Unmanned aerial vehicles (UAVs), characterized by their low cost and operational flexibility, have been increasingly deployed across civilian, military, and commercial applications. To improve the coverage and connectivity, UAVs can be utilized to realize the comprehensive spatial coverage for the sixth-generation mobile networks. However, the private data in UAV networks is easy to be exposed due to the light-of-sight links and openness of wireless transmission. Covert communication as an emerging technique has shown its superiority in hiding the transmission behavior, which can further enhance the security of UAV networks compared with the traditional physical-layer security. Therefore, in this article, we present a survey on the recent advanced research about covert UAV communications. First, the roles of UAVs for covert communications are described. Then, the covert UAV communications with different uncertainties are introduced. Moreover, the wireless techniques for covert UAV communications are explored. In addition, we point out the applications in covert UAV communications. Finally, the open research issues concerning practical scenarios and promising applications are highlighted.

Unmanned Aerial Vehicles (UAVs) have received a wide range of attention for military and commercial applications. Enhanced with communication capability, UAVs are considered to play important roles in the Sixth Generation (6G) networks due to their low cost and flexible deployment. 6G is supposed to be an all-coverage network to provide ubiquitous connections for space, air, ground and underwater. UAVs are able to provide air-borne wireless coverage flexibly, serving as aerial base stations for ground users, as relays to connect isolated nodes, or as mobile users in cellular networks. However, the onboard energy of small UAVs is extremely limited. Thus, UAVs can be only deployed to establish wireless links temporarily. Prolonging the lifetime and developing green UAV communication with low power consumption becomes a critical challenge. In this article, a comprehensive survey on green UAV communications for 6G is carried out. Specifically, the typical UAVs and their energy consumption models are introduced. Then, the typical trends of green UAV communications are provided. In addition, the typical applications of UAVs and their green designs are discussed. Finally, several promising techniques and open research issues are also pointed out.