Waveform design for low frequency tomography

There are multiple applications that would benefit from the ability to produce three dimensional, high resolution, imagery collected at low operating frequency; among them remote archeological survey of ruins through foliage, and searching for voids in collapsed structures and underground. High vertical resolution circular SAR requires the use of wide-to-ultra wideband waveforms, a problematic aspect in the modern RF spectral environment, particularly at lower frequencies. RF tomography offers the potential to yield high, 3-dimensional resolution using spectrally sparse, narrowband waveforms simultaneously with operation at frequencies that have demonstrated favorable penetration through intervening dielectric media. In this paper we explore this potential by evaluating minimal spatial support tomographic apertures combining diverse narrowband signals with the form (trajectory) of the monostatic collection aperture. Results are presented in terms of image quality metrics: those frequency combinations that jointly minimize peak and rms voxel sidelobe level, cardinal axis resolution length and voxel volume. It is shown that, generally, the frequency selection is a soft constraint in terms of the achievable resolution and image 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 form/shape of the tomographic and the waveform set.