Automatic Dependent Surveillance–Broadcast (ADS-B) has fundamentally changed how air traffic is monitored. Unlike traditional Secondary Surveillance Radar (SSR), which requires a ground-based interrogator to actively query each aircraft, ADS-B is a passive system — aircraft broadcast their own position, velocity, and identity continuously without being asked. This shift has significant implications for both infrastructure cost and coverage, particularly in remote or maritime environments where ground radar is impractical.
How ADS-B Works
At its core, ADS-B relies on a GNSS receiver aboard the aircraft to determine its position. That position, combined with other flight parameters, is encoded into a standardised message format and transmitted on 1090 MHz using a Mode S transponder. These transmissions occur spontaneously (hence "broadcast") at a rate of approximately two per second for airborne aircraft.
ADS-B Out — where the aircraft transmits — is now mandatory in most controlled airspace worldwide. ADS-B In — where an aircraft or ground station receives those transmissions — requires only a suitable 1090 MHz receiver and decoder.
The 1090 MHz Signal Structure
The physical signal is a Pulse Position Modulation (PPM) waveform. Each message consists of:
- Preamble: 8 µs synchronisation pattern (four pulses at fixed intervals)
- Data block: 56 or 112 bits, encoding message type, ICAO address, and payload
- CRC: 24-bit parity for error detection
The most important message type for surveillance is the Extended Squitter (ES) — a 112-bit message that carries the aircraft's position, altitude, velocity, identification, and status. Decoders identify these by their Downlink Format (DF=17 for ADS-B Out from Mode S transponders).
Message Types
| Type Code | Message Content |
|---|---|
| 1–4 | Aircraft identification (callsign) |
| 9–18 | Airborne position (with barometric altitude) |
| 19 | Airborne velocity (speed and heading) |
| 20–22 | Airborne position (with GNSS altitude) |
| 28 | Aircraft status (emergency, TCAS) |
| 31 | Operational status |
Decoding Pipeline
A ground-based ADS-B receiver performs the following chain of operations:
- RF front end: Low-noise amplification and bandpass filtering centred on 1090 MHz
- Demodulation: PPM detection — identifying the presence or absence of a pulse in each 0.5 µs slot
- Framing: Detecting the preamble pattern to align to message boundaries
- CRC validation: Discarding corrupted messages
- Decoding: Parsing the payload fields for position, altitude and identity
- CPR decoding: Resolving latitude/longitude from the Compact Position Reporting encoding (requires two consecutive messages or knowledge of approximate position)
Receiver Sensitivity and Range
Because ADS-B signals are line-of-sight, ground station coverage is limited by the horizon — typically 200–400 km for high-altitude aircraft. At low altitudes or over terrain, coverage drops sharply. Receiver sensitivity is therefore critical: a 3 dB improvement in noise figure can meaningfully extend the practical detection range.
Sparrow Global's ADS-B receiver is designed around a low-noise front end with careful filtering to reject out-of-band interference — a common challenge in environments shared with DME (Distance Measuring Equipment) and TCAS transponders also operating near 1090 MHz. Combined with our in-house mapping and database software, the system provides real-time airborne situational awareness suitable for both fixed and mobile deployments.
ADS-B vs. Multilateration vs. Radar
| Technology | Active/Passive | Position Source | Infrastructure Cost |
|---|---|---|---|
| Primary Radar | Active | Radar return (no cooperation) | Very high |
| SSR / Mode C/S | Active (interrogation) | Transponder reply + radar bearing | High |
| Multilateration (MLAT) | Passive (multiple receivers) | Time difference of arrival | Medium |
| ADS-B | Passive (single receiver) | Aircraft GNSS | Low |
ADS-B's low infrastructure cost makes it particularly attractive for supplementary coverage, maritime and border surveillance, and airspace monitoring in regions where radar deployment is not economically viable.
Conclusion
ADS-B is a mature, standardised technology with well-defined signal formats and a broad ecosystem of compatible transponders and receivers. For organisations requiring airborne situational awareness without the cost of active radar infrastructure, a well-engineered 1090 MHz receiver paired with capable display and logging software represents an accessible and effective solution.
For more information on Sparrow Global's ADS-B and AIS receiver solutions, visit our AIS & ADS-B Solutions page or contact us directly.
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