The ship was being steered automatically on a pre-determined route by way of the Automatic Navigation and Track Steering (ANTS) system. The master was on the bridge, but the mate had the con. The ship did not make a planned automatic turn to port and recovery from the situation required swift intervention by the bridge team to initiate the turn manually and prevent the ship grounding. The report concludes that the ARPA radar navigation system probably defaulted from the ANTS mode to autopilot mode without the change being noticed by the mate or master.
There were a number of reasons for the system to default to autopilot mode: it may have received an erroneous signal from an external input such as the DGPS due to aerial masking or incorrect differential signal reception; it may have received such conflicting information from the ground and water speeds of the Doppler log that the information was discarded as erroneous; or the parameters for the off-track jump limit
