A320 ACFT ON SID DEP AND SHORTLY AFTER TKOF FLC HAD TCASII RA DIRECTING A DSCNT WITH TARGET SUPERIMPOSED ON ACFT. FLC FOLLOWED THE RA UNTIL BREAKING OUT OF CLOUDS AT 900 FT AT WHICH TIME GPWS SOUNDED AND FLC SAW TALL BUILDINGS. THEY CONTINUED WITH ATC CLRNC AND THE FALSE WARNING CONTINUED THROUGH MOST OF THE FLT.

Narrative

UPON DEP FROM LGA RWY 13; WHITESTONE CLB; PASSING APPROX 1000 FT AGL; TCASII TA 'TFC; TFC' SOUNDED. AT THE TIME WE WERE IN IMC CONDITIONS. IMMEDIATELY AFTER THAT RA 'REDUCE VERT SPD' CAME ON WITH TCASII TARGET SUPERIMPOSED ON OUR ACFT SYMBOL. WE BEGAN REDUCING CLB WHEN RA 'DSND; DSND' SOUNDED WITH VERT SPD COMMAND OF GREATER THAN 2000 FPM ANNUNCIATED. TARGET STILL DIRECTLY ON TOP OF US WITH RELATIVE ALT DISPLAYED. WE IMMEDIATELY COMMENCED DSCNT AND EXITED CLOUDS AT 900 FT AGL AT WHICH TIME GPWS WARNING CAME ON SAYING 'TOO LOW; TERRAIN' BY THIS TIME SPD HAD BUILT UP TO 280 KTS. I THEN DECIDED IT WAS BETTER TO TAKE A CHANCE ON HITTING ANOTHER AIRPLANE VERSUS THE SURE THING OF COLLIDING WITH THE GND; AND DIRECTED THE FO TO RESUME CLB AND DEP WHILE I TURNED XPONDER TO TA ONLY. AT THIS TIME ATC GAVE US A NEW HDG AND ALT AND I RPTED THE TCASII RA MANEUVER. ALL THIS TIME EITHER A TA; RA OR GPWS WARNING HAD BEEN GOING ON AND FOR A WHILE 'DSND; DSND' AND 'TOO LOW; TERRAIN' WERE BEING BROADCAST SIMULTANEOUSLY. THE COCKPIT INDEED WAS A CACOPHONY OF BELLS; WHISTLES AND FLASHING LIGHTS. THIS SHOWS THE DANGER OF BLINDLY FOLLOWING TCASII DIRECTIONS. THE PHANTOM TARGET FOLLOWED US ALMOST ALL THE WAY TO OUR DEST AT RELATIVE ALT. IF WE HADN'T POPPED OUT WHEN WE DID AND SEEN BUILDINGS IN FRONT OF US IT COULD HAVE BEEN A MUCH DIFFERENT STORY. WE ARE TAUGHT TO IMMEDIATELY RESPOND TO AN RA AND AT THAT ALT YOU CAN LITERALLY FIND YOURSELF 'BTWN A ROCK AND A HARD PLACE.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT HAD NOT HAD FEEDBACK ON THE CAUSE OF THE FALSE TARGET; BUT WAS PLANNING TO FOLLOW UP. AND; HE SAYS HE WILL CONTACT ASRS IF HE HAS ANY INFO TO PASS ON. RPTR HAD BEEN ON VACATION AND WAS JUST GETTING BACK TO WORK; SO HE WILL TRY. HOWEVER; AS HE SAYS; SINCE THE PROB WENT AWAY ENRTE; MAINT PROBABLY WOULDN'T DO MUCH. RPTR CAPT WAS COUNSELED ABOUT THE SUPPRESSION BUS PROB IN OTHER ACFT AND A SUPERIMPOSED TARGET IS HIGHLY SUSPECT.