This paper presents a method based on the phase dierence of arrival and the non-linear least square estimation to lo- calize a radio-frequency (RF) source, e.g a wireless capsule endoscope, inside the human body. The phase dierence of arrival at multiple frequencies of a signal is used to estimate the distance of the source to a receiver. The linear least square estimation is used to estimate the initial position of the source followed by a non-linear least square method to improve the localization accuracy. The method is applied on noise free simulated data obtained from FDTD simulations in the 403.5 MHz MedRadio band of a simple homogeneous phantom and a more realistic heterogeneous phantom. The distance between the true position and the estimated po- sition is found to be within 1 cm for the simulated cases showing the possibility to use the proposed method for lo- calization of an RF source inside the human body, as long as the signal to noise ratio is high enough.
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