B737-400 CREW GOT A MSAW WHILE CONDUCTING A CAT 3 APCH AT SEA.

Narrative

WX IN SEA WAS 1/4 SM BR (MIST) VV001 30.15 RVR 16R 4500-6000-6000. I ELECTED TO DO A CAT 3 AUTOLAND APCH WITH ALL HGS (HEADS UP GUIDANCE SYS) MONITORING. WE BRIEFED THE APCH PLATE AND CAT 3 CARD PER THE FLT HANDBOOK. ALL CHKLISTS AND CALLOUTS WERE COMPLETED PER OUR PROS. THE APCH WAS NORMAL ON SPD; BOTH AUTOPOLTS ENGAGED. ALL ENUNCIATED AND BOTH NAVS SHOWING ON GS AND LOC. AT AROUND 800 FT MSL SEA TWR GAVE US AN ALT ALERT; AS I INITIATED A GAR I NOTICED WE WERE STILL ON LOC AND GS. DURING THE MISSED APCH WE AGAIN COMPARED ALTIMETER SETTINGS AND READINGS. BOTH ALTIMETERS MATCHED AND TWR GAVE US A NEW BAROMETRIC OF 30.16 BUT THAT WAS ONLY .01 OFF WHAT WE HAD SET 30.15. SINCE WE COULD NOT FIND ANY PROBS WITH ANY ACFT NAV SYS A SECOND CAT 3 AUTOLAND WAS ACCOMPLISHED. SINCE ALT RPTING IS THE ONLY SYS IN COMMON WITH ATC; I SWITCHED THE XPONDER ALT SOURCE TO THE #2 SIDE AND LEFT THE XPONDER IN THE 2 POS WHERE IT WAS IN THE FIRST APCH. NO FURTHER PROBS WERE RPTED BY ATC AND THE APCH WAS NORMAL. I WROTE UP THE PROB AND MAINT MEL'ED THE #1 XPONDER. TWR INDICATED THAT THEIR SYS IS VERY SENSITIVE. ACFT OP NORMAL SUSPECTED ALT RPTING OR ATC SYS PROB.