Reading the tracker

NORAD IDs, TLEs, elevation, magnitude and passes — every label on this site explained.

1. What is a NORAD ID? — A NORAD ID is the catalogue number given to every tracked object in orbit — the ISS is 25544 and India's Cartosat-3 is 44804.
2. What is a COSPAR ID (international designator)? — A COSPAR ID names an object by its launch — year, launch number in that year, then a letter for each piece — so Cartosat-3 is 2019-081A.
3. What is a TLE (two-line element set)? — 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.
4. What are the six orbital elements? — Six numbers — size, shape, tilt, two orientation angles and position along the orbit — are enough to pin down an orbit and where the satellite is on it.
5. What is a TLE epoch, and why does the age of the data matter? — The epoch is the exact moment a TLE describes; the further you predict from it, the less accurate the satellite's estimated position becomes.
6. What is SGP4, and how does this site turn a TLE into a position? — SGP4 is the standard mathematical model that carries a TLE forward in time to give a satellite's estimated position and velocity at any moment.
7. Why does this site say positions are 'estimated'? — Because each position is calculated from public orbital data with a mathematical model, not reported by the satellite itself, so it is a prediction rather than a measurement.
8. How accurate are the satellite positions on this site? — Typically within a few kilometres for fresh data — plenty for finding a satellite in the sky — but accuracy falls as the orbital data gets older.
9. What do elevation and azimuth mean? — Azimuth is the compass direction to face, measured clockwise from north; elevation is how high to look, from 0° at the horizon to 90° overhead.
10. What is a satellite pass? — A pass is one crossing of your sky by a satellite; this site counts it only while the satellite is above 10°, sunlit, bright enough, and your sky is dark.
11. What does 'sunlit' mean for a satellite? — A satellite is sunlit when sunlight reaches it instead of being blocked by the Earth; satellites have no lights of their own, so only sunlit ones can be seen.
12. What does magnitude mean on a pass card? — Magnitude measures brightness backwards: smaller numbers are brighter, negative numbers are very bright, and each step of 1 is about 2.5 times dimmer.
13. Why can you see satellites only after sunset and before sunrise? — You need a dark sky and a sunlit satellite at the same time, which mostly happens in the hours after sunset and before sunrise.
14. How do I use the 'Where to look' compass? — Hold your phone flat, turn until the pointer says you are aligned with the satellite's direction, then look up by the elevation angle shown.
15. Why can't you see geostationary satellites with your eyes? — They are about 36,000 km away — roughly 85 times farther than the ISS — so their reflected sunlight is thousands of times fainter than a low satellite's.
16. Is it a satellite, a plane or a meteor? — A satellite is a steady, silent point that glides across the sky in a few minutes; planes blink red, green and white, and meteors flash past in seconds.