Time-Reversal Methods in Electromagnetics and Acoustics

When dealing with propagation in a multi-path environment, a short pulse of radiation emitted at one point may yield a very complex waveform at another point in the environment, with extended temporal support. This is manifested because the original short pulse radiates in many directions, and the multiply scattered radiation that propagations in some directions arrives earlier at the receiver than other radiation. This can be viewed as a race between several runners: at the start all runners are at the same position, and the runners are therefore localized spatially; after running a marathon, the runners are typically highly distributed spatially (and they arrive to the finish at very different times) because of their different speeds. Time reversal simply turns the measured time-domain waveform around temporally, with the last arriving radiation going in first, and the fastest arriving radiation going in last. In the context of our marathon, the slowest runner starts first, the fastest runner starts last, distributed temporally exactly as determined by the original race discussed above. In this way, at the end of the marthon all runners come together spatially and temporally. This time-reversal concept may be used to implement covert communications, and it may also be used to effect high-resolution imaging. Our group has applied time-reversal imaging to measured at-sea acoustic data, and we are examining this technology for covert electromagnetic communications.

Comparison of conventional SAS (left) and time reversal combined with SAS (right) for three extended targets of interest to US Navy (labeled a-c). Note that the time-reversal cleans up the imagery, revealing scattering centers on the target (especially the resonant target represented by c). These results are based on measured data.

Time-Reversal “Movies”

Time-reversal movie for acoustic propagation in a waveguide (at sea)

This movie was developed by colleagues at NRL, and it addresses long-range time-reversal focusing of interest for propagation in a shallow water channel.

Time-reversal movie for electromagnetic propagation in a forested environment

This movie was developed using a time-domain ray-propagation model, for a forest-like environment composed of randomly placed dielectric rods (tree trunks and tree branches). The system operates from 1-3 GHz. The focusing shown in this movie occurs in a 4m times 4 m region about the receiver antenna, situated in the center of the model forest.