The Flying Phone BoxA surprise for air travelersThe strides made in satellite-communicatio

The Flying Phone Box

A surprise for air travelers

The strides made in satellite-communications technology in recent years are at last yielding benefits for air travelers, particularly executives who need to keep in touch with their offices. Five airlines— three of them based in the Asia-Pacific region—have recently equipped aircraft to offer outbound (向外的)telephone calls to anywhere in the world using a new satellite system. And in the not-too-distant future, businessmen will be able to send faxes in flight, too.

Air Canada, Qantas of Australia, United Airlines of the US and Japan Air Lines have at least one satellite-communication aircraft apiece. Hong Kong's Cathay Pacific Airways is also about to offer the service and several other airlines are expected to follow. The calls, dialed directly and charged to a credit card, will cost about US $ 9 ～ 10 per minute.

The earlier flying phones

Phone communications from aircraft are not entirely new. For some time, telephones have been available to passengers on US internal flights, for example, and on private jets.

But these systems have used cellular phones operating on very high frequency radio links. These systems—the same links that keep pilots in touch with flight controllers—have either limited range or low quality and reliability, particularly over vast oceans such as the Pacific. By contrast, the new satellite service will provide clear links instantly to destinations around the world.

Technological obstacles

Several technological problems had to be resolved before the service could be introduced. One was to define an effective technical standard, Airinc 741, that guarantees compatibility among different manufacturers' avionics systems—essential if the service was to be truly global.

Another was to make reliable antennas(天线) small enough and light enough to mount on aircraft of different types. And a third was to perfect voice-coding algorithms(规则系统), which tell machinery how to break down voice signals into digital signals for transmission at the standard rate of 9,600 bits a second.

A further drawback, for any type of call routed by satellite, is an uncomfortable time lag between words being spoken and received. Because of the distance that signals have to travel, there is a delay of about a quarter of a second. With airborne calls, there is the potential for a double delay as signals arriving via satellite at a ground station are passed on via another satellite to their destination.

To avoid this problem, ground stations have been equipped to identify calls from aircraft and route them to coaxial or fibre-optic cables for the second lag of their journey. Surface links, though long, are still considerably shorter than the 72,000-km round-trip into space. Most calls will therefore not suffer the double delay.

The International Maritime Satellite Organization (Inmarsat) approved the initial sitcom system for commercial service last November 'after two years of trials. Besides the handful of aircraft operated by the five major carriers who have signed on to the system, about two dozen company jets have fitted with the necessary equipment.

A new technology

The technology has three components: antennas and associated equipment on board planes, communications satellites, and ground stations.

In the course of development, antennas have lust both their dish shape and several kilograms. Even so, antenna and avionics weigh as much as two occupied scats. One obstacle to further weight savings, however, is that lighter antennas are more easily confused by the plethora(过多) of satellites now circling the Earth.

The satellites themselves are provided by Inmarsat, a London-based cooperative of 64 nations. One operational satellite and several 'back-ups

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