How we track the ISS in real time

When this site shows the Station crossing the Pacific, it is not reading a live GPS feed from orbit. There is no cable to the ISS. The position you see is calculated — predicted from a small bundle of numbers published a few times a day, run through a piece of mathematics older than the Station itself. Here is how that works.

The two-line element set

Everything starts with a TLE, short for two-line element set. It is exactly what the name suggests: two lines of densely packed numbers that describe an object’s orbit at a precise moment — its shape, its tilt, where the object sits along the path, and how the orbit is slowly decaying.

These are produced by the United States Space Force, which tracks thousands of objects in orbit by radar, and published openly through services such as Celestrak. The Station’s TLE is refreshed several times a day. Each one is a snapshot of the orbit, accurate around the moment it was issued.

Turning two lines into a position

A TLE on its own is just a description. To get an actual latitude, longitude and altitude for a given instant, you feed it through a propagator — a model that takes the orbit and winds it forward or backward in time.

The standard model is called SGP4. It dates back to the 1960s and 70s and was built precisely for this: taking element sets and producing positions, while accounting for the messy real-world effects that pull an orbit off a perfect ellipse — the bulge of the Earth, the faint but constant drag of the upper atmosphere. It is not glamorous code, but it is the quiet workhorse behind almost every satellite tracker on the planet.

Why it runs in your browser

This site does the SGP4 calculation on your own device, not on a server. Once your browser has the current TLE, it can work out where the Station is for any moment — right now, a minute from now, an hour from now — without asking anyone. That is what lets the live tracker move smoothly and the pass predictions be specific to your exact location: the heavy lifting happens locally, from the same handful of numbers.

The data is refreshed automatically. New orbital elements come in, schedules are pulled from The Space Devs, crew details from NASA, and the site rebuilds itself so what you read stays current.

How accurate is it

For an object the size of the Station, propagating a fresh TLE with SGP4 is good to within a few kilometres — comfortably precise enough to tell you which minute to look up and which horizon to face. Accuracy slowly drifts the further you predict from the moment the elements were issued, which is exactly why the underlying TLE is refreshed so often. Fresh numbers in, trustworthy positions out.

The short version

• A TLE describes the orbit, published several times a day.
• SGP4 turns that description into a position for any instant.
• The maths runs in your browser, so tracking and pass predictions are instant and local.
• Fresh elements keep it accurate to a few kilometres.

If you would rather just look up, our other guides cover what the Station is and how to see it with your own eyes.