An active sensor is a system which emits a signal as part of its normal functioning. Active sensors tend to be more precise than passive sensors, but they are much less useful than passives most of the time - because the square law guarantees that the signal emitted by said sensors will be detectable by an observer for a much greater range than it will be able to return usable information to the sender.

Some examples of active sensors include:

• RADAR
• SONAR
• searchlight (think about it)