Narrative:

Type of event/situation: GPS error of 5 mi plus terrain-blocked VOR. Sightseeing flight in the vicinity of class B airspace. I was using pilotage, DME, and portable GPS to keep clear of class B. I realized the GPS was reporting an erroneous position, 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 problems in that complicated airspace if I had been relying on it or if I had not recognized the error. Details: flight departed in VFR conditions for a sightseeing flight. I was monitoring sfo approach but did not contact them since it would be a very brief flight, 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 reporting that we were still 1 mi inland, about 4 mi in trail of our present position. The GPS did not report a loss of satellite lock or any other error. It was clearly incorrect, however, it continued to track us moving south over land, while we were clearly over water and further south of the reported position. I became concerned with keeping clear 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 right, located visual landmarks to keep me clear, 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 clear of class B, I elected to power-cycle the unit. When it came back, the position jumped approximately 5 mi southwest, to a position that agreed with my understanding of the aircraft's location. The DME flickered back on occasionally, and showed that we had 7.5 DME or greater at all times, clear of class B at our altitude of 1900 ft MSL. I continually monitored the GPS during the remainder of the flight, xchking it with visual landmarks and VOR/DME, and found no further discrepancy whatsoever. I have used this unit for about 10 hours in the cockpit and 20-30 hours 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 system, 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 system. Unfortunately it's not clear if the error was due to a 'bug' in the lowrance receiver, a reception problem, or with the GPS satellites. I never planned to use my GPS as a primary means of navigation, 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 clear things up for me on this flight, it caused a huge distraction that could have led to other problems. I was lucky to have noticed that the GPS was reporting erroneously -- only the magnitude of the error and the close proximity of prominent visual landmarks alerted me to the problem. 2) the boundary between 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 altitude that makes this boundary relevant.

Related reports:

Google
 

Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.