▁systems , ▁as ▁well ▁as ▁combining ▁G PS ▁position ▁with ▁GL ON ASS . ▁Ac oust ics . ▁This ▁system ▁consists ▁of ▁one ▁or ▁more ▁trans pond ers ▁placed ▁on ▁the ▁se ab ed ▁and ▁a ▁trans du cer ▁placed ▁in ▁the ▁ship ' s ▁h ull . ▁The ▁trans du cer ▁sends ▁an ▁ac oust ic ▁signal ▁( by ▁means ▁of ▁pie zo elect ric ▁elements ) ▁to ▁the ▁trans ponder , ▁which ▁is ▁triggered ▁to ▁reply . ▁As ▁the ▁velocity ▁of ▁sound ▁through ▁water ▁is ▁known ▁( pre fer ably ▁a ▁sound profile ▁is ▁taken ▁regularly ), ▁the ▁distance ▁is ▁known . ▁Because ▁there ▁are ▁many ▁elements ▁on ▁the ▁trans du cer , ▁the ▁direction ▁of ▁the ▁signal ▁from ▁the ▁trans ponder ▁can ▁be ▁determined . ▁Now ▁the ▁position ▁of ▁the ▁ship ▁relative ▁to ▁the ▁trans ponder ▁can ▁be ▁calculated . ▁Dis adv antages ▁are ▁the ▁vulner ability ▁to ▁noise ▁by ▁thrust ers ▁or ▁other ▁ac oust ic ▁systems . ▁The ▁use ▁is ▁limited ▁in ▁shall ow ▁waters ▁because ▁of ▁ray ▁b ending ▁that ▁occurs ▁when ▁sound ▁travel s ▁through ▁water ▁hor izont ally . ▁Three ▁types ▁of ▁H PR ▁systems ▁are ▁commonly ▁used : ▁Ult ra - ▁or ▁super - ▁short ▁base ▁line , ▁USB L ▁or ▁S SB L . ▁This ▁works ▁as ▁described ▁above . ▁Because ▁the ▁angle ▁to ▁the ▁trans ponder ▁is ▁measured , ▁a ▁correction ▁needs ▁to ▁be ▁made ▁for ▁the ▁ship ' s ▁roll ▁and ▁pitch . ▁These ▁are ▁determined ▁by ▁Mot ion ▁Reference ▁Units