What is a TLE and How to Read It

Every satellite tracker you have ever used — including SatFleet Live — relies on a surprisingly compact piece of data called a TLE. Two lines of plain text, each exactly 69 characters long, contain everything needed to calculate where a satellite is at any moment in time. Understanding how to read TLE data gives you a much deeper insight into how satellite orbit tracking actually works.

What is a TLE?

TLE stands for Two-Line Element set. It is a standardised data format developed by NORAD (North American Aerospace Defense Command) to describe the orbital parameters of an Earth-orbiting object. The format has been in use since the 1960s and remains the universal standard for sharing satellite orbital data today.

A complete TLE entry actually consists of three lines: a name line followed by Line 1 and Line 2. Together they encode the satellite's identity, its orbital shape, its orientation in space, and its position at a specific moment in time — called the epoch.

The Structure of a TLE

Here is what a raw TLE satellite entry looks like, using the International Space Station as an example:

ISS (ZARYA)
1 25544U 98067A   24058.51795370  .00016717  00000-0  30029-3 0  9993
2 25544  51.6413 179.2137 0003828  90.5455 269.6118 15.50041732447771

At first glance it looks like gibberish. But every single character has a precise meaning. Let's break it down field by field.

How to Read Each Field

The name line

The first line is simply the satellite's common name — in this case ISS (ZARYA). This is not part of the official two-line format but is always included for human readability. Zarya was the first module of the ISS launched in 1998.

Line 1 — Identity and orbital timing

Line 2 — Orbital shape and position

Practical Example: The ISS

Let's use the colour-coded view below to see all the fields at a glance on the actual ISS TLE. Hover over each highlighted section to identify the field:

What can we immediately tell about the ISS from this TLE? Quite a lot:

• 🌍  Orbit passes over latitudes up to ±51.6° — covering most populated areas of Earth
• ⏱️  Completes an orbit every ~93 minutes (15.5 orbits per day)
• ⭕  Nearly perfectly circular orbit (eccentricity of only 0.00038)
• 🚀  Launched in 1998, 67th launch of that year
• 🔢  Had completed 447,771 orbits by the epoch date

How TLEs Power Real-Time Satellite Tracking

A TLE alone is just a snapshot — it describes where a satellite was and how it was moving at the epoch moment. To find its current position, you need a propagation model that applies the laws of orbital mechanics to project that snapshot forward in time.

The standard model for TLE-based tracking is called SGP4/SDP4 (Simplified General Perturbations). It accounts for:

1. Atmospheric drag In low Earth orbit, trace amounts of atmosphere cause satellites to slow down and gradually lower their orbit. The BSTAR drag term in Line 1 quantifies this effect.
2. Earth's non-spherical shape Earth is slightly flattened at the poles. This causes the orbital plane to precess (rotate slowly) over time — accounted for in the RAAN and argument of perigee fields.
3. Lunar and solar gravity For higher orbits, the gravitational pull of the Moon and Sun causes measurable perturbations. SDP4 handles these for deep-space objects.
4. Solar radiation pressure Photons from the Sun exert a tiny but real force on satellites, especially those with large solar panels like the ISS.

SatFleet Live runs SGP4 calculations entirely in your browser using the satellite.js library — every dot you see on the map is a live SGP4 result, updated every second.

This is also why TLE data needs to be refreshed regularly. The further in time you propagate from the epoch, the less accurate the result. For most LEO satellites, a TLE that is more than 7–10 days old will start to show meaningful position errors. SatFleet Live updates its data every 48 hours to stay well within that window.

Frequently Asked Questions