PVT PLT OF AN SEL SMA INADVERTENTLY ENTERED CLASS C AIRSPACE BEFORE OBTAINING A CLRNC FOR LNDG DUE TO DETERIORATING WX CONDITIONS AND DISORIENTATION.

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DEPARTED BOWIE ARPT (0F2) IN VMC FOR PLAINVIEW VOR VIA V278 FOR A STOP AT HALE COUNTY (PVW). AROUND GUTHRIE VOR THE WX STARTED TO DETERIORATE RAPIDLY. I TURNED S BECAUSE THE WX LOOKED BETTER AND DECIDED TO STOP AT ABERNATHY. ABOUT 30 DME FROM LUBBOCK VOR AT 8500 FT; I DECIDED THE WX WAS DETERIORATING TO A POINT THAT I HAD TO GET ON THE GND. I WAS ABOVE A CLOUD COVER IN VFR CONDITIONS BUT UNABLE TO SEE THE GND. I CHKED LUBBOCK ATIS AND THEY WERE RPTING 800 FT CEILING AND 10 MI VISIBILITY. THE WX BEHIND ME WAS NO BETTER. I CHKED MY GPS AND IT WAS SAYING 'NO GPS COVERAGE.' I FOUND A LARGE HOLE IN THE CLOUDS AND WENT UNDER. THE VISIBILITY WAS GOOD BUT LOW CEILINGS. I HEADED FOR LUBBOCK VOR ON V102 AND CHKED LUBBOCK'S ATIS TO CALL THE TWR AND CONFESS MY SIT AND ASK FOR HELP. BEFORE I WAS ABLE TO CALL THE TWR; I FOUND MYSELF IN THEIR AIRSPACE. I CALLED THE TWR; TOLD THEM MY SIT AND LOCATION; AND ASKED TO LAND. DETERIORATING WX. LOSS OF GPS COVERAGE. TRIED TO TROUBLESHOOT PROB TOO LONG. STRESS LEVEL RISING AND MISREAD VOR'S CDI. AFTER DEPARTING LUBBOCK ARPT ON MAY/XA/99; I FOUND MY GPS STILL INOP. THE PROB WITH THE GPS IS THE EXTERNAL ANTENNA. I HAVE A GARMIN 195 HANDHELD AND IT WORKS FINE WHEN REMOVED FROM THE ACFT YOKE.