FLT CREW OF MDT UTILIZING HUD FOR AN ILS APCH TO PDX DISCOVER IT IS FOLLOWING A FALSE GS WELL BELOW THE VERT PATH.

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FIRST LEG OF THE DAY; EARLY MORNING DEP OUT OF SPOKANE INTL. ON APCH TO PDX (ILS RWY 10R) USING HUD FOR MORE PRECISE FLYING AS WX WAS MARGINAL WITH GND FOG. (TOPS APPROX 800 FT.) (PDX AT SEA LEVEL.) HAD FLT DIRECTOR PROGRAMMED TO INTERCEPT LOC AND GS. LOC CAPTURED AT 15 DME (FAF AT 8.1 DME). WAS SURE GS WAS CAPTURED THROUGH HUD (LOOKING OUT THE FRONT WINDOW). STARTED DOWN WHAT I THOUGHT WAS A CAPTURED GS BY CUE PROMPTS ON HUD. JUST INSIDE FAF (8.1 DME) I HAPPENED TO GLANCE DOWN AT MY PFD ON THE PANEL AND NOTICED GS WAS FULL SCALE DEFLECTION ABOVE OUR ALT. IMMEDIATELY INITIATED MISSED APCH PROC AND OVERFLEW THE ARPT (WHICH I COULD SEE THROUGH THE FOG LAYER (DOWNWIND VISIBILITY)). AT NO TIME DID WE ENTER THE CLOUDS AND HAD ALL TERRAIN INCLUDING I-T BRIDGE IN SIGHT FOR THE ENTIRE APCH. AS WE WERE CLBING OUT ON THE MISSED APCH; PDX TWR ISSUED A 'LOW ALT ALERT.' WE COMPLIED WITH MISSED APCH VECTORS AND CAME AROUND FOR ANOTHER UNEVENTFUL APCH TO LNDG ABOUT 8 MINS LATER. WE THOUGHT THE GS MAY HAVE BEEN COMPROMISED BY GND TFC AND ASKED TWR IF THEY WERE SHOWING ANYONE NEAR THE ILS HOLD LINE. THEY REPLIED THAT THE APCH SYS WAS PROTECTED. IN CONCLUSION; I WAS FLYING WITH A BRAND NEW FO. WE HAD AN EARLY MORNING WAKE-UP CALL THAT AND THE PREVIOUS MORNING; BUT I STILL CAN'T FIGURE OUT WHY THE HGS SYS WAS PROMPTING ME TO FOLLOW THIS 'FALSE GS' WHICH CAUSED THE FULL SCALE DEFLECTION TO OCCUR. BOTH I AND MY FO LEARNED A VALUABLE LESSON ON XCHKING MORE DILIGENTLY IN THE FUTURE.