INSTRUCTOR WITH STUDENT HAS FALSE INTERCEPT INDICATION ON VOR. RESPONDS AND ATC CORRECTS WITH VECTOR.

Narrative

WHILE DEPARTING ON AN IFR TRAINING FLT WITH MY STUDENT DOING THE FLYING IN HIS ACFT; WE WERE ASSIGNED THE SANTA ROSA 5 DEP. SCAGGS ISLAND TRANSITION AS DEPICTED HERE. THE DOTS ARE OUR FLT PATH. THE DEP CALLS FOR A CLB TO 1500 FT ON THE STS 321 THEN TURN TO HDG 070 UNTIL INTERCEPTING THE SGD 307 INBOUND TO SNUPY. THIS IS WHERE THE CONFLICT OCCURRED. DUE TO WINDS OUT OF THE N AND A DEP OFF 19; WE WERE TURNING TO THE 070 HDG OVER THE STS ARPT. WHILE PASSING THROUGH 2000 FT; THE #2 CPI NEEDLE CTRED ON THE SGD 307 RADIAL WITH A 'TO' INDICATION. DURING MY 12 YRS OF INSTRUCTING AT STS I HAVE SEEN THIS HAPPEN MOMENTARILY BEFORE; SO EVEN THOUGH I KNEW WE HADN'T COVERED THE DISTANCE NECESSARY TO GET TO THE 307 RADIAL THE STUDENT REACTED AS HE SHOULD (CTRED NEEDLE TO FLAG 127 OBS) AND MADE THE TURN SEBOUND. AT THAT POINT ATC ASKED WHAT WE WERE DOING AND ISSUED A NEW HDG OF 340 VECTORS TO V494. LATER IN MY PHONE CONVERSATION WITH THE CTR; THEY SAID THE STUDENTS EARLY TURN SEBOUND CONFLICTED WITH A VISUAL APCH TO STS REQUIRING THE ACFT TO DO A 360 DEG TURN. I FEEL THAT THE FALSELY CTRED 307 RADIAL IS CAUSED BY BOTH 'CHEAP RADIOS' ABOUT MORE OVER AN INTERFERENCE SOME WHERE IN THE SIGNAL SENT FROM SGD. MOST OF THE TIME THE NEEDLE WAVERS AND THE TO/FROM FLAG FLIPS; BUT I'VE SEEN MANY STUDENTS AND OTHER PLTS CONFUSED BY THOSE MOMENTARY DISCREPANT SIGNAL. I TOLD THE CTLR IF I SEE THIS ANY MORE I WILL DOCUMENT IT AND ADVISE THE STS TWR CHIEF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR STATES THIS HAS HAPPENED TO OTHER PLTS AS WELL. IN 'ACTUAL' IMC COULD BE A PROBLEM. USUALLY DOES NOT STAY CTRED FOR LONG PERIOD; BUT CERTAINLY CREATES CONFUSION; ESPECIALLY FOR NEW INST PLTS. BELIEVES THERE IS SOME SMALL GLITCH IN THE SGD SIGNAL THAT CAUSES THIS DISCREPANCY. WOULD LIKE FAA TO CHK THE NAVAID BUT FEELS IT MIGHT NOT HAPPEN IN THEIR FASTER TEST ACFT. THE TRAINING PLANES ARE THE ONES MOST SUSCEPTIBLE.