 2020; other regions such as Europe and Australia have their own timelines and requirements. Mandates generally specify performance (position accuracy, update rate) and interoperability with existing surveillance systems.

Limitations, privacy & security

• Line-of-sight radio propagation at 1090/978 MHz is essentially line-of-sight; terrain and buildings can block reception.
• GNSS dependency ADS-B relies on accurate position sources; GNSS faults or spoofing can lead to incorrect broadcasts.
• Open broadcast messages are unencrypted and unauthenticated. This openness enables hobbyist tracking but also raises privacy concerns and the possibility of spoofing or message injection. Operators concerned about privacy use techniques such as blocking services or using temporary identifiers where allowed.
• Spectrum congestion as ADS-B use grows, channel occupancy and packet collisions become operational concerns in dense airspaces.