The ultimate mine emergency communications system would be useable for normal operations but remain functional after a serious accident. Aside from the significant problems of propagating radio signals in an underground environment, there are other substantial Communications and Tracking Challenges.

NIOSH is currently researching a variety of potential solutions for mine emergency communications. These systems generally fall into one of two categories - hard-wired and wireless.

Hard-wired Systems

Hardwired systems use a wire physically connected to the miner's handheld device. Many underground coal mines use hard-wired telephones to communicate between the surface and the underground miners. For example:

• Mine pager phones (most common) - Each phone broadcasts to all other pager phones in the system. Pager phones work well for notifying miners about an emergency, but do not allow private or simultaneous conversations.
• Dial type phones - Similar to, but separate from commercial dial systems. Private conversations are possible. Key personnel are assigned ring codes to identify when a call is for them. Personnel must remember the phone numbers of people to contact.
• Trolley phones - Often used by mines with extensive rail haulage. Communication signals travel through the trolley power line. Phones are located in the trolley vehicles and at key stationary points. Communications to cages and elevators in vertical and slope shafts use the same system.

Hard-wired systems are generally not robust. Wires are easily broken or shorted by rock falls. Once a line is shorted, communications may be severely affected or cease altogether because the systems lack redundancy.

Wireless Systems

Wireless communications systems require no physical connection to a miner's handheld radio. Antennas couple electromagnetic energy from the transmitter (the speaker's radio) to a transmission medium (air, wires, metallic pipes, fiber optic cable, or even the ground) and then capture it at the receiver location (the listener's radio). NIOSH is developing systems based on four different wireless communication technologies. The primary difference between them is their frequency bands of operation, each of which uses a different transmission medium.

Medium Frequency Radio Systems (300-3000 KHz)

Medium frequency (MF) radio systems work by coupling on metallic structures and cables within the mine. Ranges in excess of a mile are possible, but current systems have limited portability and cannot operate in explosive atmospheres. MF systems also have limited range in mine tunnels without conductors, such as escapeways and bleeder systems. NIOSH is working with the U.S. Army and Kutta Consulting via the following interagency agreement to adapt a military system for use in underground mines:

Leaky Feeder Systems (150 MHz and 450 MHz)

Leaky feeders are two-way systems featuring a base station on the mine surface that communicates with underground radio units such as walkie-talkies. A special cable network is installed underground that is designed to "leak" signal, allowing two-way radio transmissions from underground to the surface.

Commercial leaky feeder systems are installed in many U.S. coal mines, but these are not designed to withstand serious mine fires, explosions, or roof falls. Under a contract with Pillar Innovations LLC, a leaky feeder system has been developed that can maintain mine-wide operational integrity after such an emergency. A major disadvantage of leaky feeder systems is their lack of coverage in cross-cuts and adjacent drifts unless additional splitters and cable are installed. One solution being researched by Rajant Corporation is to couple the VHF leaky feeder signal to a UHF wireless node. The Pillar Innovations and Rajant Corporation contracts are described in the following table:

Node-Based Systems (300-3000 MHz)

Node-based systems use antennas connected to small transceivers called nodes. Communications are generally achieved by passing a signal from the sender through a series of intermediate nodes which ultimately relay it to the receiver.

The nodes contain small microprocessors that can detect when another node is within range, determine its identity, and establish a wireless connection between the nodes. When a sender and receiver wish to link, the computers work in concert to determine the optimum route between the participating nodes. Thus, a node-based communications system can reestablish or reconfigure itself following an accident. NIOSH has contracted with L-3 Global Security & Engineering Solutions to design, install, and evaluate a wireless mesh communications and tracking network in an underground mine, as described in the table below.

Fiber optic cables are ubiquitous for surface applications. In underground environments, they have significant advantages over conventional systems: they are inherently immune to electrical interference and provide electrical isolation for intrinsic safety purposes. A contract has been awarded to Commonwealth Scientific and Industrial Research Organisation (Australia) to evaluate the concept of a buried fiber optic trunk communications pathway with exposed access points (nodes) along the way. Since only a small portion of the system is exposed to air, damage in the event of a mine disaster is greatly reduced. This contract is documented in the table below.

Through-The-Earth Systems (less than 10 kHz)

Through-The-Earth (TTE) is the only technology that can transmit an electromagnetic signal between underground and surface locations without relying on a network or other additional infrastructure. Most electromagnetic waves reflect off the earth or rapidly weaken as they pass into the earth to penetrate only a few feet below the surface. However, at frequencies less than about 10 kHz, waves can propagate more than a thousand feet through the earth. Several factors limit the potential applications for TTE in underground coal mines:

• Difficulties exist in designing the large antennas needed to pick up the low frequency signals of a TTE system.
• Low frequencies limit the amount of information that can be transmitted, making it difficult to use TTE for voice communication. Receipt of the message may be delayed by several minutes.
• Natural and man-made noise sources existing at these low frequencies (e.g., electromagnetic energy from power lines and naturally occurring electromagnetic noise in the atmosphere) further limit the range and information flow of a TTE system.

Given these constraints, TTE systems will most likely be practical only in emergencies. However, they are highly survivable. Three projects have been funded to investigate the use of TTE technology:

Additional Research Support

Support was also needed to resolve difficult questions related to implementing emergency communications systems in underground mines. NIOSH identified the following issues through in-house research and discussions with partners and contractors. Six contracts and two interagency agreements were awarded to address these issues: