The MINER Act of 2006 requires that the locations of underground workers be known after an emergency. While surface conditions allow the use of Global Positioning Systems (GPS), there are no readily available means to perform a similar feat underground. Also, since location data must ultimately be sent to the surface, the challenges of implementing reliable communication systems are also an issue.

Manual Systems

At the start of each work shift, the mine foreman provides the dispatcher with a list of miners and where they will be working in the mine. If a miner later needs to go to a different location, he or she notifies the dispatcher via the underground phone, and the dispatcher updates the list. This form of manual tracking has a number of limitations:

• A miner may report a location as being within a working section, which can cover a very large area.
• A miner may forget to notify the dispatcher when changing work locations.
• A dispatcher has many duties and may not be available when a miner calls.
• In an emergency, the phone system may not be operational.

Electronic tracking systems can address most, if not all, of these limitations. The following sections discuss several types of electronic tracking systems.

Reader-Based Systems

Reader-based systems are usually based on radio frequency identification (RFID) technology. An RFID tag is a small device (typically about the size of a postage stamp) which acts as a very small radio, able to transmit and receive messages. An RFID reader is a transmitter/receiver that reads the contents of RFID tags in the vicinity.

Zone-Based RFID

In a zone-based system, each miner wears an RFID tag that transmits a unique identifier. RFID readers are placed throughout the mine. Each reader has its own identification and known location. When a miner passes within range of a reader, the reader interrogates the miner's tag and relays information to a central location, usually the mine operations center, though the mine's communication system. Based on the relayed information, personnel at the center know that the miner is within a certain distance of that reader.

Reverse RFID

By comparison to zone-based RFID, as the name implies, with reverse RFID the miners wear the readers and the tags are in fixed, known locations. As a miner travels through the mine, his or her RFID reader receives identification information emitted by the stationary RFID tags and relays that information to the mine operations center through the mine's communication system. RFID tags are relatively inexpensive, especially compared to RFID readers. They can be positioned close together so that a miner's location can be accurately determined. Each tag contains a battery, which makes maintenance a concern, but the batteries can last 10 years. NIOSH funded one contract to develop a reverse RFID miner tracking system:

Radio Node-Based Systems

Radio node-based tracking systems use the same physical components as node-based communications systems (see Wireless Systems). The known locations of the fixed position nodes are used as reference points, and each handheld radio has a unique identifier. Thus, node-based communications systems often have all the necessary components to support electronic tracking.

Positioning the nodes close enough so that there is continuous communications coverage allows a miner's handheld radio to receive signals from multiple nodes and determine their signal strengths. Each signal identifies the node from which it came. In some systems, the information accumulates in the miner's radio and then transmits back to the mine operations center for analysis. In other systems, the miner's radio compares the signal strengths received from two or more nodes and automatically determines the miner's distance from each of those nodes.

An example of a radio node-based system is under development by L-3 Global Security & Engineering Systems in the following contract:

Other Systems

Inertial Navigation

"Dead reckoning" is the process of estimating one's current position by advancing a previously determined position using a known speed, elapsed time, and course. Modern inertial navigation systems (INS), which depend upon dead reckoning, are widely used in commercial and military applications.

The introduction of Micro-Electro-Mechanical Systems (MEMS) has allowed miniaturization of the sensors that provide the position tracking functions of an INS. These devices use accelerometers and gyroscopes to monitor a person's velocity and heading while walking. NIOSH is exploring this approach for underground coal miner tracking applications under the following contract:

Passive Fiber Optics

A novel approach under development by US Sensor Systems, Inc. uses a fiber optic cable strung or buried throughout the mine workings which detects acoustic signals from transmitting beacons worn by miners. A second fiber optic line in the same cable provides two-way communications using optically powered earplugs at designated breakouts. No underground electrical power is required. This work is being conducted under the following contract: