The proliferation of small unmanned aircraft systems (sUAS), miniaturization of sensor technology, and advancement of UAS swarm logic foretell that swarms of sUAS will threaten US air-bases by the 2025 timeframe. Currently fielded base defense systems are not well-suited to combat this emerging threat. Current directed energy (DE) developmental systems indicate this class of weapons is the best solution.

Three indicators necessitate US Air Force preparation for an imminent sUAS swarm threat: 1) the proliferation of sUAS, 2) the miniaturization of sensor technology, and 3) the advancement of UAS swarm logic. The first indicator necessitating US Air Force defenses against sUAS swarms is the proliferation of these systems. According to FAA estimates, the number of hobbyist sUAS in the US will increase from 1.1 million in 2016 to 3.5 million in 2021. These estimates only account for UAS weighing 55 pounds or less purchased in the US for personal use. The FAA further estimates the number of sUAS conducting commercial activities in US airspace to increase from 42,000 in 2016 to 420,000 in 2021.

The second indicator necessitating defenses against sUAS swarms is the miniaturization of sensor technology. Hyperspectral imaging technology is one example of sensor miniaturization. The Nano-Hyperspec sensor system from Headwall Photonics is only 3 inches x 3 inches x 5.1 inches, weighs just 1.32 pounds, and includes 480 GB of on-board storage capacity (approximately 130 minutes of processed hyperspectral imagery at 100 frames per second).

The third indicator necessitating defenses against sUAS swarms is the advancement of UAS swarm logic. One recent example of military swarm logic advancement was three US Navy F/A-18 Super Hornets releasing 103 sUAS during a Pentagon Strategic Capabilities Office exercise at China Lake, California. During this exercise, the Perdix UAS, developed by the Massachusetts Institute of Technology (MIT), demonstrated “collective decision-making, adaptive formation flying, and self-healing” swarm behaviors. Interestingly, MIT constructed the Perdix UAS by snapping readily available engines onto 3-D printed frames, indicating the threat may arrive sooner rather than later.

One of the few technologies capable of defending against UAS swarms is DE. There are two types of DE pertinent to military operations: lasers and microwaves. Although lasers and microwaves are both manifestations of electromagnetic energy, they are characterized by different wavelengths and, therefore, different frequencies. The relationship between wavelength and frequency is inverse, with wavelength (measured in meters) decreasing from left to right along the electromagnetic spectrum and frequency (measured in hertz) increasing.

Lasers and microwaves can be further categorized as either continuous wave or pulsed based upon how energy is emitted from their source. Whether a form of DE is continuously beamed or pulsed can fundamentally change the way the energy interacts with targets. For example, continuous wave lasers (CWL) affect targets by depositing energy, which typically results in a build-up of heat at the point of impact. Depending on the material, this heat buildup can result in burning through material layers until structural/component failure occurs. Several types of lasers are capable of producing continuous beams including chemical lasers like the YAL-1 Airborne Laser (ABL), and electric lasers such as diode-pumped or fiber lasers.

Pulsed lasers affect targets differently. The very high energy of a short pulse tends to cause ablation (stripping away molecules and atoms) at the point of impact, more than heating. These molecules can take on very high-energy states, creating plasmas at or near the point of impact, which, with proper timing, can be ignited to produce shock waves. Depending on the frequency with which a laser is pulsed, internal electronic effects are also possible. For both continuous wave and pulsed lasers, atmospheric attenuation limits the effective range and aiming precision.

Continuous wave microwaves also heat. However, microwaves heat by exciting water molecules. The Active Denial continuous wave microwave system is at a much higher frequency than a typical microwave oven, and therefore only penetrates a small distance into the skin, targeting the skin layer's pain receptors. In contrast, pulsed microwaves affect targets not by heating, but by creating an electromagnetic field that can induce currents in electrical wires. These currents can upset or destroy electronic components/systems. Unlike lasers, atmospheric attenuation, clouds, and moisture do not affect microwaves.

This work was done by David F. Pina for the Air Command and Staff College. For more information, download the Technical Support Package (free white paper) below.

This Brief includes a Technical Support Package (TSP).
Directed-Energy System to Defeat Small Unmanned Aircraft System Swarms

(reference AFRL-0295) is currently available for download from the TSP library.

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This article first appeared in the September, 2020 issue of Aerospace & Defense Technology Magazine.

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