▁number ▁of ▁sens ors , ▁q < r . ▁ ▁The ▁set ▁of ▁any ▁q ▁ste ering ▁vectors ▁is ▁linear ly ▁independent . ▁ ▁Is otrop ic ▁and ▁non - dis pers ive ▁medium ▁– ▁Un iform ▁propag ation ▁in ▁all ▁directions . ▁ ▁Zero ▁mean ▁white ▁noise ▁and ▁signal , ▁un cor related . ▁ ▁Far - Field . ▁a . ▁Rad ius ▁of ▁propag ation ▁>> ▁size ▁of ▁array . ▁b . ▁Pl ane ▁wave ▁propag ation . ▁ ▁Through out ▁this ▁survey , ▁it ▁will ▁be ▁assumed ▁that ▁the ▁number ▁of ▁underlying ▁signals , ▁q , ▁in ▁the ▁observed ▁process ▁is ▁considered ▁known . ▁There ▁are , ▁however , ▁good ▁and ▁consistent ▁techniques ▁for ▁estim ating ▁this ▁value ▁even ▁if ▁it ▁is ▁not ▁known . ▁ ▁Est imation ▁techniques ▁In ▁general , ▁parameters ▁estimation ▁techniques ▁can ▁be ▁class ified ▁into : ▁spectral ▁based ▁and ▁paramet ric ▁based ▁methods . ▁In ▁the ▁former , ▁one ▁forms ▁some ▁spectrum - like ▁function ▁of ▁the ▁parameter ( s ) ▁of ▁interest . ▁The ▁locations ▁of ▁the ▁highest ▁( separ ated ) ▁pe aks ▁of ▁the ▁function ▁in ▁question ▁are ▁recorded ▁as ▁the ▁DO A ▁estimates . ▁Param et ric ▁techniques , ▁on ▁the ▁other ▁hand , ▁require ▁a ▁simultane ous ▁search ▁for ▁all ▁parameters ▁of ▁interest . ▁The ▁basic ▁advantage ▁of ▁using ▁the ▁paramet ric ▁approach ▁comparing ▁to ▁the ▁spectral ▁based ▁approach ▁is ▁the ▁accuracy , ▁al beit ▁at ▁the ▁exp ense ▁of ▁an ▁increased ▁computational ▁complexity . ▁ ▁Spect ral – based