Looking Ahead: The Future of RF Technology, Military and Homeland Perspectives
Persistent surveillance, assuring identification and affecting the threat all sound like military terms in the protection of the free world. They are, of course, but they equally apply to other phenomena that affect the quality of life. The future pull and push of RF technology will continue to provide our military with unprecedented capabilities as well as provide opportunities for new commerce. The government pull comes from the DOD, DHS, DOC (NOAA & FAA) and DOT.
What are some of the pulls? We need systems that can observe the world around us, look closer and farther away, find smaller and hidden objects, some that move fast, and others that move slow. Persistent surveillance, assuring identification and affecting the threat are applicable in military operations and homeland security and for terrorists and severe weather. What are some of the RF technology pushes that are applicable to these pulls? We’re seeing unprecedented high power RF devices, more affordable, faster and highly integrated RF electronics, components that support higher frequencies and wider frequency bandwidths, and man-made materials with RF properties not realized previously, just to name a few. We’ve come a long way from the first transistor radio.
Today’s research and development at Raytheon is focused on RF technologies and subsystems that improve the performance and capability of current and future systems and include:
• Solid-state Active Electronically Scanned Arrays (AESA)
• High-efficiency power amplifiers
• Directed energy technologies
• Semiconductors, including CMOS, SiGe, InP, GaAs and GaN for higher levels of integration, higher power and higher speed
• High Density MMICs and TR Modules
• Frequency Agile sources
• Digital receivers and transmitters
• Software Defined Radios
• Higher bandwidth and higher sensitivity RF components
• Radar stealth coatings and materials
• Micro Electro Mechanical Structures (MEMS) RF Switches
• Ka-band frequencies for higher resolution and pointing accuracy
• Integrating multiple beams and simultaneous modes into multi-function systems
• Space-time, adaptive processing (STAP) and jammer-nulling techniques
• Netted Communications across platforms
The following sections first discuss some of the key RF technology pushes and finish with some of the novel pulls that solve critical needs for our nation. It is clear that RF technology will continue to play a critical role in providing our military and homeland the best, most affordable and reliable capabilities for the next 50 years and beyond.
The Next Generation of Active Electronically Scanned Arrays (AESA):
Wideband, Multi-function and Digital RF
How can a platform obtain more functionality given the limitations in size, weight and cost? The military began investing in architectures and technologies several years ago to improve and streamline the proliferation of aperture systems on airplanes and ships. Many platforms need to support radar, communications and electronic warfare functions. Platforms regularly have to trade off capabilities and cost because there isn’t room for everything that’s desired. The challenge is to develop architectures and technologies that may one day realize a concept whereby each platform would possess a minimal set of apertures that can be dynamically reconfigurable—in real time—to perform radar, communications and electronic warfare tasks independently and simultaneously, using only software. There are several key technologies that need to be invented and developed before wideband multi-function systems can be realized. Highly linear amplifiers, tunable channelizers, wideband apertures, high speed analog and digital converters, digital beamforming, and efficient dense packaging are just some of the challenges going forward.
The Next Generation of Microwave Solid-state RF Device and MMIC Technology
New advanced-device technologies will also play role in future AESAs. Maturation of gallium nitride (GaN) and silicon germanium (SiGe) semiconductor technologies show excellent promise. GaN is focused on achieving an order of magnitude increase in amplifier power density for the same MMIC area. SiGe is a predominant commercial semiconductor for the telecom industry.
GaN transistors have a high frequency power handling capability well beyond silicon, GaAs or any other semiconductor yet fabricated. This capability makes it the technology of choice for the monolithic MMIC power amplifiers that are the building blocks of the RF portions of next-generation defense systems. Use of GaN MMICs leads to weight, range, sensitivity, prime power, cooling and cost advantages at the system level. GaN’s material properties allow it to support device operation at much higher voltages than the GaAs that dominates today’s defense systems. GaN MMICs easily operate at 28 V, have ~2 times the maximum channel current and can produce five to 10 times the power (with comparable gain and efficiency) of an equivalently sized GaAs MMIC typically operating at less than 10 V.
So-called high voltage GaAs PHEMT MMICs can be engineered to operate at higher voltage (10 to 20 V) but at the expense of operating current, limiting power density to 1.5 to 2 times that of a typical GaAs PHEMT. Amplifiers of equivalent total power can be made more compactly using GaN because of the higher GaN power density. In addition, the higher voltage of GaN results in higher matching impedance, which enables broader bandwidth design than GaAs. Table 1 compares GaAs and GaN device properties. In terms of reliability, tests predict a mean time to failure (MTTF) of greater than one million hours at a standard transistor channel temperature of 150 degrees Celsius.
Raytheon is presently transitioning the fabrication of GaN MMICs into its high-volume 100 mm diameter wafer production fabrication (see Figure 1). GaN is a disruptive high-power semiconductor technology that will enable a new class of microwave and millimeter-wave RF systems envisioned for the near future and beyond.
UCLA, working on a Raytheon university grant, has developed an artificial dielectric structure for use in SiGe BiCMOS integrated circuits at military frequencies. Two major challenges in CMOS millimeter-wave IC designs include: (1) device noise, which is typically one or two orders higher than that of compound or SiGe HBTs; and (2) signal attenuation due to the skin effect and substrate losses, which inevitably result in low performance on-chip lumped passive components. An artificial dielectric is formed by embedding metal obstacles in a periodic pattern, as shown in the upper part of Figure 2.
A 60 GHz VCO was designed and implemented in a 90 nm CMOS to verify the effects of the embedded artificial dielectric on resonator size, loss and noise reduction. Compared to resonators without artificial dielectrics or those using conventional spiral inductors at this frequency, a much lower loss resonator can be accomplished. A 60 GHz CMOS VCO with a measured phase noise of –107 dBc/Hz and a record low F.O.M. of –200 dBc/Hz at 1 MHz frequency offset has been realized. This VCO dissipates only 1.9 mW from a 1 V power supply and occupies a chip area of 0.015 mm2, which is less than 10 percent of the prior arts (see Table 2).
Raytheon supports the Caltech Microelectronics Center in the research of silicon circuits for microwave and millimeter-wave applications. Professor Ali Hajimiri and his staff are developing novel approaches that leverage the strengths of silicon while supporting mixed signals to provide more affordable solutions for sensor applications.
A 60 GHz RF-combined 4-element phased array front-end is implemented in silicon using a novel hybrid parallel/series-fed approach that reduces on-chip phase shifter requirements. The array, which includes amplitude control as well as continuous phase adjustment, provides for simultaneous illumination of two angles of incidence. We combine the series-fed and parallel-fed array architectures to further relax the RF phase-shifter requirements to enable RF signal-combining. As shown in the simplified block-diagram of Figure 3a, discrete phase shifters (DP) in every element choose one of two phase-shift settings (e.g., 0° or 180° in Element 2).
The signals are then fed into bidirectional series phase shifters, each of which provides a certain amount of phase shift. The important point is that the signals on the series phase shifters travel in both directions, yielding the following signal summations at the two outputs providing for two concurrent receive beams. The input to each element is first amplified by a four-stage 60 GHz LNA that has variable gain to compensate for downstream gain variation. The fourth stage of each LNA provides variable gain by current steering. The output of the LNA is provided to a DP that can choose between two phase-shift settings. The front-end has a noise figure lower than 6.9 dB at 60 GHz and the array achieves full spatial coverage with better than 20 dB peak-to-null ratio. The four-element 60 GHz front-end consumes 270 mW and occupies 4.6 mm2 of die area. Figure 3b shows a die photo of the array, which was implemented in a SiGe process with a BJT cut-off frequency of 200 GHz.
Dense Low Power Radar Networks
Today’s weather forecasting and warning infrastructure uses data from high-power radars that have helped meteorologists improve forecasts significantly in the past 20-plus years.
Despite having substantial capability to measure wind and rainfall and to diagnose storms, these long-range radars have limited ability to observe the lowest and most critical part of the atmosphere owing to the Earth’s curvature. This prevents the radars from observing the behavior of tornadoes and other hazards at or near ground level. As a result, one in five tornadoes goes undetected by current technology, and 80 percent of all tornado warnings turn out to be false alarms. Raytheon, in partnership with a team of academic, government and industrial collaborators, has formed a National Science Foundation Engineering Research Center (ERC) called the Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) to address this problem. CASA is researching a new weather hazard forecasting and warning technology based on a low-cost, dense networks of radars that operate at short range, communicate with one another, and adjust their sensing strategies in direct response to the evolving weather and to changing end-user needs.
In contrast to today’s large weather radars with 10-meter-diameter antennas, the antennas in CASA networks are expected to be one meter in diameter with electronics that are about the size of a personal computer. This small size allows these radars to be placed on existing cellular towers and rooftops, enabling them to comprehensively map damaging winds and heavy rainfall from the top of storms down to the critical boundary layer region beneath the view of current technology. This approach can achieve breakthrough improvements in resolution and update times, leading to significant reductions in tornado false alarms; quantitative precipitation estimation for more accurate flood prediction; fine-scale wind field imaging; and the estimation of thermodynamic state variables for use in short-term numerical forecasting and other applications such as airborne hazard dispersion forecasting. Cost, maintenance and reliability issues, as well as aesthetics, motivate the use of small (approximately one-meter diameter, two-degree beamwidth) antennas that could be installed on either low-cost towers or existing infrastructure elements (such as rooftops or cellular communication towers).
AESA arrays are a key enabling technology in many production radars today and a desirable technology for use in dense networks since they do not require maintenance of moving parts and they permit flexibility in beam steering. A particular challenge in realizing cost-effective dense networks composed of thousands of radars will be to achieve a design that can be volume-manufactured for approximately $10,000 per array (current dollars). Several thousand transmit/receive (T/R) channels are needed to realize a phased array capable of electronically steering a two-degree beam in two dimensions over the desired scan range of these radars. The realization of such an antenna will benefit from leveraging commodity silicon RF semiconductors to achieve T/R functions, in combination with very low-cost packaging, fabrication and assembly techniques. Prototypes of the sub-panels are shown in Figure 4a (front view) and Figure 4b (rear view).
Beyond the Microwave Oven
As the pioneer and inventor of the microwave oven, Raytheon has leveraged high power RF confined in metallic boxes (or resonant cavities) to defrost, cook, cure adhesives, etc. Now and in the future, high power RF energy may be harnessed to protect airports and stop intruders in their tracks without killing them. Two particular systems being examined are called Vigilant Eagle and Silent Guardian.
Vigilant Eagle provides an invisible dome of protection around airports or airfields, offering all aircraft—international and domestic commercial flights, as well as military and private planes—protection from terrorist surface-to-air missiles including the Man-Portable Air Defense System (MANPADS). Vigilant Eagle had already been proven against real missiles in field tests. In 2006, Raytheon was awarded a $4.1 M DHS contract to demonstrate the suitability of the Vigilant Eagle airport protection system to function in a civilian environment and its ability to protect aircraft from the threat of shoulder-fired missiles.
Vigilant Eagle uses a simple technique of illuminating the missile body with electromagnetic energy tailored to divert the missile (see Figure 5). It aims a focused, precisely steered beam of electromagnetic energy at a terrorist’s missile, diverting the threat away from the targeted aircraft. Vigilant Eagle would be installed at airports, rather than on individual aircraft. The system includes a distributed missile detect and track subsystem (MDT), a command and control (C2) system and the Active Electronically Scanned Array (AESA), which consists of a billboard-size array of highly efficient antennae linked to solid-state RF amplifiers. The electromagnetic waveforms disrupt the missile and deflect it away from the aircraft. Created electromagnetic fields are well within the Occupational Safety and Health Administration (OSHA) standards for personnel exposure limits.
Another new application of military-proven technology is Raytheon’s directed-energy protection system called Silent Guardian that employs millimeter-wave energy to stop, delay, deter and turn back violent aggressors. Silent Guardian can be utilized from up to 250 meters away against would-be attackers, while enabling the operator to distinguish friend from foe in real-time without having to use lethal force. Potential applications include facility and critical asset protection, riot control, home and perimeter security, and counter-terrorism. The system emits a focused beam of millimeter-wave energy to repel individuals without causing any physical damage. The beam heats the water molecules around the skin’s pain and heat receptors (located 1/64 of an inch under the skin), creating a burning sensation intended to get the aggressors’ attention and repel them (see Figure 6). There are legislative and policy questions that must be answered before DHS is able to implement this technology.
The passions of invention and pushing performance, plus the capability and affordability needs of the government agencies will mold the next 50 years of RF technology. We can expect to see the maturation of emerging RF technologies paired with our digital, mixed signal and systems cousins that will continue to provide an exciting portfolio of diverse products and capabilities for our government customers.
This work is supported in part by the Engineering Research Centers Program of the National Science Foundation under NSF award number 0313747. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation.