The basic theory of telecommunications
Low frequency electromagnetic waves (radio waves) have been used to send information for around 100 years. Tapping out morse code messages generated electromagnetic pulses which then made the receiving device pull down its 'tapper' to repeat the pattern. The operator had to decode the message from the pattern of long and short taps.
Now we take it for granted that events taking place around the globe can be transmitted, via satellite, direct to our homes within seconds of it happening.
Early radio and television programmes were transmitted as analogue signals where the continuously varying information signal was simply added to a carrier radio wave.
Many modern systems now use digitally coded information but still require a carrier radio wave.
Satellites use a radio frequency carrier wave. For example, most of Surrey's satellites use a UHF (ultra high frequency) wave at 430 MHz (megahertz) - equivalent to a wavelength of 70 cm. These waves can be detected by the ground station (Mission Control) as soon as the satellite comes over the horizon. In fact you do not need high tech apparatus for this. A simple hand held device will be enough to track the satellite as it travels from horizon to horizon. This means that even though the satellites transmit at only a few watts power, their signal can be detected 3000 km away on Earth.