Improving near-field orthogonality sampling method for qualitative microwave imaging

In this paper, we formulate a recently proposed qualitative imaging approach named near-field orthogonality sampling method (NOSM) for a general measurement configuration. In the very first version, the NOSM was basically a backpropagation method, which analytically transforms the scattered fields, which is obtained with some specific multiview-multistatic measurement configurations (circular or linear), to reduced scattered electric fields on the sampling domain, which encapsulates the targets. The contributions of this paper are: 1) physical meaning of the reduced scattered field of the NOSM is explained by showing how it approximates the contrast source values in the sampling domain for different generic and limited aperture configurations; 2) the application of the NOSM is generalized to a generic configuration and some comments are made how to reduce the computational complexity and source requirements; 3) the reciprocity of the data is accounted for to improve the performance for limited aperture measurement scenarios; and 4) finally, a useful modified indicator is obtained by relying on the physical meaning of the indicator. The accuracy and stability of the proposed formulations are corroborated with numerical examples for which the excitation-measurement configuration is of limited aperture.