This tutorial section on satellite basics covers following sub topics:
Main Page About satellite Bands Parts Orbits Services Types capacity allocations network configurations Applications Satellite TTC Remote sensing VSAT basics
As mentioned in the "what is a satellite" page there are two main types of satellites natural satellites e.g. sun.moon,stars and man-made satellites(artificial satellites) e.g. Geostationary,LEO and MEO satellites.
Artificial satellites are mainly called as communication satellites which are categorized based on coverage,service type and their usage. Coverage varies based on beamwidth of the antenna on satellite and can be global,regional,or national. Service types are FSS(Fixed Service Satellite),BSS(Broadcast Service Satellite) and MSS(Mobile Service Satellite). Usage of the satellite can be either for military use or for commercial use.
In order to use satellite for communication purpose or relay/repeater application, Ground station(Earth station) need to track the satellite as it revolves above the Earth for short duration as per the orbit it is placed in. There will be no communication when satellite disappear and move to the other side of visible area of the earth station antenna. One solution to this problem is to launch satellite in a long elliptical orbit and second solution is to have Geosynchronous satellite at the distance of about 35,860 Km. The same is explained below.
Geostationary satellite is the most common type of satellite and more popular today. This is located at geostationary circular orbit above earth at about 35,860 Km. GEO satellite's angular speed is same as that of the Earth, it remains at the same position on the equator even if the Earth moves/rotates. Hence GEO satellite is ideal for providing continuous service 24 Hrs of the time.
Advantages/Merits of GEO Satellite
• Tracking of the satellite from Earth is easy.
• As satellite is stationary, doppler frequency change can be avoidable between satellite and antenna on the Earth.
• About three satellites separated by 120 degrees can cover entire region of the earth except north/south poles.
Dis-advantages/Demerits of GEO Satellite
• signal becomes weak after travelling such long distance
• delay is more, about 120 ms from Earth to the satellite. It will be about 240ms for double hop system i.e. from the Earth to the satellite and back to the Earth.
• some of the regions such as polar region and north and south hemispheres are poorly served.
LEO satellite is located at about 500 to 1500 Km altitude. This type of satellite is visible for about 20 minutes from a fixed location on earth. The orbit period is about 1.5-2 Hours. Coverage diameter is approx. 8000Km.
Merits of LEO satellite
• The signal power is good enough for earth terminals.
• The round trip delay also is less(about 20ms) due to its location at lesser altitude than GEO satellite.
Demerits of LEO satellite
• This LEO satellite system requires many such satellites to provide 24 Hr service on earth.
• Atmospheric drag is significant resulting in orbital deterioration gradually.
MEO satellite is located at the altitude of about 5000 to 12000 Km above earth. The orbit period is about 6 Hours. Coverage diameter is approx. 10000 -15000Km. Round trip delay is about 50ms.
Merits of MEO satellite
• This system of satellites requires much fewer handoffs compare to LEO satellites.
• Maximum visible time from earth is about few hours.
Demerits of MEO satellite
• Propagation delay is more compare to LEO satellites.
• Power required is also more than LEO satellites.