1. The SPI (Special Purpose Identification) pulse is used by air traffic controllers to confirm the identity of certain aircraft. The controller will ask the pilot to squawk ident – the pilot pressing a button on the control panel which adds the SPI pulse to SSR replies for a certain period (18±1 s).

Im Fall des Modus A wird der Oktalcode (ABCD8) am Bedienteil des Transponders im Flugzeug eingestellt.