Safety Assurance and Rescue Communication Systems ... - IEEE Xplore

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TOPICS IN DESIGN & IMPLEMENTATION

Safety Assurance and Rescue Communication Systems in High-Stress Environments: A Mining Case Study Prasant Misra and Salil Kanhere, The University of New South Wales Diethelm Ostry, CSIRO ICT Centre Sanjay Jha, The University of New South Wales

ABSTRACT Effective communication is critical to the success of response and rescue operations; however, unreliable operation of communication systems in high-stress environments is a significant obstacle to achieving this. The contribution of this article is threefold. First, it outlines those common characteristics that impair communication in high-stress environments and then evaluates their importance, specifically in the underground mine environment. Second, it discusses current underground mine communication techniques and identifies their potential problems. Third, it explores the design of wireless sensor network based communication and location sensing systems that could potentially address current challenges. Finally, preliminary results are presented of an empirical study of communication using a WSN constructed from commercially available wireless sensor nodes in an underground mine near Parkes, New South Wales, Australia.

INTRODUCTION Communication systems relying on wireless links have become integral to industry and to our daily life. They now form a core infrastructure component, which has led to great improvements in convenience, productivity, and safety. Their success has led to a desire to make their capabilities available reliably in all environments of commercial, industrial, and social importance. Some of these environments inherently present challenging technical problems, which constrain wireless communications. For example, wireless communication between mobile devices inside buildings and factories must often operate in conditions of high signal attenuation, electrical interference, and multiple reflections or echoes, which restrict range and performance. Apart from those requirements, which arise in specific applications, reliable operation requires that a communication system should be designed to survive foreseeable accidents and emergencies. These two situations might together be

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termed high-stress environments: environments in which by their nature it is intrinsically difficult to provide communications, and the extreme or abnormal conditions following an accident or disaster, which can both destroy system components and radically alter environmental conditions in which the system must operate. We shall focus in this article on one such environment, which is both physically harsh under normal operating conditions, and has significant risk of accidents that can damage communications infrastructure and disrupt communications: underground mines. Underground mines are typically extensive labyrinths of long (perhaps several kilometers) and narrow (only a few meters in width) tunnels. They may employ hundreds of mining personnel working at one time under extreme environmental conditions and distributed throughout the mine. The overall mining process is highly mobile, and mining machinery has to be repositioned as the mining operation progresses; consequently, the communication environment continually changes. The combination of ever-changing ground conditions with a dynamic mining system generates a broad profile of risks, which results in human casualties in mine accidents [1]. Management of the hazards in underground mines requires continuous monitoring of critical information: the presence and concentration of flammable and toxic gases and dust, the structural integrity and stability of the mine tunnels, water ingress, and the current locations and communication status of all underground mine personnel. In the aftermath of an accident, it can be vital to maintain communications with trapped miners and rescuers, and to establish and track their positions. A knowledge of environmental conditions through remote sensors in potential escape routes would aid the preparation, planning, and execution of rescue operations. Regardless of the specific type of high-stress environment, reliable communication is essential for successful mine operation under normal conditions, and is vital to the success of emergency response and rescue operations. Communication

IEEE Communications Magazine • April 2010

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failures in hostile environments can occur because of inadvertent destruction of network infrastructure during normal operations as well as in emergencies. Failures also occur due to an error-prone communication channel in the particular application environment, and degradation of the channel after an accident. The term communication channel here notionally lumps together all the physical components of the system between the communicating devices, typically including the material path through which signals must pass. Conventional communication equipment may never be entirely adequate in some severe high-stress environments, but it is important to identify and investigate the various characteristics that prevent satisfactory communication in such environments, both to map the range of applicability of different approaches and to indicate possible direction that may lead to future advances. This article provides a summary view of the field, which may provide practical benefits to other engineers who are working on similar problems and projects. The remainder of the article is arranged as follows. The next section presents a general study of the channel characteristics for high-stress environments, and then examines these characteristics in the specific case of underground mines. We then outline the various communication techniques used in underground mines, and provide a brief survey of the location sensing and tracking approaches for these conditions. Wireless sensor networks (WSNs) have recently been applied to this task, and we provide a concise background and overview of existing work on WSNs, and then explore the design of WSN-based communication systems. This approach is supported by an empirical study of the wireless communication characteristics of typical commercial WSN nodes deployed in an underground mine. The final section suggests potential research directions in the field of underground mine communication and concludes with a summary of the areas covered in the article.

COMMUNICATION CHANNEL CHARACTERISTICS This section first outlines the general channel properties that are common to what we have called high-stress or harsh environments, and then specializes them to the unique channel conditions prevalent in underground mines.

HIGH-STRESS ENVIRONMENTS Communication channels in high-stress environments share several characteristics that make reliable operation difficult [1–4]. Extreme Path Loss Due to Signal Absorption and Geometric Spreading — The transmitted signal is attenuated by absorption in the medium through which the signal travels, and by the geometric effect of the wavefront area expanding as it propagates away from the transmitter. Both these effects cause a decrease in signal strength with range from the transmitter. The dependence on range typically has an

inverse power law with an exponent, which depends on material properties of the medium (which may vary with operating frequency and environmental factors such as temperature and humidity) and the geometry of the channel. The absorption of electromagnetic (EM) waves in water may be so high at usable frequencies that acoustic links can be an alternative to radio or optical links. EM signals are generally strongly attenuated by the Earth at frequencies normally used for wireless communications, but can penetrate large distances at ultra-low frequencies (hertz to kilohertz). Extensive Multipath Propagation and Fading — When a transmitted signal travels by multiple routes (i.e., multipath) to a receiver (e.g., by reflection from surfaces in the environment), they get added at the receiver antenna. This sum typically ranges between a maximum corresponding to the case when all the individual signals add in phase, to a minimum, even zero, when the signals cancel. The random addition causes space and time fluctuations in signal strength which varies with receiver and transmitter position, signal frequency, and also movement of the transmitter, receiver, or reflecting surfaces (which may be, e.g., vehicles). When different paths have large length differences, their corresponding signals interfere to cause multipath fading and overlap in time, and may result in distortions causing a degradation in the link quality.

When different paths have large length differences, their corresponding signals interfere to cause multipath fading and overlap in time, and may result in distortions causing a degradation in the link quality.

Rapidly Changing Time-Varying Channels — Rapid motion of portable communication equipment, as well as variations in the intervening channel caused by the motion, can cause Doppler frequency shifts and rapid signal strength fluctuations as multipath conditions change. Underwater communication devices may also encounter equivalent acoustic conditions as a result of the motion of the ocean surface and waves in internal water strata. Large Propagation Delay and High Delay Variance — This is a prime challenge faced by very-long-range communication devices (e.g., satellites and deep space communication), as well as underwater communications where the acoustic propagation is some 200,000 times slower than EM waves in air. Variations in the effective path length of the signals due to non-homogeneous material along the path can cause changes to the total propagation time and also introduce a large variance in path delays. Noise — Noise in the communication system, whether externally or internally generated, reduces the effective system sensitivity and therefore maximum range. Some environments (e.g., the vicinity of high-power electrical motors) can have high noise levels, which can degrade radio communication. Noise levels can be severe in satellite and deep space communications because of EM radiation from transient solar storms and background astronomical sources. In the ocean storm, wave motion, shipping, and even biological activity can generate severe acoustic noise.



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attenuation. Restrictions on LOS communication arise from the normal mine arrangement of long orthogonal tunnels, support pillars, tunnel blockages, and floor undulations.

Miner 1 transmitting

Leaky feeder cable Signals are leaking along the entire length of the cable.

Ionized Air — Fires generate ionized air, which can act as a plasma and disturb EM propagation in mines. Humid and Warm Conditions — The relative humidity in mines is high, typically greater than 90 percent and the ambient temperature is commonly around 28°C.

Miner 2 receiving

Figure 1. Wireless communication mediated through fixed leaky feeder cables.

Besides these factors, stringent power constraints, topological variability, lack of interoperability, and the use of fixed communication infrastructure [1] are important characteristics of harsh environments. As a consequence of all these factors, communication systems may suffer from limited bandwidth, intermittent link connectivity, high distortion and link error and packet loss rates, unacceptable packet reception jitter, and delay (important in the case of lowlatency applications such as voice and video).

UNDERGROUND MINES Underground mines are generally structurally non-uniform, with a network of interconnected tunnels, crosscuts, shafts, escape ways, first-aid stations, alcoves, and tunnel blockages. Some of the tunnels may contain rail tracks and conveyor belts. The walls are generally rough and the ground surface uneven, and scattered regions of accumulated water may be present. Some parts of the wall and ceilings may be strengthened with bolted wooden grids and metal beams. Environmental conditions that affect communication in mines include the following [1, 5]. Dynamic Changes in Underground Topology — The location of mine walls and faces may alter continuously as a result of mining operations.

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Gaseous Hazards — The main component of the flammable gases that leak from coal seams is methane. When the concentration of methane exceeds a critical threshold, an explosive mixture is formed with a risk of gas blasts. Hence, continual ventilation is required to decrease the buildup of dangerous gases. However, in the case of a disaster, power supply to the mine equipment may be cut, possibly leading to failure of the ventilation system with the risk of dangerous gas accumulation. Equipment for use in coal mines in most jurisdictions must be certified as explosion-proof (i.e., unable to trigger an explosion in air containing any proportion of methane). Besides these natural environmental conditions, every mine is unique with its own distinct operating considerations. In addition to the above environmental properties, there are other channel characteristics specific to underground mines. Waveguide Effect — Mine tunnels can act as waveguides at certain frequencies, and allow relatively low-loss propagation, which can provide long-range communication. This behavior is discussed in more detail in the next section. Noise — The EM channel is effectively shared with all the other electrical systems in the mine, leading to background noise. Electric machinery, power cabling and other mining appliances can generate noise in some of the frequency bands used by underground communication devices, and hence can have an adverse effect on their performance. Other independent systems using wireless links can also contribute to the background noise. In a disaster response and recovery situation, noise levels may be temporarily reduced due to power shutdowns, but heavy mechanical rescue equipment and other electronic equipment may introduce additional noise.

UNDERGROUND MINE COMMUNICATION

Instability in Mine Structures — Some extraction techniques use collapse zones where there are no supports and the faces are allowed to collapse as mining operations proceed, or in the event of seismic activity.

This section describes some of the communication techniques that have been applied in underground mines, and outlines recent approaches to communication and tracking devices.

Limited Line of Sight— Having a line of sight (LOS) between transmitter and receiver can significantly improve communication, as signals can propagate directly rather than through material or around corners, both of which cause excess

Communication techniques applied in mines can be classified as one of three basic types [5]: Through-the-Wire (TTW), Through-the-Air (TTA), and Through-the-Earth (TTE).

COMMUNICATION TECHNIQUES



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Name

Type

Advantages

Disadvantages

Telephones

TTW

Easy operation

Vulnerable to damage from roof falls, mine fires, and explosions

Pager phones

TTW + TTA

Cheap; simple operation

One-way

Trolley phones

TTW

Fixed/mobile — can provide communication to all rail haulage vehicles

Limited coverage; constant vibration; warm, humid, and dusty conditions; interference from electrical machinery

Hoist phones

TTW

Simple operation

Limited to communication between the hoist cage and surface/underground stations

Walkietalkie

TTA

Wireless communication; portable; twoway; can connect to nearby communication infrastructure (e.g., leaky feeder)

Generally poor range but may have good LOS performance

Table 1. Communication devices. Through-the-Wire — As in systems deployed above ground, a fixed infrastructure can provide routine long-distance communication in harsh mine environments. Signals can be sent over electrical conductors such as twisted pair and coaxial cables, and via optical fibers [1, 2]. Cabling primarily intended for other purposes, such as to provide power to electric rail vehicles (trolleys), can also be used to carry signals. A major disadvantage of these systems is that underground personnel must use equipment that is physically connected to the cables for signaling, whereas communication with unhindered mobility is a prime requirement underground. Hybrid systems, such as those using leaky feeders, which use fixed wiring to distribute signals accessed by wireless connections to nearby miners (Fig. 1) will be discussed in the next section. Although the performance of TTW systems is satisfactory for routine operations, fixed cabling is prone to damage and breakage in accidents involving fire, earth falls, and tunnel disruptions, and is difficult to maintain [5]. In order to improve the reliability of TTW systems, various cable protection schemes have been applied, including deployment through conduit, burying the cable, feeding cables through borehole connections to main lines, and redundant cabling [1]. However, these methods are expensive, make maintenance more difficult, and increase system complexity. Fiber optic cables have a significant advantage over conventional wired communication techniques as they are not susceptible to electrical interference and generally have far lower attenuation with distance. Some existing communication devices [2, 5] that use TTW techniques are shown in Table 1. Through-the-Air — TTA systems use wireless links to allow untethered mobile communications. The environmental conditions in both metalliferous and coal mines present a unique set of challenges for wireless communications. A simple model of a wireless communication system comprises a transmitter, which generates and launches an EM signal, the communication channel through which the signal propagates, and a receiver. Apart from the practical constraints on portable transmitter and receiver

design, the main difficulties in an underground wireless system arise from the properties of the communication channel and noise sources. In general, EM propagation between two arbitrary points in a mine level requires propagation through the Earth, down tunnels, around corners and past machinery blockages. All these conditions cause strong attenuation and signal degradation, dependent on both operating frequency [1] and the specific environment. The material through which mine tunnels are constructed typically behaves as a low-loss dielectric, allowing a tunnel to act as a waveguide, with relatively low-loss EM propagation possible along it [5]. Ideal waveguides have a characteristic frequency called the cutoff frequency, below which EM waves cannot propagate. The cutoff frequency is directly related to the tunnel crosssection dimensions, and for typical mines the cutoff frequency is in the tens to low hundreds of megahertz (i.e., the very high frequency [VHF] band). Above the cutoff frequency, EM waves can propagate by essentially following paths that bounce along the tunnel walls at a grazing angle. At each reflection, some signal energy is lost by scattering from irregularities in the tunnel walls and floor, and refraction into the surrounding material. The loss tends to be greater at higher frequencies. LOS waveguide propagation can be surprisingly good at ultra high frequency (UHF) [6], with the best performance in coal mines typically at around 900 MHz in the UHF band and providing ranges of some hundreds of meters. Below the cutoff frequency, waveguide propagation is not possible, although direct LOS propagation can allow communication over a short range. Long-range across-mine communications can be implemented with hybrid systems in which signals are carried sequentially in both fixed TTW infrastructure and generally shorter wireless links. The TTW infrastructure can have translation or bridge equipment at regular spacing to convert signals from cable form to wireless UHF signals, for example, which can be used by miners with portable and handheld UHF equipment. This approach combines some of the benefits of both TTW and TTA systems, but also carries the disadvantage that the fixed infrastructure is vulnerable in mining accidents. Two common hybrid forms use either UHF or



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Extremely low frequency (ELF) (30–300 Hz)

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Very low frequency (VLF) (3–30 kHz)

Low frequency (LF) (30–300 kHz)

Medium frequency (MF) (300–3000 kHz)

Very high frequency (VHF) (30–300 MHz)

Decreases

Bandwidth

Increases

Increases

Antenna size

Decreases

Decreases

Attenuation

Increases

Increases

Noise level

Decreases

Ultra high frequency (UHF) (300–3000 MHz)

Figure 2. Influence of operating frequency. medium frequency (MF) signals at around 1 MHz. A UHF implementation typically uses leaky feeder cables mounted along selected tunnels as the fixed infrastructure. Leaky feeder cable is specially constructed to allow a proportion of EM signals traveling in the cable to both escape into the environment and enter the cable from the environment. Two separated miners, each near the leaky feeder, can communicate via this cable using UHF handsets. The signal transmitted by one is picked up by the leaky feeder cable, propagates down the cable while being partly re-radiated into the environment along the whole cable length, and the second miner can receive this signal. MF signals strongly couple to continuous metal conductors and can use them as the longrange transmission medium rather than specially constructed leaky feeders. Miners use MF equipment, generally bulkier and less portable than UHF equipment, to generate signals that are carried along purpose-deployed single metallic conductors, or suitable pre-existing structures such as lifelines or power rails. One benefit of MF systems is that, in case of an accident, it may be possible to use any available undamaged conductors to traverse blocked tunnel regions. A natural extension of the hybrid approach uses a deployment of wireless nodes to form a wireless mesh network, which can forward messages from a miner within range of any node to a destination point in the network where the message can be either delivered or forwarded through other communications systems. The availability of multiple paths in these networks gives them resistance to link failures, which are likely to occur in emergencies. Digital modulation technologies (e.g., as used in WiFi networks) have been developed to operate at high data rates in the severe multipath environments typical of mines, and hence may be able to support speech and video communications. Because of their flexibility and potential performance in a range of difficult environments, wireless mesh networks are the subject of active current research.

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All the above systems can be combined to extend the range over which communication is possible and to provide system redundancy by providing independent modes of operation. Through-the-Earth — The attenuation of EM signals through the ground strongly depends on operating frequency. Figure 2 shows the qualitative dependence of several factors: available signal bandwidth, attenuation, antenna size, and noise levels on the frequency. These trade-offs prevent a system operating in one frequency band from satisfying all operational and emergency requirements. EM signals at the operating frequencies typically used by TTA systems are unable to penetrate rock strata. However, attenuation of EM signals through the earth (TTE) decreases with frequency, and at very low frequencies, ranges can become great enough to allow even direct surface-to-underground communication [5]. TTE communication systems typically operate between 90 Hz and 4 kHz, and typically must use large loop antennae to launch EM signals efficiently at these frequencies. The data rates required for speech cannot be supported at these frequencies, so communications are limited to text messages. Efficient antennas must be large (perhaps even kilometers in diameter to support direct surfaceto-miner operation), and miners may have to deploy wire loops underground as required. The capability of direct communication with trapped miners, independent of below-ground mine infrastructure, makes provision of TTE systems particularly important in mine emergencies. TTW, TTA, and TTE communication technologies have their distinct capabilities and limitations, which makes selection of a suitable system or combination of systems strongly dependent on the particular application [2].

TRACKING SYSTEMS The majority of current tracking systems are based on radio frequency identification device (RFID) technology. RFIDs or tags are small electronic



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devices that can communicate with more complex reader nodes via wireless communications. Reader nodes can interrogate tags to exchange identity and other information. Two main systems are currently being used. In the first, miners carry RFID tags, and reader nodes placed at known fixed positions are connected to a mine communications system, which can send data back to a central collection point. When a miner passes within range of a reader, the tag identity is transmitted back to the control center, giving an indication that a particular miner is located within the reception zone of the reader. Position resolution depends on the density of reader nodes in the mine and their coverage areas. In the alternate system known as reverse RFID, the RFID devices are deployed at known positions. The miners now carry portable readers, which interrogate the static RFID devices for their identity and transmit the information back to a central site via the miners’ existing communication system. As mentioned previously, underground wireless communications can be implemented via a mesh network of fixed nodes (Fig. 3). This raises the possibility of integrating communication and positioning functions in one system. At its simplest, the identities of the nodes nearest to the miner can give zone position information. System performance can be improved by combining link information, such as signal strength and propagation timing measurements, to allow more precise localization. Information about research agencies, manufacturers, and commercially available tracking systems can be found in [1, 7].

WSNS IN UNDERGROUND MINES The general dependence on TTW systems for operational use, together with restricted environmental monitoring capabilities, is a limitation in providing safety assurance and rescue communication capabilities. This section investigates the feasibility of applying the emerging WSN technology to implement a location sensing and environmental monitoring system, and discusses related work and our own experiences in the deployment of a WSN in an underground mine in Parkes, New South Wales, Australia.

BACKGROUND WSNs provide a new option for portable wireless communication systems, by using a network of WSN nodes to provide the required network connectivity in a cheap and efficient manner. WSN devices are also well suited to distributed environment monitoring, and can report gas and dust concentrations and geological stability data over their deployment range by attaching suitable sensors.

The system reconfigures when a node in a route fails and determines a new route for communication.

Figure 3. Node-based tracking system: a wireless mesh network. for integration with deployed systems and planned enhancements • Modifications required to make WSN nodes usable in the mining environment and able to provide the desired data • Long sensor node life through use of both batteries high in energy density or rechargeable using available energy sources, and techniques to minimize node power consumption while carrying out network operations • Physical protection of the WSN nodes and sensors to prevent damage or faulty operation in normal and post-accident circumstances without adversely affecting communications • Network protocols to store, exchange, and retrieve information reliably under harsh operating conditions • System health monitoring to establish and report the functional status of the system during normal conditions as well as after the occurrence of a mine accident • System maintainability, that is, the effort required to keep the system operational in both normal and emergency conditions • Decision systems to present sensor data in a way that can be easily interpreted to assist operational and emergency planning

REQUIREMENTS FOR A WSN IN HARSH ENVIRONMENTS

AN EMPIRICAL STUDY IN AN UNDERGROUND MINE

There are a number of important factors that must be considered in order to design a WSN implementation for location sensing and environmental monitoring in underground mines: • Availability of sensors and nodes suitable

In order to assess the limitations of currently available commercial WSN nodes when deployed as a wireless communication network in underground mines, we conducted a series of experiments using off-the-shelf MicaZ [1] wireless



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0

Base station

10 8.5 m 9 1

14 m

12 m 2

8

8.8 m

12 m 3 7

11 m 4

9.5 m

4m 6

5 5m

received by the base station from a majority of the motes. Hence, a second experiment was conducted with the motes placed much closer to each other, as shown in Fig. 4. Figure 5 shows the percentage of packets received correctly at the base station from each of the individual motes. The success rate is less than 50 percent for most of the motes. The results suggest that those motes at one hop distance from the base station (motes 1 and 10) and with a clear LOS (mote 10) performed better than the other motes. Several factors were identified that could have contributed to the low packet reception rate: dynamic channel changes due to personnel motion during the tests; slight misalignment of antennas due to the mounting method, and multiple reflections from the mine walls and other metallic objects attached to them. In addition, we believe that variations in the performance of the individual mote radios also influenced packet throughput, as some motes achieved good signal strength and performance, while others failed to communicate at the same distance in a similar configuration. Our experience highlights the need for custom design of wireless sensor nodes that can provide reliable communication in harsh environments.

EXISTING WORK Figure 4. Experimental setup inside the mine. sensor nodes (motes) in an underground gold and copper mine located near Parkes, New South Wales, Australia. Deployment — The mine tunnel that was accessible for experimentation was approximately 5 m in width and 10 m in height, and had projecting bolts positioned approximately 2 m above the tunnel floor. MicaZ motes were enclosed in plastic boxes to act as a protective casing, with the antennas protruding. The plastic boxes containing the MicaZ motes could be mounted on the bolts in the tunnel walls. Mote 0 was configured as the base station, while all the other motes (numbered 1–10) were sited along both walls of the tunnel, as depicted in Fig. 4. An experiment was conducted to test whether motes 1–10 were able to successfully send packets to the base station across multiple hops. All the motes were programmed using TinyOS and nesC [1]. The packet length was fixed at 29 bytes with a simple structure comprising a header and a payload containing the mote identification code. Packets were sent at intervals of 100 ms, and approximately 6000 packets were sent from each mote over a period of 10 min. Discussion — It proved to be more difficult than expected to set up the experiment in the humid and dusty mine environment. Coordinating the deployment of sensor motes inside a dark underground mine tunnel and conducting experiments is a nontrivial task in practice, as the acoustic properties of the tunnel do not permit people to speak to each other if they are more than 50 m apart. When the motes were placed at 15 m intervals, no packets were

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We provide here a brief survey focusing on work directed to the systems design and deployment of WSNs in underground mines. Li et al. [8] present a sensor network deployment and collaborative communication strategy to detect the structural changes in the event of underground mine collapses. Field studies were conducted through the deployment of a prototype system consisting of 27 Crossbow Mica2 motes in the D. L. coal mine in China. The prospects of using ultra-wide-band (UWB) signals in conjunction with WSNs for localization in underground mines have been studied by Chehri et al. [9]. Measurement data for simulation were collected from the CANMET experimental mine in Canada. Xuhui et al. [10] describe the implementation of a methane gas sensor and propose an automatic calibration technique with the help of network connectivity. FireFly, a new sensor hardware platform based on a cross-layer solution for tracking and voice communication in harsh environments, was introduced in Mangharam et al. [11]. The experimental results reported in that work were collected in a NIOSH experimental coal mine. Xiaodong et al. [12] describe their experiences in monitoring the coal mine conditions via a wireless network consisting of Crossbow MicaZ sensor nodes equipped with custom developed multifunctional sensor boards.

RESEARCH DIRECTIONS AND CONCLUSION The performance of communication and tracking systems in underground mines has not been as actively or extensively researched as contemporary surface-based systems. There are few existing systems, and there is limited public information regarding implementation details and actual performance in mines. Ensuring safe-



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ty of mining personnel is one of the dominant issues driving the development of these systems. However, the difficult conditions in mines and the lack of practical approaches have prevented the development of a robust and generally applicable safety system. In view of experiences learned from mining accidents, there is a need to research the applicability of new technologies in mine environments. Current systems lack the capability of sensing and assessing information that could help in predicting the risk of an accident. There is a need to engineer early warning systems to bridge this gap because there is only a very limited capability to limit the intensity and impact of a disaster when it strikes. Currently available tracking systems only register that a person is within a certain region or zone. Research is needed into autonomous and robust tracking systems capable of high spatial resolution in real-time continuous tracking. Wireless systems using spread-spectrum or UWB radios that promise accurate positioning together with robust communication in strong-multipath environments, and software defined radios able to adapt to dynamic propagation conditions are other promising research areas that could address some of the challenges posed by radio propagation in mines. This article outlines features common to a range of high-stress environments and described the factors that may affect communication in deployments in these environments. It identifies those factors that present the greatest challenges to reliable communication in underground mines. Properties of the underground wireless channel, and the design and implementation of current approaches to communication and tracking using this channel are also discussed. Finally, the article discusses the emerging technology of WSN deployments in harsh environments and its applicability in underground mines, and we describe our preliminary experiments to assess the operation of WSNs in mines using generalpurpose commercial nodes.

REFERENCES [1] P. Misra et al., “Safety Assurance and Rescue Communication Systems in High-stress Environments,” tech. rep. UNSW-CSE-TR-0912, Univ. New South Wales, 2009. [2] Niosh Office of Mine Safety and Health Research, “Tutorial on Wireless Communications and Electronic Tracking,” working draft, May 2009; http://www.msha.gov/ techsupp/PEDLocating/WirelessCommandTrack2009.pdf [3] I. F. Akyildiz, D. Pompili, and T. Melodia, “ Underwater Acoustic Sensor Networks: Research Challenges,” Ad Hoc Net. J., 2005, pp. 257–79. [4] I. F. Akyildiz et al., “Interplanetary Internet: State of the Art and Research Challenges,” Comp. Net., vol. 43, no. 2, 2003, pp. 75–112. [5] L.K. Bandyopadhyay, S. K. Chaulya, and P. K. Mishra, Wireless Communication in Underground Mines, Springer, 2010. [6] A.G. Emslie, R.L. Lagace, and P.F. Strong, “Theory of the Propagation of UHF Radio Waves in Coal Mine Tunnels,” IEEE Trans. Antennas Propagation, vol. AP-23, no. 2, Mar. 1975. [7] P. Misra, D. Ostry, and S. Jha, “Underground Mine Communication and Tracking Systems: A Survey,” tech. rep. UNSW-CSE-TR-0910, Univ. New South Wales, 2009. [8] M. Li and Y. Liu, “ Underground Coal Mine Monitoring with Wireless Sensor Networks,” ACM Trans. Sensor Net., vol. 5, no. 2, 2009, pp. 1–29. [9] A. Chehri, P. Fortier, and P. M. Tardif, “UWB-Based Sensor Networks for Localization in Mining Environments,” Ad Hoc Net., vol. 7, no. 5, 2009, pp. 987–1000.

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Figure 5. Success rate vs. node ID. [10] Z. Xuhui and W. Sunan, “Design of a Wireless Sensor Network for Methane Monitoring System,” 6th IEEE Int’l. Conf. Industrial Informatics, 2008, pp. 614–18. [11] R. Mangharam, A. Rowe, and R. Rajkumar, “Firefly: A Cross-Layer Platform for Real-Time Embedded Wireless Networks,” Real-Time Sys., vol. 37, no. 3, 2007, pp. 183–231. [12] X. Wang et al., “Deploying a Wireless Sensor Network on the Coal Mines,” IEEE Int’l. Conf. Net., Sensing, Control, 2007, pp. 324–28.

BIOGRAPHIES PRASANT MISRA ([email protected]) is a Ph.D. student in the Networks Research Laboratory (NRL), School of Computer Science and Engineering, University of New South Wales (UNSW), Sydney, Australia. His research interests have been in the area of wireless sensor networks, network embedded systems, and wireless networks. He is a recipient of the Australian Leadership Awards (ALA) scholarship, awarded by the Australian Agency for International Development (AusAID), Government of Australia. He received his B.E. (Hons) in computer science and engineering from Sambalpur University, India, in 2006, and worked as a senior software engineer in Keane Inc., Bangalore, India, 2006–2008. SALIL KANHERE received his M.S. and Ph.D., both in electrical engineering, from Drexel University, Philadelphia, Pennsylvania, in 2001 and 2003, respectively. He is currently a senior lecturer in the School of Computer Science and Engineering, UNSW. His current research interests include participatory sensing, vehicular communication, and wireless mesh and sensor networks. DIETHELM OSTRY [M] ([email protected]) is a research scientist in the Wireless and Networking Technologies Laboratory, ICT Centre, CSIRO Australia. His recent research interests have been in the areas of wireless networks, data network traffic characterization, optical packet networks, and wireless sensor networks. He holds a B.Sc.(Hons) in physics from the Australian National University and an M.Comp.Sc. from the University of Newcastle, Australia. SANJAY JHA is a professor and head of the Network Group at the School of Computer Science and Engineering at UNSW. He holds a Ph.D. degree from the University of Technology, Sydney, Australia. His research activities cover a wide range of topics in networking including wireless sensor networks, ad hoc/community wireless networks, resilience/quality of service (QoS) in IP networks, and active/programmable networks. He has published over 100 articles in high-quality journals and conferences. He is the principal author of the book Engineering Internet QoS and a co-editor of the book Wireless Sensor Networks: A Systems Perspective. He is an Associate Editor of IEEE Transactions on Mobile Computing. He was a Member-at-Large, Technical Committee on Computer Communications (TCCC), IEEE Computer Society for a number of years. He has served on program committees of several conferences. He was the Technical Program Chair of the IEEE Local Computer Networks 2004 and ATNAC ‘04 conferences, and CoChair and General Chair of the Emnets-1 and Emnets-II workshops, respectively. He was also the General Chair of the ACM SenSys 2007 Symposium.



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