A C150 PLT INVADES THE CLASS D OF PDK WHILE USING A HAND- HELD GPS FOR POS AWARENESS. APCH CTLR ADVISES HIM ON HIS MONITORING RADIO THAT HE IS IN CLASS D AIRSPACE WITHOUT CLRNC.

Narrative

I DEPARTED LZU ARPT WITH THE INTENT OF TAKING AERIAL PHOTOGRAPHS OF A SITE LOCATED NEAR THE EDGE OF PDK'S CLASS D AIRSPACE. TO MAINTAIN A PROPER POS OUTSIDE OF THE CLASS D AIRSPACE; I RELIED ON MY HAND-HELD GPS TO INFORM ME OF MY ACCURATE POS (DOWN TO 1/10 MI ON THE DISPLAY). WHILE CIRCLING THE PHOTOGRAPHIC SITE; I ALSO MONITORED PDK'S TWR FREQ SO THAT I COULD MAINTAIN AN AWARENESS OF ANY DEPARTING OR ARR TFC THAT MIGHT BECOME A FACTOR. WHILE SNAPPING PHOTOS; AN ANNOUNCEMENT WAS BROADCAST BY THE TWR CTLR REQUESTING THE INTENT OF THE ACFT E OF THE ARPT AT MY ALT. I RESPONDED; AND ADVISED THE CTLR THAT I WAS TAKING PHOTOS. THE CTLR THEN ADVISED ME THAT I WAS WITHIN HER CLASS D AIRSPACE. ACCORDING TO MY GPS; I WAS STILL OUTSIDE OF THE CLASS D; BUT AS I TALKED WITH HER; I WATCHED THE NUMBERS DECREASE UNTIL HER STATEMENT WAS TRUE; EVEN THOUGH AT THE TIME; I WAS FLYING AWAY FROM THE CLASS D. AFTER RETURNING TO MY HOME ARPT; I VISITED THE LCL AVIONICS SHOP TO DETERMINE WHY MY GPS HAD RESPONDED AS IT HAD. AFTER A CONVERSATION WITH ONE OF THE TECHNICIANS; HE EXPLAINED THAT THERE IS A DELAY IN THE PROCESSING OF THE INFO REPRESENTED ON A GPS DISPLAY; AND AN ESPECIALLY LONG DELAY IN THE PROCESSING OF INFO ON OLDER HAND-HELD UNITS LIKE THE ONE THAT I OWNED. EVEN THOUGH THE GPS WILL DISPLAY DISTANCE DOWN TO 1/10 MI; THE LAG TIME GENERATED BY THE SIGNAL PROCESSING WILL PRODUCE A SIZABLE DISTANCE ERROR; WHICH CAN 'SUCKER' A PLT WITH A REAL-TIME FALSE POS; OR IN MY CASE; DISTANCE FROM AN ARPT. IN RETROSPECT; I SHOULD HAVE CONTACTED THE TWR; EVEN THOUGH I PLANNED ON REMAINING OUTSIDE OF THE CLASS D AIRSPACE. CLOSE TO THE EDGE OF A LINE DRAWN ON A MAP DOESN'T NECESSARILY GUARANTEE SAFETY; AND THE MORE PRUDENT ACTION WOULD HAVE BEEN TO 'CHK IN;' AND PROVIDE THE CTLR WITH A BETTER UNDERSTANDING OF WHY I WAS FLYING IN CIRCLES.