Communication satellites are usually put in a high altitude, geosynchronous orbit above a ground station which has a large dish for transmitting and receiving radio signal. The satellite serves as a relay station – receiving radio signals from one location and transmitting them to another. Three frequency bands have been assigned for non-military satellite communication, a lower frequency bands is chosen for the downlink because rain and atmospheric attenuation of the radio waves are reduced at lower frequency.
Geosynchronous or geostationary orbits are those that are always positioned over the same point on the Earth’s surface – so they must have an orbital period of 24 hr. As the orbit is fixed, the receiving dishes don’t need to change direction (track) in order to receive the signal from the satellite. Most geosynchronous satellite occupies a band above the equation at an altitude of about 35800 km.
Each satellite must be precisely positioned to prevent its signals from interfering with those from neighbouring satellite. However, there would be a noticeable delay between transmission and reception of the signal – because the EM wave would be travelling nearly 72000 km. * The power of EM waves spreads out with distance from the transmitted obeys the inverse square law. In order to avoid loss of power form a transmitting aerial, it’s vital that the transmitted beam is very narrow.
This is done by positioning the aerial at the focus of a parabolic reflector dish. Using the parabolic reflector means the receiving dish has a high gain (increase received signal strength due to the precise targeting of narrow beam) – and so would have a stronger signal output.
The gain can also be improved by increasing the diameter of the dish, the amount of energy collected is proportional to the area of the circle enclosed by the edge of the dish – as the area is (P D2/4), doubling the diameter of the dish would increase the power of a factor of four. If a satellite dish is composed of a mesh of spacing less than the wavelength of the waves, it could behave as a solid, This principle allows a large reflector to be built without concern about its weight (suitable for radio astronomy). However, X ray has too short a wavelength to use a mesh structure for the reflector. Therefore the weight of the dish becomes a significant factor.