How Flight Tracking Works
From ground-based radar to satellite surveillance, a complete guide to how aircraft are tracked across the globe in real-time.
The Evolution of Aircraft Surveillance
Flight tracking has evolved dramatically since the mid-20th century. What began with primitive radio direction-finding has become a global network of sensors, satellites, and software capable of tracking tens of thousands of aircraft simultaneously.
1. Primary Surveillance Radar (PSR)
The oldest form of aircraft detection. A ground station transmits powerful radio pulses and listens for reflections from aircraft. PSR can detect any object, even those without a transponder, but provides only position (range + bearing) with no identity or altitude information. Range: 60-200 nautical miles. Still used at major airports as a backup.
2. Secondary Surveillance Radar (SSR)
An improvement over PSR. The ground station transmits an "interrogation" signal, and aircraft transponders reply with coded information. Three modes exist:
| Mode | Data Transmitted | Era |
|---|---|---|
| Mode A | 4-digit squawk code (identity) | 1960s |
| Mode C | Squawk code + pressure altitude | 1970s |
| Mode S | 24-bit ICAO address + altitude + selective interrogation | 1990s |
Mode S is the foundation for modern ADS-B. Each aircraft has a unique 24-bit ICAO address (like a MAC address for airplanes), enabling individual identification without relying on squawk codes.
3. ADS-B. The Modern Standard
Automatic Dependent Surveillance. Broadcast (ADS-B) is the current global standard. Unlike radar (which requires ground infrastructure to interrogate), ADS-B is automatic, aircraft continuously broadcast their position, altitude, speed, heading, and identity on 1090 MHz. Anyone with a simple receiver can pick up these signals.
Key characteristics:
- Automatic: No interrogation needed, aircraft broadcast continuously
- Dependent: Relies on aircraft's own GPS for position (not ground radar)
- Surveillance: Used for air traffic control and safety
- Broadcast: Unencrypted, receivable by anyone within range
ADS-B transmits: ICAO hex address, latitude, longitude, barometric altitude, ground speed, track heading, vertical rate, callsign, squawk code, aircraft category, and integrity parameters (NIC, NAC, SIL). Update rate: every 0.5-2 seconds.
The FAA mandated ADS-B Out for most controlled airspace in the US as of January 1, 2020. EASA has similar requirements in Europe. This is why most commercial aircraft are now trackable via ADS-B.
4. MLAT. Multilateration
For aircraft that transmit Mode S but not full ADS-B position data, multilateration can calculate position by measuring the Time Difference of Arrival (TDOA) of signals at 4+ ground receivers. This enables tracking of older aircraft that have Mode S transponders but no GPS-based ADS-B Out. Accuracy: typically 50-500 meters.
5. Satellite-Based Tracking
The newest layer. Companies like Aireon have placed ADS-B receivers on Iridium NEXT satellites, enabling global coverage, including over oceans, poles, and remote areas where ground receivers don't exist. This closed the last major gap in flight tracking and was a direct response to the MH370 disappearance in 2014.
6. Community Receiver Networks
Platforms like AeroScope aggregate data from thousands of volunteer-operated ADS-B receivers worldwide. Anyone can set up a receiver for under $50 (RTL-SDR dongle + antenna) and feed data to community networks. This creates comprehensive coverage that rivals commercial systems. AeroScope uses data from adsb.fi, adsb.lol, and OpenSky Network.
How AeroScope Puts It Together
AeroScope queries four ADS-B aggregator APIs in parallel (adsb.fi, adsb.lol and OpenSky), merges results by ICAO hex, attaches registry metadata, and pushes the result to the browser over a WebSocket. On top of that core flow, the server runs a few derived analyzes on each aircraft.
The Analyzes, In Plain Terms
- Integrity checks: NIC, NAC, SIL fields from the ADS-B message, plus an impossible-position-jump check between consecutive updates. Either can hint at noisy reception or, very occasionally, spoofing.
- Drone-candidate scoring: Low altitude, low ground speed, low climb rate, and ICAO category B6 (UAV) each contribute to a 0–100 score. False positives on Cessnas doing pattern work happen, which is why we keep an exclusion list of common manned types.
- Route deviation: For callsigns we've seen before, we compare current position to a baseline route and flag substantial offsets (diversion, hold, off-airway).
- Formation clustering: Aircraft within a few hundred metres, matching heading, speed, and altitude, get grouped as a probable formation.
- Composite threat score: 0–100, weighted combination of integrity warnings, emergency squawks (7500/7600/7700 dominate), military flag, drone score, and proximity. Useful for sorting; not a verdict.
- Pattern-of-life: Per-aircraft running stats, typical altitudes, typical times of day, common origin/destination, that let us flag aircraft well outside their own normal envelope.
Honest Limits
None of these analyzes are ML in any deep sense: they're transparent heuristics built from the same fields you can see in the aircraft detail panel. We don't have multilateration, satellite ADS-B, or our own receiver network. International NOTAMs aren't integrated. Military aircraft that don't transmit ADS-B don't appear. The latency from the original 1090 MHz broadcast to a pixel on your screen is typically 3–15 seconds.