Military Embedded Systems

Leveraging SDR for counter-UAS applications


September 13, 2022

John Orlando

Epiq Solutions

U.S. Army photo.

The unmanned aircraft system (UAS) market is expected to grow to more than $60 billion by 2025, and with each passing year the potential for threats due to these often hard-to-detect craft is only going to increase. Successful military-focused counter-UAS (c-UAS) solutions may neutralize these threats today, but these threats continue to evolve. Software-defined radio (SDR) will continue to play a key role in c-UAS solutions for the foreseeable future.

Unmanned aircraft systems (also known as UASs or drones) are getting smaller and more sophisticated, not to mention a lot more affordable for larger number of users. They are becoming faster with increased operational capabilities and yet are easy to operate with minimal training required. Add to that their ability to access any area while under remote control and beyond line-of-sight and it is easy to see why UASs are becoming a preferred asset for both defense and commercial users. As with many other technology-enabling devices, the very benefits that make drones appealing also attract bad actors who would use them for wrongdoing. In the wrong hands, a UAS is an asymmetric threat because the cost and effort for an individual to deliver a large amount of destruction is low.

Their ability to access areas that are otherwise hard to reach, and to do so with relatively little risk of detection, have already been highlighted in a few high-profile security breaches: One notable one was an unauthorized landing on the White House lawn in 2015, which resulted in the Transportation Security Admin­is­tration (TSA) establishing a seven-mile “no-drone zone” around Wash­ing­ton; another was an assassination attempt on Iraqi Prime Minister Mustafa al-Kadhimi in November 2021, during which an explosive-laden UAS attacked his Baghdad residence, while two others were shot down. The proliferation of and increase in complexity of UASs clearly results in a growing need for counter-UAS (c-UAS) technologies and approaches.

There are multiple deployment scenarios that are critical when discussing the need for c-UAS, including airports where interference with aircraft could be catastrophic, prisons where drones can deliver contraband, and of course military/defense facilities and battlegrounds where personnel are intent on force and civilian protection. Governments and municipalities have been working on how to respond, and federal regulations have been drafted to provide guidance on recreational operation of drones. In addition to “no-fly zones,” certain classes of drones must be registered with the FAA and carry different operating requirements depending on the class of drone. For example, Part 107 in the FAA’s Code of Federal Regulations, commonly referred to as the Small UAS Rule, outlines requirements for drones lighter than 55 pounds. This covers most hobbyists as well as many commercial applications.

At the Department of Defense (DoD) level, there are more than a thousand individual R&D programs targeting UAS applications. In 2019, the Secretary of the Army was designated as the DoD Executive Agent for Counter-Small Unmanned Aircraft Systems, which is charged with coordinating all c-UAS activities.

The central challenge stated in the “U.S. Department of Defense Counter-Small Unmanned Aircraft Systems (C-sUAS) Strategy” states: “The exponential growth of sUAS creates new risks for the Department. Technology trends are dramatically transforming legitimate applications of sUAS while simultaneously making them increasingly capable weapons in the hands of state actors, non-state actors, and criminals. Small UAS may also pose hazards to DoD operations in the air, land, and maritime domains when controlled by negligent or reckless operators. The Department must protect and defend personnel, facilities, and assets in an environment where increasing numbers of sUAS will share the skies with DoD aircraft, operate in the airspace over DoD installations and be employed by our Nation’s adversaries.”

By centralizing this activity, the DoD recognizes the importance of a uniform and rapid response to a very quickly evolving threat. Even as regulations overseeing the use of UASs continue to evolve, they cannot be relied on as a first line of defense against the potential damage that can be caused by drones. C-UAS solutions need to be developed and be ready to address both current and future airborne threats.

For the rapidly evolving wireless threat landscape of c-UAS, software-defined radio (SDR) provides an ideal RF situational-awareness solution. The software in an SDR can be remotely reconfigured and upgraded, thus helping to future-proof the c-UAS solution. SDRs also often cover a wide RF frequency range, which is critical since different UAS manufacturers tend to use different RF frequencies. The benefits of SDRs for use in c-UAS are plentiful, but there are critical factors to keep in mind when selecting which SDR is appropriate for the task at hand.

Detecting UASs through RF sensing

Typical c-UAS solutions involve, at a minimum, a means to detect the presence of a UAS in an airspace of interest. Once detected, more advanced c-UAS solutions act to either disable or take control of the craft to limit the damage it can perform.

Today’s c-UAS solutions may leverage a variety of sensor inputs for detecting the presence of a UAS nearby, including audio, visual, and RF sensing. Both audio and visual sensing have a limited standoff distance for successful operation, which can limit usefulness; by the time a drone is visible or audible, it is already close enough to cause significant damage. Furthermore, there are typically limited options to defeat a UAS threat if only audio and visual means are considered.

Leveraging RF to sense and/or defeat an impending UAS threat has significant advantages: By design, most drones are wirelessly controlled by a remote operator typically located over several miles away from the drone’s location, as shown in Figure 1. There can be multiple RF data links in use between the controller and the UAS to deliver command/control messages, as well as the data feed from the craft (typically a video signal from the UAS to be displayed on the remote operator’s control terminal). These RF transmissions from both the UAS itself as well as the remote operator’s control terminal are of sufficient signal strength to enable operation over a range of several miles (or more) typically. Thus, it is also possible to perform detection and/or defeat of the UAS over a similar standoff distance, which is significantly further than what can be achieved with audio or visual sensing alone.

[Figure 1 | A diagram illustrates a typical UAS – c-UAS scenario.]

Manufacturers of UASs use a range of different RF frequencies and bandwidths to provide transmission of their command/control plus data feeds between the craft and the operator, including:

  • ISM bands (typically 900 MHz, 2.4 GHz, and 5 GHz in the U.S.)
  • Wi-Fi bands (2.4 GHz and 5 GHz)
  • 4G LTE cellular bands (600 MHz to 3.8 GHz)
  • 5G NR [new radio] cellular bands (600 MHz to 3.8 GHz)
  • Other licensed RF spectrum between 100 MHz and 6 GHz

In some cases, these RF command/control plus data links use frequency-hopping to quickly move from one RF frequency to the next at a known cadence agreed upon between the drone and the remote operator’s control terminal. This structure helps minimize interference with other RF signals operating in the same unlicensed (or licensed) bands.

The data-transport protocol that is used for the command/control plus data links also varies from one drone manufacturer to the next. Some manufacturers use the same low-cost Wi-Fi chipsets that support the standard 802.11 physical layer and protocol commonly found in commodity electronic equipment today. Others, such as DJI, use a proprietary physical layer and data transport protocol for their drones (Lightbridge or Lightbridge 2 in some systems, or OcuSync in other variants of their systems). A c-UAS solution leveraging RF needs to be able to accurately detect and identify the tell-tale RF signs of the various UAS command/control plus data links of a craft operating nearby.

Defeating UASs with RF

Once a UAS is detected and an appropriate alert is provided, the next question is whether action will be taken against the craft. For RF-based solutions, “defeat” or countermeasures can be as simple as RF jamming: By generating enough RF energy in the appropriate radio bands, the UAS can be rendered useless. Other actions apply more elegant techniques, such as emulating and overriding the commands being sent by the suspect craft’s remote operator, possibly incapacitating the UAS by commanding it to land immediately. Note that transmitting RF signals, especially at the higher power levels that may be necessary for defeat operations, is typically governed by regulations that vary depending on the country where the system is deployed.

Leveraging SDR for c-UAS platforms

When time-to-market and risk reduction are important factors, a COTS [commercial off-the-shelf] SDR can be a significant accelerator to bringing a c-UAS solution to the field that performs as expected (Figure 2). In general, there are a number of different factors to consider when exploring the right SDR solution for c-UAS, especially when exploring c-UAS solutions that need to scale from small low-power handheld c-UAS solutions all the way to fixed-site larger scale c-UAS solutions.

[Figure 2 | A diagram illustrates SDR’s function within a c-UAS scenario.]

Key SDR features for c-UAS

When evaluating SDR options for potential use in a c-UAS solution, it may not be immediately apparent which features are most important to ensure that the solution will be effective for both current and future UAS threats. The following section outlines key SDR characteristics that are important for a successful c-UAS operation. Of course, the efficacy of a final c-UAS solution is also highly dependent on the higher-level software that is performing the detection and/or defeat processing. Fundamentally, the capabilities of the SDR help set the functional foundation of the c-UAS solution.

Size, weight, and power (SWaP): The physical form factor of the c-UAS solution helps to provide an initial bounding box around the potential set of SDR solutions that could be viable. A handheld c-UAS solution that is battery-operated will focus on small physical size and low power consumption. Similarly, manpack c-UAS solutions that are intended to be carried by soldiers or patrol elements will place a premium on the final size and weight of the solution. For vehicle-mounted or static fixed-site solutions, power is typically abundant, with size and weight being significantly less of a concern. In general, with more power and size available for the solution, an increase in functionality and capability can be achieved. Ultimately, different deployment scenarios will require a different SDR solution, so this initial selection is critical to get right.

RF tuning range and frequency-hopping: As mentioned earlier, UASs may leverage a wide range of different RF frequencies for their command/control plus data links. For a c-UAS solution to be capable of detecting the RF signatures from these craft, it must be capable of tuning to the same RF frequencies being used for the command/control plus data links. In general, most current UASs are using RF frequencies that range between 600 MHz and 6 GHz. Thus, an SDR should be capable of receiving RF signals within these frequency bands for detection. If the object is defeat, the same SDR will need to be able to transmit RF signals in these same RF frequency bands.

Future UASs will almost certainly continue to expand the RF frequency bands being used, thus making detection more challenging. The ability to quickly scan through large ranges of RF spectrum to look for the telltale RF signatures of a UAS is critical. Similarly, for UASs that are employing fast frequency-hopping to rapidly move from one RF channel to the next, it is imperative that an SDR also be capable of either performing the same frequency-hopping operation, or consuming the entire RF channel through which a UAS may be hopping for digital post-processing. This action enables the SDR to keep pace with the RF transmissions being sent by the drone.

Multiple receivers: There are multiple deployment scenarios wherein having more than one RF receiver can help improve the performance of a c-UAS solution. In some use cases, leveraging multiple independently tunable RF receivers enables one RF receiver to focus on receiving/processing the RF signals from a detected UAS, while a second RF receiver sweeps through the RF spectrum looking for additional threats. In addition, it is typically beneficial to be able to ascertain the direction of origin of the RF signal being transmitted from the UAS. The most common method to determine the angle of arrival of an RF signal is to use multiple RF receivers in a phase-coherent configuration. With two or more RF receivers operating phase-coherently, a reasonable estimate for direction of an RF signal emanating from a drone can be calculated. Expanding to four or more receivers can further improve the accuracy of this direction estimate.

RF transmit power: Scenarios where a UAS-defeat operation is anticipated will typically require an RF power amplifier to increase the transmit power coming from the SDR. The RF signal must be of sufficient strength to not only reach the UAS at a desired standoff distance, but in the case of either jamming or overriding the actual command/control signal coming from the remote operator’s terminal, power may need to be at a significantly higher level than the operator’s terminal. Most SDRs that include an RF transmitter emit an RF signal at a fairly low level, typically between 0 dBm and +10 dBm (1 mW and 10 mW, respectively). Thus, an external RF power amplifier is needed to boost this level to an appropriate level (typically in the range of 0.5W to 5W depending on the use case).

SDR modules appropriate for c-UAS are available today, enabling both the ­flexible RF hardware as well as the software/FPGA [field-programmable gate array] interface layer needed to keep pace and future-proof c-UAS for tomorrow’s airborne threats.

John Orlando is CEO and co-founder of Epiq Solutions, an engineering company focused on delivering software-defined radio (SDR) products and turnkey RF sensing solutions. He is an author and presenter on all things SDR, most recently presenting at GRCon 2021 on “Breaking through the 6 GHz Barrier.”

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