  Reconfigurable intelligent surfaces (RISs) are artificial planar structures
able to offer a unique way of manipulating propagated wireless signals.
Commonly composed of a number of reconfigurable passive cell components and
basic electronic circuits, RISs can almost freely perform a set of wave
modification functionalities, in order to realize programmable wireless
environments (PWEs). However, a more energy-efficient way to realize a PWE is
through dynamically relocating static metasurfaces that perform a unique
functionality. In this paper, we employ a UAV swarm to dynamically deploy a set
of lowcost passive metasurfaces that are able to perform only one
electromagnetic functionality, but with the benefit of requiring no power.
Specifically, the UAV-mounted static metasurfaces are carefully positioned
across the sky to create cascaded channels for improved user service and
security hardening. The performance evaluation results, based on
