The electronics footprint on unmanned aircraft systems (UASs) continues to shrink. These reduced size, weight, and power (SWaP) requirements are driving radio frequency (RF) and microwave designers to come up with innovative ways to deliver the reduced SWaP while increasing performance and enhancing the platform's capability.

Military unmanned aerial systems (UASs) have become multimission platforms tackling tasks such as intelligence, surveillance, and reconnaissance (ISR), electronic warfare, and signal intelligence. Packing all that capability into each UAS payload puts pressure on RF and microwave designers who have to grapple with the reduced SWaP requirements that always accompany the demands for more capability.

“Today’s unmanned systems range from small drones to large systems – such as the Global Hawk – and bring with them a huge set of challenges,” says Doug Carlson, Vice President of Strategy for MACOM (Lowell, Massachusetts).

One major obstacle: “Next-generation sensors are collecting greater amounts of data that require data link subsystems to support increased data rates and transmission ranges,” explains Kevin Beals, Vice President and General Manager, RF and Microwave Solutions Group, Mercury Systems (Andover, Massachusetts).

RF and microwave designers take this overall platform design into account when dealing with an already SWaP-constrained system. “From a broader perspective, unmanned systems are growing increasingly prevalent and packing more capabilities into ever-shrinking form factors,” Beals states.

Another complicating element is that users “are now looking to adapt technologies once reserved for larger platforms, like electronic warfare (EW) for aircraft, for use in unmanned systems,” Beals adds.

The interesting part is that “Design trends remain relatively unchanged over the last decade – size, weight, power, and cost (SWaP-C) are key focus areas for every military customer and design,” MACOM’s Carlson points out. “Years ago, one of MACOM’s first MMIC [monolithic microwave integrated circuit] solutions was a Ku-Band device for a small UAV data link, and the objective was to make it small and lightweight.”

However, when comparing DoD requirements then and now, “Today’s requirements are much the same, but amplified – RF components must be even smaller, more highly integrated, and lighter than before,” Carlson explains. “At one end of the spectrum, a drone’s small platform creates major space and power limitations, and without highly efficient, integrated, compact and lightweight components, it can’t perform its mission.”

David Markman, Technical Director, Advanced Technology for Cobham Advanced Electronic Solutions (Lansdale, Pennsylvania), says that in addition to the space limitations, “Today, most RF and microwave signals are digitally generated and detected. This allows customers to reconfigure systems real-time. This trend is similar for commercial wireless, SATCOM [satellite communications], and military systems.”

“Trends in RF and microwave design cannot be entirely separated from the digital domain,” Beals states. “For space-constrained applications like unmanned vehicles, we’re also seeing a convergence of RF and digital technologies.”

Mercury Systems built “preintegrated RF and digital solutions around modular open system architectures, including OpenVPX and OpenRFM,” Beals says.

Power is also a big issue in unmanned systems designs. “On the other end of the spectrum, the payload on bigger platforms is greater, but still fairly challenged in terms of power,” Carlson points out. “UAV missions usually consist of some type of sensing, perhaps in the form of radar or infrared sensor, and a communications link to send information in virtual real time to a destination or decision-maker.”

Ultimately, these challenges have “implications for the RF and microwave community – SWaP-C requirements are more extreme than ever before. Our military customers are looking for first, higher performance and efficiencies; second, smaller form factors; and third, cost savings through economies of scale,” Beals states.

Cost is always a major deciding factor in designing these systems, as “adding a sensing system to a drone should make financial sense, and not cost 10 times the cost of the platform,” Carlson notes. “So highly efficient, integrated, cost-sensitive, and compact RF components continue to be the essentials for unmanned systems.”

SWaP environments lead to customized solutions

Unmanned systems design would be impossible without consideration of designing to SWaP-constrained environments. “The need to optimize the performance of a system to achieve today’s SWaP requirements is driving companies like MACOM to innovate utilizing heterogeneous integration of multiple technologies to achieve the optimal level of integration, with the ideal level of efficiency, within the footprint required,” Carlson says. “This could include, for example, merging a combination of technologies such as diode switches, GaAs LNAs [gallium arsenide low-noise amplifiers], GaN-on-Si PAs [gallium nitride-on-silicon power amplifiers], or even SOI [silicon-on-insulator]-based control components, all in one package solution.”

The realities of designing for a SWaP environment mean a customized solution. Instead of going the commercial off-the-shelf (COTS) route, “Generally in the military field, RF component solutions tend to be highly customized to specific end-system applications,” Carlson adds. (Figure 1.)

Figure 1: The realities of designing for reduced size, weight, power, and cost: Customized RF components. Photo courtesy of MACOM.

Customized solutions mean the user’s mission goals determine the design. “SWaP and cost are major considerations in most designs today. In many cases, those parameters are fixed, leading the customer to ask, ‘given these SWaP and cost limitations, what performance can we achieve?’” Markman says.

Unfortunately, this question means that cost actually does take a hit and becomes a design challenge. UASs needing such customized solutions affect “not just our design strategy but also our manufacturing processes,” Beals states. “For the latter, our designers are working more closely with our manufacturing groups to ensure a seamless, cost-effective transition to accelerate initial concept to volume production.”

In fact, “The design constraints of unmanned systems push the boundaries of innovation in the RF community,” Beals explains. “Unmanned systems are driving our design engineers to blend technology, advanced nonlinear modeling, and manufacturability together to deliver what we thought was impossible years ago. As an example, we developed a GaN solid-state power amplifier that achieves three seemingly competing priorities without compromise: high efficiency, high reliability, and extended range.

“Over time, we’ve developed and refined a unique set of technology building blocks for these extremely SWaP-constrained applications,” Beals adds referencing to an RF BGA device (Figure 2.) This will lead to cost savings: “As an example, we commercialized an advanced packaging technique using multilayer circuit boards with ball grid array technology to achieve incredibly small form factors and high packaging densities. This is an example of an innovation originally developed for a much larger platform facing SWaP constraints, but the fundamental building blocks also offer tremendous SWaP benefits to the designers of unmanned systems.”

Figure 2: Mercury Systems’ miniature ruggedized RF multichip module (MCM). Photo courtesy of Mercury Systems.

The challenge will continue as mission requirements evolve over time; “Based on mission requirements, we optimize the integration of our innovative RF and digital building blocks to address performance and SWaP constraints simultaneously,” Beals says. “Our tight linkage between design and manufacturing then assures that the ‘C’ in SWaP-C is optimized for scale throughout the program life cycle.”

ISR, EW also on the board

UAS payloads are pushing the use of RF and microwave components in certain applications. “One could argue that ISR [intelligence, surveillance, and reconnaissance] payloads generally contain the highest RF content compared to UAV communications,” Carlson says. “ISR systems often are implemented in the form of an array – driving content. UAV communications are, in essence, a radio with a single RF channel.”

Radar and EW systems also increasingly incorporate RF and microwave into unmanned systems to enable faster, multifaceted missions. “Two applications immediately come to mind – radar and EW. Unmanned vehicles need to navigate through congested environments in any type of weather conditions,” Beals says.

Markman points out that “Communications terminals produced in large volume also drive RF content,” adding that “Large active phased arrays have large amounts of component content which is directly proportional to the element count.”

“These environments include both stationary objects and potentially other unmanned systems in motion,” Beals adds. “Collision avoidance and communication between unmanned vehicles is now a requirement. For unmanned aerial vehicles, there is also the additional consideration of improving the precision of the landing approach. Embedding millimeter-wave technology into unmanned systems makes the difference between a successful and a failed mission in challenging environmental conditions and/or congested environments.”

“While we generally do not associate EW functionality with unmanned vehicles, it’s a game-changer for the future, particularly when the asset being protected is of high monetary or strategic value,” Beals says. “As just one example, self-protection capability in the form of a miniaturized digital RF memory module improves the odds of mission success when facing electronic attack. Any of the EW capabilities present on a larger platform represent the opportunity to port a miniaturized version of the technology to unmanned systems. This is entirely new RF content introduced into the platform.”