ADS-B. Automatic Dependent Surveillance–Broadcast, is how modern aircraft continuously announce who and where they are. It is the raw material behind every flight tracker, and understanding it is the key to understanding what can be trusted, what can be spoofed, and how AeroScope tells the difference.
The name is the explanation. The transmission is automatic (no interrogation needed), dependent on the aircraft's own navigation systems (typically GPS), it is a form of surveillance, and it is broadcast in the clear for anyone in range to receive. Twice a second, a transponder radiates a small packet containing position, altitude, velocity and identity.
GPS latitude/longitude plus barometric and often geometric (GPS) altitude, the two altitude sources are a key cross-check for integrity.
Ground speed, track angle and vertical rate. Enhanced-surveillance aircraft may also broadcast indicated/true airspeed, Mach and heading.
A unique 24-bit ICAO address and, usually, a flight callsign, the keys AeroScope uses to de-duplicate and to look up registry and military allocations.
NIC, NACp, NACv and SIL describe how trustworthy the position is. They are central to spoofing detection, a fabricated message often gets them wrong.
Most of the world broadcasts ADS-B on 1090 MHz (1090ES, Mode S Extended Squitter). In the United States, many lighter aircraft use 978 MHz UAT (Universal Access Transceiver). AeroScope's feeds cover the 1090 MHz picture globally and UAT where networks republish it.
| Field group | Examples | Why it matters here |
|---|---|---|
| Position | lat, lon, baro alt, geo alt | Drives the map and altitude self-consistency checks |
| Velocity | ground speed, track, vertical rate | Feeds Kalman tracking and envelope checks |
| Identity | ICAO hex, callsign, squawk | Registry, military hex, emergency-squawk alerts |
| Integrity | NIC, NACp, NACv, SIL | The backbone of spoofing / DO-260B integrity scoring |
ADS-B was designed for safety and openness, not security. The messages are not signed or encrypted, so in principle anyone with a transmitter can fabricate one. That is exactly why AeroScope treats every broadcast as a claim to be checked rather than a fact to be trusted:
Read how those checks combine on the threat & attention scoring page, or how the multi-detector layer works on behavioural baselines.
An ADS-B Extended Squitter frame is 112 bits, transmitted on 1090 MHz roughly once or twice a second. The payload is split across message types rather than sent all at once, which is why a newly appearing aircraft fills in its details over several seconds.
| Element | Purpose |
|---|---|
| ICAO 24-bit address | Unique per airframe and present in every frame. The only reliable key for joining reports across networks. See fusion. |
| Identification | The callsign, sent on its own message type and therefore sometimes missing when a track first appears. |
| Airborne position | Latitude and longitude, encoded using compact position reporting so a pair of messages is needed to resolve an unambiguous global position. |
| Altitude | Usually barometric, referenced to standard pressure. It is a flight level, not a height above the ground beneath the aircraft. |
| Velocity | Ground speed, track and vertical rate, sent separately from position. |
| Integrity and accuracy | NIC, NACp, NACv and SIL: the avionics stating how good it believes its own data to be. Central to spoof detection. |
| System | How it works | Key limitation |
|---|---|---|
| Primary radar | Transmits a pulse and listens for a reflection | Detects an object but learns no identity or altitude from it |
| Secondary radar | Interrogates a transponder, which replies with identity and altitude | Needs ground interrogators and cooperative equipment |
| ADS-B | The aircraft derives its own position, normally from GNSS, and broadcasts it unprompted | Unauthenticated and dependent on the aircraft's own position source |
| Multilateration | Solves position from arrival-time differences at several receivers | Requires four or more stations hearing the same reply |
The trade ADS-B makes is precision and cost against trust. It is far more accurate and far cheaper to deploy than radar, and it takes the aircraft's word for where it is. That single design decision is the origin of both its success and its entire security problem. See ADS-B security.