Active sensors are phased-array radars or lidar systems. They are used for hi-res target identification. Active sensors are used to track vessels whose drives are not active, and so are the primary sensors system at shorter ranges. The decision to 'go active' is a major tactical choice as the sensor system becomes visible at a considerably greater range that that at which it can detect targets.
Radar systems require much less accuracy for aperture synthesis than optical sensors, so a common strategy is to deploy the emitters and detectors on sensor drones. This provides much greater resolution and also keeps the bright radio emission away from the controlling ship.
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