# How flight trackers work, and why the position lags

> Where a dot on the map comes from, how old it is by the time you see it, and how to tell a live position from a guess.

By Derek Johnson. Updated Nov 7, 2026. Source: https://skycapapp.com/articles/overhead/how-flight-trackers-work/

A flight tracker is a map of other people's radio reports. Planes broadcast their position, receivers collect it, a server merges it, and your phone asks the server what's new. Each hop adds seconds, and over an ocean the dot may be an estimate with no report behind it at all. A good position is a few seconds old. A bad one can be minutes old and still look perfectly smooth.

Most trackers don't tell you which you're looking at. I think they should.

## Where a position comes from

| Source | What it gives a tracker | The catch |
|---|---|---|
| The plane's own broadcast (ADS-B) | Position from satellite navigation, about once a second, with speed, altitude and call sign | Needs a receiver within line of sight. Hobbyists and companies both run them |
| Satellites listening from above | The same broadcast, over oceans and poles | Usually a paid service |
| Timing the signal (multilateration) | A position for planes that don't report one, worked out from when the signal reached each receiver | Needs at least four receivers in range, so it works best around cities |
| Air traffic control | Flight data from radar and the same broadcasts | Rules and access differ by country |
| The airline | Timed messages for pushback, take-off, landing and arrival at the gate | Says when an event happened, not where the plane is |
| The schedule and flight plan | Route and times | An intention, not a sighting |

A broadcast doesn't say where a plane is going, so a tracker has to match the call sign to a schedule or flight plan to show a route.

## Why the dot is behind the plane

Follow one report. The plane sends it. A receiver hears it almost instantly, then forwards it with others. A server merges several sources, drops duplicates and publishes. Your app asks every so many seconds, then draws it.

None of those steps is long, but they add up, and the plane doesn't wait. At 500 mph, a jet covers about 730 feet a second.

| Age of the position | How far the plane has moved at 500 mph |
|---|---|
| 1 second | 730 feet |
| 5 seconds | 0.7 miles |
| 15 seconds | 2.1 miles |
| 60 seconds | 8.3 miles |

So when you match a dot against a plane in the sky, a 15-second-old position puts the dot visibly behind it. Some apps slide the dot forward between reports so it looks continuous. That's fine as long as the slide is small. It's a guess, though, and the smoothness doesn't tell you how good it is.

Update rates also vary with the source. Broadcasts arrive every second, while other systems report far less often. A tracker that merges them can have a plane that moves in small steps over land, then in big jumps elsewhere.

## The assumed path

When a plane leaves coverage, some trackers don't stop the dot. They carry it along the filed route at the expected speed, sometimes for hours, and it looks exactly like the live part of the flight.

That's reasonable for a flight across the Atlantic. The plane is very likely close to that line. But the dot isn't evidence of anything. It can't show a diversion, a hold or a return until the next real report arrives.

A few signs that you're looking at an estimate:

| What you see | What it suggests |
|---|---|
| A perfectly straight or perfectly curved line, no small wobbles | Assumed path |
| Altitude and speed flat for a long time, with no change at all | Numbers may be held, not reported |
| A jump when the plane reaches land | The real report came back |
| "Estimated" or an old timestamp in the details | The tracker is telling you, which is to its credit |

"Live" should mean someone heard this plane moments ago. It often means the app is open.

## What this means for you

If you're watching someone's flight over land in the US, the position is good to within a few seconds' flying. If you're watching over the middle of an ocean, treat the dot as the plan. Wait for landfall before you read anything into it, in either direction.

For timing, don't rely on the dot. [The landing and gate times are separate events](/articles/overhead/why-tracker-and-airline-disagree), and they come through different routes than the position does. For a person waiting at the airport, the estimate in [how long from landing to the curb](/articles/landing/how-long-from-landing-to-curb) matters more than the plane's exact spot.

The altitude trace has its own grammar, which is in [how to read a flight's path](/articles/overhead/how-to-read-a-flight-path). What can't be tracked at all is covered in [what's that plane overhead](/articles/overhead/whats-that-plane-overhead).

> When you're following a flight in Skycap, the tracker labels the difference: "Live" when a position came in, "Now" when the plane is estimated from the schedule. Live position covers US airspace, and beyond it you get schedule and weather. [How it works](/app/how-it-works)

## Sources

- [FAA Aeronautical Information Manual, 4-5-7, ADS-B services](https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap4_section_5.html)
- [14 CFR 91.227, ADS-B Out performance requirements](https://www.law.cornell.edu/cfr/text/14/91.227)
- [14 CFR 91.225, ADS-B Out equipment and use](https://www.law.cornell.edu/cfr/text/14/91.225)