Abstract: Because of the low resolution of OTHR radar and the temporal change of the ionosphere state,the measured target data of the OTHR are uncertain.Besides, the multi-mode of the ionosphere will produce multipath effect for radar target returns.It is very difficult to perform correlation and track processing. In this paper,we discuss the error which may be caused by the transformation from OTH radar slant coordinates to the ground coordinates,and present a basic correction formula in the ground coordinates.Basing on the known signals from external reference sources(e.g.beacon,responder or island, and so on),we also discuss a method that can improve radar location accuracy and eliminate the ionosphere mode ambiguity.