Page "International Regulations for Preventing Collisions at Sea" Paragraph 52

: Every vessel must at all times maintain a proper look-out by sight ( day shape or lights by eyes or visual aids ) and hearing ( sound signal or Marine VHF radio ) as well as by all available means ( e. g., Radar, Automatic Radar Plotting Aid ( ARPA ), Automatic Identification System ( AIS ), GMDSS ...) in order to make a full assessment of the situation and risk of collision.