# 7.3.4 Geostationary Orbits

### Synchronous Orbits

• A synchronous orbit is:

When an orbiting body has a time period equal to that of the body being orbited and in the same direction of rotation as that body

• These usually refer to satellites (the orbiting body) around planets (the body being orbited)
• The orbit of a synchronous satellite can be above any point on the planet’s surface and in any plane
• When the plane of the orbit is directly above the equator, it is known as a geosynchronous orbit

### Geostationary Orbits

#### Geostationary Orbit

• Many communication satellites around Earth follow a geostationary orbit
• This is sometimes referred to as a geosynchronous orbit
• This is a specific type of orbit in which the satellite:
• Remains directly above the equator
• Is in the plane of the equator
• Always orbits at the same point above the Earth’s surface
• Moves from west to east (same direction as the Earth spins)
• Has an orbital time period equal to Earth’s rotational period of 24 hours
• Geostationary satellites are used for telecommunication transmissions (e.g. radio) and television broadcast
• A base station on Earth sends the TV signal up to the satellite where it is amplified and broadcast back to the ground to the desired locations
• The satellite receiver dishes on the surface must point towards the same point in the sky
• Since the geostationary orbits of the satellites are fixed, the receiver dishes can be fixed too

#### Low Orbits

• Some satellites are in low orbits, which means their altitude is closer to the Earth’s surface
• One example of this is a polar orbit, where the satellite orbits around the north and south pole of the Earth
• Low orbits are useful for taking high-quality photographs of the Earth’s surface. This could be used for:
• Weather
• Military applications

Geostationary satellite in orbit

#### Worked Example

Calculate the distance above the Earth’s surface that a geostationary satellite will orbit.

Mass of the Earth = 6.0 × 1024 kg

Radius of the Earth = 6400 km

#### Exam Tip

Make sure to memorise the key features of a geostationary orbit, since this is a common exam question. Remember:

• Equatorial orbit
• Moves west to east
• Period of 24 hours

You will also be expected to remember that the time period of the orbit is 24 hours for calculations on a geostationary satellite.

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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