Radar systems take two basic approaches to maximizing signal coverage: by spinning a traditional parabolic reflector and through electronic control of a fixed planar array. The planar array, although a more complex and demanding technology, has several advantages over the parabolic antenna, most of which stem from its mechanical simplicity and lack of moving parts.
A parabolic reflector antenna has the shape of a parabola -- all points on the inside surface are the same distance from the antenna's focal point. A fixed parabolic antenna transmits signals in a pattern that radiates outward. To cover a greater area, a motor spins the antenna in the horizontal plane; some applications call for a 360-degree rotation, while others sweep the antenna back and forth in less than a full circle. By contrast, a planar array does not move physically. The antenna consists of a grid of smaller elements, each radiating a signal slightly out of phase with its neighbors. The radar waves interfere constructively and destructively, "steering" the composite signal in any desired direction. The technology requires sophisticated software which controls the phase of each element; the radar signal moves in accordance with a computer program.
No Mechanical Wear
A planar array antenna is more reliable than a moving parabolic dish because it has no motors, gears or bearings that can wear out. This is particularly important for radar equipment operating in harsh environments such as salty ocean air or fine desert sand -- challenging conditions for machinery. The elimination of the need for mechanical repairs is an added bonus, especially in locations far from spare parts supplies. Greater reliability tends to improve overall uptime, an important consideration for air traffic control, military applications and other situations that require continuous radar coverage.
Variety of Scan Movement
Parabolic antennas under motor control have a limited range of movement, while planar arrays have fewer restrictions. For example, in a radar installation between two mountains, technicians can program a planar array to avoid the directions in which the mountains lie in order to avoid scanning them. A parabolic antenna cannot skip segments of its rotation, however, and must complete a wedge or a full circle. For a planar array, the same flexibility applies in all three dimensions: it can scan up and down as well as horizontally, and it includes or omits any areas the operators choose.
Some radar installations communicate with satellites. A moving parabolic antenna must rotate into position before it receives a satellite signal. This is a time-consuming process that becomes more cumbersome with multiple satellites. A planar array antenna, on the other hand, can instantly switch among several satellites. It wastes no time in finding the correct locations, thereby improving the speed of communications.
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