tle

Two-line element set (TLE) is a compact data format encoding the orbital elements of an Earth-orbiting object in two text lines. Developed for NORAD tracking, TLEs are the de facto standard used to predict satellite positions and to run orbital propagations with simple atmosphere drag models. They are widely used by space agencies, satellite operators, and hobbyists for tasks such as pass predictions and conjunction analyses.

A TLE consists of two lines of fixed-field text. Line 1 starts with the digit 1 and includes the satellite catalog number, international designator, epoch, first and second derivatives of the mean motion, a ballistic coefficient term (B*), ephemeris type, element set number, and a checksum. Line 2 starts with the digit 2 and includes the satellite catalog number again, inclination, right ascension of the ascending node, eccentricity (as a decimal point-free value), argument of perigee, mean anomaly, mean motion, revolutions at epoch, and a checksum. The key orbital elements encoded are inclination (i), right ascension of ascending node (Ω), eccentricity (e), argument of perigee (ω), mean anomaly (M), and mean motion (n). Epoch expresses the time at which these values are valid.

TLEs are propagated with orbital models such as SGP4 or SDP4 to yield satellite position and velocity in Earth-centered inertial coordinates. They are valid for short time horizons and require frequent updates as perturbations like atmospheric drag and solar activity cause the orbit to evolve. Although not precise ephemerides, TLEs enable rapid, scalable tracking and are publicly distributed, for example via Celestrak and space-track databases.