Describe the difference between Active and Passive sensor operation in ADA and give an example.

• A. Active sensors emit energy to detect targets (e.g., primary surveillance radar); passive sensors do not emit and rely on external signals such as EO/IR.
• B. Active sensors emit energy to detect weather patterns only.
• C. Passive sensors emit energy to detect targets.
• D. Active sensors are used only for navigation.

Answer: (A)

Active and passive sensor operation hinges on whether the sensor itself emits energy. In air defense operations, an active sensor sends out energy—such as radio waves—and then listens for echoes to detect targets, measure range, and often track their movement. A classic example is primary surveillance radar, which transmits pulses and uses the returning echoes to locate aircraft or missiles.

Passive sensors, by contrast, do not emit energy. They rely on energy already present in the environment or energy emitted by the targets themselves. An example is electro‑optical/infrared (EO/IR) sensing, which detects visible or infrared radiation from targets (or reflected light) to form images or detect heat signatures without sending out any signals.

This distinction matters because it affects how targets are detected and tracked, how far they can be detected, and how susceptible the system is to countermeasures. The other statements don’t fit: weather or other patterns are not the defining scope of active sensing, passive sensors do not emit energy, and active sensors are not restricted to navigation.