IN AN APPARENT PED INTERFERENCE EVENT; A PAX'S PORTABLE GARMIN GPS MODEL NUVI 660 ALLEGEDLY INTEFERED WITH A B737 CLASSIC'S (NO GLASS) DME NAVIGATION UPDATE FUNCTION.

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I HAD THIS EXACT PROBLEM ABOUT 10 YEARS AGO UNDER THE SAME CIRCUMSTANCES. THIS EVENT OCCURRED IN THE SAME TYPE OF ACFT; A B737 [NON GLASS]. DURING CLBOUT AND INITIAL CRUISE; I NOTICED THE NAV RADIOS WERE IN AUTO UPDATE BUT THEY WERE NOT UPDATING. I THEN CHKED THE FMC STATUS OF THE IRS' AND FMC RADIO UPDATES. IT SHOWED DME UPDATING FAIL. BEFORE I COMPLETED A POSITION SHIFT; I (CAPT) CALLED THE FLT ATTENDANTS AND ASKED THEY DO A WALK THROUGH AND CHK FOR ANY PED'S. THEY FOUND A PAX WITH A HANDHELD GPS; GARMIN NUVI MODEL 660. ONCE THIS WAS TURNED OFF; THE FMC DME UPDATING WENT FROM FAIL TO ON AND THE RADIOS STARTED TO UPDATE AGAIN. THE FLT CONTINUED AND LANDED WITHOUT INCIDENT. THE ACFT WAS A B737 'CLASSIC' MEANING ROUND DIALS AND NOT EFIS. IT HAD TWO IRS' WITH A SINGLE FMC BUT 2 FMC HEADSETS IN THE COCKPIT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT HE WAS CONCERNED THAT BECAUSE HE HAD EXPERIENCED THIS SAME TYPE OF EVENT THREE TIMES IN THE LAST 10 YEARS AND ALL IN THE B737 ACFT WITH THE FMS SYSTEM BUT NO GLASS; THAT THERE MAY BE A COMMON THREAD. ONE PREVIOUS EVENT WAS CONFIRMED AS CAUSED BY PORTABLE GPS AND THE OTHER MAY HAVE BEEN A CELL PHONE CALL MADE JUST PRIOR TO LANDING. IN THE EVENT REPORTED HERE; THE DME FAILING TO UPDATE WAS THE FIRST INDICATION. HE NOW FLIES THE B737 CLASSIC AS WELL AS THE B737 NG'S. THE NG'S APPEAR TO BE MORE ELECTROMAGNETICALLY ROBUST.