What Is a TLE?
A TLE — two-line element set — is a compact block of text that describes exactly where a satellite's orbit is and how it's shaped. It's the format every satellite tracker, including AstroSat, actually runs on.
What's packed into two lines
Which satellite, and when
A catalog number identifies the object; the epoch is the exact moment the rest of the line describes — a TLE is a snapshot, not a live feed, which is why it needs refreshing.
The shape of the orbit
Inclination, eccentricity and the argument of perigee together describe the orbit's tilt, how stretched it is, and which way it points — everything needed to reconstruct the path.
Where on that path, right now
Mean motion (orbits per day) and mean anomaly locate the satellite along its orbit at the epoch — from there, a propagator can work out where it'll be at any other time.
How much it's dragging
The BSTAR term captures atmospheric drag, which is what makes low satellites slowly lose altitude — see orbital decay for what that adds up to.
TLEs were designed in the punch-card era to fit an entire orbit description into two 69-character lines, and the format has outlived every computer it was designed for — it's still what Space-Track and CelesTrak publish today, and still what SGP4, the standard propagation model, expects as input. AstroSat pulls a fresh TLE for every tracked object every two hours and runs SGP4 in your browser to turn that snapshot into a live, moving position — the same pipeline professional operators use, just running on your device instead of theirs.
Frequently asked questions
What does TLE stand for?
Two-line element set — a standardised, two-line text format that describes a satellite's orbit at a specific moment (the epoch).
Who publishes TLEs?
Space-Track.org (run by the 18th Space Defense Squadron) is the primary source; CelesTrak mirrors and redistributes them. AstroSat reads from both.
Why does a TLE go stale?
It describes the orbit at one epoch. Drag and other perturbations mean the real orbit drifts from that description over time — TLEs are typically republished every day or so, and AstroSat refreshes every two hours.
What turns a TLE into a position?
SGP4, a propagation model that takes a TLE and a target time and returns where the satellite actually is. It's the standard used across the industry, and it's what runs inside AstroSat's globe.