Radio Frequency Weapons - 21st Century Threat Live Fire Testing of Radio Frequency Weapons Aircraft Survivability, Summer 1998 W. MARK HENDERSON Naval Air Warfare Center, Weapons Division Electronic Systems Engineer, HPTE Test Director DAVID A. SCHRINER Schriner Engineering Chief Scientist, Directed Energy Technologies Background The U.S. Navy is concerned about the Electromagnetic Environment it must operate in. This includes both the known and evolving electromagnetic environments. The harshest known environments are created by the Navy's aircraft carrier deck, hostile termination end game and some aircraft radars. Military systems that operate within these environments are required to undergo an Electromagnetic Environmental Effects (E3) test process. E3 deals with the interaction between a system (weapon system in our case) and the in-service operational environment produced by those systems necessary to carry out the total mission. A charter for this work is currently shared by the Naval Surface Warfare Center, Dahlgren, the Naval Air Warfare Center Aircraft Division, Patuxent River and the Naval Air Warfare Center Weapons Division (NAWCWD), China Lake/Pt. Mugu. The Naval Medical Research Institute (NMRI) detachment at Brooks Air Force Base has the Department of Defense (DoD) lead for RF Human Effects also known as hazards of electromagnetic radiation to personnel (HERP). Another type of testing required for both fielded and new weapons systems in the acquisition process is Joint Live Fire Test and Evaluation (JLT&E) and Live Fire Test & Evaluation (LFT&E). JLFT&E is chartered by congress and LFT&E is a congressionally mandated requirement. The LFT&E legislation requires "...realistic survivability testing ... by firing munitions likely to be encountered in combat at the system configured for combat..." "...with the primary emphasis on testing vulnerability with respect to potential user casualties..." Title X U.S. Code Section 2366 The term "munition" includes the category known as Directed Energy. U.S. weapon systems are becoming more complex with the integration of advance computer based technologies. This provides the war fighter with a technological advantage over our adversaries, but there are trade-offs for this. The use of computer-based light weight systems must be protected from the threat of "soft" or partial kills. High Power Transient Electromagnetics (HPTE) is a revolutionary threat going through a evolution even as this article is being written. The subject of much controversy over the past several years, the remainder of this article will focus on this new Directed Energy Weapon or as the title states Radio Frequency Weapons. The Technology There are two basic types of High Power Microwave (HPM) which are narrow-band and transient wave, also known as ultra-wide-band. Narrow-band devices generate a radio frequency (RF) signal made up of sine waves either in a group or "pulse" of a few to many or, in some cases, continuous waves (CW). Because of this wave shape they will appear on a spectrum analyzer as a small width, hence the name "narrow-band. Transient wave signals do not radiate sine waves but rather an electromagnetic spike-like waveform of very narrow width. A Fourier transform of the waveform will show it to occupy a rather large instantaneous spectral bandwidth. For this reason it is generally referred to as an ultra-wide-band (UWB) signal, as is a swept CW signal of the same bandwidth. To avoid confusion in the nomenclature, such waveforms can be referred to as transient wave signals. Much information exists in both the classified and open literature concerning narrow band HPM systems and their use in vulnerability and susceptibility measurements and effects. Development and testing has been going on for many years. Several novel types of RF generators have been developed specifically for this type of RF weapon. The types of signals generated lend themselves well to conventional RF modeling and analysis approaches. Many types of antennas can be used to radiate this type of signal effectively. Examples include dish types, horns, dipoles, phased arrays, and several in the class of frequency dispursive designs due to the sine wave nature of the generated signal. Transient wave HPM systems are quite a different case. Conventional modeling and analysis techniques are difficult if not impossible to use because they cannot accommodate the UWB waveform. Only a single class of antenna can be used to radiate such signals and the effects of multi-path are quite different than for narrow band systems. Waveforms of this type applied to HPM weapons work is very new, thus there is little information in the literature. Because of this fact it is worthwhile to list a few of the peculiar characteristics of this type of waveform. 1. The radiated waveform, Figure I, is normally generated by applying a fast step up voltage to a Transient Electromagnetic (TEM) horn antenna. The horn will differentiate the applied waveform and generate the radiated signal. These applied step waveforms can be generated by switch closures, spark-gap closures, or solid state device switching. Rise times ranging from 50 to a few hundred pico seconds are typically generated with voltage swings from a few hundred volts for mechanical switches, to tens of thousand volts with comparable rise times for solid state switches, and voltage swings greater than a mega Volt with slightly longer rise times with spark-gap switches. Generally such voltage swings are produced from sources of low impedance ranging from parts of an Ohm for some solid state switches to the low tens of Ohms for spark-gap switches. 2. The extremely fast voltage swings into horn antennas produce rather high peak powers and radiated field strengths. A mega Volt per meter (or more) field strength at the antenna mouth can be obtained. Peak radiated powers in the multi Terra Watt levels could be achieved with repetition rates in the 1000 PPS range. These systems are generally specified with a figure of merit related to this field strength. A system with a figure of merit of 200 KV/m will produce field strengths of 20 KV/m at ten meters and 2 KV/m at 100 meters. 3. Antennas other than a horn will produce a radiated signal that is representative of their impulse response. A dipole, for example, will radiate a damped sine wave of many cycles while a log-periodic dipole array will radiate a chirped sine wave relative to its bandwidth. A helical antenna will radiate a sine wave of rotating polarization. 4. When transient wave signals are received by any type of antenna besides a TEM type, the output waveform will also be determined by the impulse characteristics of the receiving antenna and will result in a time-distortion of the received waveform. If, for example, a radio receiver antenna on an aircraft were illuminated by a transient wave signal, the antenna output would likely be a signal easily processed by the receiver regardless of where it was tuned. The receiver output would likely be a large audio pulse and a train of such signals would make the radio unusable. Transient wave signals make very good jamming waveforms since they occupy the entire bandwidth of the targeted receiver and their peak power is such that receiver circuit recovery (after initial receipt) further hampers receiver operation. The Threat Recent discovery of a new electromagnetic environmental condition has matured to the point that justifies its inclusion in the Live Fire Test and Evaluation process. It is called Transient Electromagnetic Device (TED) technology. Demonstrated peak power levels up to hundreds of Tera Watts are easily obtained at repetition rates up to 1000 pulses per second. Because of these extremely high physical parameters, testing fully operational systems against this threat must be done at an open air test range that provides containment of this type of directed energy. The Etcheron Valley Junction Ranch test range, located at the NAWCWD, China Lake, was selected to conduct the Live Fire Testing of Radio Frequency Weapons. The Junction Ranch facility provided the following inclusively unique parameters; 1. Low backscatter test environment. 2. Remote location from population centers. 3. Extensive controlled clear airspace. 4. Minimum of spurious electromagnetic radiation interference. 5. Infrastructure to support tests. 6. On-station technical expertise. 7. Physical security. 8. Scheduling efficiency. Preliminary testing has shown that some military weapon systems and commercial infrastructure platforms are highly susceptible to this new waveform. The primary application for TED is weaponization. This new environmental threat is not listed in the Strategic Threat Assessment Report (STAR) for new and currently deployed military systems; however, the authors and technical community expect to see it available for use in the field of battle against high technology assets within the next 2 to 3 years. Figure I Radiated Transient Electromagnetic Waveform Figure II AH-1S Test Bed The Test NAWCWD, in anticipation of the expected exposure to this threat recently participated in the first Directed Energy Joint Live Fire Test (JLFT) using both conventional and TED High Power Microwave (HPM). The test was a cooperative effort where DoD and Department of Energy laboratories (NAWCWPNS, China Lake, Air Force Research Lab, Army Research Lab, and Lawrence Livermore National Lab). The purpose was to develop and demonstrate the methodology required to perform RFW survivability testing for the Live Fire Test and Evaluation Office. The H5 high power TED was provided by the Air Force Research Laboratory (AFRL), Kirtland AFB. The H5 is one of a series of Hydrogen spark gap switch sources developed at AFRL. The RF energy radiated by this device is an E field with an ultra wide spectral bandwidth. The HPM device was provided by the Army Research Laboratory (ARL), Adelphi, MD. The HPM device provided was an L band source where the radiated RF energy is an E field with an narrow spectral bandwidth. The average power of the radiated waveform was much greater than that of the HPTED source. Lawrence Livermore National Laboratory (LLNL) was responsible for taking measurements of the incident (external) electric field power density on AH-1S during the Joint RF Live Fire Demonstration at China Lake allowing a mapping of threat, distance, and effect. Conclusion Transient wave sources and weapons are a new technology that is not well known by many in the HPM community who have been working with narrow band systems for years. The effects that were observed during this test series were supportive of the statement that "complexity equates to vulnerability". There is no data available concerning target vulnerability except that which various modeling efforts have produced. The complexity of the signal's propagation and inter-relationship with mechanical structures (both targets and antennas) makes it very difficult to model. For this reason, there is doubt regarding the utility of modeling results, particularly with regard to the operational engagement related issues for each system. Open-air testing of military systems against likely RFW threats should be used to determine problem areas. The need to test strike asset platforms first is critical. The type of test performed at China Lake for the LFT&E Office last year is the best way to resolve this issue. When problems are identified it is best to do so in a non-hostile environment where we can fix them easily. Figure III H5 High Power Transient Electromagnetic Device (HPTED) Figure IV High Power Microwave Device Figure V LLNL Instrumentation Hardware