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from:
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<A
HREF="http://www.airpower.maxwell.af.mil/airchronicles/kopp/apjemp.html">The
Electromagnetic Bomb - a Weapon of Electric
</A>
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explainatory diagrams.
Om
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The Electromagnetic Bomb - a Weapon of Electrical Mass Destruction
Carlo Kopp Defence AnalystMelbourne, [EMAIL PROTECTED]
http://www.cs.monash.edu.au/~carlo/
ABSTRACT
High Power Electromagnetic Pulse generation techniques and High Power
Microwave technology have matured to the point where practical E-bombs
(Electromagnetic bombs) are becoming technically feasible, with new
applications in both Strategic and Tactical Information Warfare. The
development of conventional E-bomb devices allows their use in
non-nuclear confrontations. This paper discusses aspects of the
technology base, weapon delivery techniques and proposes a doctrinal
foundation for the use of such devices in warhead and bomb applications.
1. 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.
2.The EMP Effect
The ElectroMagnetic Pulse (EMP) effect [1] was first observed during the
early testing of high altitude airburst nuclear weapons [GLASSTONE64].
The effect is characterised by the production of a very short (hundreds
of nanoseconds) but intense electromagnetic pulse, which propagates away
from its source with ever diminishing intensity, governed by the theory
of electromagnetism. The ElectroMagnetic Pulse is in effect an
electromagnetic shock wave.
This pulse of energy produces a powerful electromagnetic field,
particularly within the vicinity of the weapon burst. The field can be
sufficiently strong to produce short lived transient voltages of
thousands of Volts (ie kiloVolts) on exposed electrical conductors, such
as wires, or conductive tracks on printed circuit boards, where exposed.
It is this aspect of the EMP effect which is of military significance,
as it can result in irreversible damage to a wide range of electrical
and electronic equipment, particularly computers and radio or radar
receivers. Subject to the electromagnetic hardness of the electronics, a
measure of the equipment's resilience to this effect, and the intensity
of the field produced by the weapon, the equipment can be irreversibly
damaged or in effect electrically destroyed. The damage inflicted is not
unlike that experienced through exposure to close proximity lightning
strikes, and may require complete replacement of the equipment, or at
least substantial portions thereof.
Commercial computer equipment is particularly vulnerable to EMP effects,
as it is largely built up of high density Metal Oxide Semiconductor
(MOS) devices, which are very sensitive to exposure to high voltage
transients. What is significant about MOS devices is that very little
energy is required to permanently wound or destroy them, any voltage in
typically in excess of tens of Volts can produce an effect termed gate
breakdown which effectively destroys the device. Even if the pulse is
not powerful enough to produce thermal damage, the power supply in the
equipment will readily supply enough energy to complete the destructive
process. Wounded devices may still function, but their reliability will
be seriously impaired. Shielding electronics by equipment chassis
provides only limited protection, as any cables running in and out of
the equipment will behave very much like antennae, in effect guiding the
high voltage transients into the equipment.
Computers used in data processing systems, communications systems,
displays, industrial control applications, including road and rail
signalling, and those embedded in military equipment, such as signal
processors, electronic flight controls and digital engine control
systems, are all potentially vulnerable to the EMP effect.
Other electronic devices and electrical equipment may also be destroyed
by the EMP effect. Telecommunications equipment can be highly
vulnerable, due to the presence of lengthy copper cables between
devices. Receivers of all varieties are particularly sensitive to EMP,
as the highly sensitive miniature high frequency transistors and diodes
in such equipment are easily destroyed by exposure to high voltage
electrical transients. Therefore radar and electronic warfare equipment,
satellite, microwave, UHF, VHF, HF and low band communications equipment
and television equipment are all potentially vulnerable to the EMP
effect.
It is significant that modern military platforms are densely packed with
electronic equipment, and unless these platforms are well hardened, an
EMP device can substantially reduce their function or render them
unusable.
3. The Technology Base for Conventional Electromagnetic Bombs
The technology base which may be applied to the design of
electromagnetic bombs is both diverse, and in many areas quite mature.
Key technologies which are extant in the area are explosively pumped
Flux Compression Generators (FCG), explosive or propellant driven
Magneto-Hydrodynamic (MHD) generators and a range of HPM devices, the
foremost of which is the Virtual Cathode Oscillator or Vircator. A wide
range of experimental designs have been tested in these technology
areas, and a considerable volume of work has been published in
unclassified literature.
This paper will review the basic principles and attributes of these
technologies, in relation to bomb and warhead applications. It is
stressed that this treatment is not exhaustive, and is only intended to
illustrate how the technology base can be adapted to an operationally
deployable capability.
3.1. Explosively Pumped Flux Compression Generators
The explosively pumped FCG is the most mature technology applicable to
bomb designs. The FCG was first demonstrated by Clarence Fowler at Los
Alamos National Laboratories (LANL) in the late fifties [FOWLER60].
Since that time a wide range of FCG configurations has been built and
tested, both in the US and the USSR, and more recently CIS.
The FCG is a device capable of producing electrical energies of tens of
MegaJoules in tens to hundreds of microseconds of time, in a relatively
compact package. With peak power levels of the order of TeraWatts to
tens of TeraWatts, FCGs may be used directly, or as one shot pulse power
supplies for microwave tubes. To place this in perspective, the current
produced by a large FCG is between ten to a thousand times greater than
that produced by a typical lightning stroke [WHITE78].
The central idea behind the construction of FCGs is that of using a fast
explosive to rapidly compress a magnetic field, transferring much energy
from the explosive into the magnetic field.
The initial magnetic field in the FCG prior to explosive initiation is
produced by a start current. The start current is supplied by an
external source, such a a high voltage capacitor bank (Marx bank), a
smaller FCG or an MHD device. In principle, any device capable of
producing a pulse of electrical current of the order of tens of
kiloAmperes to MegaAmperes will be suitable.
A number of geometrical configurations for FCGs have been published (for
examples see REINOVSKY85, CAIRD85, FOWLER89) The most commonly used
arrangement is that of the coaxial FCG. The coaxial arrangement is of
particular interest in this context, as its essentially cylindrical form
factor lends itself to packaging into munitions.
In a typical coaxial FCG , a cylindrical copper tube forms the armature.
This tube is filled with a fast high energy explosive. A number of
explosive types have been used, ranging from B and C-type compositions
to machined blocks of PBX-9501. The armature is surrounded by a helical
coil of heavy wire, typically copper, which forms the FCG stator. The
stator winding is in some designs split into segments, with wires
bifurcating at the boundaries of the segments, to optimise the
electromagnetic inductance of the armature coil.
The intense magnetic forces produced during the operation of the FCG
could potentially cause the device to disintegrate prematurely if not
dealt with. This is typically accomplished by the addition of a
structural jacket of a non-magnetic material. Materials such as concrete
or Fibreglass in an Epoxy matrix have been used. In principle, any
material with suitable electrical and mechanical properties could be
used. In applications where weight is an issue, such as air delivered
bombs or missile warheads, a glass or Kevlar Epoxy composite would be a
viable candidate.
It is typical that the explosive is initiated when the start current
peaks. This is usually accomplished with a explosive lense plane wave
generator which produces a uniform plane wave burn (or detonation) front
in the explosive. Once initiated, the front propagates through the
explosive in the armature, distorting it into a conical shape (typically
12 to 14 degrees of arc). Where the armature has expanded to the full
diameter of the stator, it forms a short circuit between the ends of the
stator coil, shorting and thus isolating the start current source and
trapping the current within the device. The propagating short has the
effect of compressing the magnetic field, whilst reducing the inductance
of the stator winding. The result is that such generators will producing
a ramping current pulse, which peaks before the final disintegration of
the device. Published results suggest ramp times of tens to hundreds of
microseconds, specific to the characteristics of the device, for peak
currents of tens of MegaAmperes and peak energies of tens of MegaJoules.
The current multiplication (ie ratio of output current to start current)
achieved varies with designs, but numbers as high as 60 have been
demonstrated. In a munition application, where space and weight are at a
premium, the smallest possible start current source is desirable. These
applications can exploit cascading of FCGs, where a small FCG is used to
prime a larger FCG with a start current. Experiments conducted by LANL
and AFWL have demonstrated the viability of this technique [KIRTLAND94,
REINOVSKY85].
The principal technical issues in adapting the FCG to weapons
applications lie in packaging, the supply of start current, and matching
the device to the intended load. Interfacing to a load is simplified by
the coaxial geometry of coaxial and conical FCG designs. Significantly,
this geometry is convenient for weapons applications, where FCGs may be
stacked axially with devices such a microwave Vircators. The demands of
a load such as a Vircator, in terms of waveform shape and timing, can be
satisfied by inserting pulse shaping networks, transformers and
explosive high current switches.
3.2. Explosive and Propellant Driven MHD Generators
The design of explosive and propellant driven Magneto-Hydrodynamic
generators is a much less mature art that that of FCG design. Technical
issues such as the size and weight of magnetic field generating devices
required for the operation of MHD generators suggest that MHD devices
will play a minor role in the near term. In the context of this paper,
their potential lies in areas such as start current generation for FCG
devices.
The fundamental principle behind the design of MHD devices is that a
conductor moving through a magnetic field will produce an electrical
current transverse to the direction of the field and the conductor
motion. In an explosive or propellant driven MHD device, the conductor
is a plasma of ionised explosive or propellant gas, which travels
through the magnetic field. Current is collected by electrodes which are
in contact with the plasma jet [FANTHOME89].
The electrical properties of the plasma are optimised by seeding the
explosive or propellant with with suitable additives, which ionise
during the burn [FANTHOME89, FLANAGAN81]. Published experiments suggest
that a typical arrangement uses a solid propellant gas generator, often
using conventional ammunition propellant as a base. Cartridges of such
propellant can be loaded much like artillery rounds, for multiple shot
operation.
3.3. High Power Microwave Sources - The Vircator
Whilst FCGs are potent technology base for the generation of large
electrical power pulses, the output of the FCG is by its basic physics
constrained to the frequency band below 1 MHz. Many target sets will be
difficult to attack even with very high power levels at such
frequencies, moreover focussing the energy output from such a device
will be problematic. A HPM device overcomes both of the problems, as its
output power may be tightly focussed and it has a much better ability to
couple energy into many target types.
A wide range of HPM devices exist. Relativistic Klystrons, Magnetrons,
Slow Wave Devices, Reflex triodes, Spark Gap Devices and Vircators are
all examples of the available technology base [GRANATSTEIN87,
HOEBERLING92]. From the perspective of a bomb or warhead designer, the
device of choice will be at this time the Vircator, or in the nearer
term a Spark Gap source. The Vircator is of interest because it is a one
shot device capable of producing a very powerful single pulse of
radiation, yet it is mechanically simple, small and robust, and can
operate over a relatively broad band of microwave frequencies.
The physics of the Vircator tube are substantially more complex than
those of the preceding devices. The fundamental idea behind the Vircator
is that of accelerating a high current electron beam against a mesh (or
foil) anode. Many electrons will pass through the anode, forming a
bubble of space charge behind the anode. Under the proper conditions,
this space charge region will oscillate at microwave frequencies. If the
space charge region is placed into a resonant cavity which is
appropriately tuned, very high peak powers may be achieved. Conventional
microwave engineering techniques may then be used to extract microwave
power from the resonant cavity. Because the frequency of oscillation is
dependent upon the electron beam parameters, Vircators may be tuned or
chirped in frequency, where the microwave cavity will support
appropriate modes. Power levels achieved in Vircator experiments range
from 170 kiloWatts to 40 GigaWatts over frequencies spanning the
decimetric and centimetric bands [THODE87].
The two most commonly described configurations for the Vircator are the
Axial Vircator (AV) (Fig.3), and the Transverse Vircator (TV). The Axial
Vircator is the simplest by design, and has generally produced the best
power output in experiments. It is typically built into a cylindrical
waveguide structure. Power is most often extracted by transitioning the
waveguide into a conical horn structure, which functions as an antenna.
AVs typically oscillate in Transverse Magnetic (TM) modes. The
Transverse Vircator injects cathode current from the side of the cavity
and will typically oscillate in a Transverse Electric (TE) mode.
Technical issues in Vircator design are output pulse duration, which is
typically of the order of a microsecond and is limited by anode melting,
stability of oscillation frequency, often compromised by cavity mode
hopping, conversion efficiency and total power output. Coupling power
efficiently from the Vircator cavity in modes suitable for a chosen
antenna type may also be an issue, given the high power levels involved
and thus the potential for electrical breakdown in insulators.
4. 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 which 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.
4.1. 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.
�Back Door Coupling occurs when the electromagnetic field from a weapon
produces large transient currents (termed spikes, when produced by a low
frequency weapon ) or electrical standing waves (when produced by a HPM
weapon) on fixed electrical wiring and cables interconnecting equipment,
or providing connections to mains power or the telephone network
[TAYLOR92, WHITE78]. Equipment connected to exposed cables or wiring
will experience either high voltage transient spikes or standing waves
which can damage power supplies and communications interfaces if these
are not hardened. Moreover, should the transient penetrate into the
equipment, damage can be done to other devices inside.
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 [2].
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.
4.2. 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 first step in maximising bomb lethality is is to maximise the peak
power and duration of the radiation of the weapon. For a given bomb
size, this is accomplished by using the most powerful flux compression
generator (and Vircator in a HPM bomb) which will fit the weapon size,
and by maximising the efficiency of internal power transfers in the
weapon. Energy which is not emitted is energy wasted at the expense of
lethality.
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 first is sweeping the frequency or chirping the Vircator. This can
improve coupling efficiency in comparison with a single frequency
weapon, by enabling the radiation to couple into apertures and
resonances over a range of frequencies. In this fashion, a larger number
of coupling opportunities are exploited.
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.
Another aspect of electromagnetic bomb lethality is its detonation
altitude, and by varying the detonation altitude, a tradeoff may be
achieved between the size of the lethal footprint and the intensity of
the electromagnetic field in that footprint. This provides the option of
sacrificing weapon coverage to achieve kills against targets of greater
electromagnetic hardness, for a given bomb size (Fig.7, 8). This is not
unlike the use of airburst explosive devices.
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.
5. 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 [3] 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 [4].
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 [5]. 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.
6. 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 [6] 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 [7] 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% [8].
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.
7. Defence Against Electromagnetic Bombs
The most effective defence against electromagnetic bombs is to prevent
their delivery by destroying the launch platform or delivery vehicle, as
is the case with nuclear weapons. This however may not always be
possible, and therefore systems which can be expected to suffer exposure
to the electromagnetic weapons effects must be electromagnetically
hardened.
The most effective method is to wholly contain the equipment in an
electrically conductive enclosure, termed a Faraday cage, which prevents
the electromagnetic field from gaining access to the protected
equipment. However, most such equipment must communicate with and be fed
with power from the outside world, and this can provide entry points via
which electrical transients may enter the enclosure and effect damage.
While optical fibres address this requirement for transferring data in
and out, electrical power feeds remain an ongoing vulnerability.
Where an electrically conductive channel must enter the enclosure,
electromagnetic arresting devices must be fitted. A range of devices
exist, however care must be taken in determining their parameters to
ensure that they can deal with the rise time and strength of electrical
transients produced by electromagnetic devices. Reports from the US [9]
indicate that hardening measures attuned to the behaviour of nuclear
EMP bombs do not perform well when dealing with some conventional
microwave electromagnetic device designs.
It is significant that hardening of systems must be carried out at a
system level, as electromagnetic damage to any single element of a
complex system could inhibit the function of the whole system. Hardening
new build equipment and systems will add a substantial cost burden.
Older equipment and systems may be impossible to harden properly and may
require complete replacement. In simple terms, hardening by design is
significantly easier than attempting to harden existing equipment.
An interesting aspect of electrical damage to targets is the possibility
of wounding semiconductor devices thereby causing equipment to suffer
repetitive intermittent faults rather than complete failures. Such
faults would tie down considerable maintenance resources while also
diminishing the confidence of the operators in the equipment's
reliability. Intermittent faults may not be possible to repair
economically, thereby causing equipment in this state to be removed from
service permanently, with considerable loss in maintenance hours during
damage diagnosis. This factor must also be considered when assessing the
hardness of equipment against electromagnetic attack, as partial or
incomplete hardening may in this fashion cause more difficulties than it
would solve. Indeed, shielding which is incomplete may resonate when
excited by radiation and thus contribute to damage inflicted upon the
equipment contained within it.
Other than hardening against attack, facilities which are concealed
should not radiate readily detectable emissions. Where radio frequency
communications must be used, low probability of intercept (ie spread
spectrum) techniques should be employed exclusively to preclude the use
of site emissions for electromagnetic targeting purposes [DIXON84].
Appropriate suppression of UE is also mandatory.
Communications networks for voice, data and services should employ
topologies with sufficient redundancy and failover mechanisms to allow
operation with multiple nodes and links inoperative. This will deny a
user of electromagnetic bombs the option of disabling large portions if
not the whole of the network by taking down one or more key nodes or
links with a single or small number of attacks.
8. Limitations of Electromagnetic Bombs
The limitations of electromagnetic weapons are determined by weapon
implementation and means of delivery. Weapon implementation will
determine the electromagnetic field strength achievable at a given
radius, and its spectral distribution. Means of delivery will constrain
the accuracy with which the weapon can be positioned in relation to the
intended target. Both constrain lethality.
--[more at web site]--
Aloha, He'Ping,
Om, Shalom, Salaam.
Em Hotep, Peace Be,
Omnia Bona Bonis,
All My Relations.
Adieu, Adios, Aloha.
Amen.
Roads End
Kris
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