  Reconfigurable intelligent surfaces (RISs) are recognized with great
potential to strengthen wireless security, yet the performance gain largely
depends on the deployment location of RISs in the network topology. In this
paper, we consider the anti-eavesdropping communication established through a
RIS at a fixed location, as well as an aerial platform mounting another RIS and
a friendly jammer to further improve the secrecy. The aerial RIS helps enhance
the legitimate signal and the aerial cooperative jamming is strengthened
through the fixed RIS. The security gain with aerial reflection and jamming is
further improved with the optimized deployment of the aerial platform. We
particularly consider the imperfect channel state information issue and address
the worst-case secrecy for robust performance. The formulated robust secrecy
rate maximization problem is decomposed into two layers, where the inner layer
solves for reflection and jamming with robust optimization, and the outer layer
tackles the aerial deployment through deep reinforcement learning. Simulation
results show the deployment under different network topologies and demonstrate
the performance superiority of our proposal in terms of the worst-case security
provisioning as compared with the baselines.
