  Satellite networks are expected to support global connectivity and services
via future integrated 6G space-terrestrial networks (STNs), as well as private
non-geostationary satellite orbit (NGSO) constellations. In the past few years,
many such private constellations have been launched or are in planning, e.g.
SpaceX and OneWeb to name a few. In this article we take a closer look at the
private constellations and give a comprehensive overview of their features. We
then discuss major technical challenges resulting from their design and briefly
review the recent literature addressing these challenges. Studying the emerging
private constellations gives us useful insights for engineering the future
STNs. To this end, we study the satellite mobility and evaluate the impact of
two handover strategies on the space-to-ground link performance of four real
private NGSO constellations. We show that the link capacity, delay, and
handover rate vary across the constellations, so the optimal handover strategy
depends on the constellation design. Consequently, the communications solutions
of future STNs should be compliant with the constellation specifics, and the
STN standards need to be flexible enough to support satellite operation with
the large parameter space observed in the emerging private constellations.
