NAV ERROR.

Narrative

AFTER DETERMINING BOTH ADF'S WERE OFF BY 10-12 DEGS; WERE COMPENSATING TO THE ERROR AND WERE VERY NEAR BARROW INTXN AND MADE A CALL TO ZHU AND WERE TOLD TO FLY DIRECT TO SAN SABINE VOR. AFTER APPROX 30 MINS; RADAR CONTACT WAS ESTABLISHED AND WERE TOLD WE WERE 42 MI E OF COURSE. CORRECTIVE VECTOR WERE TAKEN. SUPPLEMENTAL INFO FROM ACN 214645. GALVESTON RADIO BEACON WAS THE PRIMARY NAV FACILITY. CONSISTENTLY SHOWING THIS FLT TO BE ON COURSE. A766. FIRST RADAR PLOT SHOWED US TO BE 44 NM NE OF THE AIRWAY. VECTORS RETURNED THE FLT TO THE AIRWAY. ADF AND DEAD RECKONING NAV SHOULD NOT BE THE ONLY METHODS OF NAV USED FOR EXTENDED OVERWATER FLTS. SUPPLEMENTAL INFO FROM ACN 214912. THE PLTS WERE NAVIGATING BY A RADAR VECTOR FROM THE MEXICAN CTR AND BY MEANS OF 2 ADF'S. THIS IS THE ONLY MEANS OF NAV AVAILABLE OVER THE GULF. ADF'S INDICATED EXACTLY ON TRACK. IN MY OPINION; NAV FACILITIES ON THIS RTE ARE INADEQUATE FOR ACFT NOT EQUIPPED WITH A SYS SUCH AS OMEGA. ATC SURVEILLANCE RADAR IN THE GULF SHOULD BE A MUST PRIORITY.