Passive radar can detect aircraft without transmitting a radar signal of its own. By exploiting FM radio, digital television, cellular networks and potentially satellite communications, these systems are becoming an increasingly important layer of modern air defence.

Passive infra-red and electro-optical sensors are becoming one of the least understood layers of modern air defence. They do not replace radar, but they can help air-defence networks detect, track and classify aerial threats without transmitting radar energy. This makes them harder to detect electronically and harder to suppress in contested airspace. From Israel’s Sky Spotter to European, US, Russian, Chinese and Iranian systems, passive EO/IR sensing is moving from a specialist technical niche into the centre of modern layered air defence.