 Schmidt (1977), while working at Northrop Grumman and independently (1979) were the first to correctly exploit the measurement model in the case of sensor arrays of arbitrary form.Schmidt, in particular, accomplished this by first deriving a complete geometric solution in the absence of noise, then cleverly extending the geometric concepts to obtain a reasonable approximate solution in the presence of noise.The resulting algorithm was called MUSIC (MUltiple SIgnal Classification) and has been widely studied.