An investigation of the microwave upset of avionic circuitry

Abstract

Circuit technology of the 1970-90 era appears fairly resilient to microwave radio frequency interference, with few reported occurrences of interference. However, a proposition has been developed which substantiates fears that new technologies, with their extremely high packing densities, small device p-n junctions and very high clock rates, will be very susceptible to interference throughout the microwave band It has been postulated that the mechanism for this upset is demodulation and that it will come about by either the predicted changes in the microwave RF environment by the year 2000, or by a suitable choice of phasing and frequency at high power. The postulation is studied by developing an overall ingress equation, relating incident power density at the aircraft to the load voltage at an avionic circuit component. The equation's terms are investigated to quantif' their contribution to the likelihood of interference. The operational RF environment for aircraft is studied and predictions of the current and maximum future environments are made. A practical investigation of 2-18 GH.z airframe shielding is described, with comparison of the results with those from a number of other aircraft and helicopter types. A study of ingress into avionic boxes is presented and is followed by the results of an investigation of energy coupling via the cables and connectors, including the development and practical examination of a coupling model based on transmission line theory. A study is then presented of circuit technology developments, electronic component interference and damage mechanisms, and evidence of upset of electronic equipment is given. Investigations show that there is more 1-18 GHz upset of electronic equipment than originally thought and data suggest that thermal damage of active devices may dominate over-voltage stressing of p-n junctions. Aircraft investigations have shown that incident microwave radiation is attenuated approximately 20 dB by the airframe, in a complex fashion which does not lend itself to being modelled easily. Under some conditions this value of airframe attenuation is seen to approach zero, removing any shielding of avionics by the airframe for these cases. A predictor for airframe shielding independent of air vehicle type has been developed, based on cumulative density ftrnctions of all data from each of the aircraft types examined. The cable coupling model gives good agreement with measured data except for the dependency of load voltage on cable length and illuminating antenna position along the cable, for which an empirical equation has been developed. Computer power limitations and significant variations of most of the parameters in the overall ingress equation suggest that modelling of the complex innards of aircraft and avionics at these frequencies will remain impractical for the foreseeable future and that probabilistic models are the only achievable goal.
It is concluded that all avionic circuit technologies may well be upset as postulated above or by speculative High Power Microwave weapons, but that careful use of existing aircraft and equipment design methodologies can offer adequate protection. An improved protection regime is proposed for future aircraft and a number of fUture research areas are identified to enable better understanding of the microwave hazard to aircraft. The three areas which will add most to this understanding are modelling of the precise microwave environment to be encountered, further airframe shielding measurements and analyses, from all incidence angles and on different aircraft types, and the construction and cumulative probability fUnction analyses of electronic component and equipment upset databases.