TWR RECEIVED LOW ALT ALERT ON GA SMT ON ILS APCH AND CALLED OUT PULL UP. LATER DETERMINED GLIDE SLOPE WAS MALFUNCTION OR ALERT WAS FALSE.

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ACFT WAS ON ILS APCH TO BUR BTWN BUDDE (OM) AND VINNE (LMM); WHEN BUR TWR CALLED 'ALT ALERT; PULL UP 400' IMMEDIATELY.' THIS INSTRUCTION WAS FOLLOWED EVEN THOUGH ILS G/S DISPLAYED FLY DOWN. AIRPLANE ALT WAS APPROX 2300'. TWR WAS SO ADVISED. 1BUR109.5 WAS SELECTED ON #2 NAV RECEIVER AND G/S DISPLAY AGREED WITH #1. AFTER PULL UP; BOTH G/S DISPLAYS SHOWED MAX FLY DOWN SIGNAL; AND WHEN RWY WAS SIGHTED; IT WAS APPARENT THAT AIRPLANE WAS TOO HIGH AS LMM WAS APCHED; FLAPS WERE EXTENDED FROM APCH TO LNDG AND PROPS WINDMILLED TO INCREASED RATE OF DSCNT. A NORMAL LNDG WAS MADE; SLIGHTLY LONG. DURING A FLT THE PREVIOUS DAY AND ON AN ILS APCH FOLLOWING THE RPTED APCH; THE ILS G/S DISPLAY INDICATED NORMALLY. THERE WAS NO SAFETY OF FLT INVOLVED IN THE RPTED OCCURRENCE; BUT UNDER OTHER CONDITIONS; THERE COULD BE. THIS OCCURRENCE WAS DISCUSSED WITH AN EXPERIENCED AND KNOWLEDGEABLE FAA EMPLOYEE AND THE FOLLOWING POSSIBLE CONDITIONS FOR THIS OCCURRENCE ARE SUGGESTED: 1) ELECTRONIC RADIO FREQ INTERFERENCE RADIATED FROM THE GND. 2) THE ALT ALERT SIGNAL TO THE CTLR MAY; UNDER CERTAIN CONDITIONS; BE GENERATED BY A COMPUTED VALUE; BASED ON RATE OF CHANGE OF ALT; SPD (DISTANCE); RATHER THAN AN ACTUAL INSTANTANEOUS AIRPLANE ALT.