I. Introduction

Imaging radars take different forms and have various applications, ranging from stationary radars to synthetic aperture radars, for aircraft objects or celestial ones. All of these imaging techniques are mainly developed based on the mathematical model of range-Doppler (RD) principle [1]. Moreover, Munson [2] and Mensa [3] also provided a rather different explanation of radar imaging using the tomography theory. Both of the two techniques construct high-resolution images by processing data obtained from many different perspective views of a target area. The image resolution of coherent radars is generally produced based on the fundamental processing of measuring range (time-delay) and changes in range (Doppler gradient) while the observation angle varying [1]. RD principle treats data (returned pulse) from various aspect angles as a time history series where the exhibited Doppler frequency is associated with the scatterer azimuth position. Then target images are derived via extracting information of time-delay and Doppler frequency of radar echoes accumulated in the process of observing the relative motion between the target and the radar antenna. Alternatively, tomography explains radar image formation based on the projection slice theory. Munson et al. [2] demonstrated that the radar receiving signal derived from a particular aspect angle actually is the convolution between the transmitting signal and the projection of the target scattering distribution at the same angle. Then target images could be obtained using Fourier methods based on the support area filled by the returned signals acquired over a range of frequencies and multiple aspect angles. Note that there are no essential differences between the two theories for revealing the radar imaging formulism. Actually, inverse synthetic aperture radar imaging developed typically based on the RD principle can be seen as the special case of the tomography in the conditions of a narrow aspect-angle range, high frequency, and far-field. Thus, various radar imaging techniques and algorithms could be demonstrated with the theory of either tomography or RD principle, depending on the approximations and limiting assumptions in application.