Typical radio interferometer observations are performed assuming the source
of radiation to be in the far-field of the instrument, resulting in a
two-dimensional Fourier relationship between the observed visibilities in the
aperture plane and the sky brightness distribution (over a small field of
view). When near-field objects are present in an observation, the standard
approach applies far-field delays during correlation, resulting in loss of
signal coherence for the signal from the near-field object. In this paper, we
demonstrate near-field aperture synthesis techniques using a Murchison
Widefield Array observation of the International Space Station (ISS), as it
appears as a bright near-field object. We perform visibility phase corrections
to restore coherence across the array for the near-field object (however not
restoring coherence losses due to time and frequency averaging at the
correlator). We illustrate the impact of the near-field corrections in the
aperture plane and the sky plane. The aperture plane curves to match the
curvature of the near-field wavefront, and in the sky plane near-field
corrections manifest as fringe rotations at different rates as we bring the
focal point of the array from infinity to the desired near-field distance. We
also demonstrate the inverse scenario of inferring the line-of-sight range of
the ISS by inverting the apparent curvature of the wavefront seen by the
aperture. We conclude the paper by briefly discussing the limitations of the
methods developed and the near-field science cases where our approach can be
exploited.Comment: Accepted in Publications of the Astronomical Society of Australia
(PASA). 10 pages, 7 figures, and lots of linked animation