Aviation transponder interrogation modes
http://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes
"In its simplest form, a "Mode" or interrogation type, is generally determined by pulse spacing between two or more interrogation pulses. Various modes exist from Mode 1 to 5 for military use, to Mode A, B, C and D and Mode S for civilian use"
Mode S – provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian)
Walt