Communicating with satellites

Kelvin Barnsdale explains how satellites use radio signals to communicate with ground stations and how satellite dishes are shaped to reflect these signals into a single point.

Point of interest:

• Radio signals and light are both forms of electromagnetic radiation – they differ only in terms of wavelength

Transcript

Kelvin Barnsdale

Well, all satellites communicate with Earth through radio signals, so there would be a radio transmitter on the satellite, and that sends a signal down to a receiver on the ground. The ground stations have a large aerial and a transmitter and a receiver. Quite often, the bigger ground stations will have something that can move the aerial so that, when the satellite passes over, the aerial’s always pointing at the satellite as it goes over. The ground stations have lots of computers to collect the data from the satellite, if there will be any data, or send commands up to the satellite. Quite often, you need to send commands up to satellites to control them, to make them go into a different orbit or turn them on or off or control the power systems on the satellite.

Satellites’ radio signals are very much like your WiFi or Bluetooth or your cell phone radio signals – they carry not voice but data. As an example, a TV satellite will be sending out a signal that is received by millions of satellite dishes on the roofs of people’s houses. The signal is shaped as such that it covers the correct parts of the country or even the correct countries. That signal that’s coming from the satellite contains data, which is your TV pictures. But that signal also has to get up to the satellite in the first place, so that comes from the satellite ground station controlled by the company. The TV signal will come into the ground station and then get converted into a radio signal, which goes up to the satellite and then comes back down to the users.

A large satellite dish is shaped that way to reflect the radio signals into a single point. The curve is a special curve – it’s a parabola – and the angle of the curve is such that, if the signal comes in from the satellite and hits anywhere on this dish, the reflection will always end up at this receiver down here. If you look at a torch, it has a reflector behind the bulb and that does the same thing, it takes all the rays of light from the bulb and reflects it all forward into a beam. A radio satellite dish does the same thing with radio signals.

Acknowledgements Jock Phillips, Te Ara – The Encyclopedia of New Zealand Photograph of Warkworth Satellite Earth Station, 1971, courtesy of the J. R. Diamond Collection International Polar Foundation