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Aviation History
1991
1991 - 0092.PDF
Rafale: the French leading edge Avionics marketing director Jerry Fisher. Avionics are the common weakness in many of the aircraft now in service, "...where they are inadequate", says Fisher. The retrofit of systems such as HUDs and providing a night-attack capability, is cer tainly thought to be worth considering. A number of in-service platforms have been viewed as potential opportunities for avionics retrofits, including the F-4, F-5 and A-4. As Fisher points out, however, "...fa tigue life is fairly critical". On F-4s now in service, excluding Israeli and German air craft, the remaining airframe life probably does not justify an avionics upgrade. The F-5 and A-4 provide more attractive possi bilities. Improving mission capability was the only reason until recently to upgrade avion ics. Fisher suggests reducing life-cycle costs may start to figure more prominently. Replacing an analogue with a digital system can reduce maintenance. Successful software development and in tegration is the common problem faced by those developing advanced avionics for the next generation of combat aircraft. So far software development and integration on complex systems has proved to be the rock on which many development programmes have foundered. It is reasonable to assume that some of those mentioned may yet suffer a similar fate. Others, however, will succeed at least partially in providing capa bilities considerably beyond the scope of systems currently deployed. • Finding the best bus route The European Fighter Aircraft (EFA), Rafale and the Swedish JAS39 combat aircraft offer European avionics makers, investigating databus technologies, the first chances to participate in large-scale, advanced, airborne databus projects. Until now, European experience has been confined largely to developing in terfaces to enable avionic equipment to work with buses on US aircraft such as the F-15, F-16, F-18 and C-17. New aircraft are carrying so much avionic equipment that interconnecting with discrete wire point-to-point links in the traditional way would make the avion ics impossibly heavy. Fault finding and servicing would also be difficult. These snags can be avoided if the input and output signals of each piece of equipment are digital to a common for mat, and the items are interfaced to a single length of wire to carry signals between the boxes. Transporting data on a wire bus is not a problem; the difficul ties are in developing a bus which works fast enough, including all the launching and retrieval operations, to have adequate capacity. Operation of all the avionic boxes must be integrated, so data always joins the bus at the right time, arrives at* the correct place in good condition and iaintelligible. In practice, no aircraft has/all equip ment attached to a single bus^To make the system manageable, several are used, each carrying items associated function ally in some way. On EFA there will be separate buses for: the attack sensors (radar, forward-looking infra-red) and the navigation equipment, cockpit displays and controls, weapon systems, defensive aids (electronic war fare) .utility systems, and general avionic systems. Each bus will carry computing for its local needs. Many functions on different buses are interdependent so buses also have to be interconnected to allow some data to pass from one to another. Breaking down EFA's avionics into sev eral bus-centered subsystems means that some of the subsystems will need only proven 1553B standard bus technology working at 1 Mbit/s — slow by develop ment standards — for adequate capacity, including an allowance for system growth. The 1553B is too slow to provide adequate capacity on the buses serving the attack sensors and navigation equipment and general avionics. The items on these buses are those most likely to experience increased use of data with development. Higher capacity will be provided by running an optical-fibre bus alongside the wire 1553B bus, and using the fibre to carry data at 20 Mbits/s and the wire to carry only control signals for the data channel. This idea is formalised in Europe as Stanag 3910. It originated in Germany about four years ago and is accepted by the EFA partners as the best solution within cost, technical risk and develop ment timescale limits. Except for interfaces between the two buses which have to be developed, the 3910 uses proven 1553B control tech nology. The disadvantage is that it doubles the number of carrier strands, data transmit ters and receivers, connectors, interfaces, etc, which are needed per bus system. Hence the tendency on EFA to arrange the buses so, where possible, 1553B alone will do the job. A longer-term disadvantage, according to Kevin Dawson, chief systems engineer of GEC Avionics' systems and equipment division, which is developing Stanag 3910 test equipment, is that as a hybrid it is probably a dead-end development route. New work on a single-strand, all-optical- fibre, high-speed bus will have to be done in Europe as it is being done in the USA. Dawson says all post-1553B buses (in cluding 3910) will need new optical-fibre components and new protocols for han dling data more efficiently. Light-pulse transmitters and receivers and low-loss optical fibre available probably will meet 3910, provided light losses at connectors and couplers in the fibre runs can be reduced. Some believe there is insufficient experience of optical fibre to justify its use in an operational combat aircraft. Nobody knows how robust it will be and there are doubts about maintenance. Concerns over the technology have persuaded the French not to use optical fibre in Rafale databuses. They will use the 3910 specification, but have a 20 Mbit/s wire for the high-speed channel. In EFA, for generating, receiving and handling high-speed data, a special mod ule will be located between each avionic equipment item and the local light pulse transmitter-receiver serving the high speed bus. This module will be switched on and off and into the right mode by signals from a standard 1553B controller on the parallel wire channel. Post-1553B bus work in the USA is aimed initially at incorporation in -the Advanced Tactical Fighter (ATF) and A-12 aircraft, which will generate far more digital data than will EFA. Upgrading and expanding 1553B to do the job is not feasible. 30 FLIGHT INTERNATIONAL 9 - 15 January, 1991
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