Satellite Applications

Satellites provide a variety of mobile services, which are not MSSs per se, but complement or compete with MSS offerings. Some of the more interesting ones are:

i)navigation systems;
ii)direct to individual radio broadcast systems;
iii)personal communication system by FSS;
iv)direct television broadcast to mobiles.

i).Navigation systems

Navigation is used for estimating the position of a vehicle on sea, in air or space and on land to ensure that the chosen route is followed accurately, both in the short and long term. Short term navigation is required for making instantaneous changes in direction, speed and acceleration to avoid an obstacle, and long term navigation is used for making a general correction to a route. In recent years receivers of satellite navigation systems, and in particular the GPS, have become a personal and mobile communications accessory due to a significant reduction in receiver costs, making them a part of regular gear carried by explorers, travellers, fleet managers, rally organizers, etc.

The GPS navigational system was introduced by then US military in 1978. A constellation of 18 GPS satellites can provide continuous worldwide two dimensional coverage; increasing the constellation to 24 satellites can give three dimensional position fixes world wide. Satellites are in circular 63' incline orbit at an altitude of 20,200 km and an orbital period of 12 hours; accuracy of fixes for military users is 10m, degradingto about 100m for civilian users. Satellites transmit atomic clock controlled timing signals together with their orbital parameters, which are used by receivers for a range estimation. The system operates on a similarprinciple as a Loran oe Decca ststem except that instead of hyperbola, hyperboloid of revolution are used for position estimation. A S receiver estimtes its location by measuring the range from three (or four) most favourably positioned satellites simultaneously and solving three (or four) simultaneous equations. A similar system using a different type of transmission signal was introduced by the former USSR at about the same time.

Satellite aided navigation systems can be categorized on the basis of their operating principles: Doppler signature, range determination, single satellite and multiple satellite transmissions. Chapter 10 summarizes the principles of operation of the most commonly used satellite navigation systems.

ii) Direct to individual radio broadcast systems

A number of studies have investigated the viability of satellite radio broadcast directly to individuals as they promise rapid wide area coverage, highquality Sound and niche programme channels. The current disadvantages include large investment and expensive receiver technology (but technology is evolving very rapidly) and, on the programming front, possible lack of local coverage (e.g. city events, traffic reports, etc.).

In the early 1990s, the ITU assigned spectrum for direct sound broadcast in the S band. Since then, a number of commercial systems have been proposed and at least one system is close to service introduction.

Satellite radio systems require high power satellite transmitters, robust modulation and coding schemes as shadowing and multipath countermeasures and most importantly affordable receivers. Chapter 10 addresses these issues in detail.

iii) Personal communication system by fixed satellite service

The smallest terminals of FSS, commonly known as very small aperture terminals (VSATs), have been used in low capacity routes since the 1970s. Advances in technology leading to the introduction of K,, (11 14 GHz) and K', (20 30 KHz) band high power satellites deploying spot beam antennas and VLSI have enabled a reduction in size and cost of VSATs to an extent that the distinction between VSAT and broadband MSS services is blurred at the upper end of MSS throughput ( 64 500 kbps).

Several FSS systems, specifically targeting personal and private enterprise communications, are under development. The goal is to provide broadband services to very small terminals a personal set could offer 2 Mb/s and a transportable set 155 Mb/s.

The formal definition, applicable radio regulations and frequency management methods between the services differ due to differences in application and terminals traditionally supported. At present, the majority of MSS spectrum allocations lie in the L and S bands, where propagation conditions are congenial for the service, whereas those of the FSS lie in a wide range from 3 to 30 GHz. Furthermore, MSS allocation tends to be exclusive for each operator within that area to permit users unrestricted movement within that area. In contrast, FSS allocations are shared with other terrestrial/satellite services, because terminals remain fixed at a specific geographical location. Each move ideally requires a further series of lengthy coordination meetings, making it impractical to move terminals freely. There is potentially an opportunity for unrestricted mobility within areas managed by a specific jurisdiction when the authority manages the spectrum accordingly.

Let us return to the issue of competition between these two services. Competition in core MSSs, such as communication from moving terminals, hand field services, etc., is not feasible for reasons mentioned above, For fixedsite applications which allow use of directive antennas, both services are similar from a user's perspective. The main difference lies in economics, the regulatory advantage of MSS in terms of unrestricted connection to the public network and the ease of setting up MSS terminals at will. VSAT networks have the advantage in terms of high throughput and lower cost for large volume data transfer between fixed locations. Figure 1. 11 shows a VSAT expected to be available in the near future; compare this to MSS terminals illustrated in Figure 1.9.

iv) Direct television broadcast to mobiles

Television broadcasts from direct broadcast satellites can be received on moving mobiles, such as ships where a stabilized tracking antenna may be easily deployed. Conventional direct broadcast satellite systems require several MHz of bandwidth and therefore similar transmission schemes cannot be used in mobile satellite systems which have severely limited bandwidth. The challenge is to be able to transmit good quality television within bandwidths of the order of 50 KHz. The key technologies to achieve this goal are video compression and highly efficient modulation and coding schemes. Television broadcasts to ships within a bandwidth of 400 KHz using the offset Q phase shift keying (O QPSK) modulation scheme at a modest quality were demonstrated in MSS systems.