Q 1.1 How does satellite communication work?
Quite simply, satellites are radio relays located in space. Because of their wide field-of-view satellite relays can connect widely separated regions – order of hundreds to thousands of kilometres- irrespective of nature of the intervening geography. A clear line-of-sight of satellite with benign radio conditions is desirable.
Satellites receive radio transmissions from Earth stations, amplify and re-transmit them towards the service region. The radio frequency (RF) and satellite transmission power depends on the application. In general smaller an Earth station receiver, the larger is the satellite transmitted power.
Fixed large Earth stations act as an interface between the terrestrial and satellite networks, by mapping incoming signals for the outgoing transmission medium. As such, these Earth stations are also called gateways. For personal and mobile applications the user terminals are small – portable, pocket sized, mounted on aircraft, ship, railway coach, etc.
The terrestrial network can comprise public switched fixed or mobile network, public data network, Internet or indeed a private network. Thus connectivity is possible between – fixed users, fixed-mobile users, mobile-mobile users, user-Internet, etc.
Consider a scenario where a fixed user A makes a call to another fixed user B located thousands of kilometres away. The call from user A is routed through the local terrestrial network to the satellite gateway which maps the incoming signals for the space segment and transmits them over a given satellite to the destination gateway Earth station where the call gets routed to the end user B via the destination terrestrial network. The same processes repeat in the return direction.
A majority of systems use the geostationary orbit wherein satellites appear stationary from the Earth. This is an equatorial orbit that has an altitude of about 35786 km with an inclination of 0o. At this altitude and inclination satellite motion synchronises with that of the Earth’s rotation. Geostationary satellites can cover nearly a third of the globe and hence provide instantaneous connectivity between locations separated by thousands of kilometre. We observe that the equatorial distance of 1/3rd globe is approximately 13358.4 km. [We assume a circular equator with a radius of 6378.14 km for this calculation. It transpires that geostationary satellites are not visible beyond +/- 81 deg. latitude.]
Satellite communication systems also use low and medium Earth orbit ranging between about 700 km to 24000 km. Low and medium orbit satellite systems require handovers between satellite and/or Earth stations because satellites rise and set regularly. As such, the system architecture and constellation management is more complex than geostationary satellite systems and number of satellites for a given coverage can be large but they exhibit lower propagation delay, provide full global coverage and are less demanding in power since they are closer to the Earth.