Skylane 182/RG Turbo Skylane/RG
Air Traffic Control Fully Certified
Human-Machine Interface
ATC Issue All Types
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings
A C182 called for flight following so I assigned a code. When the data block acquired onto the beacon code it began flashing conflict with what appeared to be the aircraft's previous 1200 code even though they had already switched to the newly assigned code. The altitudes were about 300 to 400 FT apart between the conflict code and the aircraft. I had keyed up to give a traffic alert but then the ghost target went away so I never gave it. This happens randomly and is very disconcerting for the pilots and controllers when you issue a traffic alert for an aircraft at the same location/merged and same altitude. It is impossible to tell the pilot an alternative course of action when the targets are merged! Which way do you tell him to go? This seems to happen about once out of every 15-20 call ups for VFR flight following. We need to find out why this happens and then prevent future occurrences.
ZSE Controller described an on going problem of an aircraft conflicting with itself and activating a 'Conflict Alert' when changed from a VFR code to an IFR code.