Satellite Communications

Ppt for Computer Networks

Topics of Presentation

How itworks

How itworks

Applications

LaunchingHow itworks

FrequencyBands

Intro

Satellites

Types

VSAT

GPS

What are Communication Satellites?

• A satellite is an object that orbits another large object like planet.

• A communication satellite is a station in space that is used for telecommuncation, radio and television signals.

• The first satellite with radio transmitter was in 1957.

Need for communication satellite.......

• Early 1950’s and 1960’s,satellite communication by mettallized weather baloons..

•Moon as an artificial satellite.In 1954, the US Naval Research Laboratory successfully transmitted the first voice message.

•Disadvantage using moon as a satellite….Communications via the Moon is limited by simultaneous visibility of the Moon by both the transmit and the receive stations  

•For further progress,had to wait for an Artificial satellite.

Early Satellites

Syncom• In October 1957, the first artificial satellite Sputnik -I was launched.

• 1963 Clark’s idea became a reality when the first geosynchronous satellite SYNCOM was successfully launched by NASA.

•India launched its first satellite Aryabhatta on 19 April 1975

Types of Satellites•In 1950’s both Active and Passive satellites were used.

• The principle of communication by passive satellite is based on the properties of scattering of electromagnetic waves from different surface areas.

•Thus an electromagnetic wave incident on a passive satellite is scattered back towards the earth and a receiving station can receive the scattered wave.

Active replaces Passive The disadvantages of passive satellites for

communications are: • The large attenuation of the signal.• Earth Stations required high power (10 kW) to transmit

signals strong enough to produce an adequate return echo. 

• Large Earth Stations with tracking facilities were expensive.

• A global system would have required a large number of passive satellites accessed randomly by different users 

• Control of satellites not possible from ground.

Active satellites

• In active satellites, which amplify and retransmit the signal from the earth have several advantages over the passive satellites.

The advantages of active satellites are: 

1.Require lower power earth station

2.Less costly

3.Not open to random use

4.Directly controlled by operators from ground.

 

How Does a Satellite Work?

• Consider the light bulb example:

Orbital Options

• A Geosynchronous satellite(GEO) completes one revolution around the world every 23 hrs and 56 minutes in order to maintain continuous positioning above the earth’s sub-satellite point on the equator.• A Medium earth orbit satellite(MEO) requires a constellation of 10 to 18 satellites in order to maintain constant coverage of the earth. •A Low earth orbit satellites(LEO) offers reduced signal loss since these satellites are 20 to 40 times closer to the earth in their orbits thus allowing for smaller user terminals/antennas.

Geostationary orbits

• What are they? Geostationary orbits is fixed position to an earth-based observer.

• When was the first use? The first truly geostationary sateliite was the SYNCOM3 in 1964.

• Why they are important in communications? - The antennas in the ground don’t need equipment to track the satellite. - Lower cost & complexity.

• Disadvantages? - Not always suitable for providing services at high latitudes. - Molniya satellite was introduced as a solution.

Geostationary Orbit (GEO)Characteristics of Geostationary(GEO) Orbit System• User terminals do not have to track the satellite• Only a few satellites can provide global coverage• Maximum life-time (15 years or more)• Above Van Allen Belt Radiation• Often the lowest cost system and simplest in terms of

tracking and high speed switching.Challenges of Geostationary(GEO) Orbit System• Transmission latency or delay of 250 millisecond to

complete up/down link • Satellite antennas must be of larger aperture size to

concentrate power and to create narrower beams for frequency reuse

• Poor look angle elevations at higher latitudes

Medium Earth Orbit (MEO)

Characteristics of Medium-Earth Orbit system• Less latency and delay than GEO (but greater than LEO)• Improved look angle to ground receivers in higher latitudes• Fewer satellites to deploy and operate and cheaper TTC&M

systems than LEO (but more expensive than with GEO) • Longer in-orbit lifetime than LEO systems (but less than

GEO)

Challenges of Medium-Earth Orbit System• More satellites to deploy than GEO (10 to 18 vs. 3 to 4)• Ground antennas are generally more expensive and complex

because of the need to track satellites. Or, one must use lower-gain, quasi-omni antennas.

• Increased exposure to Van Allen Belt radiation

Low Earth Orbit (LEO)Characteristics of Low-Earth Orbit (LEO) Systems • Low latency or transmission delay •  Higher look angle (especially in high-latitude regions) •  Less path loss or beam spreading • Easier to achieve high levels of frequency re-use • Easier to operate to low-power/low-gain ground antennas.Challenges of Low-Earth Orbit (LEO) Systems• Due to Larger number of satellites (50 to 70 satellites). Thus higher launch costs to deploy, build, and operate. • Harder to deploy, track and operate. There is higher TTC&M costs even with cross links.• Shorter in-orbit lifetime due to orbital degradation

Properties of Orbits

Satellite-Fiber Comparisoncapability Fiber optic

cable systemsGeo satellite in a global system

Meo satellite in a global system

Leo satellite in a global system

Transmission speed

10 Gbps-3.2 Terabits/second

Single sat 1Gbps-10Gbps

Single sat 0.5Gbps-5Gbps

Single sat 1Gbps-2Gbps

Quality of service

10-11 -10-12 10 -6 -10-11 10-6 -10-11 10-2 -1091

Transmission latency

25 to 50 ms 250 ms 100-150 ms 25-75 ms

System availability w/o backup

93 to 99.5% 99.8%(C-Ku band)99% (Ka band)

99.9%(C-Ku band)99% (Ka band)

99.5%(L-c-Ku band)99% (Ka band)

Broadcasting capabilities

Low to Nil High Low Low

Multi-casting capabilities

Low High High Medium

Trunking capabilities

Very High High Medium Low

Mobile services Nil Medium-to-High High High

ELEMENTS OF SATELLITE COMMUNICATIONS SYSTEM

•Space SegmentSatellite (transponders etc)Means for launching satelliteSatellite control centre for station keeping of the satellite

•Ground SegmentEarth StationsRear Ward Communication linksUser terminals and interfacesNetwork control centre

 

Elements

Transponders

•The Transponder (short-for Transmitter-responder) is the “BRAIN” of the satellite that provides connection between the satellite’s Receive and Transmit antennas. •Satellites can have 12 to 96 transponders plus spares, depending on the size of the satellite. •A transponder bandwidth can frequently be 36 MHz, 54 MHz, or 72 MHz or it can be even wider.

•A transponders function is to •Receive the signal, (Signal is one trillion times weaker then when transmitted)

•Filter out noise, •Shift the frequency to a down link frequency (to avoid interference w/uplink)•Amplify for retransmission to ground

Transponder

Satellite Power SystemsSatellite Power Systems

• Main source of power is solar cell panels - new solar cells are increasingly efficient

• The solar cell system is backed up by battery system that provides energy during solar eclipses and other periods of outages.

• Typical power levels of 2 to 5 KWs for Fixed Satellite Systems and 10 to 12 KWs for Mobile and Broadcast Satellite Systems.

Batteries• latest battery technology is represented by Lithium Ion

systems that can provide a greater power density for longer periods of time and survive a greater depth of discharge

Launching Satellites

• How does a satellite stay in it’s orbit?

Launching cntd…

The launch process can be divided into two phases:

• The Launch Phase

- Satellite placed into the transfer orbit.

• The Orbit Injection Phase

- Satellite transferred from elliptical transfer orbit to

geosynchronous orbit.

Launch Vehicle• Function-Place the satellite into the desired Orbit.

• Two types

Expandable

• Launch vehicles can be used only once.

• Most of the satellite are of this type.

Reusable

• Only one reusable launch vehicle available.

Satellite ejected from the cargo compartment.

• The Space Shuttle performs the functions of the first two stages of an expendable launch vehicle.

• The satellite and third stage assembly are ejected after reaching elevation of 150 to 200 miles.

Launching contd….• By riding on a rocket or in the

cargo bay of the Space Shuttle.

• At about 120 miles (193 km) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position.

• Inertial guidance system mechanism is used to calculate adjustments to tilt the rocket

Path followed by a satellite.

SATELLITE CONTROL CENTRE 

Satellite Control Centre performs the following function:-

1. Tracking of the satellite

2. Receiving data

3. Determining Orbital parameters from Tracking and Ranging data

4. Commanding the Satellite for station keeping

5. Switching ON/OFF of different subsystems as per the operational requirements

6. Thermal management of satellite.

7. Eclipse management of satellite

8. Communications subsystems configuration management.

9. Satellite Bus subsystems configuration management etc.

CONTROL ROOM

Satellite Frequencies•There are specific frequency ranges used by commercial satellites.

L-band (Mobile Satellite Services) 1.0 – 2.0 GHz

S-band (MSS, DARS – XM, Sirius) 1.55 – 3.9 GHz

C-band (FSS, VSAT) 3.7 – 6.2 GHz

X-Band (Military/Satellite Imagery) 8.0 – 12.0 GHz

Ku-band (FSS, DBS, VSAT) 11.7–14.5 GHz

Ka-band (FSS “broadband” and inter-satellite links)

17.7 - 21.2GHz and 27.5 – 31 GHz

Frequency Efficiency•The vital resource in satellite communications is spectrum. •As the demand for satellite services has grown, the solution has been;

•To space satellites closer together, •Allocate new spectrum in higher bands, •Make satellite transmissions more efficient so that more bits/Hz can be transmitted, and •To find ways to re-use allocated spectrum such as through geographic separation into separated cells or beams or through polarization separation

•Today the satellites systems transmit more efficiently than ever before but interference is now a bigger problem - there is a basic trade off;

•The higher the frequency the more spectrum that is available •But, the higher the frequency the more problems with interference from other users terrestrial, unlicensed, etc.

VSAT(VERY SMALL APERTURE TERMINALS)

What is VSAT???• A new development in the communication satellite

world is the development of low-cost microstations, sometimes called VSATs.

• These tiny terminals have 1-meter or smaller antennas (versus 10 m for a standard GEO antenna) and can put out about 1 watt of power.

• In many VSAT systems, the microstations do not have enough power to communicate directly with one another (via the satellite, of course). In many VSAT systems, the microstations do not have enough power to communicate directly with one another (via the satellite, of course).

Transmission process of VSAT

Working of VSAT

The uplink is generally good for 19.2 kbps, but the downlink is more often 512 kbps or more. Direct broadcast satellite television uses this technology for one-way transmission.

In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier. The trade-off is a longer delay in return for having cheaper end-user stations.

What is GPS???

• The Global Positioning System (GPS) is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.

Global Positioning System

Structure• GPS consists of three parts: the space

segment, the control segment, and the user segment.

• The U.S. Air Force develops, maintains, and operates the space and control segments.

• GPS satellites broadcast signals from space, which each GPS receiver uses to calculate its three-dimensional location (latitude, longitude, and altitude) plus the current time.[1]

Structure contd……• The space segment is composed of 24 to 32

satellites in medium Earth orbit and also includes the boosters required to launch them into orbit.

• The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations.

• The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service.

Satellite Network

Branch Offices

Corporate Data Center/HQ

Network HUB

Corporate Offices

Gas Stations

Apartment Buildings

Residential

Internet

Some large scale corporate networks Some large scale corporate networks have as many as 10,000 nodeshave as many as 10,000 nodes

Applications

• Telephony - Fixed points< earth station> Satellite> earth station> fixed points.

• Televesion & Radio - e.g. Direct broadcast satellite (DBS) & Fixed service satellite (FFS).

• Mobile satellite technology - Special antenna called mobile satellite antenna. - No matter where or how this antenna is mounted on.

Applications

• Amateur radio - Access to OSCAR satellite. - Low earth orbits.

• Internet - High Speed. - Useful for far away places.

• Military - Uses geostationary satellites. - Example: The Defense Satellite Communications System (DSCS).

Disadvantages

• The antenna noise due to energy - Unwanted radiation sources (stars – galaxies - …etc). - Worsen S/N ratio.

• Atmosphere behaves as a resistive medium - Supplies noise power to the antenna.

•Meteors - Have to be programmed to avoid any rock or any harmful thing. - Rules of orbits.

• Expensive - only for governments or large organizations.

In Conclusion

Satellites remain the best utilization Satellites remain the best utilization used for communications due to their used for communications due to their

speed and other advantages speed and other advantages mentioned in this presentation.mentioned in this presentation.