A TLE is two 69-character lines of numbers that describe a satellite's orbit at one moment, so software can calculate where it is now.
This site never receives a satellite's position from the satellite. It receives a short description of the orbit, and calculates the position itself. That description is the two-line element set, or TLE.
A TLE is two lines of exactly 69 characters each, usually with the satellite's name on a line above. Only digits, capital letters, spaces, full stops and plus or minus signs are allowed. Here is one for the ISS, published by CelesTrak with a reference time of 4 November 2026:
`1 25544U 98067A 26308.88559265 .00005352 00000+0 10457-3 0 9991`
`2 25544 51.6327 323.8448 0007212 348.6490 11.4335 15.49180468593565`
Line 1 is about identity and time: the NORAD ID (25544), the international designator (98067A), the epoch — the moment the numbers describe (26308.88559265 means day 308 of 2026) — and two terms that describe how air drag is slowing the satellite.
Line 2 is the shape and tilt of the orbit: inclination 51.6327°, the orientation angles, eccentricity 0.0007212 (the decimal point is assumed), the position along the orbit, and the mean motion — 15.49180468 orbits per day. The last digit of each line is a checksum, a simple test for typing or transmission errors. The name line above is only for people; the calculation uses the two numbered lines.
This site's own server downloads the current TLEs from CelesTrak, a public source that republishes the US satellite catalogue's data. Your browser then fetches them from this site, not from CelesTrak directly, and keeps a copy for up to six hours. Every satellite page has an Orbital Elements (TLE) block, and landing pages such as the ISS tracker have a Raw TLE Data button that shows the two lines and when they were updated.
A TLE is a snapshot. It goes slowly out of date, which is why it has to be replaced every few days — see TLE epoch.
Press Raw TLE Data: the two lines start with 1 25544U and 2 25544, and the note under them says when they were updated. Open the ISS tracker
The checksum rule from CelesTrak's TLE FAQ: add every digit on the line, count each minus sign as 1 and everything else (letters, spaces, full stops, plus signs) as 0, and keep the last digit. For the ISS line 1 above, the digits of columns 1–68 add up to a number ending in 1, and column 69 is indeed 1; line 2 ends in 5, matching its last digit. CelesTrak says this simple test catches about 90 per cent of errors.
Mean motion n gives the orbital period directly: T = 1440 / n minutes. For n = 15.49180468, T = 1440 / 15.4918 ≈ 92.95 minutes, which matches the 92.95-minute period in CelesTrak's catalogue record for the ISS.
Kepler's third law turns n into the size of the orbit. Converting to radians per second, n = 15.4918 × 2π / 86,400 ≈ 0.0011266 rad/s. With the WGS-72 value μ = 398,600.8 km³/s² used by SGP4, the semi-major axis is a = (μ / n²)^(1/3) ≈ 6,797 km. Subtract the WGS-72 Earth radius of 6,378.135 km and the mean height is about 419 km — between the 414 km perigee and 424 km apogee in the catalogue record.
The BSTAR field (10457-3, meaning 0.10457 × 10⁻³) is SGP4's drag coefficient in units of inverse Earth radii. A 1980 US report stresses that the elements are ‘mean’ values with periodic wobbles removed in a particular way, so they give good predictions only with the matching SGP4/SDP4 model.
T = 1440 / 15.2 ≈ 94.7 minutes.
0.0012177 — the decimal point is assumed to be in front of the seven digits.
TLE elements are mean values produced by the SGP4/SDP4 model, so another model will reconstruct the periodic effects differently and give worse predictions.