A320 FO REPORTS NAV/GPS POSITION DISAGREE ECAMS DURING TAXI AT SAN; WHICH ARE COMMON AND ROUTINELY DISREGARDED. AFTER TKOF AN FMGC IS DISCOVERED TO HAVE FAILED.

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WHILE TAXIING OUT AT SAN WE GOT REPEATED 'NAV FMS/GPS POSITION DISAGREE' WARNINGS FROM ECAM. THIS IS A WELL KNOWN PROBLEM THAT HAPPENS EVERY TIME WE FLY THE AIRBUS INTO SAN DIEGO (I DO NOT KNOW THE CAUSE). WE CANCELLED THE WARNING REPEATEDLY AND CONTINUED WITH THE TAKEOFF. AFTER TAKEOFF WE HAD INDICATIONS THAT ONE OF OUR 'FLIGHT MANAGEMENT GUIDANCE COMPUTERS' (FMGC'S) HAD FAILED. I'M NOT POSITIVE; BUT I BELIEVE IT MAY HAVE BEEN FAILED BEFORE WE TOOK OFF; WE MAY HAVE DISREGARDED THE 'WARNING SIGNS' WHEN WE WERE DISREGARDING THE NAV POSITION FAILURES. THERE ARE 2 PROBLEMS HERE: 1) THE GPS ACCURACY ANOMALY AT SAN NEEDS TO BE ADDRESSED SO CREWS ARE NOT 'TRAINED' BY HABIT TO JUST 'IGNORE' AN ECAM WARNING. 2) THE FMGC'S ARE NOT MONITORED BY THE ECAM SYSTEM ON THE AIRBUS. LOSS OF AN FMGC IS A VERY SERIOUS MATTER; IT SHOULD HAVE A SEPARATE CREW WARNING RATHER THAN JUST LEAVING HARD TO INTERPRET 'CLUES' FOR THE CREW TO ATTEMPT TO DECIPHER.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER BELIEVES THAT THERE MAY BE INTERFERENCE AT SAN THAT AFFECTS THE GPS RECEIVER CAUSING THE ECAM WARNINGS. HOWEVER; THE REAL PROBLEM IS THE FACT THAT FLT CREWS ARE IGNORING THE WARNINGS AND USE THE ECAM EMERGENCY CANCEL TO PREVENT THEM FROM REPEATING. THE FACT THAT AN FMGC FAILURE CAN BE SUBTLE AND SHOULD PRODUCE AN ECAM WARNING NEEDS TO BE ADDRESSED.