If the source is passive, i.e. some type of isolated reflector, an iterative technique can be used to focus energy on it.
The TRM reverses and retransmits the signal as usual, and a more focused wave travels toward the target.
An example of an ergodic cavity is an irregularly shaped swimming pool: if someone dives in, eventually the entire surface will be rippling with no clear pattern.
The TRM sends the reversed version of the impulse response back through the same channel, effectively autocorrelating it.
Even when the scatterers were moved slightly (on the order of a wavelength) in between the receive and transmit steps, the focusing was still quite good, showing that time reversal techniques can be robust in the face of a changing medium.
In addition, José M. F. Moura of Carnegie Mellon University has led a research team working to extend the principles of Time Reversal to electromagnetic waves,[8] and they have achieved resolution in excess of the Rayleigh resolution limit, proving the efficacy of Time Reversal techniques.
Their efforts are focused on radar systems, and trying to improve detection and imaging schemes in highly cluttered environments, where Time Reversal techniques seem to provide the greatest benefit.
An attractive aspect of time reversal signal processing is the fact that it makes use of multipath propagation.
When the recorded message is time reversed and played back, there is only one location to launch the waves from in order for them to focus.