This work describes an implementation method of the delay-sum back projection (DSBP) algorithm that can be used in penetration imaging radar. DSBP is one of the synthetic aperture radar imaging algorithms that reconstruct the target image based on the round-trip time of the electromagnetic (EM) signal between the antenna and the target. The round-trip time is influenced by the EM signal path length and the refractive index of the medium. In this work, the EM signal path length is determined using an interpolation method. This method precalculates the EM signal path lengths for a number of discretized incident angles. Then, EM signal path lengths for arbitrary points are calculated by interpolating the precalculated path lengths. The proposed method is applied to both the simulated and measured data and shown to produce accurate images. The performance of the proposed method is analyzed and compared with other conventional methods. The results imply that the proposed method is flexible, accurate, and reasonably fast. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2406–2411, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26277