Communication satellite

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INTRODUCTION

A communication satellite, also called COMSAT, is an artificial satellite launched into space and stationed around the earth's orbit for the purpose of telecommunications.

A mobile communication satellite system is a form of wireless communications which is not based on base transceiver stations or any terrestrial infrastructure on earth. It operates by deploying satellites into space with antennas on earth to receive the signals.

Mobile satellite communications offer us the advantage of having communication everywhere. It was a major breakthrough in the communications industry and till date research is ongoing as to how to improve its technology and services.

BACKGROUND HISTORY

The first fictional idea of launching a satellite into orbit was by Edward Everett Hale in 1869, in a story titled 'The Brick Moon'. In 1911, fiction novel writer Hugo Gernsback proposed the idea of communication by transmitting radio waves through space in his space novel 'Ralph 124C41'.[1]

It was not considered possible until 1945 when Arthur Clark wrote an article titled 'Extra Terrestrial Relays'. He described using satellites in orbits far above the earth's surface to transmit television programs and the possibility of covering the earth with 3 satellites especially in the geostationary orbit. The article had little effect at that time, though he repeated the story again in another article titled 'Exploration of Space', probably because the commercial advantages were not known.

It was when John Pierce explained in 1955 the possibilities with satellite communications, and its financial advantages, by comparing the capacity of a satellite to be 100 telephone calls that satellite technology was taken seriously. [2]

Sputnik 1

This was the first satellite to be launched into space on October 4, 1957 by the Soviet Union. Sputnik 1 orbited the earth for 3 months, and it helped to measure the density of atmospheric layers and also to detect meteoroids in space. The unanticipated success of Sputnik 1 spurred the launch of Sputnik 2 a month later with a dog named Laika on board. With the launch of Sputnik, the competition was on for countries, many considering the profits and prestige of satellite communication. USA joined the race by launching Explorer 1 in February 1958.

Signal Communication by Orbital Relay (SCORE)

SCORE was the first communication satellite. It was launched in December 18 1958. Its main aim was to prove that an atlas missile could be placed in orbit and also to show that communication repeaters could be built into a missile and it would perform as expected. SCORE was used to broadcast a Christmas message from President Eisenhower - 'Peace on earth, goodwill toward men'. It orbited for 12 days before its batteries failed.

COURIER

This was an improvement over the shortcomings of SCORE. It was designed to have a longer life time and more capacity than SCORE. It was used to test communications and techniques for traffic handling. However, the first launch was unsuccessful and was lost 2.5 minutes after lift-off because of booster failure. Another one was launched in October 1960, and it was successful. Ground stations in Puerto Rico and New Jersey were used to perform communication tests on the satellite until it failed 17 days after the launch.

WEST FORD

Lincoln laboratory was performing a study on how to transmit secure and reliable communications, and then developed the West Ford satellite. The satellite had 480 million thin copper wires with each of them about 1.5cm long and weighing 19.5kg. Voice and data were transmitted to the satellite from California, reflected by the wires and received at Massachusetts. After four months, the wires became dispersed and could only reflect low rates of data. As a result of the low data rate and increasing knowledge about satellites, West Ford was the last experimental satellite launched into space. [3]

DESIGN FACTORS OF A MOBILE COMMUNICATIONS SATELLITE SYSTEM

Designing a mobile communications satellite system involves a consideration of so many technical, administrative and economic factors. However, these decisions will be influenced by the estimated budget for the project which will also include the cost of maintaining the system after launch. Some of the factors are:

ACTIVE OR PASSIVE: Mobile communications satellite systems may either be active or passive. Active satellites are satellites that have both transmitting and receiving equipments. E.g. Telstar, Relay and Syncom. The passive satellites are satellites that simply amplify and reflect radio waves. E.g. echo 1.

SATELLITE STABILITY: A satellite can either be spin stabilised or attitude stabilised. With Spin stabilised satellites, the direction of the spin axis is substantially fixed in space. In attitude stabilised antennas, one axis of the satellite continuously points to the centre of the earth.

FREQUENCY: Satellite communications usually use frequency ranges between 1Gc/s and 10Gc/s. Lower frequencies than 1Gc/s is not usually used because they are prone to noise from the star galaxy (i.e.)cosmic noise. Frequencies above 10Gc/s are avoided because of radio noise and rain noise in the earth's atmosphere which increases as the frequency increases. Frequency choice is important because the signal level received from the satellite could be low, so noise needs to be kept to be barest minimum.

MODULATION CHOICE: A Modulation method like the wide deviation frequency modulation is suitable for low signal levels as the wider bandwidth is used to improve the signal to noise ratio in the telephone or television channels. Alternative modulation methods like pulse-code vestigial sideband phase modulation and single side-band amplitude modulation are being developed to be efficiently used for multichannel telephony, this modulation scheme will allow several high and low capacity earth stations to communicate with one another through a satellite. QPSK is also an efficient modulation choice.

STATION-KEEPING OR RANDOM-KEEPING. A station keeping satellite is controlled so as to continue in the same relative position as it rotates around in orbit. For a random-keeping satellite, the relative position of the satellite changes continuously, as a result, random systems need a large number of satellites and could be problematic. [4]

PARTS OF A MOBILE COMMUNICATION SATELLITE SYSTEM

The satellite usually consists of many parts and subsystems that must work together for the satellite to function effectively.

THE PROPULSION SYSTEM

This consists of a large rocket motor which is used to position the satellite to the right location, and also smaller thruster rockets that help to keep the satellite in the desired location.

ATTITUDE CONTROL SYSTEM

As a result of the weightlessness of satellite in space, it is vulnerable to the external forces of the earth and the sun, this system ensures that the satellite is pointing towards the desired location on the earth despite these forces. This can be done either by using spinners which spin the satellite up and down around its axis, or using a momentum wheel which consists of a solid metal disk driven by an electric motor, by spinning the disk; the stability of the satellite is maintained.

THE POWER SYSTEM

Electricity is generated from the solar cells on the satellite's panels. The cells convert solar energy to electrical energy, then stores it in the satellite batteries. This is what is used to power the electronics on board. Fuel tanks are also used to power parts of the satellite and to move it.

COMMUNICATIONS SYSTEM: This includes antennas which are used to transmit and receive radio signals to and from the earth. Transponders which amplify these signals are also included in the communications system.

THERMAL CONTROL SYSTEM:

This system keeps the electronics on board at the right temperature so that they work properly, without this, the electronics can overheat and then stop working. Liquefied gases like Helium which melt at very low temperatures can be used to cool the system. Electric heaters and actuators can also be used to control the temperature at certain ranges.

TELEMETRY AND COMMAND SYSTEM

This consists of a series of a series of antennas and computers that allow the satellite to be monitored and controlled by people in the satellite earth station.

BUS

This is the body of the satellite. It is usually made of metal; it gives the satellite a strong frame and helps the satellite survive the launch by keeping all the parts together despite the high vibrations from the launch. The bus houses the other sub-systems. Examples of satellite bus are Spacebus, Boeing 702, STAR Bus and Loral 1300.[5]

COMMUNICATION SATELLITE PAYLOAD

A mobile communications satellite payload is a major part of the satellite system that helps it fulfil its mission. The payload majorly consists of the transponders and antennas. It also has hardware and software that carry out information processing. It enables effective communication between user devices like hand- held mobile cellular telephones and other equipments which may be connected to world-wide public telephone and telegraph networks. The payload supports multiple independent antenna beams on both downlink and uplink communications for user devices using full duplex communications.

THE TRANSPONDER

A transponder is an electronic device that receives signals from the earth stations, examines, amplifies and re-transmits received signals back to the earth receiving stations.

The transponder has a component called 'interrogator' which inquires for information, then transmits a radio wave signal automatically at a given frequency. To this, first it receives the signal over a range of frequencies which could be in Hertz or Megahertz, minimises the noise in it, and filters the signal to remove out of band noise or interference. Then, it amplifies the signal by using a travelling wave tube amplifier (TWTA) or a solid state power amplifier (SSPA), and re-transmits the signals on a separate band concurrently. The transponders could either make use of bent-pipe repeaters or on-board processors.[6]

ANTENNAS

Antennas in a communications satellite are either used for transmitting or receiving microwave signals. The purpose of receiving antenna is to efficiently accept the radiated energy and convert it to guided form for detection and processing by the receiver while transmitting antenna efficiently transforms the currents in a circuit or waveguide into radiated radio or microwave energy.

The antennas act as an interface between uplinks, downlinks and the electronic circuits in the satellite. Spot beam antennas concentrate their power on a particular area on the earth while earth coverage antennas receive signals from all points on the earth and transmits to all points on the earth.

In the past, Omni-directional antennas were used. They were not so effective, so they were replaced with directional antennas which have higher gain, and so are able to cover a longer range by pointing exactly to the target area, now antennas that can be steered are being worked on. This will allow for flexibility because they can be steered to cover various areas and the beams can be re-shaped. In designing the antennas, so many factors are taken into consideration, some of which include: reciprocity, gain, directivity, efficiency, impedance, frequency, and radiation pattern. Compromise will be made with all these factors in order to design the best antenna for the satellite.

CHOICE OF ORBIT

An orbit is a gravitational curved part along which an object moves around another object.

The choice of orbit for a satellite depends primarily on the purpose of the satellite and also on the area the satellite is to serve. The orbits can either be circular or elliptical.

When satellites orbit the earth, they form a track along the earth which is defined as Ground Track. This is the point on the earth surface where the satellite is directly overhead as it orbits around. When this ground track crosses the earth, it is known as a node. An elliptical satellite has two focal points. The point where the satellite is furthest from the earth is known as the apogee, while the point where it is closest to the earth is known as the perigee.

For a communication satellite to fulfil its purpose, the ground station should be able to follow it, which means it must be visible most of the time, if not all the time.

The orbits mostly used for mobile communication satellites are explained below.

GEOSTATIONARY ORBITS

This is the orbit that completes one revolution in the same amount of time it takes the earth to do a complete rotation on its axis. It is at an angle of 00 to the earth's equator. From the earth surface, a satellite in this orbit appears to be in a particular position in the sky. This makes it easy to point the earth receiving antenna which is usually a satellite dish in just one direction while the satellite is pointing down at a fixed position on the earth surface.

Geostationary orbit is only in the area above and near the earth equator, (i.e.) the area of the earth which is equi-distant from both the North and South Pole. A satellite moving at a speed of 3 m/s at an altitude of 35,786 km will move at a speed that will match the rotation speed of the earth which is 24 hours. Most communications and direct broadcast satellites use the geostationary orbit. The handoff process has to be smooth when multiple satellites are used.

Added to its advantages, the geostationary orbit also has some disadvantages. Due to the gravity of the earth and the moon in space, the satellite will slowly lose its speed and will not be able to move against the gravitational pull. This will cause it drift from its point and get pulled down by the earth's gravitational force.

The second is that because no two satellites can share the same geostationary orbit, extra considerations have to be put in place to avoid interference from new satellites to be launched into space. Also, geostationary satellites are so far away from the earth, that results in latency in the signal transmission, during phone calls or while using data services, there is a noticeable delay.[7]

LOW EARTH ORBIT(LEO)

This is an orbit which is between 160km - 2000km above the earth's surface. It makes a complete revolution around the earth in 90 minutes; as a result, the position its pointing to on the earth's surface changes rapidly, thereby interrupting connectivity. Therefore, a large number of satellites is needed to retain connectivity in this orbit.

LEO orbit is used for communication satellites because it is less expensive to launch into, and because it is close to the earth, it does not require high signal strength. A compromise is then made between the number of satellites needed and how much they cost.

A constellation is a group of satellites working in harmony. Iridium is an example of a constellation in the LEO which consists of 66 satellites. It is used to provide satellite phone services. Globalstar is another example.

Its constraint is that, it experiences a stronger gravitational pull because it is closer to the earth, thus the satellite has to travel at 8 km/s to counter the pull. Despite this, it is easily affected by the gravitational drag.

Also, as with other orbits, it is finite, and soon it will be difficult to launch into it without collision. Collisions are already reported on the orbit. According to CBS news, on Tuesday, 9th of February 2010, an Iridium satellite collided with a German satellite. This applies that the orbit is becoming congested.

MOLNIYA ORBIT

This is a highly elliptical orbit which has an inclination of 63.4 degrees. It was named after a series of Molniya satellites launched into the orbit in the mid 1960s by the Russians. Geostationary satellites find it difficult to provide services to countries in high latitude because they will appear low in the terrain, thus affecting connectivity and causing multipath. Molniya orbit is a suitable alternative because it is designed to provide coverage to countries in the far Northern latitudes.

Satellites in this orbit spend most of their time in the apogee region which is above the earth. It spends less time in the perigee region and moves over quickly because it travels at a higher speed in that region.

Satellites in Molniya orbit are used to provide telephony services in Russsia and regions in areas of high elevation or where there are obstructions. Jumpseat and Trumpet which are intelligent satellites use the Molniya orbit.[8]

APPLICATIONS OF MOBILE COMMUNICATIONS SATELLITE

TELEPHONY

Mobile communications satellites are used for mobile telephony. These satellite phones connect to satellites rotating in their orbits instead of terrestrial base stations. Satellite telephony is used to provide voice and data communication to users located anywhere in the world, either in remote places or sparsely populated areas. This is usually used by the military when being deployed to areas without any form of network coverage.

GLOBAL MARITIME DISTRESS AND SAFETY SERVICES (GMDSS)

This is a standard international safety method of rescuing distressed ships, boats or aircrafts. They use a network of systems which include mobile communications satellites like Iridium to communicate with vessels in distress especially when beyond cellular network coverage. With the use of GPS, the exact location of the vessel is found and there is on-going communication with the vessel till it is navigated to a safe place or help is sent to it.

SATELLITE INTERNET

Satellite technology has been used to provide broadband internet connection. This has been very useful for users who live in remote places and have no access to terrestrial broadband services.

AIRCRAFT COMMUNICATION

In order not to repeat the disaster of Korean airline 007, which led to a wrong flight path as a result of lack of communication; aircrafts are equipped with communications for the whole duration of their flight, including the oceanic regions. With the antenna located in the airplane, it can communicate with the air traffic control via the satellite for the whole flight duration.[8]

EXAMPLES OF MOBILE COMMUNICATION SATELLITE PHONE OPERATORS

INMARSAT: This is a British satellite communications company which was founded as International Maritime Satellite Organisation (Inmarsat). This was the first satellite phone operator. It was launched in 1979. Inmarsat operates 12 satellites in the geostationary orbit. Inmarsat offers communication on most of the earth's surface and was initially intended to be used for ships, but now it offers services to aeronautical, maritime and land- mobile users.

Inmarsat provides services like voice call, data tracking system, data and internet services as well as safety services. Inmarsat handles its voice services through collaboration with ACes. Inmarsat services cover most of the earth except the polar regions. It also provides ISDN services which is used for reporting live events around the world.

THURAYA

This is a satellite phone operator based in the United Arab Emirates. It has 3 communications satellites in the geostationary orbit which covers Asia, Australia, Africa and most of Europe. Thuraya transceivers make use of QPSK modulation and each phone has a GPS receiver in it. Thuraya SIM cards work well in order phones and vice versa. The first satellite called Thuraya 1 had solar panel problems and was moved above Korea for testing purposes. In May 2007, it was moved to the junk orbit.

TERRESTAR

It is an American phone operator which plans to integrate satellite and terrestrial telecommunications services. It launched the largest commercial telecommunications satellite called TerreStar-1 into the geostationary orbit on July 1, 2009. It will provide data, messaging and mobile voice communications to North America.

GLOBALSTAR

It is a low earth orbit constellation of 44 active satellites which are inclined at an angle of 52 degrees and provides voice and data communications. This network system uses the Qualcomm CDMA air interface and uses standard SIM cards. It has roaming agreements with cellular network operators.

IRRIDIUM

This is a satellite constellation of 66 satellites which has coverage everywhere on the earth surface including the Polar Regions, oceans and air-space. It provides voice and data services to satellite phones and other transceivers. They are about 800km above the earth as a result, the delay times are short. The satellites communicate through Ka band inter-satellite link.

CHALLENGES OF MOBILE COMMUNICATION SATELLITES

A major disadvantage of mobile communications satellite is the data rate. Because the transmitting and receiving antennas are in motion, it allows for low data rates and does not efficiently handle high capacity applications like multimedia.

For satellites placed in the geostationary orbits, as a result of the distance between the satellite and the earth, there is a significant delay during data transmission which is noticeable during voice calls. It is therefore not ideal for video conferencing.

It requires professional support to monitor and operate the satellite.

The initial cost of building and launching the satellite is very high. Since most operators want to recover initial cost and running costs, the price of using satellite communications telephony is high.

Sometimes, the satellites get shifted from their orbits due to gravitational pull; this could eventually result in a loss in connectivity.

There is increasing concern to avoid satellite collision in space as the number of satellites in orbits keeps increasing.

FUTURE IMPROVEMENTS

Mobile satellite communications has so much room for improvements.

EFFICIENT FUEL SYSTEM: This will enable the satellites to be driven to earth and maintained when it has exceeded its life expectancy, instead of dropping it in the space junk yard.

INTEGRATION OF CELLULAR COMMUNICATION WITH MOBILE SATELLITE COMMUNICATIONS: This will be an added advantage to the user, because it will provide the user with network connectivity when he is out of terrestrial service range.

COVERAGE AREA: Satellite communications that cover developing countries which are black spots can make revenue from publicity and marketing in such areas as they will be the only source of mobile communications.

SATELLITE INTERNET: Mobile satellites need to improve the capacity available to users for internet access, especially the uplink data rate. An improved data rate will make it a preferred choice of internet access.

USER FRIENDLY GADGETS: Most of the equipments needed at the customer end require expert installation before use. Mobile satellite communication operators should provide a user friendly package which is easier to maintain by the user.

A robust package which consists of telephone, internet and TV services via satellite will be a choice package for users.

CONCLUSION

Mobile satellite communications is ideally suitable for users who are in remote areas or for emergency purposes. It is cost intensive to build and launch the satellites, but maintaining it is relatively cheaper. With so much research and improvements, the satellite systems are becoming more efficient and are lasting longer in space.

With over 3 million subscribers to satellite telephony services, the network keeps getting better and the tariffs cheaper. Services from mobile communications satellite operators keep improving, and the option of an ubiquitous network makes it a viable choice to users and organisations especially for emergency purposes.

REFERENCES

1. Dr Marc Garneau,"International communications satellite system" May 13,2002[Online]. Available: http://spacejournal.ohio.edu/pdf/Garneau.pdf [Accessed February 17,2010]
2. David Whalen,"Communications satellite short history" July 27,2007[Online]. Available: http://history.nasa.gov/satcomhistory.html [Accessed February 17, 2010]
3. Donald Martin,"A history of US Military satellite communication System" November 23,2007 [Online]. Available: http://www.aero.org/publications/crosslink/winter2002/01.html [Accessed February 18, 2010]
4. "Satellite" in Wikipedia, the free Encyclopedia. March 12,2010 [Online]. Available: http://en.wikipedia.org/wiki/Satellite [Accessed February 19,2010]
5. Federal Communication Commission "Parts of a Satellite"[Online] Availabe: http://www.fcc.gov/cgb/kidszone/satellite/kidz/parts_of_sat.html [Accessed February 20, 2010]
6. Topbits "Transponders" 2010 [Online]. Available: http:// www.topbits.com/transponder.html [Accessed February 24,2010]
7. Tega Jessa "Geosynchronous Orbits" July 20,2009 [Online]. Available: http://www.universetoday.com/guide-to-space/spaceflight/geosynchronous-orbit/ [Accessed February 24, 2010]
8. "Communication Satellite" in Wikipedia, the free Encyclopedia. March 12,2010[Online]. Available: http://en.wikipedia.org/wiki/Communications_satellite [Accessed February 24, 2010]

FIGURES

1. http://en.wikipedia.org/wiki/Sputnik_1
2. http://en.wikipedia.org/wiki/Project_SCORE
3. http://en.wikipedia.org/wiki/Courier1B
4. http://www.gma.org/surfing/sats.html
5. http://www.applicationstrategy.com/Communications_simulation.htm
6. http://www.radio-electronics.com/info/satellite/satellite-orbits/satellites-orbit-definitions.php

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