A group from MIT Lincoln Laboratory has engineered and showcased the wideband selective propagation radar (WiSPR), a system adept at detecting over varied distances at millimeter-wave (mmWave or MMW) frequencies. Normally, these elevated frequencies, spanning from 30 to 300 gigahertz (GHz), are utilized exclusively for close-range applications. Employing transmit-and-receive electronically scanned arrays comprised of numerous antenna elements, WiSPR generates narrow beams proficient in rapidly scanning an area to identify objects of interest. These tight beams can also be adjusted into wider beams for communication purposes.
“Creating a system with adequate sensitivity to function over extended distances at these frequencies for radar and communication capabilities is intricate,” states Greg Lyons, a senior staff member in the Airborne Radar Systems and Techniques Group, part of Lincoln Laboratory’s ISR Systems and Technology R&D sector. “Our group includes many radar specialists, and we all deliberated whether such a system was even plausible. Considerable innovation is occurring in the commercial field, and we utilized those improvements to create this multifunctional system.”
The significant signal bandwidth available at mmWave makes these frequencies attractive. Licensed frequency bands are becoming congested rapidly, and utilizing mmWave frequencies liberates substantial bandwidth and minimizes interference among systems. A high signal bandwidth is advantageous in a communication system to convey more information, and in a radar system to enhance range resolution (i.e., the radar’s capability to differentiate between objects in the same angular direction but at varying distances from the radar).
The phases for success
In 2019, the laboratory team embarked on evaluating the viability of their mmWave radar concept. Using commercially available off-the-shelf radio-frequency integrated circuits (RFICs), which are chips that transmit and receive radio waves, they developed a fixed-beam system (capable of only aiming in one direction, not scanning) with horn antennas. During a demonstration on a misty day at Joint Base Cape Cod, the proof-of-concept system successfully identified calibration objects at unprecedented distances.
“How do you construct a prototype for what will ultimately be a highly complex system?” queries program manager Christopher Serino, an assistant leader of the Airborne Radar Systems and Techniques Group. “From this feasibility testing, we demonstrated that such a system could indeed function, while identifying the technological challenges. We understood those challenges would demand innovative solutions, so we concentrated our initial efforts there.”
WiSPR relies on multiple-element antenna arrays. Whether fulfilling a radar or communication role, the arrays are phased, meaning the time delay between each antenna element is adjusted. This modification guarantees that all phases combine to direct the narrow beams in the intended direction. With this arrangement of numerous phased elements, the antenna becomes more focused in transmitting and receiving energy toward a single location. (Such phased arrays are becoming widespread in technologies such as 5G smartphones, base stations, and satellites.)
To allow the minuscule beams to continually search for objects, the team custom-built RFICs employing cutting-edge semiconductor technology and introduced digital functionalities to the chips. By managing the performance of these chips with tailored firmware and software, the system is capable of locating an object and, once found, maintaining its “track” while the search for additional objects carries on— all without physically moving antennas or relying on an operator to determine the next step.
“Phasing elements in an array to achieve gain in a specific direction is typical practice,” elucidates Deputy Program Manager David Conway, a senior staff member in the Integrated RF and Photonics Group. “What is not standard is having this many elements with RF at millimeter wavelengths still functioning in unison, still consolidating their energy in transmit and receive, and capable of rapidly scanning over very broad angles.”
Line ’em up and cool ’em down
For the communication feature, the team developed an innovative beam alignment methodology.
“To combine multiple antenna elements for a radar to reach beyond typical MMW operating distances — that’s unprecedented,” Serino states. “To electronically scan the beams around as a radar with effectively no latency between beams at these frequencies — that’s novel. Expanding some of those beams so that you’re not perpetually reacquiring and repointing during communications — that’s also innovative.”
Another significant advancement crucial to WiSPR’s progression is a cooling scheme that dissipates the considerable heat generated in a compact space behind the transmit elements, each possessing its own power amplifier.
Last year, the team showcased their WiSPR prototype at the U.S. Army Aberdeen Proving Ground in Maryland, collaborating with the U.S. Army Rapid Capabilities and Critical Technologies Office and the U.S. Army Test and Evaluation Command. WiSPR technology has since been transitioned to a vendor for production. By incorporating WiSPR, Army units will enhance their operational effectiveness.
“We anticipate that this system will be utilized in the near future,” Lyons states. “Our work has advanced the state of the art in MMW radars and communication systems for both military and commercial use.”
“This is precisely the type of work Lincoln Laboratory is proud of: monitoring the commercial landscape and leveraging billions of dollars in investments to develop new technologies, rather than starting anew,” remarks Lincoln Laboratory assistant director Marc Viera.
This initiative supported the U.S. Army Rapid Capabilities and Critical Technologies Office. The team includes additional members from the laboratory’s Airborne Radar Systems and Techniques, Integrated RF and Photonics, Mechanical Engineering, Advanced Capabilities and Systems, Homeland Protection Systems, and Transportation Safety and Resilience groups.