SATELLITE Communication.

Topics Covered. Introduction. Satellite Orbits. Structure. Frequency Allocation for Satellite Communication. Working. Advantages and Disadvantages. Application.

Introduction.

In satellite communication, the signal which is basically a beam of modulated microwaves is sent from the sender towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. Satellites are important in: voice communications, video & radio transmission, navigation (GPS), remote sensing (maps, weather satellites) etc.

History of Satellite Communications For the first 20 years of satellite communications analog signals were widely used. Satellite communications began in October 1957 with the launch by the USSR a small satellite called Sputnik 1 First true communication satellites (Telstar I & II) were launched in July 1962 & May 1963 10/1964 Syncom2: First GEO satellite, 7.4/1.8 GHz (one TV-channel or several 2-way telephone connections. 1987 TVSAT: First DBS-satellite (Direct Broadcast Satellite, Television broadcasts directly to home).

SATELLITE ORBITS One will always be in the sky for transmission of communication signals.

HOW DOES A SATELLITE STAY IN IT’S ORBIT?

DIFFERENT SATELLITES REVOLVING AROUND THE EARTH

TYPES OF ORBITS

LOW EARTH ORBITING SATELLITES LEO satellites are much closer to the earth ranging from 500 to 1500 km above the surface. Don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass A network of LEO satellites is necessary for LEO satellites to be useful. Eg. Envisat.

MEDIUM EARTH ORBITING SATELLITES MEO satellites have a larger coverage area than LEO satellites, ranging in altitude from 2000 km to just below GEO. A MEO satellite’s longer duration of visibility and wider footprint It’s distance gives it a longer time delay and weaker signal than a LEO satellite. Eg. Telstar

GEOSTATIONARY EARTH ORBITING SATELLITES Objects in GEO revolve around the earth at the same speed as the earth rotates. It’s long distance from earth(approx 35,790km) cause it to have both a comparatively weak signal and a time delay in the signal High transmit power needed and launching of satellites to orbit are complex and expensive Eg. Syncom 3.

Molniya Orbit Satellites Used by Russia for decades. It is an elliptical orbit. The satellite remains in a nearly fixed position relative to earth for 8 hours. Series of three Molniya satellites can act like a GEO satellite. Useful in near polar regions.

Structure of Satellite- Communication.

What are Communication Satellites composed of? Communication Payload. Engines. Station. Power sub-system. Command and control sub-system.

Communication Payload. Communication Payload is made up of transponders, antennas and switching systems.

Communication payload is made up of transponders,antennas and switching systems. Transponder is a device which recieves radio signal and automatically transmits into different signal. Transponder.

Antenna is an electrical device which converts electrical power into radio waves and vice-versa. It is usually used with radio transmitters or recievers. ANTENNA.

Engine. Engines are used to bring the satellites to the desired orbit. Station Keeping Tracking and stabilization subsystem used to keep the satellite in the right orbit, with it’s antennas pointed in the right direction, and it’s power system pointed towards the sun. Engine.

Power subsystem is used to power the Satellite system normally composed of solar cells and batteries that maintain power during solar eclipse. Power subsystem.

Command and Control subsystem. Command and control subsystems maintain communications with ground control stations. The ground control station monitors the satellite performance and controls it’s functionality during various phases of life-cycle. Command and Control subsystem.

Frequency Allocation for satellite systems

Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is carried out under the auspices of the International Telecommunication Union (ITU).  Although a given service may be allocated different frequency bands in different regions. Fixed satellite service (FSS) Broadcasting satellite service (BSS) Mobile satellite service Radionavigation-satellite service Meteorological-satellite service Amateur-satellite service

How do communications satellites work?

Communications satellites receive information from transmitters on Earth and beam it down to receivers elsewhere on the planet. Transmitters and receivers differ widely. A communications satellite might relay a signal between one sender and receiver, satellite broadcasts typically involve one or more uplinks and multiple downlinks.

Advantages of Satellite Communication Flexibility:   Mobility:   Speedy deployment:   Provides coverage over the globe: 

Disadvantages of Satellite Communication Cost:   Propagation delay:   Specialised satellite terminals required:  

Applications of communication satellites.

Global Telephone Global Telephone One of the first applications of satellites for communication was the establishment of international telephone backbones. Instead of using cables it was sometimes faster to launch a new satellite. Telephonic conversations using satellites was not fruitful as the signal needs to travel distance 10 times than the distance between 2 stations on the earth i.e. sending data from ground to satellite and from satellite to another location on earth. This cause’s substantial amount of delay and this delay becomes more prominent for users during voice calls. The downlink follows an analogous path. Improvements in submarine communications cables through the use of fiber-optics caused some decline in the use of satellites for fixed telephony in the late 20th century

Global Mobile Communication Global Mobile Communication:-The basic purpose of satellites for mobile communication is to extend the area of coverage. Satellites cover a certain area on the earth. This area is termed as a “footprint” of that satellite. Within the footprint, communication with that satellite is possible for mobile users. These users communicate using a Mobile-User-Link (MUL). The base- stations communicate with satellites using a Gateway-Link (GWL).

Satellite Internet access Satellite Internet access is Internet access provided through communications satellites. Satellites have brought Internet access to places where IP communications seemed impossible.

Radio and TV Broadcast Radio and TV Broadcast:- These dedicated satellites are responsible for making 100s of channels across the globe available for everyone. They are also responsible for broadcasting live matches, news, world-wide radio services. The signals are received via an outdoor parabolic reflector antenna usually referred to as a satellite dish and a low- noise block down converter (LNB). Receivers can be external set-top boxes, or a built-in television tuner.

Amateur radio Amateur radio (also called ham radio) describes the use of radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radio sport contesting, and emergency communication. The international symbol for amateur radio. The diamond holds a circuit diagram featuring components common to every radio: an antenna, inductor and ground.

Navigation Navigation Satellites The system allows for precise localization world-wide, and with some additional techniques, the precision is in the range of some meters.

Connecting Remote Areas Connecting Remote Areas:- Due to their geographical location many places all over the world do not have direct wired connection to the telephone network or the internet (e.g., researchers on Antarctica) or because of the current state of the infrastructure of a country. Here the satellite provides a complete coverage and (generally) there is one satellite always present across a horizon.

Military applications. Communication satellites are used for military communications such as Global Command and Control System. Military system that uses satellites for communication are MILSTAR, DSCS, FLTSATCOM(USA), NATO satellites, Skynet(UK), and satellites from Soviet Union.

India has also launched a satellite for communication purpose of military. The name of the satellite is GSAT-7. Transponders of this satellite operate in UHF, F, C an Ku band.

THAnK YOU. Prepared by: Tejas Shah (15IT129)