ELECTROMAGNETIC PULSE (EMP)
10. General. A significant threat to the survivability of electronic
systems used by the Department of Defense (DoD) is the effect of
electromagnetic pulse (EMP). EMP is one of the destructive components of a
nuclear detonation. Unlike the other destructive components (shook wave,
thermal energy) whose effects are restricted to a region near the detonation,
EMP, since it contains electrical and magnetic components, can be collected
by conductors and transmitted great distances from the site of detonation.
The EMP can disrupt communications, upset equipment operation, or cause
equipment component destruction.
10.1 EMP generation. When a nuclear detonation occurs, gamma rays are
produced which radiate outward from the burst at the speed of light (see
figure C-1). As the gamma rays collide with air molecules, electrons are
dislocated, creating Compton electrons. These electrons are affected by the
magnetic fields of the earth, which create an electromagnetic (EM) wave that
propagates toward the earth. To be effective. the burst must take place
above the earth, at altitudes as low as 20 km. A burst 560 km above the
geographic center of the continental United States would completely blanket
10.2 EMP effects. The effects of EMP upon electronic equipment and
components can be likened to the effects of lightning - stress by high
currents and voltages. However, EMP produces higher currents and voltages,
covers a greater frequency spectrum, and has a faster rise time (see figure
C-2). Equipment may be subjected to state changes which cause improper
operation (upset) or component failure (burnout). Upset is the introduction
of spurious signals which temporarily disrupt normal operation or which may
be misinterpreted by the equipment processor. Burnout is permanent damage to
system components to the extent that the equipment is inoperative until
replacement is effected.
20. Protection requirements. Certain DoD facilities, having time-urgent
missions, mandate a high degree of assurance of immunity to EMP-induced upset
or burnout. Other supporting missions may allow lesser degrees of
protection, while accepting momentary upset. The objective then, of any EMP
protection, is to ensure the currents and voltages induced by the EMP into
electronic devices, or at other sensitive locations in a system, are smaller
than a current or voltage which could reasonably be expected to produce
damage or upset. The applied physical principles of EM compatibility may be
used to develop such protection. Three fundamental approaches may be used:
The interfering signal source level may be reduced.
The receptor susceptibility may be reduced.
The attenuation of the path or paths over which interference is
transmitted from the source to the receptor may be increased.
These approaches are implemented by increasing equipment robustness and
isolating equipment from the EMP environment.