PVT PLT OF A C152 INADVERTENTLY ENTERED CLASS C AIRSPACE WITHOUT CONTACTING ATC OR TURNING ON MODE C ALT XPONDER MODE. HE BELIEVED THAT HE WAS AT AN ALT BENEATH THE SECOND CIRCLE OF THE CLASS C AIRSPACE. HOWEVER; HE WAS ACTUALLY CUTTING ACROSS THE BOUNDARY OF CLASS C INNER CIRCLE; WHICH ALT STARTS AT THE SURFACE. THIS CAUSED A POTENTIAL CONFLICT WITH AN ACR ACFT TURNING FINAL APCH.

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I AND A PAX WERE IN TRANSIT BTWN 7G0 AND 3G7 AT 1900 FT MSL USING GPS NAV AND UNDER THE FLOOR OF THE ROC CLASS C AIRSPACE AND JUST OUTSIDE OF THE 5 MI CIRCLE. I WAS LISTENING BUT NOT IN CONTACT WITH ROC APCH CTL 123.7 AND SQUAWKING 1200 WHEN I HEARD ROC TELL ANOTHER AIRPLANE THAT HE WAS CLRED FOR THE ILS APCH TO RWY 22 AT ROC. ABOUT THE SAME TIME MY PAX AND I SAW WHAT LOOKED LIKE AN ACR MAKING A TURN TO THE SW ABOUT 5 MI AHEAD OF US AND 1000 FT ABOVE US. AT THIS TIME I THOUGHT IT PRUDENT TO CONTACT ROC. WHEN I CONTACTED ROC MY FIRST CALL WAS ANSWERED BUT I DID NOT HEAR IT. THE SECOND CALL I ACKNOWLEDGED AND THE CTLR ASKED IF I KNEW I WAS INSIDE HIS AIRSPACE. I ANSWERED IN THE AFFIRMATIVE THINKING THAT YES; I WAS INSIDE HIS LATERAL BOUNDARY BUT UNDERNEATH THE FLOOR OF HIS AIRSPACE. HE THEN ASKED ME IF I KNEW THERE WAS AN AIRPLANE ON FINAL APCH TO ROC AND I ANSWERED THAT YES I HAD THE AIRPLANE IN SIGHT. HE REPLIED THAT DIDN'T DO THE PLT OF THE OTHER ACFT ANY GOOD BECAUSE HE DID NOT HAVE ME IN SIGHT. I THEN WAS ASKED TO IDENT; WHICH WAS FOLLOWED BY THE QUERY WHETHER I HAD MODE C ON MY XPONDER. I REPLIED IN THE AFFIRMATIVE AND THEN NOTICED THAT I HAD FAILED TO SWITCH IT TO THE ALT POS. WHEN I SWITCHED IT TO ALT THE CTLR THEN CAME BACK WITH 'RADAR CONTACT; 7 MI FROM ROC.' PROB: QUESTIONABLE JUDGEMENT IN FAILING TO CONTACT ROC WHEN TRANSITING THEIR CLASS C AIRSPACE EVEN WHEN UNDER THE FLOOR OF THE AIRSPACE; FAILURE TO HAVE THE XPONDER IN THE ALT POS. HAD I CONTACTED ROC BEFORE ENTERING THE CLASS C AIRSPACE; THE ABSENCE OF A MODE C READOUT WOULD HAVE BEEN NOTICED; THE PLT OF THE ACR WOULD HAVE BEEN TOLD OF MY POS; AND ALT AND ADJUSTMENTS TO COURSES OF ONE OR BOTH OF THE PLANES COULD HAVE BEEN MADE. THIS WOULD HAVE AVOIDED THE UNCERTAINTY OF THE PLT OF THE ACR AND CTLR NOT KNOWING WHAT MY POS; ALT AND INTENTIONS WERE AND MADE FOR A SAFER APCH.