  Gravitational wave science is a new and rapidly expanding field of
observational astronomy. Multimessenger observations of the binary neutron star
merger GW170817 have provided some iconic results including the first
gravitational-wave standard-siren measurement of the Hubble constant, opening
up a new way to probe cosmology. The majority of the compact binary sources
observed in gravitational waves are however without bright electromagnetic
counterparts. In these cases, one can fall back on the ``dark standard siren''
approach to include information statistically from potential host galaxies. For
such a measurement, we need to be cautious about all possible sources of
systematic errors. In this paper, we begin to study the possible errors coming
from the galaxy catalogue sector, and in particular, look into the effect of
galaxy redshift uncertainties for the cases where these are photometry-based.
We recalculate the dark standard siren Hubble constant using the latest GWTC-3
events and associated galaxy catalogues, with different galaxy redshift
uncertainty models, namely, the standard Gaussian, a modified Lorentzian, and
no uncertainty at all. We find that not using redshift uncertainties at all can
lead to a potential bias comparable with other potential systematic effects
previously considered for the GWTC-3 $H_0$ measurement (however still small
compared to the overall statistical error in this measurement). The difference
between different uncertainty models leads to small differences in the results
for the current data; their impact is much smaller than the current statistical
errors and other potential sources of systematic errors which have been
considered in previous robustness studies.
