The three major sub-systems of an aerospace platform are Structure (airframe), Propulsion and Avionics. Traditionally, avionics, the electronics for control and communication, was assigned a relatively less important role. In the modern aerospace vehicles, however, avionics and its integration into the overall system, has become as important if not more, particularly for some defense systems, e. g. ATF (Advanced Tactical Fighter)1 ,2 For space platforms e. g. the Shuttle and the Space Station, the on-board avionics will play a crucial role. Avionics itelf is undergoing a conversion from the analog to digital systems and the associated networks3 are also expected to be entirely digital. One of the penalties one pays in the conversion to digital systems is that the bandwidth needed in digital networks can be an order of magnitude higher than the analog bandwidth. Digital systems also tend to be not as robust as analog systems. A single spike in an analog system may go unnoticed, however, in the absence of any error correcting mechanism, the corruption of a single bit in a digital stream could have disastrous consequences. Adoption of error correcting codes again increases the bandwidth. In view of the high bandwidth, immunity to Electromagnetic Interference and other factors, serious consideration is being given to digital optical communication, based on Fiber Optic Local Area Networks (FOLAN), on-board aerospace vehicles4. In principle a FO-LAN can be implementing by just replacing the copper coax wire by an optical fiber and one can immediately expect the benefits of increased bandwidth, noise immunity etc. There are, however, several problems that attend this transition including;
-- very few, if any, fiber optic systems are space qualified. Even the standards for flight and space worthiness are not well established. -- the effect of space environment, e. g. vacuum, UV (and other radiation), atomic oxygen, temperature excursions, acceleration cycles etc., on fiber optic components is not well understood. -- some mundane looking problems, such as connectors and passive T's, turned out to be formidable obstacles, particularly for space applications where in situ repair and maintainence can become critical issues. -- the intrinsic bandwidth of the fiber is indeed quite high (in Gigabits/sec), however, the pointto-point transfer of mission oriented information from one node to another, i. e. the information capacity, depends on many other factors that can adversely effect the overall performance of the network.
Here we shall address only the last problem, namely, the transfer of information on FOLAN aboard space systems. In particular we will discuss the factors that effect FOLAN performance in the context of Bandwidth, Throughput and Information Capacity.