https://newatlas.com/underground-communication-system-a-potential-lifesaver/7989/
Underground communication system a potential lifesaver
May 2, 2015 - The Cap Lamp is designed to ensure that miners can instantly and easily be contacted with potentially life saving information about emergencies and evacuation...
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https://journals.open.tudelft.nl/ejtir/article/view/4275
A Novel Communication and Radar System for Underground Railway Applications | European Journal of...