What is a TLE — orbital elements explained
Two lines of numbers tell a satellite tracker everything it needs. Here is what each part of a Two-Line Element set means, in plain language.
Every satellite tracker on the internet — this one included — starts from the same source: a small block of numbers called a Two-Line Element set, or TLE. It is the recipe that describes an orbit, and without it no tracker would know where anything is. Our guide on how we track the ISS introduced it briefly; here we unpack it properly.
What it looks like
A TLE for the ISS looks something like this (simplified):
ISS (ZARYA)
1 25544U 98067A 26177.51234567 .00012345 00000+0 12345-4 0 9991
2 25544 51.6420 123.4567 0001234 12.3456 347.6543 15.49012345123456Line 0 is the name. Lines 1 and 2 carry the actual orbital data. Every column is tightly defined — there is no room for wasted space.
The key elements, in plain English
You do not need to read a TLE raw. But knowing what the fields mean helps you understand why predictions shift and how accurate they are.
Epoch
The date and time the element set describes. Everything else in the TLE is a snapshot of the orbit at this moment. When you see a propagator like SGP4 at work, it is winding the clock forward (or backward) from this point.
Inclination (51.64° for the ISS)
The tilt of the orbit relative to the equator. The ISS’s 51.6° means it passes over everywhere between 51.6°N and 51.6°S — which is why most of the world’s population can see it. A higher number would carry it over higher latitudes; a lower one would keep it closer to the tropics.
RAAN — Right Ascension of the Ascending Node
Where the orbit crosses the equator heading north, measured against the stars. It precesses slowly because Earth is not a perfect sphere — the equatorial bulge tugs on the orbit and rotates this crossing point westward over time. This is the main reason your pass time shifts from week to week.
Eccentricity
How circular or elongated the orbit is. For the ISS this number is nearly zero (~0.0001), meaning the orbit is almost a perfect circle. A higher eccentricity would make it an ellipse — close to Earth at one end, far at the other.
Argument of perigee
Where in the orbit the satellite is closest to Earth. For a near- circular orbit like the ISS’s it barely matters, but for highly eccentric orbits it determines where the satellite speeds up and slows down.
Mean anomaly
Where the satellite is along its orbit at the epoch. Combined with the other elements, this lets the propagator place the object at any future instant.
Mean motion (15.49 revs/day for the ISS)
How many orbits per day. Divide 1440 minutes by this number and you get the orbital period — about 92.9 minutes for the ISS. A higher mean motion means a lower orbit; a lower one means a higher, slower orbit.
Drag term (B* / BSTAR)
A measure of atmospheric drag. The Station flies low enough that the thin upper atmosphere slows it, and without periodic reboosts it would descend. The drag term lets SGP4 model that decay.
Why it matters
A TLE is small, public, free, and machine-readable. That is what makes a site like this possible: we fetch the latest ISS elements from CelesTrak several times a day, run them through SGP4 in your browser, and produce a live position and pass predictions for your location — all without a server in the loop.
For the bigger picture, read how we track the ISS in real time. To see the tracker at work, jump to the live map.
Frequently asked
What is a TLE?
A TLE, or two-line element set, is a compact block of numbers that describes a satellite's orbit at a precise moment: its shape, its tilt, where the object sits along the path, and how the orbit is slowly decaying. Every satellite tracker starts from one.
What is the epoch in a TLE?
The epoch is the exact date and time the element set describes. Everything else in the TLE is a snapshot of the orbit at that moment, and a propagator like SGP4 winds the clock forward or backward from it.
Why do ISS pass times shift from week to week?
The main reason is that the orbit's crossing point with the equator (its right ascension of the ascending node) precesses westward over time, because the Earth's equatorial bulge tugs on the tilted orbit. That steady drift changes when your location lines up with a sunlit pass.
What is mean motion and the orbital period of the ISS?
Mean motion is how many orbits the Station completes per day, about 15.49 for the ISS. Divide 1440 minutes by that number and you get the orbital period, roughly 92.9 minutes per lap.
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