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And in less than 6 months people will be able to go anywhere in the world and carry one mobile phone with one phone number . They will be able to be reached in the middle of Los Angels or at the top of Mount Everest . A new system of low orbiting satellites that will allow cellular phones to work in any spot on the globe is now being launched . Geo-synchronous earth orbiting satellites Until recently, geo-synchronous earth orbiting satellites (GEOS) have dominated the skies. Big and expensive, they orbit the earth at an altitude of 22,300 miles and at the same speed as the earth's rotation. That enables them to remain over the same spot on earth all the time. Hence, to a person standing on earth they would appear stationary . The big advantage of GEOS is that one satellite can cover a huge region, thanks to its great distance from the earth . Three satellites , for example , placed in geo-synchronous orbit and at equal distances from each other around the equator can cover the entire globe, with the exception of the poles . (1 , p 23 ) However, that great distance is also a disadvantage when it comes to relaying voice communications. It takes a signal travelling at the speed of light about half a second to reach a GEO satellite and return to earth. That quarter-second delay makes telephone conversations remarkably annoying. Further more this remote distance from earth makes it impossible to establish a direct link between a GEO satellite and a hand held cellular like phone . It is for this reason that cellular phones rely on ground stations to transmit calls to satellites .This makes phone calls in remote areas, were there is no network terrestrial coverage impossible . Although GEOS still transmit significant amounts of international telephone traffic, extensions of terrestrial telephone networks have the big market share. Nearly 60% of GEOS capacity is now used for television and video services . ( 2 p 56-57 ) The solution : bring the satellites closer to earth. New companies are spending billions of dollars to launch constellations of low earth-orbiting satellites (LEOS) that will offer digital voice and data services on a global basis. There are currently six proposed LEO systems, two of which are scheduled to start commercial operations by late next year. Fig 1 : A LEO in comparison to a GEO LEO satellites are small communications transceivers , much smaller and lighter than GEOS .They are put in continuous motion at heights of 500-1200 miles above the earth .(Check Fig 1 ) LEOS can provide 100 percent geographic coverage, but unlike GEOS , LEO systems require less power for messages to reach their orbiting satellites , making it possible for specialised portable phones to transmit signals directly to the satellites . This makes phone calls independent of land based servers ; allowing people to be reached anywhere in the world .( Fig 2 ) Furthermore , less power requirements translate into lower service and equipment costs for users. ( 3 p. 27 ) Fig 2 : Satellites in low orbit transmitting signals to potential receivers ( 5) On the other hand LEO systems face a serious problem . Over 35 million bits of debris primarily pieces of old satellites, launch vehicles and solid rocket fuel currently orbit the earth, at speeds of up to seven kilometers per second. In comparison, a bullet fired from a high-speed rifle travels at only 0.8 kilometers per second. This forces LEO companies such as Iridium and Globalstar to worry not only about making sure their satellites work but that flying garbage doesn't knock them out of commission. Studies conducted by the National Aeronautics and Space Administration show that a collision with a fragment the size of a marble would do serious damage to a satellite. A 10-centimeter piece would be enough to permanently put the satellite out of order . (5) Collisions with such debris are not rare events ; space shuttles, for example, regularly return to earth with cracked windows and pockmarks presumed to have been caused by encounters with space junk. In July 1996, a French military satellite was destroyed by a fragment from the Pegasus rocket that had carried it into space (4) . The problem is particularly acute for LEO satellites . Lower orbits must pass through the debris from the rockets that carry higher-orbiting satellites farther into space. The lower altitudes are also strewn with pieces of other satellites 130 of which have broken up in space. To make matters worse, as the numbers of satellites taken to the skies increases , the probability of collisions increases, which in turn creates more junk to threaten increasingly crowded orbit paths. Already high insurance premiums for these enterprises will get worse. Figuring out ways to protect satellites and space vehicles from space junk is not an easy task . Shields and insulation material are standard parts of satellites in low earth orbit . Furthermore companies such as Iridium and Globalstar have factored in the risk of space junk by launching extra satellites that can replace disabled ones. This raises the multibillion-dollar price tag of network deployment, but it also protects against downtime, and thus lost revenue. ( 5 ) Fig 3 : A prototype of a mobile phone that will be used for communicating through LEOS . ( 5) LEO proposals have so far been put forward by six different companies ( two of which are discussed in detail ) Iridium , Globastar Odyssey , Aries , Inmarsat and Ellipsat. (check table for details ) Each company is using a proprietary network of satellites, launching methods, and implementation strategies. Iridium is the company that according to industry sources will make it to market first. In 1989 Motorola announced a proposal to launch a series of 77 satellites in low earth orbit starting in 1994. Iridium Inc. was formed to manage the programme. It aimed to offer world-wide mobile phone services. On the commercial front, Iridium plans to sign up as many operators as it can world-wide. Service would be marketed through existing cellular suppliers , networks of distributors and dealers. It even has plans to lease capacity to one of its arch rivals, Inmarsat (consists of a consortium of 65 nations ). This proposal might be intended to deflect criticism of a single US company being granted a world-wide monopoly of a part of the radio spectrum. The target users would be those breed of managers who spend many days of the month circling the globe . The cost of an Iridium handset ( Fig 3) would be $2500 and a $3 per minute tariff would be charged. Furthermore, the design of the handsets is very simple . It uses a stripped-down version of time division multiple access, TDMA, as used in the GSM cellular standard. Clearly, the use of GSM standards will be of great benefit to European and Middle East markets, as the same handsets could be used to access both terrestrial and satellite GSM cellular networks. When a user makes a call with such a dual-mode handset, it will first check for a local terrestrial signal. If it is unable to find one, then it switches to Iridium mode. In 1992 a frequency spectrum between 1610 and 1626.5 MHz was allocated for the use by LEO satellites . Iridium would utilise a 10.5 MHz part of that spectrum . Motorola gained access to this part by purchasing the bankrupt Geostar Inc. earlier that year. This company exclusively owned the licence for this part of the spectrum. This licence authorises positioning data , paging messages and voice communication to be sent via satellites. Iridium made variations on the way the satellites are to be launched. At one time, one dedicated space-craft was to be used, but now , to minimise risks , the satellites are being launched a few at a time by different launch vehicles such as the US Delta 2 , or the Russian Proton rocket . Furthermore Iridium shunted down the number of satellites from 77 to 66 , with each having a 48 channel capability instead of 37 . That means each satellite will be able to process more calls at the same time. They will be deployed in six different orbital planes with eleven satellites in each plane. Each satellite is triangular in shape , about 4.5 m long and 1 m across each base . (Fig 4 ).This shape allows for more than one satellite to be placed on a single launch vehicles . Including fuel the satellite weighs 690 kg and has an orbit of 780 Km above the earth's surface . The total cost of the Iridium project would be US $ 4,370 million. The Globalstar proposal is from Loral Qualcom Satellite Services Corporation, LQSS. Heavily backed by French interests, including Aerospaciale, Alcatel, and Alenia, LQSS will initially launch 24 satellites to initially provide an optimised service to continental USA. International operations will follow, after deployment of an additional 24 satellites . Each satellite should cover an area of 400 miles in diameter. Two kinds of user equipment have been proposed . The first, a vehicle mounted terminal based on a car radio design, will be easily removable and easy to operate. The second is a hand held terminal that the user can take anywhere. Each terminal has an average transmitting power less that 1 watt . Like Iridium, LQSS plans to provide transmission services to organisations which will resell capacity to other resellers . Globalstar is intended to operate in conjunction with existing terrestrial networks through gateways. These consist of equipment which interface signals from a Globalstar user to a local cellular network.. Globalstar will be offering a single world-wide communications solution for voice and data. Digital data can be transmitted up to 2,400 bit/s and position location services with better than 300 metre accuracy should be available nearly everywhere. Other services will include global paging, messaging, and international roaming. Unlike Iridium , Globastar satellites are trapezoidal in shape , which also allows many of them to fit on one launch vehicle . They are lighter than their Iridium counter parts , and weigh only 450 Kg . However , they have a relatively high orbit of 1414 Km . This allows each satellite to cover a greater distance on earth .Hence less satellites are needed . The total cost of the Globastar project is US $ 2,600 million . Table : summary of the major satellite systems . Globastar Ellipsat Odyssey Aries Inmarsat Start of service 1998 1999 2000 2000 2001 2001 billion $2.6 billion $1.1 billion $1.8 mobile terminal cost $2500 $750-$2500 N.A. $400 N.A. N.A. per minute $3.oo $0.55 $0.5 0.65 N.A. N.A. orbit , Km 780 1400 8040 9100 1450 3600 Primary services Voice, data , fax Voice, data , fax Voice, data , fax Voice, data , fax Voice, Teleglobe Nokia consortium of 45 nations Despite impediments and problems , LEOS still have many advantages over GEOS . Most important of which they are not confined by ground coverage of cellular phone relays , allowing people to communicate anywhere at any time . In the near future most human communication will be made through low earth orbiting satellites . Companies such as Iridium have already constructed a bridge to that future . Bibliography: 1. Richharia, M. (1995) . Satellite communications design. 3rd ed. New York , Mc Millan Press . 2. Roddy, D. (1989) . Satellite communications. 2nd ed. New York , Mc Growhill Press. 3. Miller, Barry . (March , 1998) . Satellites free the mobile phone . Spectrum . p 26-35 . 4. Microsoft . (1997) Encarta 97 [ CD-ROM ] 5. Vermillion, J. and Jimenez, F. (1997). Low Earth Orbit Satellites. [On-line].Available: http://www.satphone.com/lowearthorbit.html Word Count: 2060
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