ADS-B Explained
Automatic Dependent Surveillance-Broadcast · Sources: ICAO · FAA · EUROCONTROL · SKYbrary
ADS-B (Automatic Dependent Surveillance-Broadcast) is a surveillance technology where aircraft transmit their position (derived from GNSS), altitude, velocity, and identification on a fixed schedule. Ground stations, satellites (via Aireon), and other aircraft receive these broadcasts. ADS-B Out (transmit) is mandatory for most commercial aircraft globally as of 2020 (FAA) and 2017–2020 (EU). ADS-B is the foundation of modern air-traffic surveillance, enables apps like FlightAware and FlightRadar24, and provides the public-data signal used to detect anomalies including GPS spoofing patterns.
What ADS-B is — and isn't
ADS-B is:
- ✓Automatic: the aircraft transmits without controller request
- ✓Dependent: depends on the aircraft's own navigation (GNSS) for position
- ✓Surveillance: provides position and identification to ATC and other users
- ✓Broadcast: open transmission, any receiver in range can decode
ADS-B is not: radar (radar bounces signal off aircraft; ADS-B has the aircraft self-report). It's not encrypted (anyone with a $20 USB SDR receiver can decode aircraft signals).
ADS-B Out vs ADS-B In
Mandatory for most commercial aircraft. Transmits position, altitude, velocity, callsign, integrity flags. Equipment cost ~$5K–20K per aircraft.
Optional capability. Receives ADS-B transmissions from nearby aircraft for cockpit traffic awareness. Required for some advanced ATC services.
1090ES vs UAT — the two link types
| Link | Frequency | Use |
|---|---|---|
| 1090ES | 1090 MHz | Global standard. Required for international flights. Built on Mode S transponder. |
| UAT | 978 MHz | US only, below 18,000 ft. Used by some general aviation. Offers FIS-B (weather/NOTAM uplink). |
Commercial airliners worldwide use 1090ES.
What an ADS-B message contains
Per the 1090ES protocol (DO-260B for current generation), an ADS-B Out message includes:
- →ICAO 24-bit address — unique aircraft identifier (hex, e.g.
A12345for a US registration) - →Position — latitude and longitude derived from GNSS
- →Altitude — barometric and/or geometric
- →Velocity — ground speed and track
- →Vertical rate — climb or descent rate
- →Callsign — flight identification (e.g.
BAW123) - →Emergency status, squawk code, special-purpose indicators
- →Integrity quality indicators — NIC, NACp, NACv, SIL, GVA (next section)
Integrity quality indicators — NIC, NACp, NACv, SIL
Critical for understanding ADS-B reliability: each transmission includes flags indicating how confident the aircraft is in its data.
Indicates containment radius for horizontal position. NIC=7 means position is within 0.2 NM; NIC=0 means position integrity is unknown or position is invalid.
Indicates 95% accuracy of horizontal position. NACp=8 means accuracy better than 0.05 NM.
Velocity accuracy. NACv=2 means velocity accurate to 3–10 m/s.
Probability that reported position is within NIC containment radius. SIL=3 is the highest standard level.
Why this matters for GPS interference detection: when an aircraft's GNSS receiver experiences degradation (jamming, spoofing, multi-path), the aircraft itself reports degraded NIC/NACp values in its ADS-B Out. This makes ADS-B telemetry a public-data signal for detecting widespread GNSS interference patterns.
Ground stations vs space-based ADS-B
Networks of ground stations receive ADS-B transmissions. Coverage limited to line-of-sight from receivers. Operated by ANSPs (FAA, EUROCONTROL), public crowdsourced networks (OpenSky Network, ADS-B Exchange, Flightradar24), and commercial providers.
Aireon hosts ADS-B receivers on the Iridium NEXT satellite constellation, providing global 100% coverage including oceans and polar regions. Operational since 2019. Used by NAV Canada, NATS (UK), ENAV (Italy), and other ANSPs for oceanic surveillance.
Mandates and timeline
| Region | Mandate |
|---|---|
| Australia | Mandate since 2017 (above FL290) |
| Europe (EU) | ADS-B Out mandate phased 2017–2020 for IFR aircraft over 5,700 kg MTOW |
| United States (FAA) | Mandate since 1 January 2020 for most controlled airspace |
| Other regions | Various national-level mandates aligned with ICAO recommendations |
Where ADS-B data is publicly available
- →OpenSky Network: open research-oriented platform with bulk historical access for non-commercial use
- →ADS-B Exchange: unfiltered public ADS-B data including military/sensitive flights
- →Flightradar24, FlightAware: commercial consumer-facing tracking with proprietary data integration
- →Crowdsourced aggregators (e.g., RadarBox, plane.live): build on community-contributed ground stations
Limitations and known issues
- !Unencrypted, unauthenticated: ADS-B Out signals are open. Spoofing (broadcasting false aircraft data) is technically feasible and has been demonstrated in research settings.
- !GNSS dependency: ADS-B position is only as accurate as the aircraft's GNSS receiver. GPS jamming/spoofing degrades ADS-B utility — though the NIC/NACp flags surface this.
- !Coverage gaps: ground-station coverage is uneven globally. Some regions (oceanic, Africa, sparse-population areas) historically relied on space-based or other surveillance.
- !Military/sensitive flights: state aircraft, some sensitive operations, and certain military/diplomatic flights are filtered or de-identified on commercial trackers.
Sources
- ICAO Annex 10 Vol. IV — Surveillance Radar and Collision Avoidance Systems
- RTCA DO-260B — ADS-B 1090ES technical standard
- FAA AC 20-165B — Airworthiness Approval of ADS-B Out Systems
- EUROCONTROL — ADS-B implementation guidance
- SKYbrary — ADS-B reference articles
- Aireon — space-based ADS-B operational data