Impersonation with the Echo Protocol
  The Echo protocol tries to do secure location verification using physical limits imposed by the speeds of light and sound. While the protocol is able to guarantee that a certain object is within a certain region, it cannot ensure the authenticity of further messages from the object without using cryptography. This paper describes an impersonation attack against the protocol based on this weakness. It also describes a couple of approaches which can be used to defend against the attack. 
Yoo Chung and Dongman Lee