OrbitLive — glossary

Name: OrbitLive live feed
Creator: OrbitLive

Plain-language definitions of the terms used across OrbitLive (CelesTrak).

Tracking the sky

Two-Line Element set (TLE): A compact, standardized two-line text record of a satellite’s orbit at a given moment, published in the public catalog. The starting point from which a satellite’s future position is calculated.
SGP4 model: The standard mathematical model that propagates a TLE forward in time, accounting for atmospheric drag and Earth’s shape, to predict where a satellite will be. OrbitLive uses it to compute live positions.
Ground track: The path traced on the Earth’s surface directly beneath a satellite as it orbits. Because the planet rotates underneath, each successive orbit’s track shifts westward.
Visual magnitude: A measure of how bright an object appears in the sky, on a backwards scale where smaller and negative numbers are brighter. A bright ISS pass can reach about magnitude −3.

Low Earth Orbit (LEO): Orbits from roughly 160 to 2,000 km altitude, circled in about 90 minutes to two hours. Home to the ISS, Earth-observation satellites and megaconstellations like Starlink.
Medium Earth Orbit (MEO): Orbits between LEO and the geostationary belt. Navigation constellations live here — GPS orbits near 20,200 km with a roughly 12-hour period.
Geostationary Orbit (GEO): A ring at about 35,786 km above the equator where an orbit takes one day, so the satellite appears fixed over a single longitude. Used by communications and weather satellites.
Orbital decay: The gradual loss of altitude caused by faint atmospheric drag in low orbits. Left uncorrected it ends in reentry, where most objects burn up on the way down.

Pass: A single crossing of a satellite over your local sky, defined by its appearance time, the direction and maximum elevation it reaches, and when it fades from view. Pass predictions are specific to your location.
Elevation: How high above the horizon an object is, in degrees — 0° at the horizon, 90° straight overhead. Higher-elevation passes are brighter and easier to see.
Starlink train: The string of closely spaced, evenly bright dots seen shortly after a Starlink batch launches, before the satellites disperse and raise their orbits.
Space debris: Defunct satellites, spent rocket stages and collision fragments orbiting Earth. Tens of thousands of pieces are tracked; small, fast fragments can still damage working spacecraft.