PA28 PLT ENRTE VFR TO HGR PENETRATES THE P-40 EXTENDED TFR EVEN THOUGH BRIEFED BY FSS. PILOT PLANNING AVOIDANCE WITH GPS EQUIP ONLY DISPLAYING 5 MI RADII. CTLR ADVISED GPS SIGNAL DEGRADED IN THAT AREA.

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I LEFT 39N (PRINCETON; NJ) VFR ON MAY/SAT/04 AT ABOUT XA00. I HAD CALLED MILLVILLE FSS PRIOR TO DEP AND OBTAINED A BRIEFING; WHICH INCLUDED THE FACT THAT P-40 HAD BEEN EXPANDED TO 10 MI. MY DEST WAS HGR; A TRIP I DO REGULARLY SINCE WE HAVE A FAMILY LIVING NEAR HAGERSTOWN. I HAD FLT FOLLOWING FROM JUST OUTSIDE TTN FROM PHL APCH; WITH HDOFS TO READING AND HARRISBURG. BEFORE REACHING GETTYSBURG; PA; RADAR SVC FROM HARRISBURG DROPPED OFF. I DID NOT HAVE FLT FOLLOWING FROM GETTYSBURG TO THE END OF THE FLT. MY ACFT IS EQUIPPED WITH A GARMIN 430 GPS. THE GPS HAS A 3 MI AND A 5 MI RING FOR P-40. I ESTIMATED WHAT A 10 MI RING WOULD BE; AND FLEW THAT LINE. I CONTACTED HGR WHEN I HAD CROSSED THE RIDGE ABOUT 11 MI FROM THE FACILITY; PRIOR TO REACHING WAYNESBORO; PA. THEY ADVISED ME TO MAKE AN IMMEDIATE R TURN TO THE N; TO AVOID PENETRATING P-40'S EXTENDED TFR. I IMMEDIATELY COMPLIED; AND THEN WAS ADVISED BY HGR WHEN TO TURN TOWARD THE ARPT; AND LANDED WITHOUT INCIDENT. I WAS ADVISED TO CALL THE TWR AFTER LNDG FOR A POSSIBLE VIOLATION OF THE SUA. I CALLED THE TWR; AND THEY ADVISED ME THAT I HAD PENETRATED THE 10 MI RING BY APPROX 1/4 MI; AND THAT A GOV ORGANIZATION HAD OBTAINED MY TAIL NUMBER. THE TWR PERSONNEL ADVISED ME THAT I COULD NOT RELY ON GPS NAV AS THE MIL DEGRADED THE SIGNAL SO AS TO OBSCURE THE LOCATION OF P-40. WHAT I COULD HAVE DONE DIFFERENTLY. I COULD HAVE GOTTEN ADDITIONAL FLT FOLLOWING FROM ZDC. THE CTLR AT HARRISBURG SEEMED PRETTY BUSY AND IT WAS A BEAUTIFUL DAY FOR FLYING; SO I DIDN'T THINK IT NECESSARY TO CONTACT WASHINGTON. IN RETROSPECT; THAT WAS A MISTAKE. I COULD HAVE TAKEN A SECTIONAL AND DRAWN A LARGER RING AROUND P-40. THERE ARE ENOUGH GND REF POINTS THAT I COULD HAVE MADE OUT A 12-15 MI RING AROUND THE AREA; BUT THE GPS HAS BEEN SO RELIABLE AND HELPFUL THAT I DID NOT THINK IT NECESSARY. I AM A LITTLE SURPRISED THAT THERE IS A POLICY OF DEGRADING THE GPS SIGNAL; SINCE MOST GA PLTS ARE RELYING MORE ON THESE NAV DEVICES THAN ON OTHER NAVAIDS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE 'DEPENDED ON HIS GPS' TO AVOID P-40; ESTIMATING A 10 MI RADIUS OF THE TFR; EVEN THOUGH HIS GARMIN MODEL 430 HAD ONLY A 3-5 MI RING DISPLAY INDICATORS. TO DATE; HE HAS NOT RECEIVED ANY ADDITIONAL NOTIFICATION ABOUT THE EVENT FROM ANY GOV ORGANIZATION. RPTR ALLEGES THAT NEXT TIME HE'LL 'ADD A COUPLE EXTRA MI' FOR INSURANCE.