  A milestone of multi-messenger astronomy has been achieved with the detection
of gravitational waves from a binary neutron star merger accompanied by
observations of several associated electromagnetic counterparts. Joint
observations can reveal details of the engines that drive the electromagnetic
and gravitational-wave emission. However, locating and identify an
electromagnetic counterparts to a gravitational-wave event is heavily reliant
on localization of the source through gravitational-wave information. We
explore the sky localization of a simulated set of neutron star mergers as the
worldwide network of gravitational-wave detectors evolves through the next
decade, performing the first such study for neutron star -- black hole binary
sources. Currently, three detectors are observing with additional detectors in
Japan and India expected to become operational in the coming years. With three
detectors, we recover a median neutron star -- black hole binary sky
localization of 60 deg$^2$ at the 90\% credible level. As all five detectors
become operational, sources can be localized to a median of 11 deg$^2$ on the
sky.
