Directed-energy weapons (DEWs) show great promise for the U.S. Warfighter in that they are a speed-of-light, all-weather weapon that can generate a relatively unlimited number of low-cost shots and produce scalable target effects that range from temporary to permanent depending upon the target and the separation distance (range). There are three main types of DEWs: high-energy laser, high-energy particle beam, and high-power radio frequency (RF)/microwave (HPM) weapons. This article provides a basic introduction to HPM DEWs and their effects on electronic targets, including why HPM DEWs are important to the Warfighter and how they are like and unlike traditional electronic warfare/electronic attack (EW/EA) and electromagnetic pulse (EMP) weapons. Also discussed is how HPM energy couples into an electronic target and produces effects that range from temporary upset to permanent damage. Finally, ways to estimate/measure HPM effect levels on systems as well as to harden a system to mitigate the effects are presented.
HPM DEWs DEFINED
HPM DEWs are electromagnetic (EM) sources that can generate and direct RF/microwave pulses at a target. Typically, these weapons have peak effective radiated power of >100 MW, or >1 J per pulse. HPM DEWs can radiate energy at frequencies ranging from high-frequency to microwaves/millimeters. They can couple RF energy into a target via intentional and unintentional antennas (i.e., front doors and back doors, respectively) and produce long-term effects that last long after the HPM is gone. HPM DEWs are also known as RF weapons, EM weapons, and non-nuclear-generated EMPs.
THE IMPORTANCE OF HPM DEWs TO THE WARFIGHTER
HPM DEWs are of interest to the Warfighter because they provide the potential for “speed-of-light” engagements of multiple targets, with instantaneous fly-out times and no lead angle required. However, it must be understood that even though DEWs can engage targets at light speeds, the effects on the target are typically not instantaneous and require some dwell time on the target.
Because DEWs typically use fuels to generate energy pulses, they also represent a weapon with “deep magazines,” which can produce a relatively unlimited number of shots without needing to reload ammunition. This capability represents reduced logistics and the associated cost. DEWs can also produce “scalable effects,” ranging from temporary to permanent based on the target’s vulnerability level and the separation distance between the DEW and the target (i.e., range).
One advantage that HPM DEWs have over kinetic energy weapons (and lasers) is that they have wide beams that can cover large target areas and therefore produce a high probability of target hit (although it should be noted that the probability of target kill ultimately depends upon how much energy can be coupled into the target’s electronics and their failure levels).
THE HPM DEW SYSTEM
Figure 1 shows a block diagram of an HPM DEW and its major subsystems. On the far left is the prime power source to provide the power/energy needed to produce the HPM pulses. The prime power can be provided by an electrical generator or, in some cases, a battery pack. It can also be provided by explosives that convert the explosive energy into electrical energy. However, explosively driven sources tend to be less efficient than traditional generators.
Next to the prime power source is the pulse power conditioning section, which transforms the prime power into electrical pulses. This action may be accomplished in several ways. Typically, some form of pulse modulator, consisting of pulse-forming networks and high-power switches, converts the prime power into the electrical pulses with specific pulse durations or “pulse widths” and pulse repetition frequencies.
The RF source (shown in the middle of Figure 1) converts the electrical pulses from the modulator into RF energy. Here, the source can be one that generates pulse-modulated sinewaves (or a so-called “narrowband source”) or one that generates transient electrical pulses (or a “wide-band” source).
For narrowband sources, high-power tubes, such as magnetrons, or high-power amplifiers, such as klystrons or traveling wave tubes, are typically used. These sources use the high-power pulses from the modulator to produce high-power RF pulses. The magnetron is known as a power oscillator, and the klystron and traveling wave tube are known as power amplifiers. For the wide band sources, high-power switches and spark gaps are typically energized and then switched to produce a transient pulse. Ferrite lines can also be used to sharpen the pulses and increase the bandwidth of the transmitted signal.
Next, the antenna radiates the RF energy into space toward the target of interest. For narrowband sources, aperture-type antennas, such as horns and/or large parabolic reflectors, are typically used. These types of antennas can handle the high-power output of the RF source and provide directional gain for the energy transmitted toward the target. For the wideband sources, dipoles and transverse electromagnetic (TEM) horns are typically used. Because wideband signals have lower frequency content, their antennas tend to have less gain and directivity.
Next, the RF energy propagates through the atmosphere to the target. The energy radiated by the antenna spreads out in range and decreases as one over the range squared. The RF energy is also attenuated by the atmosphere and weather. However, one of the advantages of RF weapons over lasers is that they have a high probability of hitting the target because they have a much wider beam. Furthermore, they are typically not affected by weather unless they are radiating energy that is higher in frequency than 10 GHz (i.e., x band) and there is heavy rain or snow.
Lastly, on the far right of the figure, the target itself is represented by RF ports of entry to critical components. When the HPM DEW illuminates the target, the RF energy couples from the outside of the target interior via intentional antennas (often called “front doors”) and unintentional antennas, such as cracks, seams and cables, (often called “back doors”). When the RF reaches the internal components, the pulse modulation can be rectified or stripped off by the semiconductor junctions, producing a modulated signal that can interfere with the target’s operation and cause temporary interference or upset. If the energy is high, then it can overpower the semiconductors and produce permanent damage.
Figure 2 shows some examples of HPM DEW systems (although they are technology demonstrators and not fielded systems). Each picture shows the location of the generator or prime power supply, P, the RF transmitter, T, and the antenna, A. The system on the far left is a technology demonstrator designed and built by the Army Research Laboratory (ARL) for the Joint Non- Lethal Weapons Directorate (JNLWD) to demonstrate that commercial vehicles can be radiated by HPM pulses at tactically significant ranges to produce an engine stall. The center photo is another technology demonstrator designed and built by ARL for the Naval Surface Warfare Center Dahlgren Division (NSWC-DD) to demonstrate that an HPM source could be installed in the back of a small truck and used for EA on computers in buildings. The photo on the far right shows a wideband RF system developed by the Air Force Research Laboratory (AFRL) using its impulse-radiating antenna for EA experiments.
DEVELOPING HPM DEW SYSTEMS
Figure 3 shows a flow chart for developing HPM DEWs. Starting with the left-most block, the targets of interest and the desired engagement ranges are identified. This identification is often performed in collaboration with the Warfighter and intelligence communities, as they know the targets of interest and target details. During this process, it is important to try to identify the critical components of the target and RF ports of entry that lead to these components.
Warfighter and intelligence communities, as they know the targets of interest and target details. During this process, it is important to try to identify the critical components of the target and RF ports of entry that lead to these components.
Next is the estimation of the RF power density required on the target to produce an effect, S, or the “target vulnerability level.” The target’s RF effect level is a combination of how much power is required to affect the critical components and the effective coupling areas of the components. It must be understood that these are only target effect estimates and must be verified by target vulnerability tests in which the entire target is instrumented and irradiated at increasing levels of HPM energy to see if there is an effect (and if so, at what level).
Once the target vulnerability level and desired engagement range, R, are known, the one-way radar equation can be used to calculate the effective radiated power required for an HPM weapon to irradiate a target with the appropriate power density .
Next, the size of the HPM platform is considered, and the largest antenna that will fit is determined. Based on the antenna’s size and efficiency, the maximum gain available can be determined. By dividing the source’s effective radiated power by the gain of the antenna, G, the transmitted power, P, required of the HPM source can be obtained. Once the required transmitter power and antenna gain are known, a commercial technology survey to find appropriate subsystems can be conducted. If the subsystems are not available commercially, then they must be developed.
Finally, the subsystems to produce an HPM demonstrator are integrated, and tests are conducted against targets of interest to determine its effectiveness.
APPLICATIONS OF HPM DEWs
HPM DEWs can be used to attack all forms of electronic weapons, sensors, and communication systems, not just RF receivers. The effects are often subtle and difficult to diagnose quickly, thus providing plausible deniability of an EA. The operation and maintenance of HPM DEWs are like those of radar systems and therefore should not require personnel to develop new military occupational specialties.
HPM DEWs also represent non-lethal weapons to humans that typically will not cause permanent damage for short dwell times. An important exception is an HPM system that produces millimeter-wave energy and can cause temporary pain by stimulating the nerves in the skin. This system is called the Active Denial System because it non-lethally denies access to controlled places. The system was developed by AFRL and its contractors for JNLWD. The system produces an effect only while the millimeter waves are illuminating a human. There is a large safety margin between temporary pain and permanent damage. The Active Denial System has been tested several times over the years by AFRL and JNWLD and has met all safety, legal, and treaty requirements.
Finally, it should be mentioned that shielding against the effects of HPM (i.e., countermeasures) is theoretically possible with metal wrap (i.e., a Faraday shield); however, this shielding may be difficult in practice since the energy coupling to the target’s electronics can increase depending upon the type and placement of the shielding material.
Figure 4 illustrates some DEW applications. The red lines represent lasers, and the curved lines represent HPMs. Both airborne and ground-based DEWs are considered. One of the major applications for DEWs is counter air (i.e., air defense) and counter command, control, communications, computers, and intelligence. Rockets, artillery, mortars, missiles, and unmanned air systems are major threats to the Services that can possibly be handled by kinetic energy weapons, but at great cost. Because DEWs produce a relatively unlimited number of low-cost shots (i.e., energy pulses), it is hoped that they could be a more economic and effective means for countering these types of threats.
Also shown is the use of HPMs for countermines/improvised explosive devices, which represent a serious threat to U.S. forces and our allies. Because the power on target from an HPM DEW is greater for short ranges, these types of targets may be well suited for defeat by HPMs.
RELATING HPM TO EW AND NUCLEAR-GENERATED EMP
Figure 5 illustrates EW’s three main pillars: Electronic Protect (previously known as electronic counter-countermeasures), Electronic Support (previously known as electronic support measures), and EA (previously known as electronic countermeasures). Under EA (on the right side) is traditional EA with jammers that can attack only targets with RF receivers and produce temporary interference while the RF is on. On the left side, HPM DEWs can attack targets with and without RF receivers and produce long-term effects. Thus, HPM represents an “unconventional EA” (UEA) capability that can address classes of targets vs. a single RF receiver.
Figure 6 shows the relationship between traditional EA and HPM DEWs in another way. If one plots target knowledge vs. power required for effect, it can be seen that jammers can use little power if one knows the target receiver’s operating frequency and modulation. However, the jamming effect is only when the RF is on and is temporary. On the other side of the curve, it can be seen that a high-power single pulse can be used to produce permanent damage in an electronic target. However, it may take hundreds to thousands of watts per square centimeter to produce the effects. If a repetitive HPM pulse is used, the power required can be reduced to some degree; but a lot of power is still required. The middle of the curve appears to be the most promising area for EA since it requires less target knowledge and uses moderate power to produce long-term upset.
An HPM DEW is also similar to a nuclear-generated EMP, but different in terms of the frequency range and other parameters. Both EMP and HPM involve EM energy coupling from the outside of a target to sensitive interior components. Figure 7 shows the frequency range for EMP vs. HPM. Note that the frequency content of EMP is much lower than HPM and has much longer wavelengths. Table 1 compares the typical frequencies and characteristic wavelengths for EMP and wideband and narrowband RF.
|TYPICAL FREQUENCIES||CHARACTERISTIC LENGTH|
|Nuclear EMP||DC to 100 MHz||3 m or more|
|Wideband RF||~30 MHz to ~3 GHz||~10 cm to ~10 m|
|Narrowband HPM||~1 GHz and up||up to 30 cm|
Another major difference for an EMP is that it is a well-defined waveform with known field strengths. HPM is not as well defined and can span a large range of frequencies, pulse widths, and pulse repetition frequencies. Both EMP and HPM require complex coupling codes and testing to determine the RF coupling to the component (i.e., the “stress”) and the component’s failure level (i.e., the “strength”). For both, the stresses and strengths are best represented by statistical quantities, resulting in a probability of effect vs. an effect threshold or level. Today, it is possible to use a source that will generate large EMP pulses with amplitudes greater than tens of kilovolts/meter without a nuclear blast. Therefore, an HPM DEW is sometimes called a “non-nuclear EMP.”
As previously mentioned, HPM can couple into a target’s electronics through intentional antenna or “front doors” and through unintentional antennas or “back doors” (as shown in Figure 8). When the HPM enters through the front door, often the entry path is the normal signal path to the first sensitive component. If the HPM is in band to the receiver, then it gets amplified by the target’s antenna gain and experiences low path loss. If the HPM is out of the band, then it is attenuated by the lower antenna gain and higher path loss. For back door HPM, the energy is coupled into the circuit boards and components by reradiated energy inside the target. Front door paths lend themselves to more accurate predictions of HPM levels because one typically knows more about the path to the component and the losses.
HPM COUPLING INTO TARGETS AND THE RELATED EFFECTS
Once the HPM reaches a critical component, if the stress is greater than the component’s strength, then the component can fail (as shown in Figure 9). The diagram shows each of the key parameters in an EA scenario and the difference between jamming and HPM effects. If the component is critical to the target’s operation, the component failure can lead to system failure. Component failure can occur if the RF power at the component is greater than the semiconductor’s junction failure level. In some cases, over-voltages in which the electrical current punctures the semiconductor device can occur. Because both the HPM energy coupled to a component and the component’s failure level are statistical in nature, the failure level is best described in terms of a probability of failure.
Figure 10 shows a scale of HPM effects and the associated definitions. This scale has been proposed to try to standardize the meaning of effect levels throughout organizations doing HPM tests. The scale includes temporary effects, such as interference and upset, all the way to permanent damage.
HPM MODELING AND SIMULATION (M&S)
M&S is extremely important in predicting/estimating HPM effectiveness against electronic targets. It can also be used in performing tradeoffs to optimize HPM DEWs and their effects. M&S tools are also useful for conducting sensitivity studies to identify critical parts of a problem and areas where experiments are needed. Figure 11 shows the M&S structure for HPM DEWs.
The base of the pyramid represents the underlying physics and engineering models that are used to determine the HPM coupling inside a target. The next level up is “one-on-one engagement models,” which estimate the probability of failure of a target as a function of the incident HPM energy and range. These models include AFRL’s Radio Frequency Propagation and Target Effects Code (RFPROTEC) and ARL’s Directed RF Energy Assessment Model (DREAM). In addition, the Directed Energy Panel for the Joint Munitions and Effectiveness Manual has recently developed the Joint RF Effectiveness Model (JREM), which is a combination of the best attributes of RFPROTEC (realistic RF generation and propagation models) and DREAM (target vulnerability model). The model manager for RFPROTEC, JREM, and now DREAM is the AFRL DE Directorate at Kirtland AFB, NM.
The next level up in the pyramid comprises the mission models, or “few-on-few models,” such as Suppressor and the Extended Air Defense Simulation (EADSIM). These models use the probability of target failures from JREM or other models to determine measures of effectiveness, such as probability of mission success and loss exchange ratios.
At the top of the pyramid are the campaign, or “force-on-force,” models, such as the Army’s Combined Arms and Support Task Force Evaluation Model (CASTFOREM) and the Air Force’s Thunder model. These models can be used to look at the effectiveness of HPM DEWs on the battlefield. Typically, the results of each level are aggregated and passed up the pyramid as inputs to the next level.
PROTECTING SYSTEMS AGAINST HPM DEWs
To protect our systems against an adversary’s HPM DEW, a combination of robust components, filters, and limiters must be used to reduce the amount of energy that gets to the component. New semiconductor technologies, such as silicon carbide and gallium nitride, can handle higher junction temperatures and thus show promise of being more robust to HPM pulses. In addition, the filters reduce the out-of-band energy, and limiters reduce in-band high-power pulses. For back door entry paths, grounding, bonding, and shielding appears to be the most practical solution.
In 1992, the U.S. Army Harry Diamond Laboratories (which is now part of ARL) developed a “High-Power-Microwave Hardening Design Guide for Systems .” The objective of the guide was to help system program managers and offices better understand HPM threats and how to mitigate them. The document consists of four volumes and is accessible through the Defense Technology Information Center.
Note that hardening against HPM energy is theoretically easy but may be difficult in practice due to changes in coupling. HPM effects on a target may also be subtle and difficult to determine unless there is some way of monitoring the target’s behavior. That said, it is much cheaper to build in the hardening at the design stage of a system (estimated to be about 1 to 15% of the systems cost based on EMP hardening studies) as opposed to doing retrofit hardening after the system is built.
Although HPM DEWs used to be thought of as a weapon of the future, with all the recent advances in technology and engineering, the future is now. These weapons offer the potential for speed-of-light engagements of multiple targets in all-weather with a high probability of hit, and they can produce scalable target effects from temporary to permanent. In addition, they can provide a relatively unlimited number of low-cost shots that are limited only by their fuel supply. And because HPM DEWs can attack targets with and without antennas and produce effects that last long after the energy is gone (dependent on the dwell time), they represent a unique UEA capability.