Originally Posted by
Jan Olieslagers
Could it be that, out of the theoretical 4096, two can not be selected? Or, if they could be selected, they'd make no sense? 0000 and 7777, perhaps?
The code 'Alpha 00' (aka 0000) was commonly used in some areas when identifying traffic operating in the airspace above FL245 when I was controlling civil traffic at Northern Radar (RAF Lindholme) up until mid 1973.