Considerations On The Use Of Airborne X-Band Radar As A Microwave Directed-Energy Weapon

Introduction

The risk to digital infrastructure and military platforms posed by electromagnetic directed-energy weapons (DEW) is well established and well documented [1,9,12,14,44]. Microwave weapons are a specific category of electromagnetic weapon, all of which are intended to inflict electrical damage upon electronic hardware, whether it is used for computational, communications, or other purposes. Microwave weapons operate in the microwave bands, and typically encompass operating frequencies above 1 GHz.

Since the advent of the earliest open-source publications in the area during the early and mid 1990s [24,25,26], considerable research effort has been invested in experimental and analytical assessment of vulnerability [15,19,20,22,46,47], as well as in the development of high-power microwave (HPM) sources [2,37,39].

Much of the existing research and related literature is focused on the effects of HPM devices, pulsed or continuous wave, as these are well established as in a range of potential military applications. HPM devices for such applications can be broadly divided into ‘one-shot’ weapons, which emit a single short high-intensity pulse, and repetitively pulsed weapons, which emit a stream of pulses, typically at a regular pulse repetition frequency (PRF). HPM devices may be narrowband, wideband, or ultrawideband, the latter typically comprising the category of transient electromagnetic devices (TED) [22].

More recently, the advent of high-power active electronically steered array (AESA) radar technology has seen a range of proposals for the use of such radars as microwave directed-energy weapons. Concurrently we have observed comparable growth in the peak power ratings of some conventional airborne electronically steered array (ESA) X-band radars, raising also the prospect of these devices being used in such applications [4,5,11].

AESA radars are designs in which an array of wideband or narrowband transmit-receive elements is steered electronically, by controlling the phase angle and amplitude of individual elements. Precise control of beam direction, shape, and taper function can be achieved in this fashion [42]. These radars are typically multifunctional designs, with a wide range of digitally controlled modes [31].

ESA radars do not employ solid state transmitters embedded in their array of transmit-receive elements. Instead ESAs rely on a high-power microwave device, such as a travelling wave tube (TWT) to generate output power, but retain the beamsteering agility of the AESA[38].

AESAs operating in the X-band have been specifically identified as candidates for DEW applications—these include the MP-RTIP radar planned for a range of US platforms [11], the APG-77 in the F-22A fighter [13,14], and the dedicated Vigilant Eagle system, intended to protect civilian airports against man-portable air defence systems (MANPADS), or ‘shoulder-launched missiles’, launched by terrorists [9,33].

In all of these applications it is proposed that X-band microwave radiation of significant intensity will be used to illuminate the target and cause disruption or electrical damage, rendering the target inoperative. Intended target types include aircraft, cruise missiles and surface-to-air missiles, all of which are functionally dependent upon internal electronic hardware, especially computer hardware [3].

The aim of this paper is to explore the available research, identify practical constraints on achievable effect and target survivability, and identify required radar adaptations for this purpose.

The source, transmission, coupling, lethality, and survivability problems

The problem of transmitting and coupling power into targets and the resulting lethality of microwave weapons remains the subject of active research. Research in this area cannot be regarded as mature at this time, although considerable progress has been achieved over the last decade.

The basic model for all such engagements is that of a transmitting antenna producing a flow of microwave power through a narrow pencil-beam mainlobe, across some distance to the target, where this power couples into the target and produces a damaging or disruptive effect.

Emitted power is a function of the design of the AESA or ESA design. In terms of the parameters of interest, such radars will emit continuous pulsed power ratings of the order of 2–5 kW continuous, and >10 kW peak, in the X-band, and given antenna aperture sizes, be capable of transmitting with a mainlobe –3 dB (half-power) beamwidth of the order of 2.5–3º, in a fighter radar, refer Figure 1. Mainlobe width will be similar for most designs as it is constrained by airframe geometry. Aperture foreshortening effects will increase mainlobe width with increasing mainlobe off-boresight angle [38,42].

Representative AESA transmit receive model power ratings are of the order of 2 W continuous, and 10 W peak, with incremental development expected to result in progressive growth of power output over time. The principal limitation arises from the capacity of the liquid cooling system to extract waste heat from the AESA, and current density in semiconductor driver transistors [6].

Conventional X-band ESA fighter radars with peak power ratings of 20 kW have been reported, using ganged travelling wave tube transmitters and hybrid ESA antenna arrays. Given similar antenna aperture sizes, cardinal parameters will be similar to AESA radars of equivalent power ratings [4,5].

The emitted power must propagate over the distance between the radar and the target, and will experience propagation losses with increasing distance. While inverse square law or Friis loss will be dominant, power transfer efficiency is an issue and other loss components must be considered. In the X-band, gaseous losses due to water vapour and oxygen will increase with decreasing operating altitude, and cloud absorption losses are likely to dominate where cloud must be penetrated [21].

In practical terms, for any AESA or ESA design and target type, useful engagement range will be a function of both altitude and weather conditions in the volume of space between the targets.

The propagation science to assess such losses is mature, and many simulation programs exist which can be employed to produce quite accurate estimates [29].

Power arriving at the target must be coupled into the internal cavities, cabling and, where applicable, waveguides in the target to produce disruptive or lethal effect. Coupling effects into targets can be broadly divided into ‘front-door coupling’ and ‘back-door coupling’ effects [41].

Front-door coupling effects occur when impinging radiation enters via existing antenna apertures on a target.

These may be associated with radar, navigation equipment, communications equipment, or passive radar warning or homing equipment. Where the antenna aperture in question provides a good impedance match at X-band, and its aspect permits a large projected area, high coupling efficiency may be achieved. Under such conditions an effective aperture gain of the order of >0 dB is feasible, and in the instance of an on boresight X-band radar antenna, the full gain of the order of 30 dB may be realised.

Indirect front-door coupling may arise where the geometry of the target permits the coupling and propagation of surface travelling or creeping waves [35]. In such a situation, impinging power, in which an electric field component exists in the plane of incidence, forms a travelling wave which propagates along the skin of the target and couples in via any antenna apertures which are able to couple at that wavelength. This effect can potentially yield a high coupling efficiency due to the large projected area of the target’s skin from many aspects, and will be limited primarily by the design of antenna apertures, which if implemented well, should exhibit low coupling from the surrounding skin. To date there is no open research covering indirect front-door coupling.

Back-door coupling arises when microwave radiation enters via apertures which are not intended for the reception or transmission of microwave radiation. Such apertures typically include cooling grilles, inlet and exhaust ducts, gaps between panels, drain holes, and any other discontinuities which exist in the skin of the target, and which provide a path into internal cavities. From a physics perspective, such apertures (if of a size comparable to or greater than the wavelength of the impinging X-band radiation) will behave as slot radiators, with a gain dependent on the geometry and impedance matching between the exterior and the internal cavity. Such apertures may be approximated as slot radiators in an infinite sheet, where the orientation of the aperture and relative polarisation of the impinging radiation determine the coupling mode [30].

Indirect back-door coupling effects are analogous to indirect front-door coupling effects, and arise where surface travelling waves might couple into an aperture. This may arise where the orientation of the aperture permits a circumferential current to flow through the skin surrounding the aperture [35].

Analytical solution of both front-door and back-door coupling effects can be unusually difficult, due to the complex geometries involved and difficulty in estimating coupling efficiency of most apertures, especially those not intended for X-band signal reception.

Once power has entered the cavity behind an aperture, it can couple and propagate into any electronic hardware which may be located in close proximity, to produce disruptive or damaging effects. Cables, where these have suitably low-loss behaviour, can also propagate power deeper into the target, resulting in further damage or disruption.

Considerable research has been recently performed in modelling the behaviour of cavities containing complex internal structures. This is representative of computer equipment, but also other electronic equipment, in a typical back-door coupling situation. The approach which has yielded best results to date is that of modelling the cavity as a chaotic scattering cavity, where power entering the cavity experiences multiple internal bounces and resonances, which are a function of the internal geometry of objects inside the cavity. To date published work covers both single-port and multiport cavities. The former represent coupling into equipment, the latter into equipment or airframe cavities from whence power can propagate further into the target [15,16,17,18,19,46,47].

From a coupling perspective, there is clearly considerable merit in using circular polarisation at the radar antenna, where the opportunity exists to do so, as this maximises the statistical opportunities for coupling. A further advantage can be realised by chirping the radar waveform over at least one octave, again to maximise coupling opportunities [25].

Damage and disruption effects have been well researched, recently. Disruptive effects arise when the microwave radiation produces unwanted voltages on analogue and digital signal lines, and state changes in digital logic have been observed. Typically, chaotic voltages are superimposed on the signals flowing through circuits, as a result of non-linear interactions between semiconductor components and the microwave signal. Damage effects arise due to dielectric breakdowns and junction breakdowns, where the microwave radiation forces voltages above safe limits for specific semiconductor devices [40].

Some empirical work has been performed which identifies lethal field strengths. Commercial electronics are destroyed by exposure to magnitudes between 3–10 kV/m [22].

Figure 6 plots the achievable field strength in V/m versus distance, for representative AESA and ESA configurations. Lethal effect can only be produced at very short distances, but disruptive effect is feasible at distances comparable to those for gun engagements, of the order of 200–300 m for a field strength of 250 V/m, refer Figure 6. Hardware which is susceptible to lower field strengths would extend effective range further.

Military targets, such as combat aircraft or missiles in flight, contain also electro-explosive initiators for warheads, and often other pyrotechnics for emergency functions. These can also be vulnerable to microwave radiation. There is a large pool of extant literature and standards dealing with hazards of electromagnetic radiation to ordnance (HERO) and to fuel (HERF), which identifies unsafe power levels [7,23]. Unsafe power levels at X-band are of the order of 60–600 V/m (1–10 mW/cm²) for ordnance, and 50 kV/m (5 W/cm²) for exposed aviation fuel. There is no known research to date on microwave induced ignition in fuel ullage spaces, should these be penetrated, but the power demands suggest this is the domain of HPM devices rather than AESAs or ESAs.

The model in Figure 6 indicates that electro-explosively initiated pyrotechnics are an attractive target for microwave attack by AESA or ESA. Assuming a 600 V/m field strength, effect could be produced at ranges of 100–150 m. If we assume a 60 V/m lethal field strength, ranges extend to the order of 1 km.

In practical terms, should a disruptive field strength be achieved and coupled into onboard electronics, complex airborne targets such are aircraft are apt to experience transient failures in mission computers, stores management computers, embedded computers in subsystems (which may include engines and flight control systems) and various other items of mission avionics, including radar, electronic warfare, and communications equipments. These have the potential to render the target unflyable or non-mission capable. Less complex targets such as cruise missiles and surface to air missiles may experience loss of control, or loss of navigational position. A disruptive field strength thus may be adequate to destroy the target.

Lethal field strengths, capable of inflicting permanent damage on avionics, initiating pyrotechnic devices, or even igniting fuel vapours in ullage spaces, would likely result in the immediate destruction of the target, or rendering it incapable of performing its mission until lengthy repairs are performed.

In summary, a significant number of constraints exist in attempting to inflict disruptive effects or lethal effects using a high power AESA or ESA radar on an airborne, or other, target. For characteristic fighter radar performance, useful effect is produced at ranges comparable to those seen in gun engagements.

Engagement cycles and radar adaptations

Engagement cycles are a characteristic feature of all weapons, and AESA or ESA radars adapted as microwave weapons are no exception.

A typical engagement cycle involves the initial detection of a target, tracking of the target, identification, firing of the weapon, and kill assessment—see Figure 7.

X-band AESA and ESA radars are used primarily as acquisition, tracking, and fire control sensors in air combat. In this regime, these devices may be cued to a target by an external information source, such as a ground-based search radar, or an airborne early warning radar, or they may acquire the target by using one of several search modes, examples including velocity-search, range-while-search, or track-while-scan [31].

These radar modes are typically optimized for long- or medium-range missile engagements, and will typically be designed to gather target position and velocity information with accuracy commensurate with that required to support the kinematics of a missile shot.

Once the target is within the kinematic envelope of the missile, one or more missiles may be fired, and these will be provided with mid-course datalink updates, transmitted by the radar, until the missiles’ guidance seekers can acquire the target and initiate terminal homing.

If the initial engagement has failed, and the aircraft closes with the target, short range radar modes will be invoked to permit engagement with short range missiles, typically infrared or optically guided, or an onboard gun. Radar modes for such close in engagements are yet again optimized around the gathering of kinematic data, to permit calculation of engagement envelopes for weapon shots. In particular, range and closure rates are critical given the limited kinematic envelope of short-range missiles and guns. Given the potentially high closure rates, update rates for position measurements may be high.

In an electronic countermeasures environment, it is likely that the opposing aircraft will actively jam the radar to deny accurate range and velocity information. The effectiveness of such short-range jamming will depend on the X-band radar cross section (RCS) of the target, jammer-emitted power, and the power-aperture performance of the radar being jammed. In many instances, burn-through will occur [36,43].

In short-range engagements it is not uncommon for the radar to employ monopulse angle tracking techniques, due to their high resistance to amplitude based jamming techniques, and difficulty in implementing wavefront angle jamming techniques. A typical mode would be a dual-plane monopulse angle track mode, with a high PRF to permit a high update rate [38,42].

This characteristic engagement cycle for kinematic weapons is not ideal for a microwave weapon, and will require some adaptation.

The effective range at which a microwave weapon can achieve a lethal or disruptive effect upon the electronics of a target will depend on emitted power levels, distance between the target and emitting radar, propagation losses, target coupling characteristics, and the electromagnetic hardness of the target electronics, ordnance, and fuel systems. The range at which a viable effect on the target may be produced could easily vary by orders of magnitude, and not be known a priori.

Calculating or otherwise modelling the complete power transfer relationship between the emitting AESA and the vulnerable components of a target will present genuine difficulties, due to the complexity of the propagation and especially coupling and damage-effect mechanisms. Variations in build quality between targets may also impact lethal or disruptive ranges. It is therefore expected that empirical measurement will be required to establish both ranges, for specific target types. From an order-of-magnitude perspective, the range can be estimated as comparable to gun engagement ranges. From an operational perspective this presents some interesting constraints. It is clear that additional radar modes will be required to support engagements using the AESA or ESA as a microwave weapon.

As the AESA and ESA can effect engagements at the speed of light, kinematics of the target are only of interest in terms of range to the target being within an expected or estimated lethal or disruptive level. The target must however be tracked, and dwell time for the engagement budgeted for.

This indicates that a specific mode for ‘microwave attack’ will be required, which will combine a single target track precision tracking regime, interleaved with maximum power emission bursts to effect disruption or damage, the latter keyed by pilot or operator trigger. In operation, this mode would initiate a fine track, preferably using a dual-plane monopulse mode, and continuously measure range to the target, and propagation losses between the AESA or ESA and target. The latter can be estimated by measuring amplitude of the return and comparing it against known RCS performance for the target and its aspect.

Once disruptive range is achieved, an audible or visual cue similar to existing schemes for short-range missiles would be activated to alert the pilot or operator. The cue would then change, as the range to target decreases and lethal range is approached. The pilot or operator would then initiate firing with a trigger, upon which the AESA or ESA would cycle between illuminating the target at maximum power levels, and continuing its tracking. The illumination mode would be chirped, and if the radar has capability for circular polarization, it would be used.

Further observations apply. The first is that uncertainties in target vulnerability dictate that microwave attack should not be attempted by an aircraft which lacks the capability to defeat the opponent in a short-range engagement as, should the microwave attack fail, the target could engage and destroy the attacker.

The other is that the potential of fighter AESAs or ESAs to be operated for very short periods at peak power ratings exceeding acceptable limits for conventional radar modes should be explored. If the current density limits of the radars modules, or TWT power ratings, permit higher peak powers for very short durations, these would increase the useful disruptive and lethal ranges.

Kill assessment will present no difficulty, where the target experiences loss of control, or ignition of fuel or ordnance is achieved. Disruptive or avionic damage to mission systems may be more difficult to determine, and may only be discernable by the kinematic behaviour of the target, such as a major change in direction, or cessation of radio-frequency emissions.

Conclusion

This paper has explored a range of issues arising in the application of AESA and ESA X-band radars as DEW.

The application of circular polarisation at the radar antenna, a technique shown to be useful for HPM weapons, will maximise statistical opportunities for coupling power into targets. Chirping the radar waveform over at least one octave also maximises coupling opportunities.

It will be necessary to design radar modes specifically for the purpose of microwave attack, to provide the capability to interleave operator-controlled high-power emissions with necessary tracking waveforms.

Order-of-magnitude estimates of achievable effect using a representative radar configuration indicate that a microwave attack regime will produce useful effect only at ranges comparable to those found in gun engagements.

Future research remains to be performed in exploring indirect coupling mechanisms, where the skin of the target contributes to the capture and coupling of incident power.

References

Author

Dr Carlo Kopp graduated in 1984, from the University of Western Australia, in Electrical Engineering with First Class Honours. After more than a decade in communications and computer industry engineering positions he completed a research Masters in Computer Science in 1996, and a PhD in 1999, dealing with long-range microwave data links and airborne ad hoc digital networks. Both degrees were completed at Monash University in Melbourne, where he currently lectures graduate students in digital communications. His current research activities at Monash are centred in ad hoc networking, and evolutionary impact of information warfare. His other research interests include military strategy, doctrine and the fundamentals of information warfare. His work in this area has been published by the Royal Australian and the United States Air Forces. Kopp is a Research Fellow in Regional Military Strategy at the Monash Asia Institute since 2005. He is a senior member of the AIAA, and a member of the IEEE, and AOC professional societies, and maintains his registration as a professional engineer. Email: carlo@csse.monash.edu.au, Ph: +61-3-9905-5229.