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Home Academy Tracking & TLEs What a TLE really is
LESSON 01 OF 6

What a TLE really is

Beginner ~8 min Slide deck Free

TLEs are how the global space surveillance community shares orbital data. They're a 1970s format that persists because every piece of orbital software in the world can read them — and that ubiquity beats any technical improvement.

TLEs are how the global space surveillance community shares orbital data. They're a 1970s format that persists because every piece of orbital software in the world can read them — and that ubiquity beats any technical improvement.

What this lesson covers

What a TLE Actually Contains

Each TLE is exactly two 69-character lines (plus an optional title line). Fixed-width format, designed for punch-card-era computing.

Epoch Age: The First Thing to Check

A TLE is only accurate at its epoch. Error accumulates as time passes — and accumulates faster in some orbits than others.

SGP4: The Model Every TLE Depends On

TLEs are not raw measurements — they are fitted parameters calibrated specifically for the SGP4 propagation model.

Where TLEs Come From

The TLE ecosystem has multiple tiers of sources, each with different update frequency, coverage, and accuracy.

Key facts

💡The TLE format was standardised in the 1970s. NORAD, Space-Track, and Celestrak all distribute data in this format today — 50+ years of backward compatibility.
💡A TLE with a 7-day-old epoch for a satellite at 400 km altitude during solar maximum may have position error exceeding 100 km along-track. It is not a useful precision tool at that point.
💡SGP4 is intentionally simplified for speed and universal compatibility — it can propagate thousands of objects in real time. High-fidelity special perturbations models are more accurate but orders of magnitude more computationally expensive.
TLE epoch age is always the first thing to check before trusting any prediction.

Orbital Radar ingests TLE feeds continuously and runs them through SGP4 to compute positions, passes, and conjunctions. The freshness indicator next to any object tells you how confident to be in what you're seeing — and when to treat outputs as approximate rather than authoritative.

All lessons in Tracking & TLEs
01What a TLE really is~8 min02Epochs, drag & decay~7 min03Propagation vs "accuracy"~8 min04The NORAD catalog (who assigns IDs)~7 min05Data sources (Space-Track, CelesTrak)~7 min06Reading a TLE line by line~8 min
All 6 LessonsEpochs, drag & decay →
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