SATELLITE COMMUNICATION

SATELLITE COMMUNICATION

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Contents Introduction to satellitecommunication Applications of satellitesSpace segmentEarth segment & DBSAccess techniques VSAT

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MODULE I Introduction to Satellite Communication

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Introduction

Why satellite communication?

Long distance communications using conventional techniques like coaxial cables or M/W radio relay links involve a large number of preparation and other means of propagation also has limitations.

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TWO WIRE TRANSMISSION LINE

Limitations

Bandwidth is lessRadiation losses are largeFrequency attenuation is high

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2. COAXIAL CABLE

Coaxial cable

Outer conductor

Inner conductor LIMITATIONS Dielectric loss Acts as a low pass filter

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3. WAVE GUIDE (RECTANGULAR OR CIRCULAR)

LIMITATIONS Bulky (practically not feasible for long distance communications)

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4. OPTICAL FIBRESLIMITATIONSVery thinProblem in connectivity for long distance and even short distance communication.

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LIMITATIONSFor radio relay links 8 GHz repeater spacing is limited by LOS and is of the order of few Kms (50-60). As number of repeaters increases system performance and reliability are degraded.Troposcatter propagation can cover several hundred Kms, but their channel capacity is limited and costs are high due to necessity of large antennas and high transmit power.HF communications is subject is ionosphere disturbances and channel capacity is severely restricted due is limited bandwidth available

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Earth

Equator

Geostationary orbit

36000 KmPeriod 24 hours Global Coverage with Geostationary Satellite

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IDEA TO USE A TRANSMISSION LINE IN HEAVEN WHICH HAS NO LIMITS AND THAT IS SATELLITE

Since fiction writer Arthur C. Clarke in an article in Wireless World in 1945 proposed that world wide coverage can be obtained by using 3M/W repeaters if the height of the M/W repeater could be increased by putting on board an artificial earth satellite.

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Contd..Clarke proposed to put them in a geo-stationary orbit at the height of about 36000Km above earths surface with a span period of 24 hrs.At that time German rocket V2 had launch capacity of 200 Km.First artificial satellite SPUTNIK, was launched by USSR in 1957.

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What is a satellite?

A satellite is any object that orbits or revolves around another object. For example, the Moon is a satellite of Earth, and Earth is a satellite of the Sun.

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What Keeps Objects in Orbit?

The first of Newton's laws, proposed that every bit of matter in the universe attracts every other bit of matter with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between the two bits. That is, larger masses attract more strongly and the attraction gets weaker as the bodies are moved farther apart.

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If the sun and the planets are pulling on each other with such a large force, why don't the planets fall into the sun?

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The Earth No Longer Orbits the Sun if Gravity is Switched Off

The gravitational force between the sun and the earth holds the earth in its orbit. The force of gravity if suddenly to be turned off, the object in question would instantly leave its circular orbit, take up a straight line trajectory.This is shown in the drawing below, where the earth was happily orbiting the sun until it reached point A, where the force of gravity was suddenly turned off.

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Keplers Laws

Keplers law describes the planetary motionKeplers three laws apply to any two bodies in space which interact through gravitation.The more massive of the two bodies is referred to as the PRIMARY, the other the SECONDARY or SATELLITE.

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Round Trip Time

In a satellite network, Round Trip Time (RTT) is the time required for a signal to travel from a terrestrial system up to the satellite and back, or for a signal to travel from a satellite down to a terrestrial system and back up to the satellite again.It is given by t = 2d/c t : round trip time d : distance b/w the earth & satellite c : speed of light

Definition of terms for Earth Orbiting Satellites

Apogee. The point farthest from earth.Perigee. The point of closest approach to earth.Line of apsides. The line joining the perigee and apogee through the center of the earth.Ascending node. The point where the orbit crosses the equatorial plane going from south to north.Descending node. The point where the orbit crosses the equatorial plane going from north to south.Inclination. The angle between the orbital plane and the earth's equatorial plane.

Definition of terms for Earth Orbiting Satellites contd

Line of nodes : The line joining the ascending & descending nodes through the center of earth.Prograde orbit : An orbit in which satellite moves in the same direction as earths rotation, also known as Direct orbit, inclination lies between 0 & 90 degree.Retrograde orbit : An orbit in which satellite moves in the opposite direction as earths rotation, inclination lies between 90 & 180 degree.

Satellite orbital terms

PerigeeApogeeEarth S

Line of apsides

Prograde orbit

Retrograde orbit

Types of orbitLow Earth Orbit

Medium Earth Orbit

GeoSynchronous Equitorial Orbit

Elliptical Orbit

Polar orbit

Low Earth OrbitWhen a satellite circles close to Earth ,it's in Low Earth Orbit (LEO). Satellites in LEO are just 200 - 500 miles (320 - 800 kilometers) high. Because they orbit so close to Earth, they must travel very fast so gravity won't pull them back into the atmosphere. Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes.

Low Earth Orbit contd.

A Low Earth Orbit is useful because its nearness to Earth gives it spectacular views.Satellites that observe our planet, like Remote Sensing and Weather satellites, often travel in LEOs because from this height they can capture very detailed images of Earth's surface.

MEO (Medium Earth Orbit)

Sometimes called Intermediate Circular Orbit (ICO), is the region of space around the Earth above low Earth orbit and below geostationary orbit

The orbital periods of MEO satellites range from about two to 12 hours.

MEO AdvantagesMEO enables a satellite provider to cover the earth with fewer satellites than Low Earth Orbit, but requires more satellites to do so that geostationary orbit.MEO terrestrial terminals can be of lower power and use smaller antennas than the terrestrial terminals of GEO satellite systems. Communications satellites that cover the North and South Pole are also put in MEOMedium Earth Orbit satellite systems offer better Round Trip Time (RTT) than geosynchronous orbit systems, but not as low as Low Earth Orbit systems.

GeoSynchronous Equitorial Orbitgeo = Earth + synchronous = moving at the same rate A satellite in GEO is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth.

Advantages

Remain almost stationary in respect to a given earth station so expensive tracking equipment not required at earth station.Available to all earth stations within their shadow 100% of the time.No need to switch from one geo satellite to another as they orbit overhead. So no transmission breaks due to switching times.

Disadvantages

Require Sophisticated and heavy propulsion devices on board to keep them in a fixed orbit.Require high transmit powers & more sensitive receivers because of longer distances & greater path losses.Introduce longer propagation delays (round trip propagation delay b/w 2 earth station through a geo synchronous satellite is 500 & 600 ms).High precision spacemanship reqd. to place into the orbit and keep it there.

Elliptical OrbitA satellite in elliptical orbit follows an oval-shaped path. One part of the orbit is closest to the center of Earth (perigee) and the other part is farthest away (apogee). A satellite in this orbit takes about 12 hours to circle the planet. Like polar orbits, elliptical orbits move in a north-south direction.

Polar Orbit

A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction.

Why use Polar Orbit?

Polar orbits are useful for viewing the planet's surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the the entire surface.

Polar Coverage

While most communications satellites are in Geosynchronous orbit, the footprints of GEO satellites do not cover the polar regions of Earth. So communications satellites in elliptical orbits cover the areas in the high northern and southern hemispheres that are not covered by GEO satellites.

Comparison Between LEO, MEO and GEO

Orbit type

Low Earth Orbit (LEO)

Medium Earth Orbit (MEO)

Geostationary Earth Orbit (GEO)

Orbit Characteristics

Altitude (km)

700 to 1400

10,000 to 15,000

36,000

Satellites Needed For Global Coverage

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10 to 15

3 to 4 (1)

Phased Start Up Possible

No (global coverage)Yes (if phase start up region by region)

Yes

Yes

Link Characteristics

Delay

0.05 s

0.10 s

0.25 s

Elevation Angle

Low

Medium to High

Low to Medium

Call Handover

Frequent

Infrequent

Never

Operations

Complex

Medium

Simple

Building Penetration

Poor

Poor

None

Low Earth Orbit (LEO)Medium Earth Orbit (MEO)Geostationary Earth Orbit (GEO)

Satellite Characteristics

Space Segment Cost

High

Low

Medium

Satellite Lifetime (years)

3 to 7

10 to 15

10 to 15

Telephony Network Characteristics

Terrestrial Gateway Costs

High

Medium

Low

Hand Held Terminal Possible

Yes

Yes

Yes

Hand Held Terminal Costs

Low

Low

Low

Mobile Terminal Costs

Medium

Fixed Terminal Costs

Low

Low

Low to Medium

Low Earth Orbit (LEO)Medium Earth Orbit (MEO)Geostationary Earth Orbit (GEO)

Data Network Characteristics

Store and Forward Possible

Yes

Not Required

Not Required

Point to Point Connections Possible

No

No

Yes

VSATs Possible

Yes (2)

Yes (2)

Yes (3)

TV Network Characteristics

Broadcast TV Possible

No

No

Yes

SNG Possible

Yes

Yes

Yes

Low Earth Orbit (LEO)Geostationary Earth Orbit (GEO)Medium Earth Orbit (MEO)

Keplerian Element SetEarth orbiting artificial satellites are defined by six orbital elements: Keplerian Element Set. These are:Semi major axisEccentricityMean anomaly : gives position of satellite in its orbitArgument of perigee : gives rotation of orbits perigee point relative to the orbits line of nodes in the earths equatorial plane.Inclination Right ascension of the ascending node

Gives the shape of ellipse

Relates orbital planes position to the earth

Why satellites are used for communication?By the end of World War II, the world had had a taste of "global communications. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required to transmit television pictures required that they operate at much higher frequencies than radio stations. For example, the very first commercial radio station (KDKA in Pittsburgh) operated ( and still does) at 1020 on the dial. Television signals, however required much higher frequencies because they were transmitting much more information - namely the picture.

Why satellites are used for communication?Radio and television frequency signals can propagate directly from transmitter to receiver. The mode of propagation for long distance radio communication was a signal which traveled by bouncing off the charged layers of the atmosphere (ionosphere) and returning to earth. The television signals did not bounce off the ionosphere and as a result disappeared into space in a relatively short distance.

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Why satellites are used for communication?Adding this increase to the demands of the new television medium, existing communications capabilities were simply not able to handle all of the requirements. By the late 1950s the newly developed artificial satellites seemed to offer the potential for satisfying many of these needs.

Principal SatelliteTransmission BandsC band: 4(downlink) - 6(uplink) GHzThe first to be designated Ku band: 12(downlink) -14(uplink) GHzRain interference is the major problem Ka band: 19(downlink) - 29(uplink) GHzEquipment needed to use the band is still very expensive

ADVANTAGE OF SATELLITE COMMUNICATIONLARGE COVERAGE: Almost footprint of one satellite covers 1/3of the earth with the exception of polar regions from geo-stationary orbits.It is possible to cover about 10,000Km.The satellite can from the star point of communication net linking together many users simultaneously, who may be widely separated geographical.To provide communication links in remote areas which are sparsely populated ,which is otherwise difficult to access.Ignores political as well as geographical boundaries.

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Contd..2. Cost is distance Insensitive:The constriction and launch cost of satellite is very high-few hundreds million dollars. But it cost is distance insensitive meaning that it costs about the same to provide a satellite communication link over a short distance as it does over a large distance.So a satellite comm. System (SCS) is economical only where the system is in continuous use and costs can be reasonably spread ever a large number of users.

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Contd..3. High Quality:Satellite links can be designed for high quality performance. The links performance is highly stable since it is free from ionospheric disturbances ,multi path fading etc.High Reliability: Reliability is high since only one repeater is involved in the link.

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Contd... 5. High Capacity: With M/W freq, wide bandwidths are available and large comm. Capacity can be obtained.

6. Flexibility:In terrestrial systems,communication is tied down to the links installed but SCS is well suited for changing traffic requirements,location and channel capacities.

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Contd..7. Speed of installation:Installation of earth terminals can be achieved in a short time as compared to lying of cables or radio relay8. Mobile short term or emergency communications:With airliftable or road transportable terminals short term or emergency communication can be quickly provided.Reliable long distance land mobile,marline mobile and aeronautical mobile services are feasible only by means of satellite.

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Contd..Broadcast nature of transmission:SCS is ideally suited for point to multi-point distribution or broadcasting over large areas.so its application in TV broadcasting , audio video distributions,teleconferencing,facsimile,data comm.is increasing rapidly.

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Contd..Remote Sensing:Widely used for detection of water pollution,monitoring and reporting of weather conditions, search and resume operation for downed aircraft and the like that many other applications.

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International systems (INTEL Sat)2. Domestic satellite systemINSAT (Indian)DOM Sat (US)3. U.S. nationalOceanographic and atmospheric administration (used for environmental monitoring search and rescues.)

Satellite communication applications

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DOMSAT & SARSAT DOMSAT Domestic satellites are used to provide various telecommunication services, such as voice, data, and video transmission (T.V channels), with in a country.

SARSAT SARSAT is one type of Polar orbiting satellites. Polar-orbiting satellites orbit the earth in such a way as to cover the north and south polar-regions. Infinite number of polar polar satellite orbits are possible Polar satellites are used to provide environmental data , and to help locate ships and aircrafts in distress .This service known as SARSAT, for search and rescue satellite.

Whose Satellite Was the First to Orbit Earth?

The Soviet Sputnik satellite was the first to orbit Earth, launched on October 4, 1957. Sputnik 1, the first satellite, shown with four whip antennas