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Bandwidth

The range of frequencies in a radio signal is its bandwidth. An FM radio station requires a bandwidth of 250 kHz while analogue television needs 8MHz for all the picture information and the sound; satellite television uses microwaves with a frequency of around 30GHz and a bandwidth of 1 GHz per channel.

The bandwidth of radio stations limits the number of channels that can 'fit' onto the range of radio waves available. For satellite television the problem is less acute as more 'bandwidths' fit into the frequencies available.

Satellites use a variety of bandwidths depending on the amount of information they are sending. Typically satellite might send 9600 bits per second (the bit rate). Some may go as fast as 2 million bits per second. You need 1 Hz bandwidth for every bit per second so the 9600 bits per second bit rate is accommodated on a 10 kHz bandwidth.

However there's another problem. As the satellite moves towards and away from the ground station, its signal is Doppler shifted. The satellite is travelling at 7 or 8 km/s, so the frequency shift because of this is considerable. This means you actually need a receiver with extra bandwidth to take account of the Doppler shift: it needs to detect frequencies even further above and below the carrier frequency. Surrey Space Centre uses a different technique. It tracks the signal as it changes from blue shifted (higher frequency as the satellite moves towards the ground station) to red shifted (as is moves away) and then notes how much it has changed.

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Listen to the Doppler effect