  Networked robotic systems, such as connected vehicle platoons, can improve
the safety and efficiency of transportation networks by allowing for high-speed
coordination. To enable such coordination, these systems rely on networked
communications. This can make them susceptible to cyber attacks. Though
security methods such as encryption or specially designed network topologies
can increase the difficulty of successfully executing such an attack, these
techniques are unable to guarantee secure communication against an attacker.
More troublingly, these security methods are unable to ensure that individual
agents are able to detect attacks that alter the content of specific messages.
To ensure resilient behavior under such attacks, this paper formulates a
networked linear time-varying version of dynamic watermarking in which each
agent generates and adds a private excitation to the input of its corresponding
robotic subsystem. This paper demonstrates that such a method can enable each
agent in a networked robotic system to detect cyber attacks. By altering
measurements sent between vehicles, this paper illustrates that an attacker can
create unstable behavior within a platoon. By utilizing the dynamic
watermarking method proposed in this paper, the attack is detected, allowing
the vehicles in the platoon to gracefully degrade to a non-communicative
control strategy that maintains safety across a variety of scenarios.
