 In the TACAN system, signal strength variation due to rotation of the transmitting beam (amplitude modulation) determines the course bearing function.This function would be lost without a constant 2700-4800 pulse-pairs per second, carrier-like signal in cases of low or no interrogating aircraft.In the Mode S secondary surveillance radar system, the term is used to describe messages that are unsolicited downlink transmissions from an automatic dependent surveillance-broadcast (ADS-B) Mode S transponder system.