What Is a TLE (Two-Line Element Set)?

Overview

The Two-Line Element set (TLE) is a compact text format that describes the orbit of an Earth-orbiting object at a specific point in time (the epoch). Developed by NORAD in the 1960s and maintained by the US Space Force's 18th Space Defense Squadron, TLEs are distributed via Space-Track.org and form the basis of most public satellite tracking, including Orbital Radar.

Format

A TLE consists of a title line (optional) followed by exactly two lines of 69 characters each:

Example TLE (ISS)

ISS (ZARYA)
1 25544U 98067A   26059.50000000  .00016717  00000-0  10270-3 0  9993
2 25544  51.6400 123.0010 0005000  90.0000 270.0000 15.50000000000000

Key Fields

NORAD Catalog Number (25544 for the ISS) — the unique identifier. Epoch — the date/time the orbital parameters are valid. Inclination — the tilt of the orbit relative to the equator. RAAN — right ascension of the ascending node, orienting the orbital plane. Eccentricity — how circular or elliptical the orbit is. Mean Motion — revolutions per day (15.5 for the ISS, meaning ~92 min per orbit).

How Orbital Radar Uses TLEs

Orbital Radar ingests fresh TLEs from Space-Track.org and propagates them forward using the SGP4 algorithm to calculate real-time satellite positions. Because TLEs degrade in accuracy over time (atmospheric drag and other perturbations), they must be regularly refreshed — Orbital Radar updates its catalogue multiple times per day.

For well-tracked objects, TLEs are typically accurate to within 1–2 km at epoch and degrade at roughly 1–5 km per day. High-drag objects in very low orbits degrade faster. Precision applications use SP (Special Perturbations) data instead.