How Things Work: Phased-Array Radar
It takes a big eye to see a missile coming.
- By Sam Goldberg
- Air & Space magazine, July 2006
(Page 2 of 3)
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.”