Near-Field w-Projection for Radio Interferometric Imaging

摘要

This paper serves to elucidate the mathematical steps needed to apply near-field imaging over a significant (angular) area of regard, with specific application to the receiving sites of the Air Force Space Surveillance System (AFSSS, 'Space Fence'). Cornwell has developed the mathematical formalism to merge near-field imaging with imaging over a wide field of view. The traditional far-field, narrow field-of-view imaging developed in radio astronomy can be extended to wide fields of view by taking into account the three-dimensional nature of the receiving array (the so-called 'w term'), leading to a method called 'w-projection.' This w-projection method has the effect of retaining a Fresnel term in the imaging process, and Cornwell sketched how this w-projection might also be applicable to near-field imaging. For the Space Fence receiving sites, the near field is located at an altitude of approximately 1000 km. I illustrate how a near-field, w-projection imaging could be implemented for determining the direction cosines of radar reflections from a system like the Space Fence.