Narrative:

Took off iza heading for halfmoon bay (haf), with san carlos (sql) as my alternate. Shortly after leaving iza, was informed that mode C inoperative/intermittent, but had good primary target. At request of ATC, recycled it numerous times. Unable to get consistent mode C operation, and was told to report altitude changes. Was informed by ATC that would not be able to enter class B airspace. I acknowledged this, but asked if I could enter the mode C veil, and fly below class B. He admitted he didn't know ('a good question' was the actual reply), but that he didn't deal with that area of airspace, and that norcal would make that decision. Shortly after, contacted norcal approach, who restated no entry into class B airspace without mode C transponder. Asked about entering the mode C veil below class B, and was told 'squawk 1200, radar services terminated.' I circled down to 3500 ft MSL, above eugen, and proceeded to haf, which was IFR conditions, so headed to sql at same altitude, landed without incident, and without further communication with norcal. Later, I left sql, and followed the hayward departure at 3500 ft MSL, remaining outside class B, and on reaching eugen, climbed to 7500 ft MSL. Obtained flight following again, mode C was intermittent, with good primary target. Refueled at iza, and shortly after, obtained flight following, this time with perfect mode C response, which lasted all through the la basin, back through march approach to F70. From the response of norcal, I believe that this is a known problem with the mode C. It is too much of a coincident that the mode C stopped working at iza, and then started working at exactly the same place on the return journey. I flew the aircraft 2 days later and the mode C worked perfectly. Spoke with other pilots, who informed me that they have also had problems in that area. I feel that the problem lies with ATC radar system between iza and the general area of san fran.

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Original NASA ASRS Text

Title: C172 PLT, ENRTE FROM IZA TO HAF, CONCERNED WITH POOR ZOA AND NCT RADAR COVERAGE ALONG HIS RTE OF FLT.

Narrative: TOOK OFF IZA HEADING FOR HALFMOON BAY (HAF), WITH SAN CARLOS (SQL) AS MY ALTERNATE. SHORTLY AFTER LEAVING IZA, WAS INFORMED THAT MODE C INOP/INTERMITTENT, BUT HAD GOOD PRIMARY TARGET. AT REQUEST OF ATC, RECYCLED IT NUMEROUS TIMES. UNABLE TO GET CONSISTENT MODE C OP, AND WAS TOLD TO RPT ALT CHANGES. WAS INFORMED BY ATC THAT WOULD NOT BE ABLE TO ENTER CLASS B AIRSPACE. I ACKNOWLEDGED THIS, BUT ASKED IF I COULD ENTER THE MODE C VEIL, AND FLY BELOW CLASS B. HE ADMITTED HE DIDN'T KNOW ('A GOOD QUESTION' WAS THE ACTUAL REPLY), BUT THAT HE DIDN'T DEAL WITH THAT AREA OF AIRSPACE, AND THAT NORCAL WOULD MAKE THAT DECISION. SHORTLY AFTER, CONTACTED NORCAL APCH, WHO RESTATED NO ENTRY INTO CLASS B AIRSPACE WITHOUT MODE C XPONDER. ASKED ABOUT ENTERING THE MODE C VEIL BELOW CLASS B, AND WAS TOLD 'SQUAWK 1200, RADAR SVCS TERMINATED.' I CIRCLED DOWN TO 3500 FT MSL, ABOVE EUGEN, AND PROCEEDED TO HAF, WHICH WAS IFR CONDITIONS, SO HEADED TO SQL AT SAME ALT, LANDED WITHOUT INCIDENT, AND WITHOUT FURTHER COM WITH NORCAL. LATER, I LEFT SQL, AND FOLLOWED THE HAYWARD DEP AT 3500 FT MSL, REMAINING OUTSIDE CLASS B, AND ON REACHING EUGEN, CLBED TO 7500 FT MSL. OBTAINED FLT FOLLOWING AGAIN, MODE C WAS INTERMITTENT, WITH GOOD PRIMARY TARGET. REFUELED AT IZA, AND SHORTLY AFTER, OBTAINED FLT FOLLOWING, THIS TIME WITH PERFECT MODE C RESPONSE, WHICH LASTED ALL THROUGH THE LA BASIN, BACK THROUGH MARCH APCH TO F70. FROM THE RESPONSE OF NORCAL, I BELIEVE THAT THIS IS A KNOWN PROB WITH THE MODE C. IT IS TOO MUCH OF A COINCIDENT THAT THE MODE C STOPPED WORKING AT IZA, AND THEN STARTED WORKING AT EXACTLY THE SAME PLACE ON THE RETURN JOURNEY. I FLEW THE ACFT 2 DAYS LATER AND THE MODE C WORKED PERFECTLY. SPOKE WITH OTHER PLTS, WHO INFORMED ME THAT THEY HAVE ALSO HAD PROBS IN THAT AREA. I FEEL THAT THE PROB LIES WITH ATC RADAR SYS BTWN IZA AND THE GENERAL AREA OF SAN FRAN.

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.