Satellite motion and tracking
A satellite is an object that orbits a larger body; artificial satellites are spacecraft placed deliberately around the Earth for purposes such as communication, navigation, weather observation and scientific research. A satellite stays in orbit because it is moving sideways fast enough that, as gravity pulls it toward the Earth, the surface curves away beneath it at the same rate. In effect the satellite is continually falling toward the Earth but never reaching it. Animations show this balance clearly, illustrating why a certain speed is needed at a given altitude.
Satellites are placed in different orbits to suit their jobs. In low Earth orbit, a few hundred kilometres up, a satellite circles the planet in roughly an hour and a half, passing quickly overhead; this orbit is used for imaging satellites and crewed stations. Higher up, at the geostationary altitude above the equator, a satellite takes exactly one day to complete an orbit and so appears to hover over a fixed point on the surface, which is ideal for communications and broadcasting. Other orbits are tilted or elongated to give particular coverage of the Earth.
Because their motion follows the known laws of orbital mechanics, satellite positions can be calculated and predicted in advance. Tracking software and applets use orbital data to compute where a satellite is and to plot its ground track — the path traced on the Earth's surface beneath it. This allows ground stations to point antennas at a passing satellite and lets observers know when one will be visible overhead. The same principles of orbital motion that govern planets and moons also govern these artificial satellites, making their paths predictable and trackable.
Frequently asked questions
- Why does a satellite stay in orbit instead of falling?
- A satellite moves sideways fast enough that the Earth's surface curves away beneath it as fast as gravity pulls it down, so it keeps falling around the planet without hitting it.
- What is a geostationary satellite?
- It is a satellite in an orbit above the equator that takes one day to circle the Earth, so it appears to stay fixed over a single point, useful for communications.
- What is a ground track?
- A ground track is the path on the Earth's surface directly beneath a satellite's orbit, used to predict when and where the satellite will pass overhead.
