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nodes. Node-based systems may be wired or wireless. As the names imply, wired systems primarily use physical connections between nodes for transmission of communication signals, while wireless systems primarily employ radio links between nodes. Although not common before the MINER Act, node-based systems currently account for 56% of the systems installed in U.S mines. Furthermore, a little more than 60% of these node-based communications sys- tems are wireless. These systems usually do not rely on any physical wired connection for communication from underground to the sur- face. Also, among node-based systems in service are examples that use both wired and wireless links between nodes, as well as some systems capable of operating as a wired network under normal operating conditions and automatically transitioning to wireless operation if the wired connections are compromised. Electronic Tracking Most electronic tracking systems determine location by using either communications system nodes or dedicated "readers" installed at known locations underground. These nodes, or read- ers, detect and recognize uniquely identified electronic devices worn by miners. These electronic identification devices are gen- erally one of two types — "tags" or "integrated handsets." Tags provide a continuously repetitive signal, or "beacon," which is detected by a node or reader. Many handsets used for post-accident communication sys- tems have tag capability integrated into the device itself, thus eliminating the need for a dedicated tag. If a tag or handset is detected by more than one node or reader simultaneously, soft- ware algorithms use the detected signal strength or differential signal time of arrival between the nodes or readers to approxi- mate the location of the tag or handset. Tracking information from the readers is typically transmitted to the surface over some part of a communications system. About half of U.S. underground coal mines use the same man- ufacturer for electronic tracking as they do for post-accident communications (Figure 2). In many of the instances where the mine is using the same manufacturer for both CT requirements, the tracking technology is often integrated directly with post- accident communications through the use of a handset with tracking built right in. System Manufacturers This review showed there are 13 manufacturers of post-accident communications systems currently used in U.S. underground coal mines; five produce leaky feeder systems, four make wired node- based systems, and four supply wireless node-based systems. A total of 12 manufacturers supply electronic personnel tracking systems used in U.S. underground coal mines. Five manufacturers use tracking tags in conjunction with a leaky feeder communication infrastructure, and three use either tracking-capable handsets or tags as part of a wired node- based system. The remaining four systems provide tracking through an integrated wireless node-based communications system using either tags or handsets with tracking capability. The accompanying table (Table 1) lists the manufacturers of currently used emergency CT systems, and includes the names under which their systems are marketed. The chart provided (Figure 3) shows the number of systems in service for each manufacturer. Eight years after passage of the MINER Act, post-accident CT in U.S underground coal mines has evolved to a point where a number of systems based on a variety of CT approaches and technologies are available to mine operators. These advances are expected to greatly enhance miner safety, and it is reasonable to expect that emergency CT systems will continue to change and improve as the underlying technology improves, and as manu- facturers adapt their systems to better serve the needs of the mining industry. The findings and conclusions in this review are those of the authors and do not necessarily represent the views of NIOSH. The mention of any company or product does not constitute an endorsement by NIOSH. Nicholas Damiano, Gerald Homce and Ronald Jacksha are researchers with the National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research. c o m m s & t r a c k i n g c o n t i n u e d Table 1 — All Underground Emergency CT Systems November 2014 www.coalage.com 35 Figure 3: All emergency communications and electronic tracking systems by number of installations.