INTRODUCTION
The prosecution of a successful Information Warfare (IW) campaign against an industrialised or post industrial opponent will require a suitable set of tools. As demonstrated in the Desert Storm air campaign, air power has proven to be a most effective means of inhibiting the functions of an opponent's vital information processing infrastructure. This is because air power allows concurrent or parallel engagement of a large number of targets over geographically significant areas [SZAFRANSKI95].
While Desert Storm demonstrated that the application of air power was the most practical means of crushing an opponent's information processing and transmission nodes, the need to physically destroy these with guided munitions absorbed a substantial proportion of available air assets in the early phase of the air campaign. Indeed, the aircraft capable of delivery laser guided bombs were largely occupied with this very target set during the first nights of the air battle.
The efficient execution of an IW campaign against a modern industrial or post-industrial opponent will require the use of specialised tools designed to destroy information systems. Electromagnetic bombs built for this purpose can provide, where delivered by suitable means, a very effective tool for this purpose.
The Lethality of Electromagnetic Warheads
The issue of electromagnetic weapon lethality is complex. Unlike the technology base for weapon construction, which has been widely published in the open literature, lethality related issues have been published much less frequently. While the calculation of electromagnetic field strengths achievable at a given radius for a given device design is a straightforward task, determining a kill probability for a given class of target under such conditions is not.
This is for good reasons. The first is that target types are very diverse in their electromagnetic hardness, or ability to resist damage. Equipment that has been intentionally shielded and hardened against electromagnetic attack will withstand orders of magnitude greater field strengths than standard commercially rated equipment. Moreover, various manufacturer's implementations of like types of equipment may vary significantly in hardness due the idiosyncrasies of specific electrical designs, cabling schemes and chassis/shielding designs used.
The second major problem area in determining lethality is that of coupling efficiency, which is a measure of how much power is transferred from the field produced by the weapon into the target. Only power coupled into the target can cause useful damage.
Coupling Modes
In assessing how power is coupled into targets, two principal coupling modes are recognised in the literature:
Front Door Coupling occurs typically when power from a electromagnetic weapon is coupled into an antenna associated with radar or communications equipment. The antenna subsystem is designed to couple power in and out of the equipment, and thus provides an efficient path for the power flow from the electromagnetic weapon to enter the equipment and cause damage.
A low frequency weapon will couple well into a typical wiring infrastructure, as most telephone lines, networking cables and power lines follow streets, building risers and corridors. In most instances any particular cable run will comprise multiple linear segments joined at approximately right angles. Whatever the relative orientation of the weapons field, more than one linear segment of the cable run is likely to be oriented such that a good coupling efficiency can be achieved.
It is worth noting at this point the safe operating envelopes of some typical types of semiconductor devices. Manufacturer's guaranteed breakdown voltage ratings for Silicon high frequency bipolar transistors, widely used in communications equipment, typically vary between 15 V and 65 V. Gallium Arsenide Field Effect Transistors are usually rated at about 10V. High density Dynamic Random Access Memories (DRAM), an essential part of any computer, are usually rated to 7 V against earth. Generic CMOS logic is rated between 7 V and 15 V, and microprocessors running off 3.3 V or 5 V power supplies are usually rated very closely to that voltage. Whilst many modern devices are equipped with additional protection circuits at each pin, to sink electrostatic discharges, sustained or repeated application of a high voltage will often defeat these [MOTO3, MICRON92, NATSEMI86].
Communications interfaces and power supplies must typically meet electrical safety requirements imposed by regulators. Such interfaces are usually protected by isolation transformers with ratings from hundreds of Volts to about 2 to 3 kV [NPI93].
It is clearly evident that once the defence provided by a transformer, cable pulse arrestor or shielding is breached, voltages even as low as 50 V can inflict substantial damage upon computer and communications equipment. The author has seen a number of equipment items (computers, consumer electronics) exposed to low frequency high voltage spikes (near lightning strikes, electrical power transients), and in every instance the damage was extensive, often requiring replacement of most semiconductors in the equipment.
HPM weapons operating in the centimetric and millimetric bands however offer an additional coupling mechanism to Back Door Coupling. This is the ability to directly couple into equipment through ventilation holes, gaps between panels and poorly shielded interfaces. Under these conditions, any aperture into the equipment behaves much like a slot in a microwave cavity, allowing microwave radiation to directly excite or enter the cavity. The microwave radiation will form a spatial standing wave pattern within the equipment. Components situated within the anti-nodes within the standing wave pattern will be exposed to potentially high electromagnetic fields.
Because microwave weapons can couple more readily than low frequency weapons, and can in many instances bypass protection devices designed to stop low frequency coupling, microwave weapons have the potential to be significantly more lethal than low frequency weapons.
What research has been done in this area illustrates the difficulty in producing workable models for predicting equipment vulnerability. It does however provide a solid basis for shielding strategies and hardening of equipment.
The diversity of likely target types and the unknown geometrical layout and electrical characteristics of the wiring and cabling infrastructure surrounding a target makes the exact prediction of lethality impossible.
A general approach for dealing with wiring and cabling related back door coupling is to determine a known lethal voltage level, and then use this to find the required field strength to generate this voltage. Once the field strength is known, the lethal radius for a given weapon configuration can be calculated.
A trivial example is that of a 10 GW 5 GHz HPM device illuminating a footprint of 400 to 500 metres diameter, from a distance of several hundred metres. This will result in field strengths of several kiloVolts per metre within the device footprint, in turn capable of producing voltages of hundreds of volts to kiloVolts on exposed wires or cables [KRAUS88, TAYLOR92]. This suggests lethal radii of the order of hundreds of metres, subject to weapon performance and target set electrical hardness.
Maximising Electromagnetic Bomb Lethality
To maximise the lethality of an electromagnetic bomb it is necessary to maximise the power coupled into the target set.
The second step is to maximise the coupling efficiency into the target set. A good strategy for dealing with a complex and diverse target set is to exploit every coupling opportunity available within the bandwidth of the weapon.
A low frequency bomb built around an FCG will require a large antenna to provide good coupling of power from the weapon into the surrounding environment. Whilst weapons built this way are inherently wide band, as most of the power produced lies in the frequency band below 1 MHz compact antennas are not an option. One possible scheme is for a bomb approaching its programmed firing altitude to deploy five linear antenna elements. These are produced by firing off cable spools which unwind several hundred metres of cable. Four radial antenna elements form a "virtual" earth plane around the bomb, while an axial antenna element is used to radiate the power from the FCG. The choice of element lengths would need to be carefully matched to the frequency characteristics of the weapon, to produce the desired field strength. A high power coupling pulse transformer is used to match the low impedance FCG output to the much higher impedance of the antenna, and ensure that the current pulse does not vapourise the cable prematurely.
Other alternatives are possible. One is to simply guide the bomb very close to the target, and rely upon the near field produced by the FCG winding, which is in effect a loop antenna of very small diameter relative to the wavelength. Whilst coupling efficiency is inherently poor, the use of a guided bomb would allow the warhead to be positioned accurately within metres of a target. An area worth further investigation in this context is the use of low frequency bombs to damage or destroy magnetic tape libraries, as the near fields in the vicinity of a flux generator are of the order of magnitude of the coercivity of most modern magnetic materials.
Microwave bombs have a broader range of coupling modes and given the small wavelength in comparison with bomb dimensions, can be readily focussed against targets with a compact antenna assembly. Assuming that the antenna provides the required weapon footprint, there are at least two mechanisms which can be employed to further maximise lethality.
The second mechanism which can be exploited to improve coupling is the polarisation of the weapon's emission. If we assume that the orientations of possible coupling apertures and resonances in the target set are random in relation to the weapon's antenna orientation, a linearly polarised emission will only exploit half of the opportunities available. A circularly polarised emission will exploit all coupling opportunities.
The practical constraint is that it may be difficult to produce an efficient high power circularly polarised antenna design which is compact and performs over a wide band. Some work therefore needs to be done on tapered helix or conical spiral type antennas capable of handling high power levels, and a suitable interface to a Vircator with multiple extraction ports must devised. A possible implementation is depicted in Fig.5. In this arrangement, power is coupled from the tube by stubs which directly feed a multi-filar conical helix antenna. An implementation of this scheme would need to address the specific requirements of bandwidth, beamwidth, efficiency of coupling from the tube, while delivering circularly polarised radiation.
In summary, lethality is maximised by maximising power output and the efficiency of energy transfer from the weapon to the target set. Microwave weapons offer the ability to focus nearly all of their energy output into the lethal footprint, and offer the ability to exploit a wider range of coupling modes. Therefore, microwave bombs are the preferred choice.
Targeting Electromagnetic Bombs
The task of identifying targets for attack with electromagnetic bombs can be complex. Certain categories of target will be very easy to identify and engage. Buildings housing government offices and thus computer equipment, production facilities, military bases and known radar sites and communications nodes are all targets which can be readily identified through conventional photographic, satellite, imaging radar, electronic reconnaissance and humint operations. These targets are typically geographically fixed and thus may be attacked providing that the aircraft can penetrate to weapon release range. With the accuracy inherent in GPS/inertially guided weapons, the electromagnetic bomb can be programmed to detonate at the optimal position to inflict a maximum of electrical damage.
Mobile and camouflaged targets which radiate overtly can also be readily engaged. Mobile and relocatable air defence equipment, mobile communications nodes and naval vessels are all good examples of this category of target. While radiating, their positions can be precisely tracked with suitable Electronic Support Measures (ESM) and Emitter Locating Systems (ELS) carried either by the launch platform or a remote surveillance platform. In the latter instance target coordinates can be continuously datalinked to the launch platform. As most such targets move relatively slowly, they are unlikely to escape the footprint of the electromagnetic bomb during the weapon's flight time.
Mobile or hidden targets which do not overtly radiate may present a problem, particularly should conventional means of targeting be employed. A technical solution to this problem does however exist, for many types of target. This solution is the detection and tracking of Unintentional Emission (UE) [HERSKOWITZ96]. UE has attracted most attention in the context of TEMPEST surveillance, where transient emanations leaking out from equipment due poor shielding can be detected and in many instances demodulated to recover useful intelligence. Termed Van Eck radiation [VECK85], such emissions can only be suppressed by rigorous shielding and emission control techniques, such as are employed in TEMPEST rated equipment.
Whilst the demodulation of UE can be a technically difficult task to perform well, in the context of targeting electromagnetic bombs this problem does not arise. To target such an emitter for attack requires only the ability to identify the type of emission and thus target type, and to isolate its position with sufficient accuracy to deliver the bomb. Because the emissions from computer monitors, peripherals, processor equipment, switchmode power supplies, electrical motors, internal combustion engine ignition systems, variable duty cycle electrical power controllers (thyristor or triac based), superheterodyne receiver local oscillators and computer networking cables are all distinct in their frequencies and modulations, a suitable Emitter Locating System can be designed to detect, identify and track such sources of emission.
A good precedent for this targeting paradigm exists. During the SEA (Vietnam) conflict the United States Air Force (USAF) operated a number of night interdiction gunships which used direction finding receivers to track the emissions from vehicle ignition systems. Once a truck was
Identified and tracked, the gunship would engage it. because UE occurs at relatively low power levels, the use of this detection method prior to the outbreak of hostilities can be difficult, as it may be necessary to overfly hostile territory to find signals of usable intensity. The use of stealthy reconnaissance aircraft or long range, stealthy Unmanned Aerial Vehicles (UAV) may be required. The latter also raises the possibility of autonomous electromagnetic warhead armed expendable UAVs, fitted with appropriate homing receivers. These would be programmed to loiter in a target area until a suitable emitter is detected, upon which the UAV would home in and expend itself against the target.
The Delivery of Conventional Electromagnetic Bombs
As with explosive warheads, electromagnetic warheads will occupy a volume of physical space and will also have some given mass (weight) determined by the density of the internal hardware. Like explosive warheads, electromagnetic warheads may be fitted to a range of delivery vehicles.
Known existing applications involve fitting an electromagnetic warhead to a cruise missile airframe. The choice of a cruise missile airframe will restrict the weight of the weapon to about 340 kg (750 lb), although some sacrifice in airframe fuel capacity could see this size increased. A limitation in all such applications is the need to carry an electrical energy storage device, eg a battery, to provide the current used to charge the capacitors used to prime the FCG prior to its discharge. Therefore the available payload capacity will be split between the electrical storage and the weapon itself.
In wholly autonomous weapons such as cruise missiles, the size of the priming current source and its battery may well impose important limitations on weapon capability. Air delivered bombs, which have a flight time between tens of seconds to minutes, could be built to exploit the launch aircraft's power systems. In such a bomb design, the bomb's capacitor bank can be charged by the launch aircraft enroute to target, and after release a much smaller onboard power supply could be used to maintain the charge in the priming source prior to weapon initiation.
An electromagnetic bomb delivered by a conventional aircraft can offer a much better ratio of electromagnetic device mass to total bomb mass, as most of the bomb mass can be dedicated to the electromagnetic device installation itself. It follows therefore, that for a given technology an electromagnetic bomb of identical mass to a electromagnetic warhead equipped missile can have a much greater lethality, assuming equal accuracy of delivery and technologically similar electromagnetic device design.
A missile borne electromagnetic warhead installation will comprise the electromagnetic device, an electrical energy converter, and an onboard storage device such as a battery. As the weapon is pumped, the battery is drained. The electromagnetic device will be detonated by the missile's onboard fusing system. In a cruise missile, this will be tied to the navigation system; in an anti-shipping missile the radar seeker and in an air-to-air missile, the proximity fusing system. The warhead fraction (ie ratio of total payload (warhead) mass to launch mass of the weapon) will be between 15% and 30%.
An electromagnetic bomb warhead will comprise an electromagnetic device, an electrical energy converter and a energy storage device to pump and sustain the electromagnetic device charge after separation from the delivery platform. Fusing could be provided by a radar altimeter fuse to airburst the bomb, a barometric fuse or in GPS/inertially guided bombs, the navigation system. The warhead fraction could be as high as 85%, with most of the usable mass occupied by the electromagnetic device and its supporting hardware.
Due to the potentially large lethal radius of an electromagnetic device, compared to an explosive device of similar mass, standoff delivery would be prudent. Whilst this is an inherent characteristic of weapons such as cruise missiles, potential applications of these devices to glidebombs, anti-shipping missiles and air-to-air missiles would dictate fire and forget guidance of the appropriate variety, to allow the launching aircraft to gain adequate separation of several miles before warhead detonation.
The recent advent of GPS satellite navigation guidance kits for conventional bombs and glidebombs has provided the optimal means for cheaply delivering such weapons. While GPS guided weapons without differential GPS enhancements may lack the pinpoint accuracy of laser or television guided munitions, they are still quite accurate (CEP \(~~ 40 ft) and importantly, cheap, autonomous all weather weapons.
The USAF has recently deployed the Northrop GAM (GPS Aided Munition) on the B-2 bomber [NORTHROP95], and will by the end of the decade deploy the GPS/inertially guided GBU-29/30 JDAM (Joint Direct Attack Munition)[MDC95] and the AGM-154 JSOW (Joint Stand Off Weapon) [PERGLER94] glidebomb. Other countries are also developing this technology, the Australian BAeA AGW (Agile Glide Weapon) glidebomb achieving a glide range of about 140 km (75 nmi) when launched from altitude [KOPP96].
The importance of glidebombs as delivery means for HPM warheads is threefold. Firstly, the glidebomb can be released from outside effective radius of target air defences, therefore minimising the risk to the launch aircraft. Secondly, the large standoff range means that the aircraft can remain well clear of the bomb's effects. Finally the bomb's autopilot may be programmed to shape the terminal trajectory of the weapon, such that a target may be engaged from the most suitable altitude and aspect.
A major advantage of using electromagnetic bombs is that they may be delivered by any tactical aircraft with a nav-attack system capable of delivering GPS guided munitions. As we can expect GPS guided munitions to be become the standard weapon in use by Western air forces by the end of this decade, every aircraft capable of delivering a standard guided munition also becomes a potential delivery vehicle for a electromagnetic bomb. Should weapon ballistic properties be identical to the standard weapon, no software changes to the aircraft would be required.
Because of the simplicity of electromagnetic bombs in comparison with weapons such as Anti Radiation Missiles (ARM), it is not unreasonable to expect that these should be both cheaper to manufacture, and easier to support in the field, thus allowing for more substantial weapon stocks. In turn this makes saturation attacks a much more viable proposition.
In this context it is worth noting that the USAF's possesion of the JDAM capable F-117A and B-2A will provide the capability to deliver E-bombs against arbitrary high value targets with virtual impunity. The ability of a B-2A to deliver up to sixteen GAM/JDAM fitted E-bomb warheads with a 20 ft class CEP would allow a small number of such aircraft to deliver a decisive blow against key strategic, air defence and theatre targets. A strike and electronic combat capable derivative of the F-22 would also be a viable delivery platform for an E-bomb/JDAM. With its superb radius, low signature and supersonic cruise capability an RFB-22 could attack air defence sites, C3I sites, airbases and strategic targets with E-bombs, achieving a significant shock effect. A good case may be argued for the whole F-22 build to be JDAM/E-bomb capable, as this would allow the USAF to apply the maximum concentration of force against arbitrary air and surface targets during the opening phase of an air campaign.
Conclusions
Electromagnetic bombs are Weapons of Electrical Mass Destruction with applications across a broad spectrum of targets, spanning both the strategic and tactical. As such their use offers a very high payoff in attacking the fundamental information processing and communication facilities of a target system. The massed application of these weapons will produce substantial paralysis in any target system, thus providing a decisive advantage in the conduct of Electronic Combat, Offensive Counter Air and Strategic Air Attack.
Because E-bombs can cause hard electrical kills over larger areas than conventional explosive weapons of similar mass, they offer substantial economies in force size for a given level of inflicted damage, and are thus a potent force multiplier for appropriate target sets.
The non-lethal nature of electromagnetic weapons makes their use far less politically damaging than that of conventional munitions, and therefore broadens the range of military options available.
This paper has included a discussion of the technical, operational and targeting aspects of using such weapons, as no historical experience exists as yet upon which to build a doctrinal model. The immaturity of this weapons technology limits the scope of this discussion, and many potential areas of application have intentionally not been discussed. The ongoing technological evolution of this family of weapons will clarify the relationship between weapon size and lethality, thus producing further applications and areas for study.
E-bombs can be an affordable force multiplier for military forces which are under post Cold War pressures to reduce force sizes, increasing both their combat potential and political utility in resolving disputes. Given the potentially high payoff deriving from the use of these devices, it is incumbent upon such military forces to appreciate both the offensive and defensive implications of this technology. It is also incumbent upon governments and private industry to consider the implications of the proliferation of this technology, and take measures to safeguard their vital assets from possible future attack. Those who choose not to may become losers in any future wars.
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