A geostationary orbit lets a satellite appear to hover over one fixed point on Earth's equator. This is useful because ground antennas can point in one direction without tracking a moving satellite. The orbit is very high, about 35,786 km above Earth's surface, so the satellite's orbital period matches Earth's rotation.
Geostationary satellites are essential for television broadcasting, weather monitoring, navigation support, and long-distance communications.
The key idea is that gravity provides the centripetal force needed to keep the satellite moving in a circular path. At the geostationary radius, the satellite completes one orbit in one sidereal day, about 23 hours 56 minutes. The orbit must be circular, above the equator, and moving in the same direction as Earth's rotation.
If any of these conditions are not met, the satellite will appear to drift north, south, east, or west in the sky.
Key Facts
- Geostationary altitude above Earth's surface is about 35,786 km.
- Geostationary orbital radius from Earth's center is about 42,164 km.
- Orbital period must equal one sidereal day: T = 23 h 56 min 4 s.
- Gravity supplies centripetal force: GMm/r^2 = mv^2/r.
- Circular orbit speed at geostationary radius is about v = 3.07 km/s.
- A geostationary orbit must be circular, equatorial, and prograde.
Vocabulary
- Geostationary orbit
- A circular equatorial orbit in which a satellite appears fixed above one point on Earth.
- Geosynchronous orbit
- An orbit with a period equal to Earth's rotation period, but not necessarily fixed over one point.
- Orbital period
- The time required for a satellite to complete one full orbit around Earth.
- Centripetal force
- The inward force required to keep an object moving in a circular path.
- Sidereal day
- The time Earth takes to rotate once relative to the distant stars, about 23 hours 56 minutes.
Common Mistakes to Avoid
- Using 24 hours instead of a sidereal day, because geostationary motion must match Earth's rotation relative to the stars rather than the Sun.
- Placing the satellite over any latitude, because a truly geostationary satellite must orbit directly above the equator.
- Thinking the satellite is motionless in space, because it is actually moving around Earth at about 3.07 km/s while matching Earth's rotation.
- Confusing altitude with orbital radius, because 35,786 km is height above Earth's surface while about 42,164 km is distance from Earth's center.
Practice Questions
- 1 A geostationary satellite is about 35,786 km above Earth's surface. If Earth's radius is 6,378 km, what is the satellite's orbital radius from Earth's center?
- 2 A satellite in geostationary orbit travels at about 3.07 km/s. Estimate how far it travels in one sidereal day of 86,164 s.
- 3 Explain why a satellite in an inclined orbit with the same period as Earth's rotation is geosynchronous but not geostationary.