AIR TAXI PLT GETS BELOW GS ON ILS APCH TO RWY 15L AT BWI. HE HAD FAILED TO NOTE THE 'OFF' FLAG. CTLR ISSUES A LOW ALT ALERT.

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I WAS EXECUTING THE ILS/DME RWY 15L AT BWI; ALL APPLICABLE NAVAIDS WERE TUNED AND IDENTED. I WAS BEING VECTORED TO JOIN THE FINAL APCH COURSE OUTSIDE OF SLATR INTXN. BOTH GS AND LOC NEEDLES WERE SHOWING NORMAL READINGS AND NO FLAGS. UPON INTERCEPTION OF THE GS AND LOC; LNDG GEAR WAS LOWERED; AND I CONTINUED DOWN BEAM (GS). APCH TURNED ME OVER TO TWR. JUST AFTER SWITCHING FREQS I NOTICED MY GS 'FLAGGED;' HOWEVER; I DID NOT CATCH IT IMMEDIATELY. UPON CHKING IN WITH TWR; WHICH WAS BARELY READABLE (PROBABLY DUE TO MY LOW ALT); THEY ADVISED 'LOW ALT ALERT.' I WAS AT 1500 FT MSL AND THE STEP-DOWN FIX DSNDED FROM 2500 FT MSL TO 1600 FT MSL; THEY ADVISED A CLB; THEN QUERIED IF I HAD DME; I ADVISED 'AFFIRMATIVE' AND ALSO STATED GS INDICATION WAS LOST. I CONTINUED THE APCH UNDER LOC MINIMUMS (GS OUT) AND LANDED UNEVENTFULLY. CONTRIBUTING FACTORS: SINGLE PLT IFR/IMC; DETERIORATING WX CONDITIONS; BUSY TERMINAL ENVIRONMENT; FIXATION ON ONE INST; AND THE 'GS NEEDLE' REMAINED IN AN 'ON-GLIDE PATH' INDICATION AFTER FAILING.