What Is a TLE? Two-Line Elements, Explained
Every satellite position WatchSat draws starts as two lines of fixed-width text — a Two-Line Element set (TLE) — turned into a live position by the SGP4 model.
A whole orbit in 138 characters
A TLE is a snapshot of one object’s orbit at one epoch. Each line is 69 characters. Into them go inclination, RAAN, eccentricity, argument of perigee, mean anomaly, mean motion, drag terms, catalog number, and checksums. The format was designed for punch cards and still powers modern tracking.
Who publishes them
Unclassified TLEs are issued by the US Space Force and redistributed by CelesTrak. WatchSat loads the active catalog through an edge-cached proxy and ships a recent snapshot for instant load.
Why SGP4 matters
A TLE is not a Keplerian orbit you can plug into a simple formula. Its numbers are defined inside SGP4/SDP4, which accounts for Earth’s oblateness and atmospheric drag. WatchSat runs that model in your browser for every active satellite.
Why element sets age
Drag and manoeuvres make old TLEs drift. Fresh sets are typically good for visualization for hours to a few days in LEO. WatchSat refreshes from CelesTrak about every 30 minutes.