  Here we describe an autonomous way of producing more accurate prompt XRT
positions for Swift-detected GRBs and their afterglows, based on UVOT
astrometry and a detailed mapping between the XRT and UVOT detectors. The
latter significantly reduces the dominant systematic error -- the star-tracker
solution to the World Coordinate System. This technique, which is limited to
times when there is significant overlap between UVOT and XRT PC-mode data,
provides a factor of 2 improvement in the localisation of XRT refined positions
on timescales of less than a few hours. Furthermore, the accuracy achieved is
superior to astrometrically corrected XRT PC mode images at early times (for up
to 24 hours), for the majority of bursts, and is comparable to the accuracy
achieved by astrometrically corrected X-ray positions based on deep XRT PC-mode
imaging at later times (abridged).
