Most Mode S transponders such as the Honeywell MST-67A and Collins TDR-94D include a feature that allows it to automatically determine the aircraft Registration Number (Tail Number) of U.S. registered (N numbered) aircraft. If the aircraft is U.S. registered and operated where the tail number is used as the call sign (Flight ID), then the operator does not have to manually enter the Flight ID. Therefore, a control head with Flight ID input such as the Honeywell PS-578 or Collins CTL-92E is not required.
The transponder automatically reads the Mode S address assigned to the aircraft. If the Mode S address falls within the range of addresses assigned to the U.S. by the International Civil Aeronautics Organization (ICAO), the MST-67A will automatically convert that Mode S address to the tail number of the aircraft and place it in the Flight ID register for compliance with Elementary Surveillance.
If the aircraft is operated where the Flight ID is not the tail number or the aircraft is not registered in the U.S., then a Flight ID control head is required to allow for manual entry of Flight ID. The flight ID may contain the company-assigned number for that particular flight such as an Airline name and Flight number.