Deep Field

Starting

Deep Field
29 September 2026 2518 stories tracked

Glossary · Satellites & Orbit

Two-line element set

The compact, ancient text format that describes where a satellite is.

A TLE packs an orbit into two 69-character lines, a format designed for punched cards and still in universal use. Feed it to an SGP4 propagator and you get a position and velocity for any nearby time — accurate to a kilometre or so for a few days, then degrading as drag and thrusting take their toll, which is why fresh elements are published constantly. Every satellite tracking app, including the pass predictions on this site, starts from one.

Related terms

Sun-synchronous orbit A near-polar orbit that crosses the equator at the same local time every day. Geostationary orbit The one orbit where a satellite appears to hang motionless over a fixed point. Kessler syndrome The runaway scenario where collisions in orbit generate debris that causes more collisions. Low Earth orbit Everything from about 160 to 2,000 km up, where almost all spacecraft and all people are.

Questions

What is Two-line element set?

The compact, ancient text format that describes where a satellite is.

Why does it matter?

Feed it to an SGP4 propagator and you get a position and velocity for any nearby time — accurate to a kilometre or so for a few days, then degrading as drag and thrusting take their toll, which is why fresh elements are published constantly. Every satellite tracking app, including the pass predictions on this site, starts from one.

Written and computed by . Corrections and errors to hello@cantinalabs.dev.