PROMPT ACTION BY AN ALERT PDX APCH CTLR AVERTS POSSIBLE CFIT AT HIO WHEN B1900 FLT CREW DSND BELOW AUTH ALT PRIOR TO FAF ON NDB GPS-B APCH.

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PRIOR TO BEING CLRED FOR ADF APCH; THE SECOND OF 2 CTLRS ADVISED; 'THERE IS AN AIRPLANE ON THE APCH TO RWY 30 AND 2 AIRPLANES HAVE TRIED THE APCH AND MISSED FOR RWY 12 AND RWY 30 AND THERE IS NEW ATIS INFO AT HILLSBORO -- WHAT ARE YOUR INTENTIONS?' WE ADVISED WE WOULD LIKE TO CONTINUE FOR RWY 12 ADF APCH. I ADVISED THE CAPT TO GUARD THE APCH WHILE I CHKED THE NEW WX. BEFORE I COULD FINISH GETTING ATIS AT HIO; THE PDX CTLR CLRED OUR AIRPLANE FOR THE ADF APCH. WHEN I RETURNED; WE WERE IN A STEEP DSCNT APCHING 2900 FT FAST (FINAL APCH ALT PRIOR TO FIX). AS HE CONTINUED HIS DSCNT; APCH ADVISED US TO CONTACT THE HIO TWR. IT WAS THEN I REALIZED HE WAS NOT YET AT THE FIX -- ADF! -- AND WE WERE DSNDING INTO A LARGE VALLEY WITH GND AND HILLS VISIBLE; CONFIRMING HE WAS NOT YET TO THE ADF AND SHOULD NOT BE DSNDING AND ADVISED HIM. AT THE SAME TIME; THE APCH CTLR ADVISED THERE WAS A TERRAIN WARNING ON HIS SCREEN AND TO CANCEL THE APCH CLRNC. WE IMMEDIATELY EXECUTED THE DIRECTED MISSED APCH. OUR CORRECTIVE ACTION PREVENTED A CFIT. LATER; CONVERSATIONS REVEALED THE PIC MISINTERPED HIS POS ON THE APCH BECAUSE OF THE GPS HE WAS WATCHING. MY ATTN WAS DIVERTED BY INFO FROM ATC WHO LED ME BELIEVE ANOTHER AIRPLANE ON APCH WOULD CAUSE US TO HOLD OR RECEIVE A DELAY VECTOR AND THAT NEW WX; WHICH COULD PREVENT US FROM FLYING THE ADF APCH.