A80 CTLR VOICED CONCERN REGARDING PRM LIMITATIONS; IE; NOT ABLE TO DISPLAY PRIMARY RADAR TARGETS; SECONDARY ONLY.

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ATL LNDG E; WITH TRIPLE ILS PRM APCHS IN USE. ALL 3 FINALS WERE BUSY. I WAS WORKING THE PRM MONITOR WHEN THE APCH SUPVR CAME TO US AND POINTED OUT AN ACFT THAT WAS NOT DISPLAYED ON OUR SCOPES; BUT WAS FLYING ERRATICALLY THROUGH ALL 3 FINALS AT A HIGH RATE OF SPD (UP TO 180 KTS) WITH A MODE C THAT INDICATED ANYWHERE FROM BELOW 2000 FT TO ABOVE 23000 FT. THE ACFT CIRCLED AROUND AND FLEW RIGHT AT OTHER ACFT WITHOUT OUR HAVING ANY IDEA OF HIS ACTUAL ALT OR INTENTIONS. AT FIRST THE ACFT APPEARED TO HAVE CRASH LANDED W OF ATL; BUT A FEW MINS LATER; ONE OF THE PLTS RPTED A HIGH WING ACFT CIRCLING A COUPLE HUNDRED FT ABOVE THE TREETOPS. THE ACFT TARGET WAS NEVER DISPLAYED ON ANY OF THE 3 PRM DISPLAYS. THE PRM SCOPES WERE COMPLETELY USELESS IN THIS SITUATION. AS FAR AS I KNOW; NO RECORD WAS KEPT OF THIS INCIDENT. WITHOUT SOUNDING A LITTLE PARANOID; WHAT IF THIS PLT HAD SOME BAD INTENTIONS? AT THE VERY LEAST; THE PLT FLEW THROUGH 3 FINALS FULL OF ACFT WITHOUT ANY REPERCUSSIONS. I DON'T BELIEVE THE FAA IS FULFILLING ITS SAFETY CRITICAL MISSION BY HAVING CTLRS USE A PIECE OF EQUIP THAT DOESN'T DISPLAY PRIMARY TARGETS. WITHOUT THESE TARGETS BEING DISPLAYED; CTLRS CANNOT PROVIDE TFC INFO TO THE PLTS ON UNKNOWN TARGETS FLYING THROUGH THE FINALS.