Recent mine disasters have shown the need for improved communications underground. While broadcasters face challenges getting signals into urban canyons, sending RF through a thousand feet of rock and earth is even more difficult. But David Reagor, who works at Los Alamos National Laboratory, has a solution.
Reagor's system uses very low frequency (VLF) signals -- only a few kilohertz -- and advanced digital signal processing to recover the weak signals in the presence of interference from harmonics of equipment working from AC power lines. Digital audio compression techniques including linear predictive coding and multi-band excitation are used to reduce the required bandwidth to less than one kilohertz, allowing voice transmission over this very low frequency carrier. The system can also transmit text data.
A prototype version of the system has been found to work 500-feet below ground. Reagor is quoted in a recent Duluth News Tribune article saying, "It works everywhere we've taken it."
Additional information on this technology is available at TechCommJournal.org and the Vital Alert Communications Web site. Vital Alert is working with Los Alamos National Laboratory to make the technology commercially available.
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