▁compens ating ▁thrust ▁to ▁be ▁applied . ▁All ▁forces ▁not ▁att rib utable ▁to ▁direct ▁measurement ▁are ▁l abeled ▁" current ", ▁as ▁this ▁is ▁what ▁they ▁are ▁assumed ▁to ▁be , ▁but ▁in ▁reality ▁this ▁is ▁a ▁combination ▁of ▁current , ▁waves , ▁sw ell , ▁and ▁any ▁errors ▁in ▁the ▁system . ▁As ▁is ▁traditional ▁in ▁the ▁mar itime ▁industry , ▁D P ▁" current " ▁is ▁always ▁recorded ▁in ▁the ▁direction ▁that ▁it ▁is ▁flow ing ▁towards . ▁ ▁Control ▁systems ▁ ▁In ▁the ▁beginning ▁P ID ▁controllers ▁were ▁used ▁and ▁today ▁are ▁still ▁used ▁in ▁the ▁simpler ▁D P ▁systems . ▁But ▁modern ▁controllers ▁use ▁a ▁mathematical ▁model ▁of ▁the ▁ship ▁that ▁is ▁based ▁on ▁a ▁hyd rod ynamic ▁and ▁aer od ynamic ▁description ▁concerning ▁some ▁of ▁the ▁ship ' s ▁characteristics ▁such ▁as ▁mass ▁and ▁drag . ▁Of ▁course , ▁this ▁model ▁is ▁not ▁entirely ▁correct . ▁The ▁ship ' s ▁position ▁and ▁heading ▁are ▁fed ▁into ▁the ▁system ▁and ▁compared ▁with ▁the ▁prediction ▁made ▁by ▁the ▁model . ▁This ▁difference ▁is ▁used ▁to ▁update ▁the ▁model ▁by ▁using ▁Kal man ▁filtering ▁technique . ▁For ▁this ▁reason , ▁the ▁model ▁also ▁has ▁input ▁from ▁the ▁wind ▁sens ors ▁and ▁feedback ▁from ▁the ▁thrust ers . ▁This ▁method ▁even ▁allows ▁not ▁having ▁input ▁from ▁any ▁PR S ▁for ▁some ▁time , ▁depending ▁on ▁the ▁quality ▁of ▁the ▁model ▁and ▁the ▁weather . ▁This ▁process ▁is ▁known ▁as ▁dead ▁reck oning . ▁ ▁The ▁accuracy ▁and ▁precision ▁of ▁the ▁different