Satellites don't broadcast their GPS position the way aircraft and ships do. Instead, live trackers predict where each satellite is right now using a published orbit and a 60-year-old propagation algorithm. It sounds rough, but it's accurate to within a few kilometers — good enough to spot the ISS passing overhead.
What's a TLE?
A Two-Line Element set (TLE) is a tiny text file that describes a satellite's orbit at a specific moment in time. The US Space Force publishes fresh TLEs daily for tens of thousands of objects — active satellites, dead satellites, and debris. Every live satellite tracker on the internet starts with TLE data from the same source.
SGP4: the math
A TLE on its own is just numbers. To turn it into a position, trackers run it through SGP4 — Simplified General Perturbations 4 — an algorithm that accounts for Earth's oblateness, atmospheric drag, and the gravitational pull of the Sun and Moon. SGP4 takes a TLE and a target time and returns latitude, longitude, and altitude.
Why Starlink is harder
Starlink satellites maneuver constantly to avoid collisions and de-orbit at end of life. Their TLEs go stale fast — sometimes within hours. Trackers refresh Starlink TLEs more aggressively, and even then a satellite's shown position might be off by tens of kilometers right after a maneuver.
What you see on Aether Track
The satellite layer plots positions calculated live in your browser from the latest TLEs. ISS, Hubble, and weather satellites are accurate enough to predict visible passes. The Starlink group shows the constellation structure clearly — those neat strings of dots are recently launched batches still drifting into their final orbital planes.