INSTRUCTOR WITH STUDENT PLT ON PHASE CHK IS UNFAMILIAR WITH TYPE OF GPS IN USE REGARDING THE GND REF USED FOR NAV (VOR VERSUS ARPT) AND HAS UNAUTH AIRSPACE ENTRY INTO CLASS C.

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I WAS PERFORMING A STAGE CHK FOR ANOTHER INSTRUCTOR'S STUDENT. WE WERE ON THE XCOUNTRY PORTION. THE FLT PLANNING CALL FOR US TO PASS CLOSE TO THE LAN CLASS C AIRSPACE. AS THIS WAS A STAGE CHK I WAITED TO SEE IF THE STUDENT WOULD CALL LAN. THE STUDENT WAS W OF HIS PLANNED COURSE WHICH WOULD PLACE US IN THE CLASS C AIRSPACE. I WAS FOLLOWING OUR COURSE WITH A KING 135 GPS UNIT. THIS UNIT HAS A FEATURE YOU CAN USE TO IDENT YOUR POS RELATIVE TO A GIVEN FACILITY. HOWEVER; THE UNIT GIVES YOUR POS IN RELATION TO THE CLOSEST VOR. ANOTHER GPS UNIT WE USE GIVES YOUR POS RELATIVE TO ARPT. DUE TO THIS I BELIEVED THE POS BEING GIVEN TO ME ON THE GPS WAS IN REF TO THE ARPT; BUT IT WAS REALLY IN REF TO THE VOR; WHICH IN THIS CASE WAS W OF THE ARPT. THEREFORE; I THOUGHT WE WERE FARTHER AWAY FROM THE LAN ARPT AND CLASS C AIRSPACE THAN WE REALLY WERE. THE STUDENT FINALLY DECIDED TO CALL LAND AND THE 2-WAY COM WAS ESTABLISHED AT 9 OR 10 NM. THE ATC CTLR GAVE US A SQUAWK AND AT NO TIME STATED WE HAD VIOLATED CLASS C AIRSPACE. HOWEVER; MISREADING THE GPS MADE THIS A CLOSE CALL. I BELIEVE THAT IT IS IMPORTANT WHEN WORKING WITH SEVERAL DIFFERENT GPS UNITS TO LEARN ALL THE DIFFERENCES IN THESE UNITS AND TO REVIEW THE OP OF THESE UNITS ON A REGULAR BASIS. I HAVE REVIEWED THE DIFFERENCE OF THESE 2 UNITS AND HAVE LEARNED HOW TO SET UP THE GPS SO THAT IT WILL GIVE THE POS TO CLOSEST ARPT AS WELL AS VOR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR PARTICIPATED IN STRUCTURED CALLBACK. HE WAS FLYING A KATANA. THERE HAS BEEN NO FOLLOW UP ON THE INCIDENT. HE IS QUITE PLEASED WITH THE USE OF GPS AND CONSIDERS IT VERY HELPFUL NAV.