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The SBX, shown here on a cargo vessel in Texas, practiced two days of "weather avoidance" when Hurricane Emily arrived in the Gulf of Mexico during 2005 testing. The range of the array inside the dome is limited only by Earth's curvature. (Boeing)

How Things Work: Phased-Array Radar

It takes a big eye to see a missile coming.

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.

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