Mining Contract: Through-the-Earth (TTE) Communications: Range Reliability Improvements

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@cdc.gov.

A variety of systems have demonstrated through-the-earth (TTE) voice communication for underground coal mines that facilitate escape and rescue during a mine emergency. Current technologies include magnetic and electric field. One such system is the Lockheed Martin MagneLink® Magnetic Communication System (MCS). However, the range in a specific mine is decreased by natural ambient noise and noise from mining equipment and the mine’s electrical infrastructure. All of the above noise interferes with signal transmission and processing. Therefore, the Office of Mine Safety and Health Research (OMSHR) contracted with Lockheed Martin to leverage advanced encoding techniques for the waveforms used in MCS to mitigate these issues. The contract specified the program objective as extended range and reliability of the current magnetics-based technologies by improving bit error rate and signal-to-noise performance.

The first work element was waveform analysis. This involved building a test bench in MATLAB to perform Monte-Carlo simulations over a large set of waveform parameters. Much emphasis was placed on providing a realistic channel model for the analysis. The model includes an additive white Gaussian noise (AWGN) channel, along with two mine channels based on autoregressive models, and two mine channels based on actual acquired noise samples. The data used to produce the mine channels was captured under a previous OMSHR contract at a mine in eastern Kentucky, approximately 400 ft. deep, and a mine in Northern West Virginia, approximately 1,000 feet deep.

Based on the laboratory analysis, eight waveform upgrades were developed and incorporated into the MCS system. The enhanced MCS system was then field tested at three coal mines. The new candidate waveforms were transmitted and captured from both surface-to-underground and underground-to-surface at each mine test site. Each waveform was transmitted multiple times at stepped power levels so that coarse bit-error-rate curves could be generated.

The evaluation of the waveforms in field tests showed a performance improvement over the original MCS waveform. The three mine tests provided a sampling of different environments with individual interference and attenuation characteristics. The first test mine was operational with power lines close by, which was reflected in the 60 Hz harmonics present in the noise. The second test took place in a retired mine whose noise environment more closely represented an emergency situation. The third test took place in another operational mine but with a distinct noise environment from the first mine.

With a higher-level protocol development, it would be possible to implement a detection of an infrequent dropped packet due to impulsive noise, and initiate a re-transmission if necessary. In all these environments, the newly developed waveforms showed improved bit error performance with at least two times less transmission energy when compared to the original MCS waveform.