I. Introduction
Localization and identification of objects such as aviation aircrafts or underwater vehicles have significant applications in automated surveillance and security systems [1]–[3]. Compared with the traditional active techniques such as radar, laser, and sonar, the passive detection techniques using magnetic anomaly or magnetic gradient tensor (MGT) have many advantages, such as fast response, well defined theory, and convenient operation [4]. And they are efficient in many engineering applications, such as medical investigation, unexploded ordinance localization, and magnetic tracking [4]–[6].