The spinning radar on a boat is a unit that usually sits at the highest part of the structure. It scans the horizon to pick up any radiomagnetic signals from objects within range over a 360-degree pattern.
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As the antenna turns, it sends out signals that reach an object. The elapsed time for the signals to return indicates the distance traveled. As the boat proceeds, the antenna picks up changes in the angle and distance. A computer uses this information to plot the boat's relative position to the object. The length and strength of the antenna determine the area covered. Standard sizes are 4 feet and 6 feet.
An open-array radar unit is the most common one used on boats of all sizes. The system consists of a horizontal bar containing the scanning hardware. The bar sits on a pedestal connected to a gearbox that controls the direction, or "spinning," of the antenna. The cables then continue below deck to the display unit, which is generally at the helm.
Most larger ships use antennas contained in a dome to protect them from the weather. The same spinning bar is present, but it's encapsulated and therefore not visible. Domes for smaller craft are available in diameters of 18 and 24 inches.