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That's one way of doing it. Another way is to have two or more sensors mounted on the same mast, rotating around the same point and sweeping at the same time. You would need each transmitter receiver to be on a slightly different frequency and at a slightly different angle from each other. This would eliminate the need for a second servo (assuming the existing one could support that movement of larger mass).There's broadly two types of 3D radars, phased array (like SPY-1) which are very complex, and older ones like I mention above. Look up SPS-49, it has one antenna, but the triangular feed horn shoots out the RF at 4-5 different frequencies, and due to the angle it bounces off the antenna, corresponds to a different look angle in space.