Listening for Danger Signals
An SwRI-developed system helps warfighters detect and locate the distinctive radio signature of a fired weapon
By Thomas C. Untermeyer
Since the days of the slingshot and the arrow, military officials responsible for the safety of warfighters in the field have sought new ways to detect incoming weapons, and trace their origin, as soon as possible after their firing or launching.
On the modern battlefield, systems based on optical, infrared (IR) and acoustic technologies have been developed to detect the firing of a variety of weapons. However, each of those technologies has limitations to its operational performance. Optical and IR detection methods do not work well during obscured environmental conditions such as fog, rain, clouds, smoke or dust. Acoustic systems, meanwhile, are hampered by limited range and relatively slow response time.
Since the 1950s, the open literature has reported the possible generation of distinctive radio frequency (RF) emissions associated with the launching of a variety of weapons. Passive RF detection of weapon launches could provide a benefit over optical, IR and acoustic systems by providing fast detection through obscured environments over extended ranges. Consequently, in 2003 a team of engineers from Southwest Research Institute (SwRI) carried out an internally funded research program to investigate the generation of RF signals during the firing of small arms. Electrical engineers worked with ballistics engineers and technicians to equip SwRI's enclosed range for small-arms ballistic testing with the necessary RF test equipment and high-speed video cameras to collect RF and video data during the firing of multiple pistols and rifles. The SwRI team then presented its findings to various government organizations to determine further interest.
Based on these findings, the U.S. Army Space and Missile Defense Command (USASMDC) established and funded the All Weather RF Launch Detection (AWRFLD) program starting in 2005 under the direction of a commercial client, which subsequently issued task orders to SwRI to develop and deploy sensors to measure RF emissions generated by firing small arms, rocket-propelled grenade (RPG) launchers, mortars, artillery and rockets.
In the ensuing four years, the multidisciplinary makeup of SwRI allowed the engineering team to add expertise in chemical engineering, microwave engineering and microwave component fabrication. Chemical engineers used their laboratory equipment to analyze gun- powder and weapon propellants for use in model development. Microwave engineers designed custom circuits that allowed miniaturization and cost reduction of the passive RF sensor prototypes. The Institute's unique microwave fabrication facilities allowed the custom assembly of microwave components using wire and ribbon bonding techniques. The project team has supported data collection trips to Redstone Arsenal, White Sands Missile Range, Fort Sill and Yuma Proving Ground. During these trips, the team designed the test setups, acquired the necessary sensors and test equipment, transported them to and from the military ranges, set up and operated the data collection system during military exercises and then documented its findings. Field test sensors developed by the AWRFLD team have demonstrated the feasibility of building a deployable passive RF sensing system to detect weapon launch events on the battlefield.
RF sensor design
The SwRI team began the AWRFLD program by investigating the RF signals generated during the firing of small arms at the SwRI ballistics facility in San Antonio to establish accurate test procedures and to better understand the associated RF phenomenology. The Institute's high-speed video camera was able to pinpoint the timing of captured RF signals and verify they were caused by firing the weapon. RF signal data and video were collected from a variety of pistols and rifles such as the .357-caliber handgun and the Russian-made Kalazhnikov AK-47 rifle.
Procedures and techniques developed during small-arms testing allowed SwRI engineers to move on to collecting RF data during the launch of larger ordnance. The team used a variety of standard and custom-designed sensors to collect data over frequencies ranging from 30 MHz to 100 GHz. These sensors included various commercial and custom antennas for the lower frequencies along with commercial and custom radiometers for the higher frequencies. SwRI consistently and reliably detected RF energy during the launching of RPGs, mortars, artillery and rockets while using the custom-designed radiometers centered on frequencies of 10 GHz, 35 GHz and 94 GHz. After collecting data with individual radiometers, the AWRFLD team decided to develop a 35 GHz scalable proof-of-concept radiometer array, called the Multi-Antenna Radiometer Sensor (MARS) prototype. Unlike previously developed sensors, an array of radiometers would allow a determination of target bearing. The MARS prototype used 45 radiometers that populated an array of 5 rows by 9 columns. The MARS prototype included a dish antenna that focused the RF energy toward the array as well as both visible and IR cameras. The project team used computer-aided design software tools and rapid prototyping machines available at SwRI to develop a metal-coated plastic horn array face for MARS that would simplify assembly and reduce its overall cost. Developing the individual radiometer designs required the use of microwave modeling software and computer-aided design software. The SwRI team also developed custom user interface software that provided a composite display of captured visible and IR video along with a graphical representation of the captured RF data taken during weapon launches.
Future developments may include the detection of weapon launches using platforms based on the ground, in the air or in space. Space-based sensors in particular could provide detailed launch locations and discrimination of tactical and strategic rockets in obscured conditions. Achieving this objective will require demonstrating the ability to produce RF focal plane arrays that are very compact and lightweight and that use lower power. Likewise, data processors capable of handling data from a large array of sensors are required, along with software algorithms for processing the data.
Many events other than weapon launches cause the generation of RF signals, and these can contribute to confusion when interpreting RF signals. The project team took great care in developing the proper test setups and procedures to make sure that any RF energy they detected actually resulted from the firing or launching of weapons rather than from some coincidental signal from another source. High-speed video coupled with high-bandwidth test equipment and post-event data analysis provided the essential tools to pinpoint the origin of the detected signals. Also, advances in component technology allowed SwRI to design and develop the receivers required for sensing previously undetectable signals.
After extensive testing, the project team theorized that the RF signals generated during the firing of pistols and rifles result mainly from the triboelectric effect of charged dissimilar metals making and breaking contact. On the other hand, larger weapons generate most of their RF energy in the form of black-body radiation, at lower-than-IR and visible frequencies, emanating from their fireball during launch. The project team consistently detected RF emissions at appreciable distances during the launch of the larger weapons.
By design, the MARS prototype collected RF emissions data from 45 individual radiometers along with IR and visible images at the same time. This capability allowed the MARS prototype to detect the RF emissions from the simultaneous launching of RPGs, mortars and artillery located within its field of view. It also allowed the MARS prototype to detect the RF emissions from rockets as they crossed its field of view and allowed it to determine their line of bearing. Normalized RF signals received during the firing of various weapons proved unique to the weapon type, and therefore could allow detection and discrimination of specific weapon types.
The AWRFLD team collected video and RF data during the firing of small arms at SwRI and the launching of RPGs at Redstone Arsenal, rockets at White Sands Missile Range, artillery and rockets at Fort Sill, and mortars and rockets at Yuma Proving Ground. During this testing, the AWRFLD team collected RF data from listening stations located more than a kilometer from the launch sites. Using this data, the AWRFLD team calculated and graphed the maximum projected detection ranges of RPGs, mortars, artillery and rockets for different antenna beamwidths and for selected integration times. The data for creating this projected capability came from field test measurements before any signal processing had occurred. Increasing the integration time, or increasing the collecting antenna size, could potentially increase the detection range.
The previous collection of data using passive RF sensors during the launching of various weapons at military ranges indicates that a passive deployable weapon launch detection system using similar technology would allow the consistent detection of weapon launches from adequate stand-off distances. RF detection of these events could provide a benefit over acoustic and optical systems by providing detection through obscured environments. The multidisciplinary technical team at SwRI is continuing to work with its commercial client and the USASMDC toward eventually developing and fielding deployable passive detection systems that can potentially save warfighters' lives.