When you open Flightradar24 and see a plane icon moving across the map, it looks like magic. Real-time, global, down to a few hundred feet of precision. But the technology behind it is well-understood, and knowing how it works helps you interpret what you're seeing — and why the map sometimes lies.
ADS-B: The Transponder That Talks
The foundation of modern flight tracking is ADS-B, which stands for Automatic Dependent Surveillance–Broadcast. The name is awkward, but the concept is straightforward.
Every commercial aircraft in the US (and most worldwide) is required to have an ADS-B Out transponder. This transponder broadcasts a signal roughly twice per second containing the aircraft's GPS-derived position, altitude, ground speed, heading, and a unique aircraft identifier (the ICAO hex code — essentially a fingerprint for that specific airframe).
"Automatic" means it broadcasts continuously without being prompted. "Dependent" means it depends on GPS for position data. "Surveillance" means it's designed to let air traffic control track the aircraft. "Broadcast" means the signal goes out in all directions — anyone with a receiver can pick it up.
This is the key point: ADS-B is a one-way broadcast. The plane sends the signal. It doesn't know who's receiving it or how many. The FAA receives it. Other planes nearby receive it. And so does anyone on the ground with an ADS-B receiver, including enthusiasts with a $30 USB dongle attached to a Raspberry Pi.
The Ground Receiver Network
FlightAware and Flightradar24 built their global coverage by deploying ground-based ADS-B receivers at thousands of locations, supplemented by a crowdsourced network of enthusiasts who run their own receivers and feed the data to the companies in exchange for free premium accounts.
Flightradar24 has over 35,000 receiver stations worldwide. FlightAware has a similarly large network. The coverage over the continental US is essentially complete — at typical cruising altitudes (30,000-40,000 feet), there's almost no location where a plane isn't within range of multiple receivers.
At lower altitudes, coverage thins. Departing aircraft are visible from the ground but may not reach enough receivers below 5,000 feet in rural areas. This is why flight trackers sometimes show a gap at the beginning or end of a flight.
MLAT: When There's No GPS
ADS-B depends on GPS for position data. But older transponders (Mode S, Mode C) don't broadcast GPS position — they only broadcast altitude and a unique identifier. How do trackers locate these aircraft?
The answer is Multilateration, or MLAT. If four or more receivers pick up the same transponder signal at slightly different times, the system can use those timing differences to triangulate the aircraft's position. The math is the same principle as GPS itself: the difference in signal arrival times tells you the distance ratio between receivers, and four points of data pin down the position in three-dimensional space.
MLAT is less precise than GPS-based ADS-B (accuracy of a few hundred meters versus tens of meters), and it requires dense receiver coverage. Over the US, MLAT fills in coverage gaps for older aircraft.
Why There's Always a Delay
The plane is there right now. But the map shows where it was 30-60 seconds ago. Here's where the delay comes from:
**GPS signal processing:** The aircraft's GPS receiver computes position and hands it to the transponder. This is fast — under a second.
**ADS-B broadcast interval:** The transponder broadcasts twice per second, so the position data is at most 500ms old when it goes out.
**Receiver processing:** The ground receiver picks up the signal and sends it to the aggregation server. Modern networks handle this in under a second.
**Server aggregation and processing:** FlightAware and Flightradar24 aggregate data from multiple receivers, reconcile conflicts, and compute the best position estimate. This takes a few seconds.
**Website and app delivery:** Your browser or app polls the server on some interval — typically every 5-15 seconds. Even if the data is fresh on the server, your client doesn't know about it until the next poll.
**Regulatory delay:** In the US, the FAA has historically required that consumer-facing trackers apply a position delay for non-certified users. FlightAware and Flightradar24 have applied for exemptions on their consumer products, but free tiers may have different delay rules than paid tiers.
The net result: what you see on the map is typically 10-60 seconds behind reality for paid tiers, and potentially several minutes behind for some free services.
What Happens Over the Ocean
Fly over the continental US and you're in wall-to-wall ADS-B coverage. Cross the Atlantic or Pacific and the picture changes.
Ground-based receivers obviously can't cover the ocean. Until recently, oceanic coverage was essentially zero — ATC tracked transoceanic flights using position reports over HF radio (the pilot reads their position every 10 degrees of longitude) and radar for the first and last few hundred miles of the route.
Satellite-based ADS-B changed this. Aireon (a joint venture using Iridium satellites) deployed a network of ADS-B receivers on satellites in low Earth orbit, providing coverage everywhere on the planet. Both FlightAware and Flightradar24 receive this data, though the update interval over oceans is typically 8-15 seconds rather than near-real-time.
Position Data vs. Status Data
This is an important distinction that affects how landing alerts work.
Position data is what the map shows: where is the plane right now, based on ADS-B signals. When a plane lands and slows to taxi speed, the position data shows it moving on the airport surface.
Status data is different: it comes from the airport and airline systems. "Landed" as a status event is generated by the airline's operations system when the plane checks in at the gate or when the flight operations computer marks the flight as in-blocks. This is what flight trackers display as "Landed" in the status field.
There's a reason SkyText uses status data rather than map position for landing detection. A plane on the taxiway is still technically on the runway system, not at the gate. The status data — coming from the airline or airport operations — is the authoritative signal that a flight has completed.
This also explains why the "Landed" status sometimes appears on FlightAware before the map has updated, or vice versa. They're different data sources with different latencies.
The 30-Second Delay in Context
Understanding these layers helps calibrate expectations. When your flight tracker shows the plane 50 miles out, it's actually closer to 47. When it shows the plane on the taxiway, it may already be at the gate. The data is always a few steps behind reality.
For most use cases — watching a flight cross the country, knowing roughly when someone will land — this lag is irrelevant. A 30-second delay doesn't change your airport pickup plan.
Where it matters is when something unusual happens: a diversion, a go-around, an emergency. These events may show up on the map before the airline status system updates. In those cases, the map is actually more current than the status data, and knowing the difference tells you which source to trust.
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FAQ
Frequently asked questions.
Why is my flight tracker showing the wrong position?
Most likely a data delay. The position on the map is typically 10-60 seconds behind the aircraft's actual position. In some cases, a receiver gap or ADS-B signal dropout can cause the last-known position to remain on the map for longer. The map isn't wrong so much as it's slightly old.
Does FlightAware show real-time data?
FlightAware's paid tiers show near-real-time data with a delay of a few seconds to a minute. The free tier historically had a 5-minute delay mandated by the FAA, though this policy has been updated over the years. Check the current terms for the specific delay on each tier.
Why do planes disappear over the ocean?
Ground-based ADS-B receivers don't cover the ocean. Satellite-based ADS-B (via Iridium's Aireon network) now provides near-complete oceanic coverage, so most transoceanic flights are trackable on paid tiers of FlightAware and Flightradar24. The update interval is slower over oceans — typically every 8-15 seconds rather than a few seconds.
What's ADS-B?
ADS-B stands for Automatic Dependent Surveillance–Broadcast. It's a system where aircraft transponders automatically broadcast their GPS position, altitude, speed, and identity twice per second. Anyone with a receiver — including the FAA, airlines, and hobbyists — can pick up this signal. It has replaced most radar-based tracking for domestic flights.
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Founder, SkyText
Aviation lover who built SkyText because families deserve to know when someone lands safely. Has tracked more flights than he'd like to admit.