▁of ▁all ▁the ▁position ▁and ▁heading ▁need ▁to ▁be ▁known . ▁a ▁control ▁computer ▁to ▁calculate ▁the ▁required ▁control ▁actions ▁to ▁maintain ▁position ▁and ▁correct ▁for ▁position ▁errors . ▁thrust ▁elements ▁to ▁apply ▁forces ▁to ▁the ▁ship ▁as ▁demanded ▁by ▁the ▁control ▁system . ▁ ▁For ▁most ▁applications , ▁the ▁position ▁reference ▁systems ▁and ▁thrust ▁elements ▁must ▁be ▁carefully ▁considered ▁when ▁design ing ▁a ▁D P ▁ship . ▁In ▁particular , ▁for ▁good ▁control ▁of ▁position ▁in ▁ad verse ▁weather , ▁the ▁thrust ▁cap ability ▁of ▁the ▁ship ▁in ▁three ▁axes ▁must ▁be ▁ade qu ate . ▁ ▁M aint aining ▁a ▁fixed ▁position ▁is ▁particularly ▁difficult ▁in ▁polar ▁conditions ▁because ▁ice ▁forces ▁can ▁change ▁rapidly . ▁Sh ip - bor ne ▁ice ▁detection ▁and ▁mit ig ation ▁is ▁not ▁sufficiently ▁developed ▁to ▁predict ▁these ▁forces , ▁but ▁may ▁be ▁prefer able ▁to ▁sens ors ▁placed ▁by ▁hel ic opter . ▁ ▁Reference ▁systems ▁ ▁Position ▁reference ▁systems ▁There ▁are ▁several ▁means ▁to ▁determine ▁a ▁ship ' s ▁position ▁at ▁sea . ▁Most ▁traditional ▁methods ▁used ▁for ▁ships ▁navigation ▁are ▁not ▁accurate ▁enough ▁for ▁some ▁modern ▁requirements . ▁For ▁that ▁reason , ▁several ▁position ing ▁systems ▁have ▁been ▁developed ▁during ▁the ▁past ▁dec ades . ▁Produ cers ▁of ▁D P ▁systems ▁are : ▁Marine ▁Techn ologies ▁L LC , ▁Kong s berg ▁Mar itime , ▁Nav is ▁Engineering ▁O y , ▁G E , ▁DC NS , ▁W arts ila ▁( ex ▁L - 3 ), ▁M T - div . Ch ouest