FAULTY LOC AND GS SIGNAL ON CAPT INST -- BOTH CTRED; SO CAPT STARTED DSCNT; THINKING HE WAS ON THE GS. RPTR FO ALERTED CAPT THAT HIS GS WAS STILL DEFLECTED FULL UP; BUT THE CAPT CONTINUED DSNDING; EVEN WHEN THE CTLR INTERVENED 3 TIMES OF LOW ALT ALERT. AFTER BREAKING OUT OF CLOUDS; THE CAPT LEVELED AND CONTINUED VISUALLY.

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ON ILS APCH TO RWY 28L AT PITTSBURG; WE HAD BEEN CLRED FOR THE APCH. THE CAPT WAS FLYING THE ACFT (DH DASH8). THE CAPT LEFT THE INITIAL ALT OF 4000 FT WHILE I WAS STILL SHOWING FULL ILS DEFLECTION LOW. I RECHKED MY FREQ AND IDENT AND NOTIFIED THE CAPT AT ABOUT 3700 FT THAT I DID NOT HAVE A GS. THE CAPT ACKNOWLEDGED AND I RECHKED BOTH L AND R FREQS AND IDENTS. BOTH WERE GOOD. I CHKED THE MONITORS. BOTH SHOWED LOC AND GS. THE CAPT'S STEERING BARS AND GS WERE CTRED FOR APCH AND MY GS WAS DEFLECTED FULL UP. AT APPROX 2500 FT; THE CTLR ADVISED THAT HE HAD A LOW ALT ALERT. I TOLD THE CAPT THAT WE WERE LOW AND SOMETHING WAS WRONG. BEFORE I COULD ACKNOWLEDGE THE CTLR CALLED ANOTHER ALERT. WE WERE BREAKING OUT BELOW THE CLOUDS. I ADVISED THE CAPT THAT I HAD GND CONTACT AND HE LEVELED AT 2200 FT; APPROX 300 FT BELOW THE CLOUDS. I TOLD THE CTLR WE WERE LEVEL. THE CTLR XMITTED A THIRD WARNING ADVISING TO CLB. THE CAPT STAYED BELOW THE CLOUDS AND COMPLETED A VISUAL APCH. I ADVISED THE CAPT THAT MY ILS WAS CORRECTED AND WE WERE 2 1/2 TO 3 MI SHORT OF THE GS AND LOW.