Abstract: The ill-condition in locating and tracking a moving source from the passive sensors on board is discussed- A simple modified form of Least Square without additional computation, which can improve the estimation accuracy evidently, has been discovered by engineers. In this paper the justification of this modified form of least square is proved. Moreover, through the theoretical analysis on the ill-condition of least square, more general results are presented. These results can be used to ease the ill-condition and improve the estimation accuracy.