Squitter is a term relating to random pulses, pulse-pairs and other non-solicited messages used in various aviation radio systems. Squitter pulses were originally, and are still, used in the DME/TACAN air navigation systems. Squitter pulses, because of their randomness, appear the same as unsolicited/unsynchronised replies to other interrogating aircraft, as they are identical.
Primarily, squitter is used to maintain a regular signal from the ground beacon. In the TACAN system, signal strength variation due to rotation of the transmitting beam, determines the course bearing function. (This function would be lost without a constant 2700-4800 pulse-pairs per second) in case of a lack of interrogating aircraft.
In the Mode S secondary surveillance radar system, 'squitter' is a term used to describe messages that are unsolicited downlink transmissions from an automatic dependent surveillance-broadcast (ADS-B) Mode S transponder system. Mode S transponders transmit acquisition squitter (unsolicited downlink transmissions) to permit passive acquisition by interrogators with broad antenna beams, where active acquisition may be hindered by all-call synchronous garble. Examples of such interrogators are an airborne collision avoidance system and an airport surface system.