B757 CREW DSNDING INBOUND TO SAN HAS PROB WHEN THE ARMED LOC STARTS THE ACFT IN A R TURN AWAY FROM INTENDED TRACK.

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INBOUND TO SAN ON THE BARRT 4 ARR (STAR); WE WERE OUTBOUND FROM KARRO INTXN DSNDING THROUGH FL180. WE WERE CLRED THEN 'DIRECT TO SWATT INTXN TO INTERCEPT THE LOC FOR RWY 27; DSND AND MAINTAIN 12000 FT.' CONTINUING OUR DSCNT THROUGH 15000 FT ABOUT 35-40 NM NE OF SWATT INTXN; THE CAPT DEPRESSED THE LOC BUTTON ON THE MCP AND CONTINUED TO FLY IN ACCORDANCE WITH OUR CLRNC. APPROX 35- 40 MI FROM SWATT AROUND 15000 FT DSNDING; ABOUT 1 MIN AFTER ARMING THE LOC; THE ACFT LOCKED ONTO THE LOC AND TURNED R IN AN EFFORT TO CAPTURE THE INBOUND COURSE. IT WAS AS IF THE LOC AT SAN WAS CAPTURING OUT THIS FAR. I IMMEDIATELY INSTRUCTED THE CAPT TO TURN L AND HEAD DIRECT TO SWATT AS WE WERE TOO FAR OUT TO BE ON THE LOC. I TURNED OFF THE FLT DIRECTORS AND AUTOPLT AND REPROGRAMMED THE FMS DIRECT TO SWATT. ARMED LNAV ON THE MCP; TURNED ON THE FLT DIRECTORS AND AUTOPLT. WHILE DOING THIS; ATC ADVISED US THAT WE LOOKED LIKE WE WERE NOT HDG TO SWATT. I ADVISED THEM OF THE NAV EQUIP PROB AND THAT WE WERE ENRTE NOW. THEY GAVE US A HDG (VECTOR) TO INTERCEPT; WHICH WE DID. APCH AND LNDG WERE UNEVENTFUL. ON NOV/XA/98 I FLEW THE SAME LEG AND CONFIRMED A LOCK 35 NM OUT OF SAN ON FREQ 110.9. IN IMC THIS COULD BE A DANGEROUS SIT AND WHERE EVER THE SAME ILS FREQ IS COMING FROM; IT SHOULD BE CHANGED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR IS CONVINCED THAT THEY WERE RECEIVING A LOC SIGNAL THAT WAS NOT SAN. HE SAID HE DUPLICATED THE PROB A FEW DAYS LATER ON ANOTHER FLT AND GOT THE SAME RESULTS. HE AGREED THAT THEY MAY HAVE ARMED THE LOC FARTHER OUT AND HIGHER THAN THEY SHOULD HAVE.