Waveform and aperture design for low-frequency RF tomography

Multiple applications would benefit from low frequency, three-dimensional (3-D), high resolution, imagery, among them remote archeological survey through foliage, and detecting voids in collapsed structures and underground. High vertical resolution circular SAR requires ultrawideband waveforms, a problematic aspect in the modern RF spectral environment, particularly at lower frequencies. RF tomography offers the potential to yield high, 3-D resolution using spectrally sparse, narrowband waveforms simultaneously with operation at frequencies that have demonstrated favourable penetration through intervening dielectric media. In this paper, we explore this potential; evaluating minimal spatial support tomographic apertures, combining diverse narrowband signals with 2- and 3-dimensional monostatic and bistatic apertures. Results are presented in terms of image quality metrics: resolution and sidelobe levels. It is shown that, generally, frequency selection is a soft constraint in terms of the achievable resolution and sidelobe levels, that the tomographic aperture with spatial sampling that is linearly continuous and substantially less than hemispherical yields high spatial resolution, and that there is interaction between the tomographic aperture and the waveform set as evidenced in the point spread function. Additionally the bistatic contribution of the collection aperture Fourier surface is demonstrated to enhance image quality when combined in a hybrid monostatic-bistatic aperture.