GA PLT USES PILOTAGE; VOR AND A WEAK DME TO AVOID CLASS B AIRSPACE THAT HIS PORTABLE GPS ERRONEOUSLY INDICATED HE HAD PENETRATED.

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TYPE OF EVENT/SIT: GPS ERROR OF 5 MI PLUS TERRAIN-BLOCKED VOR. SIGHTSEEING FLT IN THE VICINITY OF CLASS B AIRSPACE. I WAS USING PILOTAGE; DME; AND PORTABLE GPS TO KEEP CLR OF CLASS B. I REALIZED THE GPS WAS RPTING AN ERRONEOUS POS; AND THE VOR/DME BECAME BLOCKED BY TERRAIN. I INCREASED MY SEPARATION WITH THE CLASS B BASED ON VISUAL LANDMARKS AND I AM CONFIDENT THAT NO AIRSPACE VIOLATION OCCURRED. THE GPS DISCREPANCY WAS QUITE LARGE AND COULD HAVE EASILY CAUSED PROBS IN THAT COMPLICATED AIRSPACE IF I HAD BEEN RELYING ON IT OR IF I HAD NOT RECOGNIZED THE ERROR. DETAILS: FLT DEPARTED IN VFR CONDITIONS FOR A SIGHTSEEING FLT. I WAS MONITORING SFO APCH BUT DID NOT CONTACT THEM SINCE IT WOULD BE A VERY BRIEF FLT; REMAINING BELOW AND OUTSIDE OF CLASS B AIRSPACE; THEN RETURNING. AFTER FLYING ABOUT 4 MI ALONG THE SHORE; I LOOKED DOWN AT MY GPS (A LOWRANCE AIRMAP 100) TO SEE THAT IT WAS RPTING THAT WE WERE STILL 1 MI INLAND; ABOUT 4 MI IN TRAIL OF OUR PRESENT POS. THE GPS DID NOT RPT A LOSS OF SATELLITE LOCK OR ANY OTHER ERROR. IT WAS CLRLY INCORRECT; HOWEVER; IT CONTINUED TO TRACK US MOVING S OVER LAND; WHILE WE WERE CLRLY OVER WATER AND FURTHER S OF THE RPTED POS. I BECAME CONCERNED WITH KEEPING CLR OF SFO CLASS B. SINCE THE EDGE OF CLASS B IS OVER WATER AND IS DEFINED BY A DME ARC OF 7 NM FROM SFO VOR IT'S A BIT TRICKY TO AVOID. I TURNED R; LOCATED VISUAL LANDMARKS TO KEEP ME CLR; AND ESTABLISHED I WAS MAINTAINING 7.5 NM AND INCREASING ON MY DME. AT THAT MOMENT; MY DME FLAGGED OFF; APPARENTLY BLOCKED BY THE HILLS. AFTER ASSURING MYSELF THAT WE WERE WELL CLR OF CLASS B; I ELECTED TO PWR-CYCLE THE UNIT. WHEN IT CAME BACK; THE POS JUMPED APPROX 5 MI SW; TO A POS THAT AGREED WITH MY UNDERSTANDING OF THE ACFT'S LOCATION. THE DME FLICKERED BACK ON OCCASIONALLY; AND SHOWED THAT WE HAD 7.5 DME OR GREATER AT ALL TIMES; CLR OF CLASS B AT OUR ALT OF 1900 FT MSL. I CONTINUALLY MONITORED THE GPS DURING THE REMAINDER OF THE FLT; XCHKING IT WITH VISUAL LANDMARKS AND VOR/DME; AND FOUND NO FURTHER DISCREPANCY WHATSOEVER. I HAVE USED THIS UNIT FOR ABOUT 10 HRS IN THE COCKPIT AND 20-30 HRS ON LAND AND THIS IS THE FIRST ERROR OF SUCH A MAGNITUDE THAT I'VE SEEN. I'M FAMILIAR WITH THE SELECTIVE AVAILABILITY OF THE GPS SYS; BUT THE BEHAVIOR I OBSERVED DOES NOT APPEAR TO BE DUE TO SITUATIONAL AWARENESS BECAUSE OF THE MAGNITUDE OF THE ERROR AND THE FACT THAT THE ERROR WENT AWAY WHEN I RECYCLED THE UNIT. THERE SEEMS TO BE 2 PERTINENT ISSUES HERE: 1) I'VE LOST ANY FAITH I MIGHT EVER HAD IN MY GPS SYS. UNFORTUNATELY IT'S NOT CLR IF THE ERROR WAS DUE TO A 'BUG' IN THE LOWRANCE RECEIVER; A RECEPTION PROB; OR WITH THE GPS SATELLITES. I NEVER PLANNED TO USE MY GPS AS A PRIMARY MEANS OF NAV; BUT IT DOES DO WONDERS FOR RESTORING ONE'S SITUATIONAL AWARENESS AT TIMES. NOW I WILL ALWAYS WONDER IF IT EVEN HAS ME IN THE CORRECT TIME ZONE. RATHER THAN CLR THINGS UP FOR ME ON THIS FLT; IT CAUSED A HUGE DISTR THAT COULD HAVE LED TO OTHER PROBS. I WAS LUCKY TO HAVE NOTICED THAT THE GPS WAS RPTING ERRONEOUSLY -- ONLY THE MAGNITUDE OF THE ERROR AND THE CLOSE PROX OF PROMINENT VISUAL LANDMARKS ALERTED ME TO THE PROB. 2) THE BOUNDARY BTWN SFO CLASS B SECTORS AT 21-80 FT MSL AND 15-80 FT MSL IS DEFINED ON A DME ARC THAT IS NOT TRIVIAL TO LOCATE VISUALLY; AND THE VOR IS BLOCKED IF YOU ARE AT AN ALT THAT MAKES THIS BOUNDARY RELEVANT.