TRENDS IN SATELLITE COMMUNICATION & ITS RELEVANCE TO INFORMATION TECHNOLOGY

S PalGroup Director

Communication Systems GroupISRO Satellite Centre

Bangalore 560 017INDIA

SEPTEMBER 1999

SATELLITE COMMUNICATIONS

*Everyday we manage to bounce radio waves off “Artificial repeater” that we have put into space at around 36000 kms height over the Earth. It is no magic that all these “Artificial Radio Repeaters” termed as “Communication Satellites” are found at that altitude. For at that distance from Earth, these objects orbit this planet with an orbital period of 24 hours, precisely matching the rotation of the Earth and these objects appear fixed over a single spot on the Equator. It is this aspect due to that we term these objects as geosynchronous or geostationary satellites.

* With these satellites in one spot, the reception and transmission was a great feat in itself and done using large ground stations with antenna diameters ranging from 30M to 10M. This was needed to get the telephone/TV or any other signal around the globe or to large distances. In such cases the users never come in direct contact with the satellite.

* The situation changed considerably in eighties and the communications requirements became really nerve-racking. Imagine the transmission reception of signals to satellites from:

* A ship pitching and rolling on the high seas

* An aeroplane flying at a speed of >800kms per hour

* An orbiting space vehicle/satellite with speeds >7 to 8 kms per second

* An aeroplane or a ship in distress

* A mountaineer/A trekker/A forest guard/An elephant/A tiger/A bird/ A turtle/a dolphin.

* A man always on move/A motor vehicle

If using satellites to communicate is a great marvel, doing the same without stopping or even slowing down is almost miracle.

VARIOUS SATELLITE COMMUNICATION SERVICES

* Telephony/TV Broadcasting/Data reception and distribution/Direct Television broadcasting/Disaster warning/Continuous weather monitoring/Spacecraft Vehicle Tracking and Commanding/ Intersatellite links/ Mail transmission

* Position (GPS) and time determination / Moving motor vehicle tracking etc.

MOBILE PERSONAL COMMUNICATIONS

*A user to exchange various kinds of information e.g.: voice, data, video and image with any one, any time and anywhere. The envisaged key element that forms the basis of this mobility is a small inexpensive handheld communicator with a single telephone number independent of location and geopolitical boundaries. The main requirement is every conceivable degree of freedom for the user.

*One can draw a conclusion that the future communication will be dominated by the MOBILE PERSONAL COMMUNICATION SERVICES and the overall system design will be done around the main dictating factor: spacecraft G/T. The ground stations which are becoming smaller and smaller day by day will be of pocket size calculator.

In the foreseable future new equipments and techniques will be used in satellite technology which will extend and improve the possibilities of satellite communications beyond our present imaginations.

* Greater Transmit power and more complex antennas on the satellites will make operations possible with a large number of very small Earth stations.

* Efficient signal processors and switching equipment will enable signal processing to be performed onboard right down to operations similar to switching in exchanges.

* Direct connections between satellites will shorten the transmission routes.

* The use of higher frequency bands will enlarge the available bandwidths and thus the transmission capacity.

* Miniaturization of electronics

With the emergence of optical communications the majority of the existing satellite communication services burden will be taken over by the optical links and the tasks to be performed by future satellites will be:

* Tasks for which it will only be essential to use the satellites to limited basis (Transmission of Telephones and TV to inaccessible places or thin traffic routes etc).

* Tasks for which it will remain advantageous to use satellites (New services with different requirements with respect to BW, partner stations, transmission times. This could include Data Collection and Distribution over a large area, mobile radio services to inaccessible areas, ship, aircrafts, Mobile communication/PCS).

FUTURE SATELLITES

*The future Telecommunication spacecrafts will be developed from transmission in to Information Satellites (INFOSAT). They will be given many of the properties of terrestrial telephone exchanges and signal processing equipments and it will be possible to integrate them directly in to future global networks. They will thus permit immediate applications of many existing and future services. Because of their inherent built up flexibility, these satellites will be able to support and speed up the initial construction phase of many new services before their trial on terrestrial networks. These type of satellites will enable new services to be tried out over a large area before being launched on to market and optimally adapted to suit the most appropriate transmission medium.

*The future INFOSATS will be of three type:

* National/Regional

* International

* Relay

Future technologies will enable the construction of an INFOSAT network in which the above three will be connected to each other. The onboard processors will ensure that the signals to be exchanged between the satellites and the terrestrial subscribers are combined using TD and SD multiplexing techniques and distributed in accordance with the user requirements. Special coding techniques will ensure the security of transmissions.

DESIGN OF INFOSAT

* The satellite platform will have multiple reconfigurable antennas/transmitters with dynamic power sharing / Receivers in various frequency ranges / large reconfigurable switching matrices at baseband and at RF level / intersatellite links permitting signals to be exchanged between satellites according to changing requirements / complex and efficient analog / optical / digital signal processors / New modulation technique and multiple access techniques.

*The satellites besides their autonomous control and power generation equipment will have sensors to observe the earth’s atmosphere and pass the data to an appropriate station after processing.

Such a system will obviously have Geo synchronous spacecrafts with some orbiting satellites to take care of North/South pole regions.

The INFOSAT Network is to be an integral component of the planned world-wide broadband telecommunication network. Therefore it will be necessary for planning of the satellite network and the terrestrial network to be closely coordinated. Until now satellites have connected the terrestrial network to be closely coordinated. Until now satellites have connected the terrestrial networks of various countries and organizations.

SERVICES TO BE OFFERED BY INFOSAT

* Radio and data distribution services to many users which are spread over a large area.

* Data collection services for large areas with many data transmitter stations (multi point-to-point operation : weather, oil, electricity & water meter reading etc.).

* Telecommunication services for thin routes

* Telecommunication services with ships, aeroplanes, space vehicles, etc).

* In the area of business communications demand is growing for broad band communication facilities which can be applied flexibly using satellites.

* Worldwide Radio paging

* Video conferencing and high resolution TV broadcast trials.

* Mobile radio services can be combined with location finding services and be used for automatically locating subscribers.

* Earth observation with special warning mechanism and environmental protection services in close coordination with terrestrial sensors.

By integrating INFOSAT into terrestrial network, it will be possible to supply all the subscribers of a future global network with all essential information. This will make the location of the user quite irrelevant.

OPTICAL FIBRE VS SPACE COMMUNICATIONS

*Much is talked about the competition between satellites and optical fibre technology. A closer examination of the specific properties of each transmission medium shows that they could complement each other very well and future world wide terrestrial networks will need to be complemented by a satellite network.

* The TV and heavy traffic trunk routes will be taken over by optical fibre.

* The satellites will serve: thin traffic routes extremely large distances/inaccessible areas/weather monitoring/disaster warning / Data collection and Distribution etc.