About
Our data & how the predictions work
SpotTheISS is an independent, free tracker for the International Space Station. Everything on the site — the live map, the pass predictor, and the email alerts — is computed from the same public orbital data professional satellite trackers use. This page explains exactly how.
Where the orbital data comes from
The station's orbit is published as two-line elements (TLEs) — the standard format for NORAD's satellite catalog. We fetch the latest ISS TLE from CelesTrak, with independent mirrors of the same NORAD elements as automatic backups, and refresh it every hour. TLEs describe the orbit precisely enough that positions computed from a fresh element set are accurate to within a few kilometers — more than enough to know when and where to look up.
How the live position is computed
The map propagates the TLE with SGP4, the standard orbital model used across the satellite industry, via the open-source satellite.js library. Your browser recalculates the station's latitude, longitude, altitude and speed every second — the movement you see is real orbital mechanics, not an animation.
How visible passes are predicted
A pass is listed only when you could actually see it. For every upcoming orbit we check three conditions at your exact coordinates: the station climbs at least 10° above your horizon, your sky is dark (after dusk or before dawn), and the station itself is still lit by the Sun. All three together are what make the ISS shine like a bright moving star. Times are shown in the searched location's local timezone, and the same calculation powers the email alerts, which are checked against fresh orbital data before each send.
Accuracy and limitations
Pass times are typically accurate to well under a minute for the next few days. Further out, small orbit adjustments (the ISS regularly boosts its altitude) shift predictions slightly — which is why we recompute everything from the latest data on every visit rather than caching old answers. Clouds, of course, remain out of our jurisdiction.
Who runs this
SpotTheISS is built and maintained independently. It is not affiliated with NASA or Roscosmos. It's free, has no ads, and exists because watching a football-field-sized laboratory glide silently across the sky never gets old.