  The Earth's ionosphere refracts radio waves incident on an interferometer,
resulting in shifts to the measured positions of radio sources. We present a
method to smoothly remove these shifts and restore sources to their reference
positions, in both the catalogue and image domains. The method is applicable to
instruments and ionospheric weather such that all antennas see the same
ionosphere. The method is generalisable to repairing any sparsely-sampled
vector field distortion to some input data. The code is available under the
Academic Free License (https://opensource.org/licenses/AFL-3.0) from
https://github.com/nhurleywalker/fits_warp
