PLT OF C172 DSNDED PREMATURELY ON VOR RWY 23 APCH TO GON. PVD APCH CTLR RECEIVED MSAW ALERT; ADVISED GON LCL CTLR WHO ALERTED PLT. PREMATURE DSCNT WAS CAUSED BY INCORRECT VOR RADIAL BEING SET TO IDENTIFY FAF.

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I WAS DIVERTING TO GROTON; CT (GON); BECAUSE OF LOW CEILINGS AND VISIBILITY AT MY INTENDED DEST ARPT; AND I MADE THE DECISION TO DIVERT AS I WAS PASSING THE FIELD AT 8000 FT. I WAS BEING CTLED BY PROVIDENCE APCH CTL; AND REQUESTED THE DIVERSION FROM THEM. THEY ACCEPTED MY REQUEST; AND GAVE ME VECTORS TO FINAL FOR THE VOR 23 APCH TO GROTON. SURFACE WINDS AT GROTON WERE FROM 150 DEGS AT 6 KTS. CEILING WAS DESCRIBED AS 1100 FT BROKEN AT THE FIELD; AND THE ALTIMETER WAS 29.91. AT A HDG OF 210 DEGS; PVD APCH TOLD ME TO MAINTAIN 2000 FT UNTIL ESTABLISHED ON FINAL AND CLRED ME FOR THE APCH. I INTERCEPTED THE FINAL APCH COURSE AND DSNDED TO 1700 FT OUTSIDE THE FAF (BABET INTXN). I NOTICED THAT THE XWIND WAS SUBSTANTIALLY GREATER ON FINAL THAN I HAD ANTICIPATED; AND THAT I NEEDED MORE CORRECTION TO MAINTAIN PROPER FINAL APCH COURSE. I WAS RELEASED BY APCH TO CHANGE TO THE TWR FREQ AFTER BEING ESTABLISHED ON FINAL. I BEGAN A DSCNT FROM 1700 FT TO 650 FT AT WHAT I THOUGHT WAS BABET; WHICH IS DEFINED BY THE FINAL APCH RADIAL FROM TMU AND R183 FROM ORW. I BELIEVE I MAY HAVE ENTERED THE RADIAL FOR ORW INCORRECTLY; CLOSER TO 177 DEGS OR 180 DEGS; AND CONSEQUENTLY; MY DISTANCE FROM TMU WAS GREATER THAN IT SHOULD HAVE BEEN. I HAD A GPS UNIT IN THE AIRPLANE; AND IT WAS RPTING MY DISTANCE FROM GON; BUT IN CORRECTING FOR THE XWIND; I TOOK LONGER TO DOUBLE-CHK MY DISTANCE THAN I SHOULD HAVE; AND BEGAN MY DSCNT BEFORE MAKING THIS DOUBLE-CHK. AS I WAS NOTICING THE GPS DISTANCE AND REALIZING THAT I MIGHT HAVE BEEN DSNDING EARLY; THE TWR CALLED TO SAY THAT PVD APCH HAD RPTED THAT I WAS BELOW THE PROPER ALT FOR THE SEGMENT AND SHOULD CHK MY ALTIMETER SETTING IMMEDIATELY. AT THE TIME OF THIS CALL; I WAS AT APPROX 1450 FT MSL. I CLBED TO 1700 FT UNTIL I WAS SURE I WAS PAST THE FAF; AND THEN CONTINUED TO APCH; DSNDING TO 650 FT AND FLYING AT THAT ALT UNTIL I COULD SEE THE ARPT AND COMPLETE A LNDG. I BELIEVE THERE WERE 2 HUMAN PERFORMANCE FACTORS THAT MAY HAVE BEEN CONTRIBUTORY TO THIS PROB. FIRST; AS A RELATIVELY INEXPERIENCED INST PLT; I FOUND MYSELF PRESSED TO QUICKLY BRIEF THE APCH; DSND THE AIRPLANE SMOOTHLY; AND MAINTAIN HDG PROPERLY WHILE BEING VECTORED. IN THE FUTURE; I WILL ASK FOR MORE TIME TO PREPARE MORE THOROUGHLY. SECOND; I HAD SPENT ABOUT 40 MINS AT 10000 FT BEFORE ASKING FOR LOWER ABOUT 30 MINS BEFORE PASSING GROTON AND DECIDING TO DIVERT THERE; AND ON REFLECTION; I MAY HAVE BEEN SUFFERING SOME RESIDUAL EFFECTS OF MILD HYPOXIA. IN THE FUTURE; I WON'T ASK FOR ALTS ABOVE 8000 FT WITHOUT USING OXYGEN IF THERE IS ANY CHANCE OF FLYING IN IMC AFTERWARDS.