Primary & secondary radar

Primary radar (PSR) transmits a pulse and listens for the echo, so it sees every reflective object (aircraft, ground clutter, weather) but only gives range and bearing. Secondary radar (SSR) interrogates transponders, which reply with identity (Mode A), altitude (Mode C) and more, but it cannot see aircraft without a working transponder. Green squares are echoes; amber boxes are SSR replies with data blocks. Three aircraft here have no transponder, so only PSR shows them.