UNAUTH PENETRATION OF CLASS D AIRSPACE; PROX OF ATA.

Narrative

WE WERE ON AN IFR FLT FROM SCOTTSDALE; AZ; TO CONCORD; CA; AND HAD BEEN CLRED FROM NEAR LIDAT INTXN DIRECT CCR. ISOLATED TSTMS WERE EVIDENT WITH CUMULUS AND STRATUS CLOUD SLOPING DOWN FROM 28000 FT TO 6000 FT OVER THE VALLEY. ABOUT 50 MI OUT I WAS CLRED TO 6000 FT. APCHING THAT ALT I WAS VFR AND WAS APCHING SOME CUMULUS WHICH LOOKED PRETTY ROUGH. SINCE CONCORD ATIS WAS RPTING 2500 FT SCATTERED AND THE CEILING IMMEDIATELY AHEAD LOOKED AT LEAST THAT HIGH; I CANCELED MY IFR AND DSNDED BELOW THE CLOUDS; KNOWING THAT THERE WERE NO OBSTRUCTIONS BTWN MY POS AND CCR. AT THE TIME OF MY CANCELLATION; STOCKTON APCH SWITCHED ME TO 127.75. I CALLED APCH BUT DID NOT RECEIVE A REPLY. THE CEILING IN FACT TURNED OUT TO BE 2000 FT AT MY LOCATION AND UPON LEVELING AT 1500 FT I SAW THAT I WAS JUST S OF STOCKTON ARPT AND PROBABLY WITHIN THEIR CLASS D AIRSPACE. I WAS OUT BEFORE I COULD FIND THE TWR FREQ AND RECEIVED NO CALLS FROM STOCKTON APCH; WHICH MAKES ME THINK IN RETROSPECT THAT I MAY HAVE FAILED TO TUNE 127.75 CORRECTLY. I PROCEEDED TO CONCORD AND LANDED. I SAW NO OTHER TFC IN THE STOCKTON AREA AND PERCEIVED NO ACTUAL CONFLICT. THIS SIT WAS CAUSED BY A LAST MIN DECISION TO CANCEL IFR AND DSND WITHOUT KNOWING MY EXACT POS. I THOUGHT I WAS BEYOND STOCKTON. SINCE I WAS CLRED TO CCR BOTH VOR AND GPS NAVS WERE ON THAT NAVAID. THE ARPT WAS BLOCKED BY THE CLOUDS AHEAD UNTIL IT WAS TOO LATE TO PREVENT AN INCURSION. I ALSO SHOULD NOT HAVE ASSUMED THE CEILING TO BE HIGHER THAN IT WAS.