The research will create a technological first – a small, lightweight, low-cost phased-array radar that uses silicon-germanium (SiGe) chips in tandem with radio-frequency micro-electromechanical systems (RF MEMS). The system being developed could be mounted on aircraft or satellites to enable high-quality mapping of ice and snow formations.
Traditionally, research on frozen areas has required bulky radar equipment that must be operated on the surface, said John Papapolymerou, a professor in Georgia Tech's School of Electrical and Computer Engineering who is principal investigator on the project. The lightweight radar approach could allow unmanned aerial vehicles (UAVs) to gather information by flying over a large area such as Greenland, using the radar system to map ice sheets in three dimensions.
"This aerial approach would greatly facilitate environmental remote sensing of ice, allowing us to map larger areas of interest to better understand location, quantity and composition," said Papapolymerou, who is teamed with another Georgia Tech professor, John Cressler, and Ted Heath, a Georgia Tech Research Institute (GTRI) senior research scientist. "This mapping ability is very important because we need to know about ice accumulation, consistency and stability.”
Phased-array radar technology uses fixed, interconnected antenna elements to send and receive multiple radar signals almost simultaneously. This approach employs a technique called phase-shifting to electronically steer the radar-signal beam.
By contrast, a conventional radar antenna changes the direction of the signal beam mechanically; the antenna moves physically among set positions, sending and receiving signals at each position. The serial approach used by conventional radar generally offers slower and less-effective performance than the more parallel technique of phased-array radar.
The basic sub-array unit under development consists of a flat grid with eight antenna elements on a side – 64 elements in all. These sub-arrays, measuring about 8.5 by 7 inches, can be combined to create a far larger radar array capable of high-quality 3-D mapping.
The sub-arrays are constructed using polymers as the substrate, which is the board-like structure in which the electronics are embedded. Polymers have numerous advantages; robust and flexible, they are also low in cost and offer good electrical performance.
To date, the researchers have produced and successfully tested an eight-by-two-element sub-array mounted on a multi-layer substrate. This substrate consists of a layer of liquid crystal polymer (LCP), which is a robust organic polymer, and a layer of a composite material called Duroid.
The LCP/Duroid substrate houses integrated circuits made from silicon-germanium (SiGe). The SiGe chips transmit and receive the radar signals via the sub-array's multiple interconnected antenna elements.
The researchers chose silicon-germanium because it offers high-performance signal amplification that is also low in noise and in power consumption, said Cressler, who is a Ken Byers Professor in the School of Electrical and Computer Engineering. SiGe chips are also robust, low in cost and highly resistant to weather and to radiation encountered in space.
"Using silicon-germanium allows much higher levels of integration, which older radar systems don’t give you," Cressler said. "It enables you to go from a system which is much larger and more expensive, and less robust, to a chip that is only a few millimeters on a side and costs far less."
Silicon-germanium circuits also interface well with RF-MEMS systems, which are tiny micro-electromechanical devices capable of movement on a very small scale. The team is using RF-MEMS devices, embedded in the substrate, to perform two functions -- switching between the transmit and receive circuits, and activating phase-shifters that electronically guide the radar signals sent by the sub-array's 64 antenna elements.
Using MEMS devices for electro-mechanical switching results in less signal loss than integrating the transmit-receive switching function within a SiGe chip electronically, Cressler said. And while MEMS switching is a bit slower than a purely electronic approach, it offers both better signal performance and the ability to handle higher signal-output power.
The system under development uses the X band -- microwave frequencies between 8 and 12 gigahertz (GHz). This band is especially effective for scanning within ice and snow deposits and remotely mapping them in three dimensions.
GTRI's Heath and his team are developing the hardware that controls the electronic components, such as the field-programmable gate arrays used by the phase-shifters to electronically steer the signal beam. The GTRI team is also designing the power supplies required by the system.
In addition, the Georgia Tech team is using the radar range at GTRI's Cobb Country Research Facility for testing.
"GTRI is tasked with taking the silicon-germanium / MEMS transmit-receive elements and putting them into a functioning radar system," Heath said. "These back-end electronics supply the power to those chips, as well as provide the signal processing and conditioning that steer the signals, and the processing of the raw data coming back."
Papapolymerou added that this approach to phased-array technology is expected to have uses in a variety of defense and commercial applications.
This research was conducted under contract number NNX-08AN22G. Any opinions, findings and conclusions or recommendations expressed in this article are those of the researchers, and do not necessarily reflect the views of the National Aeronautics and Space Administration.
Technical Contact: John Papapolymerou (404-385-6004)(firstname.lastname@example.org).
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