How Things Work: Phased-Array Radar
It takes a big eye to see a missile coming.
- By Sam Goldberg
- Air & Space magazine, July 2006
At 282 feet in height, two football fields in area, and 50,000 cubic tons in volume, the Sea-Based X-Band Radar inspires description in the mold of “Your mama” jokes: SBX is so fat it couldn’t fit through the Panama Canal. SBX is so tall it could straddle the Goodyear Blimp. SBX is such a superstar, it has its own self-propelled, semi-submersible oil-drilling platform as a ride.
The near-$900 million structure, operated by the U.S. Missile Defense Agency (MDA), is by far the largest phased-array radar system on Earth. It is 16 times more powerful than the previous champ—its own prototype—and capable of determining if a baseball-size object thrown into space from another continent is a slider, a curve, or a knuckleball.
This summer it will leave Pearl Harbor, where it is being painted, and voyage to its home port of Adak, Alaska, for the first time. After being integrated into the battle management systems of long-range interceptor missiles located in Alaska and California, the SBX will be able to move throughout the Pacific Ocean, or anywhere else it’s welcome, for training or actual defensive operations.
Once active, it will identify enemy missiles outside the atmosphere, at the highest point of their ballistic trajectories, so that the interceptors can take them out.
With the same frequency of radiation that your microwave oven uses to warm Lean Cuisines, SBX probes the nooks and crannies of “threat complexes”—the cloud of warheads, decoys, and debris such as loose nuts and bolts, spent booster stages, and unburned fuel that surround an enemy missile.
“We can differentiate between very tightly spaced objects, small and large objects, and the like,” says Army Colonel John Fellows, MDA’s project manager for X-band radars. “We can tell [which] is the threatening object.”
At the frequencies used by X-band radars, ranging from 8 to 12 gigahertz, relatively short wavelengths enable sharp, high-resolution radar images.
“You might be able to see rivets and seams and joints and fins, and [that] allows you to form a very accurate representation of what is there,” says Larry
Briggs, who as Raytheon’s program director for ground-based radars oversaw the design and construction of the SBX’s radar array.
Before catching a ride to Hawaii on the back of the Blue Marlin, the world’s largest cargo vessel (which had to be widened for the job), the SBX spent the summer of 2005 on a 52-day shakedown cruise in the Gulf of Mexico. After the platform outran hurricanes, it tested its 10-story array by turning it skyward to track satellites.
Like all radars, the SBX works by broadcasting a pulse of radio waves, then watching for the reflections. The radio waves are produced by tiny antennas called radiating elements. SBX has roughly 45,000.
Radio rays build upon or cancel each other when they cross paths. But just how waves interfere with each other depends on the phase of each contributing wave—whether the wave is at its crest, its trough, or somewhere in between.
A map of the interference between radio waves is called a radiation pattern. It is the radiation pattern that allows one to see where waves constructively overlap and where waves destructively overlap to cancel each other. The main beam is formed at the line where the greatest number of waves projected by the radar emitters constructively overlap to form a composite wave front.
A conventional radar tracks targets by physically turning its main beam 360 degrees and then measuring how reflective items—“blips”—have moved since previous sweeps.
But phased-array radars work differently; they steer the main beam by manipulating the pattern emanating from an array of hundreds or thousands of radiating elements, nearly instantaneously moving the location of the overlapping waves instead of an actual dish.
“You don’t change [the antenna’s] properties when you scan,” explains Larry Corey, chief engineer of Georgia Tech Research Institute’s Sensors and Electromagnetic Laboratory. “You just change how the energy from every one of those elements adds up either constructively or destructively.”
To point the SBX’s powerful main beam, computers command each of its radiating elements to slightly shift the initial phase of the radio waves they shoot.
Thus, each element emits a radio wave with crests and troughs that are slightly out of sync with the crests and troughs of the radio waves emitted by its neighbors. For example, a wave being radiated from element A may start at a crest, while the wave emanating from element B begins life as a trough.
The effect is that the beam swings from the center to the right or left (see diagram, opposite). With the new elements added, the beam can be pointed up or down as well. The direction of the beam can be changed in 20 microseconds or less.
The main advantage to this approach is that the radar can keep a constant eye on a target—it can shoot and watch for radio reflections thousands of times per second instead of going blind until the next rotation sweeps the main beam past the target again.
Since the main beam can be pointed almost instanteously, it can jump from object to object as they come into range.
Phased-array radars are not without disadvantages. Most are functional through a cone of just 120 degrees, because the width of the main beam diminishes the farther it gets from broadside. As an example, think of how narrow your wide-screen television looks when you’re in an adjacent room.
For this reason, at least four radars are needed to cover a hemisphere. To compensate for the narrow field of view, the SBX’s main array rotates and tilts; it’s one of the few phased arrays to do that.
Although the initial cost is 100,000 times more expensive than a conventional radar with the same beam width, a phased-array device may be cheaper long-term because the system will still function as needed even if many of its smallest components fail.