▁active ▁mode . ▁In ▁this ▁mode ▁the ▁anten na ▁array ▁based ▁system ▁radi ates ▁p uls es ▁and ▁list ens ▁for ▁the ▁returns . ▁By ▁using ▁the ▁returns , ▁the ▁estimation ▁of ▁parameters ▁such ▁as ▁velocity , ▁range ▁and ▁DO As ▁( direction ▁of ▁arrival ) ▁of ▁target ▁of ▁interest ▁become ▁possible . ▁Using ▁the ▁pass ive ▁far - field ▁listening ▁arrays , ▁only ▁the ▁DO As ▁can ▁be ▁estimated . ▁Son ar ▁systems ▁( Sound ▁Navigation ▁and ▁R anging ) ▁use ▁the ▁sound ▁waves ▁that ▁propag ate ▁under ▁the ▁water ▁to ▁detect ▁objects ▁on ▁or ▁under ▁the ▁water ▁surface . ▁Two ▁types ▁of ▁son ar ▁systems ▁can ▁be ▁defined ▁the ▁active ▁one ▁and ▁the ▁pass ive ▁one . ▁In ▁active ▁son ar , ▁the ▁system ▁em its ▁p uls es ▁of ▁sound ▁and ▁list ens ▁to ▁the ▁returns ▁that ▁will ▁be ▁used ▁to ▁estimate ▁parameters . ▁In ▁the ▁pass ive ▁son ar , ▁the ▁system ▁is ▁essentially ▁listening ▁for ▁the ▁sounds ▁made ▁by ▁the ▁target ▁objects . ▁It ▁is ▁very ▁important ▁to ▁note ▁the ▁difference ▁between ▁the ▁rad ar ▁system ▁that ▁uses ▁radio ▁waves ▁and ▁the ▁son ar ▁system ▁that ▁uses ▁sound ▁waves , ▁the ▁reason ▁why ▁the ▁son ar ▁uses ▁the ▁sound ▁wave ▁is ▁because ▁sound ▁waves ▁travel ▁farther ▁in ▁the ▁water ▁than ▁do ▁rad ar ▁and ▁light ▁waves . ▁In ▁pass ive ▁son ar , ▁the ▁receiving ▁array ▁has ▁the ▁cap ability ▁of ▁detect ing ▁distant ▁objects ▁and ▁their ▁locations . ▁De form able ▁array ▁are ▁usually ▁used ▁in ▁son