▁used ▁to ▁determine ▁heading . ▁ ▁More ▁advanced ▁methods ▁are : ▁Ring - L aser ▁gy ro scop es ▁F ib re ▁opt ic ▁gy ro scop es ▁Se ap ath , ▁a ▁combination ▁of ▁G PS ▁and ▁in ert ial ▁sens ors . ▁ ▁S ens ors ▁Besides ▁position ▁and ▁heading , ▁other ▁variables ▁are ▁fed ▁into ▁the ▁D P ▁system ▁through ▁sens ors : ▁Mot ion ▁reference ▁units , ▁vertical ▁reference ▁units ▁or ▁vertical ▁reference ▁sens ors , ▁V R Us ▁or ▁MR Us ▁or ▁V RS s , ▁determine ▁the ▁ship ' s ▁roll , ▁pitch ▁and ▁he ave . ▁Wind ▁sens ors ▁are ▁fed ▁into ▁the ▁D P ▁system ▁feed forward , ▁so ▁the ▁system ▁can ▁anticip ate ▁wind ▁g ust s ▁before ▁the ▁ship ▁is ▁blow n ▁off ▁position . ▁Dra ug ht ▁sens ors , ▁since ▁a ▁change ▁of ▁dra ug ht ▁influ ences ▁the ▁effect ▁of ▁wind ▁and ▁current ▁on ▁the ▁h ull . ▁Other ▁sens ors ▁depend ▁on ▁the ▁kind ▁of ▁ship . ▁A ▁pip el ay ▁ship ▁may ▁measure ▁the ▁force ▁needed ▁to ▁pull ▁on ▁the ▁pipe , ▁large ▁c rane ▁vessels ▁will ▁have ▁sens ors ▁to ▁determine ▁the ▁c ran es ▁position , ▁as ▁this ▁changes ▁the ▁wind ▁model , ▁en abling ▁the ▁calculation ▁of ▁a ▁more ▁accurate ▁model ▁( see ▁Control ▁systems ). ▁Some ▁external ▁forces ▁are ▁not ▁directly ▁measured . ▁In ▁these ▁cases , ▁the ▁offset ▁force ▁is ▁dedu ced ▁over ▁a ▁period ▁of ▁time , ▁allowing ▁an ▁average ▁value ▁of