Why are Surveillance Identifier (SI) codes required?
An Identifier Code (IC) is the means by which an aircraft’s Mode S transponder identifies a particular interrogating station and, subsequently, can selectively respond to it. Initially, there were only 15 Interrogator Identifier (II) Codes available for this purpose, however, amendment change 73 to ICAO Annex 10 resulted in an additional 63 codes being made available in the form of Surveillance Identifier (SI) Codes. Due to the number of Mode S stations that will be operational in Europe, it is essential for aircraft to be equipped with Mode S transponders that support both II and SI code functionality.
