  Besides conventional geostationary (GSO) satellite broadband communication
services, non-geostationary (NGSO) satellites are envisioned to support various
new communication use cases from countless industries. These new scenarios
bring many unprecedented challenges that will be discussed in this paper
alongside with several potential future research opportunities. NGSO systems
are known for various advantages, including their important features of low
cost, lower propagation delay, smaller size, and lower losses in comparison to
GSO satellites. However, there are still many deployment challenges to be
tackled to ensure seamless integration not only with GSO systems but also with
terrestrial networks. In this paper, we discuss several key challenges
including satellite constellation and architecture designs, coexistence with
GSO systems in terms of spectrum access and regulatory issues, resource
management algorithms, and NGSO networking requirements. Additionally, the
latest progress in provisioning secure communication via NGSO systems is
discussed. Finally, this paper identifies multiple important open issues and
research directions to inspire further studies towards the next generation of
satellite networks.
