AQA A Level Physics

Revision Notes

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 orbit satellite, downloadable AS & A Level Physics revision notes

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

Worked example - radius of geostationary orbit, downloadable AS & A Level Physics revision notes

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.

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