How Metamaterials Are Reinventing 3-D Radar Imaging
Synthetic aperture radar is the imaging technique behind many spy satellites. Now exotic metamaterials have made it even better.
Synthetic aperture radar is a remarkable imaging technique that produces high-resolution 2-D and 3-D pictures from radar reflections. Because it relies on radio or microwaves rather than visible light, it can see through haze, clouds, and sometimes even walls. For that reason, it has become the go-to technique for Earth sensing, security screening, and state-sponsored spying.
There is a problem, however. Synthetic aperture radar systems tend to be big, power hungry, and mechanically complex when they have steering mechanisms to point them. All that makes them expensive, too. Which is why synthetic aperture radar is used mainly by the kind of military and government organizations that can afford it.
So any way to make these synthetic aperture radar systems smaller, cheaper, and more efficient would be hugely significant.
Today, Timothy Sleasman at Duke University in Durham, North Carolina, and a few pals unveil just such a system. Their synthetic aperture radar is built from an exotic new substance called a metamaterial, which makes it more flexible, more efficient, and cheaper than anything built before—while maintaining the same image quality as traditional synthetic aperture radar systems.
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Radar systems create images by emitting a series of pulsed radio waves and then recording the signal that is reflected from the environment. The resolution of this technique is limited by the size of receiver. One way to gather more of the returning waves is to use a reflecting dish with a bigger surface area than a simple antenna. This increases the resolution of the radar.
But in the 1950s, American aerospace engineers realized that there is another way to improve the process of signal gathering—by moving the antenna while it is receiving.
In this scenario, the antenna is on board an aircraft or spacecraft. It emits a radio pulse that spreads out and reflects off a range of objects on the ground. The reflected signal returns to the antenna, which has moved. The distance it travels during this time effectively increases the size of the receiving aperture and therefore the resolution of the system.
Of course, there needs to be some powerful signal processing to number crunch the signal when it returns to create 2-D and 3-D images. But this is relatively straightforward these days. The result is a “synthetic aperture radar” with a much higher resolution than a stationary antenna.
Since the 1950s, this technique has been significantly improved and fine-tuned. For example, the resolution can be increased further by gimballing the transmitter as it moves, to point it toward a specific target. Another technique for beam focusing is to use an array of antennas that all produce pulses that interfere in a way that points the overall signal in a specific direction.
But these techniques are power hungry, mechanically complex, and expensive.
Enter Sleasman and co and their metamaterial. This is a periodic structure made from tiny electronic components that each interact with an electromagnetic field. Together these components give the material exotic bulk properties that are otherwise never found in nature.
Various groups have built metamaterials that bend electromagnetic waves, including visible light, in strange ways. They’ve even built invisibility cloaks in this way. (Indeed, the leader of this team, David Smith, built the first invisibility cloak like this at the turn of the century.)
Their radar aperture consists of a narrow strip of printed electronic resonating circuits working at microwave frequencies. Each resonator receives and broadcasts at a specific frequency, which can be varied by tuning its electronic properties, like a radio tuner. “The overall radiation pattern generated by this aperture is thus the superposition of the radiation from each single radiator,” say Sleasman and co.
The team calls this antenna a dynamic metasurface. It is significant because by tuning each radiator appropriately, the team can control the pattern of radiation precisely. This gives Sleasman and co control over the direction of the beam, its overall shape, and, within certain limits, its frequency.
That gives them a wide range of impressive abilities. “The flexibility offered by dynamic metasurfaces may be used to steer directive beams for enhanced signal strength, create nulls in the pattern to avoid jamming, probe a large region of interest with a wide beam, or even interrogate multiple positions at once with a collection of beams,” says the group.
By itself that’s a significant step forward but Sleasman and co go further by testing an entirely new form of synthetic aperture radar. Dynamic metasurfaces allow Sleasman and co to produce a series of pulses that vary in direction entirely at random. So as the dynamic metasurface moves through space, it picks up the reflections from these random beams.
The big advantage of this technique is in the way these signals are processed. Because they vary in direction at random, they cover a much wider area than a conventional beam, which points in just one direction.
With a single beam, it is possible to create high resolution images of a single subject. But with a series of random beams, it is possible to produce high-resolution images of many subjects at the same time. It is even possible to reprocess the data later to focus on a new subject of interest. “In this sense, the aperture probes many parts of the scene’s spatial content simultaneously and investigates each location multiple times,” say Sleasman and co.
The main part of their work is in building this device and then characterizing its performance. And the results are impressive.
The team shows that the new imaging technique produces images that are just as good as traditional synthetic aperture techniques, but with the added benefits described above. What’s more, the dynamic metasurface is so versatile and easy to control that it can be used in the traditional ways as well. “We demonstrate high-quality imaging in both 2-D and 3-D,” say Sleasman and co.
That’s impressive work that could have significant implications for the way synthetic aperture radar is used. Having better, high-resolution imaging techniques is obviously useful. But the biggest advantage is probably its cost. Dynamic metasurfaces can be printed en masse at low cost.
That suddenly makes them potentially useful for a wide range of applications. As Sleasman and co put it: “Dynamic metasurface aperture is poised to make important contributions across the entire field of microwave sensing.”
Ref: arxiv.org/abs/1703.00072: Experimental Synthetic Aperture Radar with Dynamic Metasurfaces
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