Communication networks are all set to be revolutionised by third generation satellite systems
a number of third generation satellite systems for mobile communications have been proposed in recent years. By integrating with terrestrial telecommunications systems, these systems support the vision of 'universal personal telecommunications'.
Three generations of mobile satellite communication systems can be identified. The first, such as immarsat-a, were characterised by geostationary earth orbit (geo) satellites, with single global beams and large, very expensive groundstations. The second generation mobile satellite systems are characterised by geostationary satellites with multiple beams and smaller, less expensive terminals. (Electronic Engineering, London, October 1996)
By the year 2000, it is expected that third generation mobile satellite communication systems will be merged with land-based communications systems. The expected cost of third generation mobile satellite systems is in excess of us $1 billion. These multi-satellite systems are more complex and more expensive than existing commercial satellite systems. This has prompted the formation of various consortia (Iridium, Globalstar) to disperse the cost -- and risk -- among several member- companies. In addition to voice telephony, third generation systems may provide other services such as database access, location information and video conferencing.
The data rates and bit error rates (ber) required to support these various applications vary considerably. Whether third generation mobile satellite systems will actually be able to support these extremes of data and ber is still uncertain. In any event, third generation systems are likely to be characterised by multiple, compact, low Earth orbit (leo) satellites, with multiple beam antennae and small, low-cost Earth terminals.
Where real-time (like voice communication) operations are specified, leo systems require 10 to several hundred orbiting satellites to guarantee that at least one craft is overhead at all times. On the other hand, the lower altitude has the advantage of shorter transmission delay and lower link attenuation .
leo systems are further divided into the Big-leo and Little-leo. Big-leo systems offer voice and data service and operate over the l and s bands, whereas Little-leo systems offer data service only and operate over the vhf,uhf bands.
The exact manner in which mobile satcom systems will integrate with -- or bypass altogether -- the terrestrial mobile system remains uncertain. Different proposals envision different methods. Of course, the higher the cost per minute of satellite communications, the more its role will be limited to that of supplementing the terrestrial system. This factor also influences the handset requirements: dual-mode handsets will be required if the satellite system merely supplements the terrestrial one.
Some market studies have projected as many as 80 million or more mobile satellite service users by 2010.
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