Friday, September 9, 2016

What is Satellite Communication?

Meaning of Satellite & Basic concepts:

Satellite:

"satellite is object which rotates around some body due to the effect of gravitational forces" .

OR

"A satellite is an object which has been placed into orbit by human endeavor" .




•A satellite communications system uses satellites to relay radio transmissions between earth terminals.
The two types of communications satellites are ACTIVE and PASSIVE.
A passive satellite only reflects received radio signals back to earth.
An active satellite acts as a REPEATER; it amplifies signals received and then retransmits them back to earth.
This increases signal strength at the receiving terminal to a higher level than would be available from a passive satellite.A typical operational link involves an active satellite and two or more earth terminals.•One station transmits to the satellite on a frequency called the UP-LINK frequency.
•The satellite then amplifies the signal, converts it to the DOWN-LINK frequency, and transmits it back to earth.





History of Satellite in Short Timeline:      

 1957-1990 :


           •USSR launched the first artificial earth satellite, Sputnik in 1957.
           •US launched the first GEO satellite, Relay-1 in 1962.
            •First trans pacific TV traffic distribution in 1963.
     •First satellite packet radio network in 1970. [ALOHA (Application Layer Optimization and High Availability) Net].
    •Satellites connected the American continent and UK in 1975 [SATNET (satellite network)].•First demonstration of interconnection between ARPANET (Advanced Research Projects Agency network), SATNET (satellite network) and PRNET (packet radio network ) in 1977.•First TCP (Transmission Control Protocol) specification was released in 1982.•VSAT (very small aperture terminal) technology (1980-1990):  Ku band (uplink 14ghz, downlink 12ghz). Both way communication capability. 1.8m-2.4m dish size.Point to point communication. Star topology.Mesh topology. 


1990-till now: 


•Big success in the digital TV broadcasting-
DirecTVB-Sky-B [British Sky Broadcasting (TV broadcaster)], SKY-perfec-TV [Japanese company for direct broadcast satellite (DBS) service], etc.
One way communication platform based on MPEG (Motion Picture Expert's Group) 2 system.
UDLR (Uni-directional Link Routing)
TCPSAT(TCP over satellite)
Satellite link as one-way communication: UDLR
Satellite link as a two-way communication: DVB-RCS (Digital Video Broadcasting - Return Channel via Satellite)



Satellite Communication System:
Orbit types
Orbit types



Satellite communications system
Satellite communications system











The basic design of a satellite communications system depends to a great degree upon the characteristics of the orbit of the satellite.

•A special type of orbit is a SYNCHRONOUS ORBIT, In which the period (time required for one revolution) of the orbit is the same as that of the earth.
An orbit that is not synchronous is called ASYNCHRONOUS. A period of orbit that approaches that of the earth is called NEAR SYNCHRONOUS (sub-synchronous).












Classification of orbit by Altitude:



Classification of orbit by Altitude
Classification of orbit by Altitude 





Additional characteristics of LEO satellites:

A period /time to revolve around the earth is about 90 minutes

These satellites are only visible from within a radius of roughly 1000 kilometers from the sub-satellite point

They change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity

They are less expensive to position in space than geostationary satellites

Because of their closer proximity to the ground, require lower signal strength (Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic)

So there is a trade off between the number of satellites and their cost!











MEO-Medium Earth Orbit:

MEO-Medium Earth Orbit
MEO-Medium Earth Orbit




GEO-Geostationary Orbit:
GEO-Geostationary Orbit
GEO-Geostationary Orbit
Earth turns from west to east
Earth turns from west to east





Geostationary orbits :

Geostationary orbits
Geostationary orbits