Optical fibres

Optical fibres, hair-thin strands of pure glass carrying information as pulses of light, have been described as "probably the biggest breakthrough in telecommunications since the invention of the telephone". All kinds of communications can be carried along the same optical fibre cable — speech, texts, photos, drawings, music, computer data, etc. — at higher speeds than have been previously possible.

The fibres, made from glass so pure that a block of it 20 km thick .would theoretically be as transparent as a window pane, have many advantages over metal wires. Small, light and easy to handle, they are made from an abundant raw material, sand. They can carry the same number of telephone calls as metal cables ten times as thick — dozens of fibres, carrying around, 100,000 telephone calls, could all pass through the eye of a needle, at the same time — and they are immune to electrical interference which affects the quality of calls. An optical fibre cable the thickness of a finger could bring a hundred TV channels to a receiver.

The tiny strands are playing a key role in the digital revolution which is sweeping through modern telecommunications. The telecommunications network developed for the telephone used a system which turned the air pressure waves created by speech into continuous and variable "analogues" of electrical waves and turned them back to speech at the receiver. Expensive conversion equipment or separate networks were needed to handle a text, TV or computer data. In the digital world, however, all forms of information are translated into bits, the standard international language of today's computers, and represented as pulses of light. Information in this form can be processed easily and sent anywhere in seconds in a single multi-purpose network. Optical fibres are ideal for digital working and open the door to a host of services not possible on an analogue system.

Each strand of fibre consists of an inner core to channel the light and an outer cladding to keep it in by reflecting it back along the core. To make the glass for the fibres, the ingredients are deposited as gases on the inside of a hollow silica tube at temperatures of around 2000°C. The tube is collapsed under intense heat to form a solid glass rod about 1cm in diameter which already has the structure of the fibre which will be drawn from it. The rod is then loaded into a furnace, drawn into fibre and coated with resin to protect it and increase its flexibility. Tiny crystals the size of a grain of salt are used to produce the light which carries information along the fibres. This passes through a lens into the fibre. At the other end a receiver reverses the process and turns each light pulse into an electrical sign. Optical fibres will have countless applications in tomorrow's "information society".

TEXT 16

Reliability of missiles and space vehicles

Reliability is above all a design parameter; it must be thought of as a physical property of a device which behaves in accordance with certain physical laws. In other words, reliability starts with engineering and is a basic property which must be designed into the equipment by engineers. It is true that there are other major factors which influence the performance in the final application such as manufacturing, quality control, and handling and checkout in the field. If manufacturing process is not carried out with the proper precision and skill, if the inspection and testing in the factory are not done with proper care, and if the field crews at the launch site do not checkout, test, and launch the vehicle in accordance with proper procedures, the net result will certainly be mission failures. To be sure, no amount of manufacturing precision care in the inspection and testing, and proficiency of the launch crews can make a missile or space mission succeed if the basic design is not right in the first place.

Although reliability is one of the primary parameters in determining the capability of the missile or space system to perform itsoverall mission, it must nevertheless be kept in balance with other systems parameters. Therefore, as part of the systems design, a trade-off between reliability and other systems parameters such as weight, accuracy, speed, and orbital precision must be made. Considerable gain in over-all system effectiveness can sometimes be obtained by sacrificing some accuracy or performance of the system for the sake of an improvement in reliability. Conversely, gains may also be realized by sacrificing some reliability in favour of improvements in accuracy and reduction of weight. The important point here is that a balance must be struck between reliability and other systems parameters.

To illustrate the severity of the reliability problem in satellites and space vehicles the Table presents some relative reliability requirements for a typical subsystem, say, a 25-watt UHF (ultra-high frequency) transmitter which might be used in any one of three applications.

Although the mean time to failure (MTTF) for the transmitter in a missile application is only slightly higher than the MTTF required in an aircraft application, the MTTF requirements for space are several orders of magnitude greater than those for either missile or aircraft.

Hence, the resulting reliability problem is different in nature and much more severe in the case of space vehicles.

Typical Reliability Requirements for Electronic Subsystem,