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9.4    Information Systems Notes - Section 4

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# Geostationary satellites relay and transmit information from the other side of the world

Word File

Section 4             Geostationary Satellites

 9.4.4.a Explain why the satellite must be at a height where its revolution period is the same as that of the Earth’s period of rotation

 9.4.4.b Explain why the Earth-based satellite dish must face a fixed direction if it remains in the same location with respect to the geostationary satellite

 9.4.4.i Gather, process and analyse information from secondary sources to identify the satellites used for ‘live’ telecasts from other regions of the world to Australia and vice versa and to present reasons why communication satellites have different aerials and positional orbits

Geostationary Satellites

 9.4.4.a Explain why the satellite must be at a height where its revolution period is the same as that of the Earth’s period of rotation

Satellites

A Satellite, in astronomy, is a body that revolves around a planet.  Our moon, which circles the earth, is a natural satellite.  Artificial satellites also orbit the earth.  They stay in orbit for varying lengths of time.  They can be classified according to the job they do - weather satellites, communication satellites, navigation satellites, scientific satellites and military satellites.

Where Satellites Orbit

There are three basic types of satellite systems:

Ř      Geostationary orbit - at high altitude (35 700kms)

Ř      Medium orbit satellites - at altitudes between 1500 – 8000kms and

Ř      Low orbit satellites - at low altitudes (between 500 -1500kms)

Geostationary satellites orbit the earth at a very high altitude (35 700 km).  These satellites remain above the same point on the equator all the time.  They orbit above the equator and can effectively "see" about one third of the earth's surface.  For this reason they make very good communications or weather forecasting satellites.

Height and Period of Rotation of Satellites

The Earth rotates (spins on its axis) once every 24 hours (ie one day).  Therefore the earth has a period of rotation of 24 hours.

Satellites revolve around the earth.  The time they take for one revolution is called their period of revolution.

To remain above the same point above the equator a satellite must revolve around the earth in the same time it takes for the earth to rotate.  This only occurs at a height of ~35,700 kilometres above the equator.  Such a satellite is called a geostationary satellite.

 9.4.4.b Explain why the Earth-based satellite dish must face a fixed direction if it remains in the same location with respect to the geostationary satellite

Communication satellites use microwaves.  Microwaves, reflected from transmitting dishes, travel in straight lines and are therefore ‘very directional’.  This means the aerials that send and detect microwaves must be pointing in the correct direction.  The Earth-based satellite dish is constantly receiving signals from the communication satellite.  To maintain the communication link, the Earth dishes and the satellite dish must face each other.  Therefore the satellite dish must face the same direction and must remain at the same location with respect to the surface of the geostationary satellite.

[Because the Geostationary satellite remains above the same point of the earth, the Earth-based satellite dish must face a fixed direction if it remains in the same location with respect to the geostationary satellite]

 9.4.4.i Gather, process and analyse information from secondary sources to identify the satellites used for ‘live’ telecasts from other regions of the world to Australia and vice versa and to present reasons why communication satellites have different aerials and positional orbits

### Research              Satellites and Live Telecasts

Communications satellites might relay signals from Sydney to New Zealand.  Sometimes the microwave signal is transmitted from one satellite to another before it is transmitted to the earth.

## What to do

Use an Internet search engine (eg Google)

Type in words such as

• Australian communication satellites

• Geostationary communication satellites

• AUSSAT

• INTELSAT

• GSOC satellites

• Satellite aerials

• Satellite footprint

Use the sites identified by the search engine to:

• identify the satellites used for ‘live’ telecasts from other regions of the world to Australia.

to present reasons why communication satellites have different aerials and positional orbits