  An unmanned aircraft system (UAS) consists of an unmanned aerial vehicle
(UAV) and its controller which use radios to communicate. While the remote
controller (RC) is traditionally operated by a person who is maintaining visual
line of sight with the UAV it controls, the trend is moving towards long-range
control and autonomous operation. To enable this, reliable and widely available
wireless connectivity is needed because it is the only way to manually control
a UAV or take control of an autonomous UAV flight. This article surveys the
ongoing Third Generation Partnership Project (3GPP) standardization activities
for enabling networked UASs. In particular, we present the requirements,
envisaged architecture and services to be offered to/by UAVs and RCs, which
will communicate with one another, with the UAS Traffic Management (UTM), and
with other users through cellular networks. Critical research directions relate
to security and spectrum coexistence, among others. We identify major R\&D
platforms that will drive the standardization of cellular communications
networks and applications.
