  Air transportation is essential for society, and it is increasing gradually
due to its importance. To improve the airspace operation, new technologies are
under development, such as Unmanned Aircraft Systems (UAS). In fact, in the
past few years, there has been a growth in UAS numbers in segregated airspace.
However, there is an interest in integrating these aircraft into the National
Airspace System (NAS). The UAS is vital to different industries due to its
advantages brought to the airspace (e.g., efficiency). Conversely, the
relationship between UAS and Air Traffic Control (ATC) needs to be well-defined
due to the impacts on ATC capacity these aircraft may present. Throughout the
years, this impact may be lower than it is nowadays because the current lack of
familiarity in this relationship contributes to higher workload levels.
Thereupon, the primary goal of this research is to present a comprehensive
review of the advancements in the integration of UAS in the National Airspace
System (NAS) from different perspectives. We consider the challenges regarding
simulation, final approach, and optimization of problems related to the
interoperability of such systems in the airspace. Finally, we identify several
open challenges in the field based on the existing state-of-the-art proposals.
