How satellite tracking works: TLEs, SGP4 and pass predictions explained

Step 1: the orbit, as a TLE

Military and civil radars track tens of thousands of objects in orbit. The results are published as orbital elements, most famously in the two-line element set (TLE) format: two lines of 69 characters that describe a satellite's orbit at one moment, called the epoch. They include the inclination, the shape of the orbit, its orientation, how many orbits it makes per day, and a term for atmospheric drag.

Sources such as Space-Track and CelesTrak publish fresh elements several times a day. Because satellite numbers will soon run past what five digits can hold, the same data is moving to a newer, more flexible format (OMM), but the idea is the same.

Step 2: SGP4 moves the satellite forward in time

A TLE is not a simple snapshot of position. Its numbers are 'mean' elements, averaged in a particular way, and they only give correct positions when used with the model they were made for: SGP4 (Simplified General Perturbations 4). SGP4 was published by the US in 1980 and revised in 2006; it accounts for Earth's flattening, atmospheric drag and, for higher orbits, the pull of the Sun and Moon.

Given a TLE and a time, SGP4 returns the satellite's position and velocity. Its accuracy is around a kilometre near the epoch and worsens by a few kilometres per day, more for satellites low in the atmosphere where drag is unpredictable. That is why trackers update their data daily.

Step 3: from space to your sky

SGP4's output is in an Earth-centred frame that doesn't rotate with Earth. To find where to look, the tracker rotates that position into Earth-fixed coordinates using the exact time, then subtracts your location to get a line of sight. From that come the two numbers you see on screen: azimuth (the compass direction) and elevation (the height above the horizon), plus the distance (range).

A pass is the stretch of time when elevation is above zero (or above a minimum such as 10°, because buildings and trees block the horizon). Trackers step through time, find the rise, the highest point and the set, and repeat for days ahead.

Step 4: will you actually see it?

Satellites shine by reflected sunlight. You see one with your eyes only when three things are true at once: the satellite is above your horizon, it is in sunlight (not in Earth's shadow), and your sky is dark enough, usually with the Sun at least about 6° below the horizon. That is why visible passes cluster in the hours after sunset and before sunrise, and why a satellite can vanish mid-pass as it enters Earth's shadow.

Brightness depends on the satellite's size, distance and the angle between Sun, satellite and you. Trackers estimate a magnitude from a standard brightness for each satellite and those geometric factors.

How OrbitNight does it

OrbitNight runs SGP4 in your browser on the latest public elements, so the live map and pass lists are computed on your own device for your exact location. Our methods page explains the data sources, the checks we run against reference software, and the known limits, such as satellites that have just manoeuvred and not yet received new elements.