  Owing to the ubiquity of cellular communication signals, positioning with the
fifth generation (5G) signal has emerged as a promising solution in global
navigation satellite system-denied areas. Unfortunately, although the widely
employed antenna arrays in 5G remote radio units (RRUs) facilitate the
measurement of the direction of arrival (DOA), DOA-based positioning
performance is severely degraded by array errors. This paper proposes an
in-situ calibration framework with a user terminal transmitting 5G reference
signals at several known positions in the actual operating environment and the
accessible RRUs estimating their array errors from these reference signals.
Further, since sub-6GHz small-cell RRUs deployed for indoor coverage generally
have small-aperture antenna arrays, while 5G signals have plentiful bandwidth
resources, this work segregates the multipath components via super-resolution
delay estimation based on the maximum likelihood criteria. This differs
significantly from existing in-situ calibration works which resolve multipaths
in the spatial domain. The superiority of the proposed method is first verified
by numerical simulations. We then demonstrate via field test with commercial 5G
equipment that, a reduction of 46.7% for 1-${\sigma}$ DOA estimation error can
be achieved by in-situ calibration using the proposed method.
