What was found
(CAN) WHILE CONDUCTING A POLLUTION SURVEILLANCE FLIGHT, ILLUMINATION OF FLIGHT MISCOMPARED WITH 15 SECONDS INTERVALS OCCURRED MULTIPLE TIMES. COMPASS DRIFT OF MORE THAN 20 DEGREES OCCURRED WITHIN 5 MIN AFTER HEADING RESET IN HEADING PRESET MODE (FMS-45) WITH ERRONEOUS GROUNDSPEED VELOCITY AND DIRECTION. THIS SITUATION REOCCURRED MANY TIMES THROUGHOUT THE FLIGHT. THE ERRORS SEEMS WORSE AND THE OCCURRENCE MORE FREQUENT ON FLIGHT WITH A LOTS OF TURNS. (TC# 20190704010)