In the aviation industry the secondary surveillance radar (SSR), as it's called, passively interrogates transmitting air traffic in ATC's airspace (1090 MHz ADS

In the aviation industry the secondary surveillance radar (SSR), as it's called, passively interrogates transmitting air traffic in ATC's airspace (1090 MHz ADS-B and 978 MHz UAT). It's not the primary radar, which is active radar and what most people think of when they hear radar. It's a terminology issue which we're stuck with now given that SSR is part of the nomenclature. The SSR receiver is usually a long conic rotating dish, the length provides lateral isolation, as the dish itself is tilted up to match the runway glideslope (inbound and outbound track for most transmitting aircraft). The SSR can receive information well beyond the range of the primary radar. For an air traffic controller, a commercial aircraft shows up on the SSR first, and can be directed right away. The primary radar mainly catches anomalous behaviour, foreign objects and GA aircraft that may not be equipped with UAT.