Preventing collisions involving surface mining equipment: 

a GPS-based approach 

Todd M. Ruffa- " Thomas P. J3oldenb 

"Spokane Research Laboratory National Institute for Occupational Safety and Health 

Abstract 

Problem: An average of three workers a year are killed in surface mining operations when a piece of haulage equipment collides with 

another smaller vehicle or a worker on foot. Another three workers are killed each year when haulage equipment backs over the edge of a dump 

point or stockpile. Devices to monitor the blind areas of mining equipment are needed to provide a warning to operators when a vehicle, 

person, or change in terrain is near the equipment. Method: A proximity warning system (PWS) based on the global positioning system (GPS) 

and peer-to-peer communication has been developed to prevent collisions between mining equipment, small vehicles, and stationary 

structures. Results: A final system was demonstrated using one off-highway haul truck, three smaller vehicles, and various stationary structures 

at a surface mining operation. The system successfully displayed the location of nearby vehicles and stationary structures and provided visual 

and audible warnings to the equipment operator when they were within a preset distance. Summary: Many surface mining operations already 

use GPS technology on their mobile equipment for tracking and dispatch. Our tests have shown that it is feasible to add proximity waming to 

these existing systems as a safety feature. Larger scale and long-term tests are needed to prove the technology adequately. Impact on Industry: 

A PWSs that incorporates a combination of technologies could significantly reduce accidents that involve collisions or driving over an edge at 

surface mining operations. 

Keywords: Proximity warning system; Collision; Global positioning system; Haulage equipment; Surface mining; Blind spots 

1. Introduction 

Each year, there are an average of 20 accidents and three 

fatalities involving collisions between a piece of surface 

mining haulage equipment and either a smaller vehicle or a 

worker on foot or some other object. Another 21 accidents 

occur and three mining equipment operators are killed each 

year when their equipment backs over the edge of an embankment, stockpile, or durnp point (Fesalc, Breland, & Spadaro, 

1996; Mine Safety and Health Administration [MSHA], 

2002). These accidents are caused by the operator's limited 

visibility 'fi-om the cab of the equipment. In mining operations, these accidents most often involve large, off-highway 

dump trucks. The areas that an equipment operator cannot see 

while seated in the cab of these trucks can be extensive, 

depending on the size and type of equipment. Fig. 1 shows the 

blind areas around a SO-ton-capacity dump truck common in 

construction and sand and gravel operations. The gray shaded 

area outside of the truck outline shows those areas where the 

truck operator cannot see a 1.8-m-tall person. Larger trucksup to 3 60-ton capacity-are common in mining, and the blind 

areas for these trucks can extend 12 m in fi-ont of the truck. 

Blind areas to the rear and right side can be even larger. 

Researchers at the National Institute for Occupational 

Safety and Health (NIOSH) are investigating methods to 

reduce accidents attributed to the lack of visibility around 

mining equipment. Many technologies exist that can provide an operator with information on unseen objects or 

workers near the equipment, including video cameras, 

sensors, and mirrors. Many of these technologies have been 

popular in other industries, such as ultrasonic sensors in the 

automotive industry and video cameras on recreational 

vehicles, but very few have been successfully applied to 

mining equipment. Other technologies are being developed 

to address this problem and include electromagnetic signal 
Fig. 1. Gray areas indicate where driver cannot see a 1.8-m-tall person fiom cab of a 50-ton-capacity dump truck. 

detection and radar (Ruff, 2001). All of these technologies 

show promise for use on mining equipment; however, 

further development is needed to overcome the challenges 

associated with the harsh environment of mining and the 

size of the equipment being used. 

Global positioning system (GPS) technology also shows 

promise for this application. Many surface mines already use 

GPS on equipment for tracking, dispatch, and control. A 

logical next step for this technology is to use it to track 

equipment, workers, and stationary structures and provide a 

warning when the possibility of a collision exists. The 

NIOSH Spokane Research Laboratory, Spokane, WA7 in 

cooperation with ~rimble, ' Sunnyvale, CA, has developed 

a new system based on GPS technology that will provide an 

equipment operator with information on all other vehicles, 

stationary obstacles, and dump points near the machine. 

2. System concept 

The concept for GPS-based proximity warning for mining equipment entails the use of differential GPS receivers 

Mention of. specific products or manufacturers does not imply 

endorsement by NIOSH. 

and radios on all equipment having reduced visibility, all 

smaller vehicles on the mine site, and all workers on foot. 

As illustrated in Fig. 2, the location of all moving objects 

must be determined and updated in real time, and this 

information must be transmitted to all nearby equipment 

+ GPS satellites 

Base station for 

9, 

differential correction 

/ target 

\ 

Fig. 2. The PWS concept. 
so that the equipment operators are aware of other vehicles 

or workers nearby. In addition, the location of stationary 

structures, such as buildings, utility poles, and dump points, 

are stored in a database of potential obstacles. An alann 

interface in the cab is required to provide a visual and 

audible warning when another vehicle, worker, or stationary 

obstacle is within a preset danger zone around the equipment. 

The advantages of using GPS technology for proximity 

warnings at mining facilities include (a) the ability to use the 

existing GPS infrastructure at many mines, (b) the system's 

accurate location and tracking abilities, (c) low-to-zero 

occurrence of false alams, (d) the capability of the system 

to identify obstacles, and (e) the ability to customize the user 

interface and warning zones. 

Development of a GPS-based proximity warning system 

(PWS) by NIOSH and Trirnble began in 2000. Prototypes 

were tested in an outdoor laboratory setting on passenger 

vehicles (Holden & R~lff, 2001). Development has progressed over the last 2 years, resulting in a mine-ready 

system that was demonstrated at the Phelps Dodge Morenci, 

copper mining operation in April of 2002. 

3. Prototype system 

3.1. System description 

A prototype system was constructed to demonstrate that 

the idea of GPS-based proximity warning was feasible. 

Readily available components were used to keep costs at a 

minimum. Each system consisted of a laptop computer to: 

(a) collect, process, and transmit data, (b) run the PWS 

software, and (c) provide a display for the vehicle operator. 

A PCMCIA wireless network card (IEEE 802.1 1 b) was used 

to communicate between laptops. An off-the-shelf, 12- 

channel, differential GPS receiver and antenna were used 

to determine location, A Coast Guard beacon was used to 

provide differential correction. Two complete systems were 

mounted in two different passenger cars for dynamic tests. 

3.2. Test description and results 

As described in Holden and Ruff (2001), the prototype 

system went through a series of operational and performance tests using two vehicles-a local vehicle and a remote 

roving vehicle. The goal of the operational tests was to 

verify the operation of the various pieces as compared to the 

defined specifications of the system. These specifications 

included the ability to set up, control, and monitor the GPS 

receiver properly, and the ability to send and receive 

information over a wireless local area network (LAN) 

connection. 

One key factor was to determine the reliable transmission 

range of the wireless LAN. Maximum (11 Mbps) and 

minimum (1 Mbps) signaling rates were tested using the 

PWS software running on two laptops with wireless LAN 

cards installed. Each LAN card had a dual-patch diversity 

antenna directly mounted on it. The system fknctioned very 

well and had no packet losses when the two vehicles were 

separated by distances under 60 m. Beyond 60 m, performance declined. The ranges where transmission completely 

stopped were 120 m for the 11-Mbps signal and 220 m for 

the 1-Mbps signal. It was evident that the quality of signal 

reception was a fknction of range, antenna properties, and 

line-of-sight to the transceiver. Note that the wireless network antennas were connected to the PCMCIA cards, so 

antenna type and placement was limited. Signal reception 

can be made more reliable by using a better antenna 

mounted on the exterior of the vehicle. 

Another important test of the wireless communications 

was the time-to-associate measure for a new vehicle entering a local area. At ranges of up to 60 m, the new vehicle 

associated, or was recognized by the PWS, in less than 1 s. 

Outside 60 m, the vehicle's time-to-associate was related to 

signal quality. 

A second set of tests evaluated the performance of the 

system and covered the following items: 

1. ability of the PWS to transfer info'rmation accurately, 

which was measured by matching received data from a 

remote vehicle and data Erorn the local vehicle using GPS 

time tags, 

2. latency of the remote vehicle information, 

3. accuracy of the real-time vehicle position, 

4. response to various dynamics of the remote vehicle, and 

5. response to various dynamics of the local vehicle. 

Provided that the comunications link between the 

vehicles was functioning, the local vehicle's PWS was able 

to follow the trajectory of the remote vehicle according to 

the transmitted information. Errors were determined by 

matching real-time data stored by the remote system with 

the perceived remote data recorded by the local PWS using 

a GPS time mark corresponding to the transmitted information. Essentially, the information was matched in time so all 

latency errors were removed. These results showed that the 

errors introduced to the system by corrupt data transmissions were negligible, as no errors of significance were 

observed. 

The latency of the information presented to an operator 

corresponds to errors in the actual position of the remote 

vehicle. Latency-induced error is dependent upon the velocity of the remote vehicle. Latency can be determined by 

special methods to roughly 0.2 s, assuming a broadcast rate 

of 4 Hz. In the tests, observed latency correlated well with 

this value. Additional sources of latency could be attributed 

to radio and processing delay. Overall, the system was 

measured to have a latency of less than 0.5 s. 

Fig. 3 shows that radio coverage for these particular 

wireless network cards was excellent within a 100-111 range. 

The position of the stationary local vehicle is near the 
Local Geodetic Position of Vehicle 0:30 Computed at Ve!IIcle 0:20 , 

-1 50 1 I 

-200 -150 -100 -50 0 50 100 

Normalized Easting (m) 

Fig. 3. Top view of remote vehicle's path as perceived by local stationary 

vehicle. 

middle right of the figure (black dot). The thin lirie is the 

actual trajectory of the remote vehicle, and the dots are the 

perceived positions. Areas where the line is not covered 

resulted from communications interference from large 

obstacles. This demonstrates the line-of-sight nature of the 

short-range radios. Note that the communication gaps 

occurred over 100 m from the origin of the grid. 

Fig. 4 shows the computed position errors of the moving 

remote vehicle as perceived by the stationary local vehicle. 

Errors of less than 2 m are evident. The graphs show that the 

errors were very small when the remote vehicle was stationary (flat line), but larger when it was in motion. The 

errors can be attributed to position update latency, but are 

within the desired specifications. 

Geodetic Position Error of Vehicle 0:30 Computed at Vehicle 0:20 

2 1 

-0.51 I 

59069 59369 59669 59969 60269 60569 60869 

16:24 16:29 16:34 16:39 16:44 16:49 1654 

Time into Day (s) UTC Time (hrs) 

4. Mine-ready system 

4.1. System description 

Tests of the prototype system showed that the concept of 

a GPS-based PWS was feasible; however, the system had to 

be redesigned using components that could be used on 

mining equipment. The mine-ready PWS consisted of the 

following Trimble components: (a) a GPS antenna, (b) a 

Windows CE-based computer with LCD display to run the 

PWS software, (c) an eight-channel, single-fkequency, differential' GPS receiver (integrated into the computer enclosure), and (d) a SiteNet 900-MHz Internet Protocol (IP) 

radio. All of these components were designed for mounting 

on heavy equipment. 

The mine-ready PWS operates in a similar manner to the 

prototype system, but with a few modifications. As before, 

GPS is used to determine the location of the vehicle on 

which a system is mounted. Differential correction inforrnation from a base station is also received by the PWS. The 

corrected location of that vehicle is then transmitted once 

per second via the IP radio to all other vehicles in the area 

equipped with a PWS. The locations of other vehicles are 

also received by the IP radio and shown on the computer's 

display if they are within a specified range. The location of 

stationary obstacles, such as dump points, power lines, and 

mine buildings, does not have to be transmitted. Their 

coordinates can be entered into the system database so that 

they show up on the vehicle's display. 

4.2. Test description and results 

For tests at the Phelps Dodge Morenci Copper Mine, a 

complete PWS was installed on each of the following 

equipment: Caterpillar 797 360-ton capacity haul truck 

(Fig. 5), Caterpillar rubber-tire dozer (Fig. 5), and two 

service trucks (pickups). A base station was also installed 

Fig. 4. Geodetic position error of moving vehicle computed at local vehicle. Fig. 5. PWS equipment installed on a Caterpillar haul truck and dozer. 
on a nearby hill to provide differential correction information to the individual systems on the vehicles. 

The GPS antennas and IP radios were temporarily, but 

securely, mounted on the mining equipment and service 

trucks in typical locations, usually on or near the cab roof. 

The computer was securely mounted in each vehicle in a 

fashion similar to a final, permanent installation. The PWS 

software ran on this computer and displayed a screen for the 

equipment operator that showed his/her equipment in the 

center, the detection zone radius, the warning zone radius, 

system status, and icons representing other vehicles or 

stationary obstacles in the area (Fig. 6). 

Each vehicle's warning and detection zones were adjusted 

according to the vehicle's size. The display in Fig. 6 was 

mounted in the Caterpillar 797 haul truck and had a 30-mradius warning zone and a 60-m-radius detection zone. The 

zones for the dozer and service trucks were set at 20 and 40 

m. Audible alarms were generated whenever another vehicle 

or stationary obstacle was detected in either zone. In addition, the color of another vehicle's icon changed from green 

(outside both zones), to yellow (inside detection zone), to red 

(inside warning zone) as it approached the center of the 

screen. 

The demonstration and tests were held in an active area of 

the Phelps Dodge Morenci Mine where production traffic 

was at a minimum. The test area consisted of a simulated 

loading area at the bottom of a small pit, a haul road, a dump 

area, and a large open area. A truck loading and dumping 

cycle, described below, was repeated several times to evaluate the reliability of the system. 

1. Haul truck parked in staging area. 

2. Haul truck drives down into pit, passing a simulated 

utility pole to show detection of a single stationary 

obstacle. 

3. Service truck follows. 

Fig. 6. PWS computer display. 

4. Haul truck pulls into simulated loading area with the 

shovel represented by a set of three stationary obstacles. 

5. Dozer works around pit area to show detection of a 

moving vehicle. 

6. Service truck enters pit to show detection of multiple 

moving vehicles. 

7. Haul truck leaves pit and drives up ramp to dump. 

8. Service truck follows within 60 m to show tracking of 

two moving vehicles. 

9. Haul truck backs into dump represented by two 

stationary obstacles to show detection of dump point. 

10. Service truck pulls up and parks in haul truck's blind 

spot to show detection near the haul truck. 

11. Repeat as necessary. 

Another separate test was conducted to demonstrate the 

accuracy of the system. The dozer and haul truck were 

parked in the test area so that each was out of the detection 

zone. The dozer then slowly drove toward the haul truck and 

stopped when the haul truck's PWS indicated that the dozer 

reached the 30-m warning zone. The distance was then 

measured to check system accuracy. The distance between 

the GPS antennas, accounting for differences in antenna 

height, averaged around 28 m for this test. 

Each system successfully tracked three other mobile 

vehicles and six stationary obstacles. Expected accuracy of 

the position of a vehicle or obstacle shown on the PWS 

display was 2-5 m using the computer's internal receiver 

with differential correction. Accuracy depends on many 

factors, including satellite positions (positional dilution of 

precision or PDOP), multipath interference, the status of 

Selective Availability (SA), and the type of GPS receiver 

used, to name a few. Observed accuracy was 2-3 m during 

the tests. Higher position accuracies could be obtained using 

higher quality, external GPS receivers, like those used with 

the prototype system. 

One instance of multipath interference caused an error in 

vehicle location during preliminary tests. Multipath interference occurs when a satellite range signal reflects off 

objects and takes several paths before it reaches the 

receiver. This interference resulted in a service truck 

location that was briefly shifted by 15 m on the haul 

truck's screen. This was corrected as soon as the vehicle 

moved. However, methods to reduce multipath problems 

may need to be studied, including improved antenna 

designs and mounting locations. No other multipath errors 

were seen during the demonstration, and good location 

solutions were obtained even when a smaller vehicle was 

parked immediately next to the haul truck. This meant that 

the satellite constellation was adequate; the number of 

satellites visible to a vehicle never dropped below four, 

even when a larger vehicle blocked some of the satellites. 

This may change at different mine locations or at different 

times of the day because the constellation depends on these 

two factors. 
No problems with satellite visibility were seen in the test 

area because of mine structures such as pit walls; however, 

in deep pits this may be an issue. Pseudolites (ground-based 

transmitters that simulate satellites) are being tested in 

another area of the Phelps Dodge Morenci Mine to supplement satellite coverage for GPS-assisted drilling equipment 

(Sl~ields, Flinn, & Obregon, 2000). The use of pseudolites in 

any mine would increase the number of range transmissions 

used to calculate position, thereby increasing the accuracy 

and reliability of a PWS or any other system using GPS. The 

existing PWS would need to be modified to allow the use of 

pseudolites. 

Some problems were seen that involved the other 

vehicle icons occasionally and briefly shifting position by 

a few meters on the haul truck's display while the truck 

was moving. Part of this was caused by the movement of 

the GPS antenna. The antenna on the haul truck was 

mounted on a long pole, which vibrated and whipped back 

and forth during abrupt truck movement. The sudden 

direction and velocity changes of the antenna sometimes 

confused the PWS, causing the position of the other 

vehicle icons to shift slightly. The shift was brief and 

could be remedied with a more rigid antenna mounting 

scheme or filtering algorithms. Slight errors in vehicle 

heading also contributed to the occasional position shifts 

seen during truck movement. Vehicle heading was calculated by comparing the current position solution with the 

previous one. Any error in position caused errors in the 

calculated heading. This could be remedied by integrating 

dead-reckoning methods and better algorithms for determining heading. These improvements will be made to 

future systems. 

The system that was demonstrated did not have an input 

available for a reverse gear sensor or switch. In order for the 

display to always be aligned so that the top of the screen 

pointed forward, the system had to detect whether the truck 

was moving in forward or reverse. A temporary solution 

was implemented that required the driver to press a button 

when reverse gear was selected. This would be automatic in 

a final version of the PWS. 

5. Discussion 

In order for a mine-wide, GPS-based PWS to be effective, all vehicles, mining equipment, and workers on the 

mine property would need to be outfitted with a system. 

Functionality and cost of each system could vary with each 

type of vehicle. For instance, service trucks and contractor 

vehicles could be outfitted with a simple system that would 

not require the current computer/display. Such a system 

could use an off-the-shelf GPS antenna and receiver, a lowcost processor, and an IP radio all packaged in a single 

enclosure that attached quickly to the vehicle's roof. A 

simple audible warning would be generated in the cab of 

the vehicle when another vehicle or piece of equipment was 

nearby. The projected cost for this system would be around 

US$2,500 per vehicle. 

The reduced visibility associated with larger mining 

equipment would require a more expensive and more functional system. A graphics display would be needed to allow 

the operator to locate and identify nearby obstacles. The 

PWS could stand alone like the mine-ready system 

described here, or it could be integrated into existing 

dispatch and control systems. The projected cost of a 

stand-alone system would be around US$10,000 for each 

.piece of large mining equipment. 

One obvious element missing from these tests was a 

system to protect a worker on foot. This would require a 

personal PWS that consisted of miniature GPS equipment, 

a small processor, and IP radio equipment. The system 

would need to fit on the belt or in the vest pocket of a 

worker. Hardware for a personal system is available, and 

sokware development is planned to begin next year at 

NIOSH. 

If a GPS-based PWS were implemented now, some 

method of redundancy would be required to ensure that 

the equipment, smaller vehicles, and workers were protected 

100% of the time, regardless of satellite visibility. Existing 

technology, such as cameras, radar, or a radio-frequency 

identification (RFID) tag system, also has limitations when 

used alone (Holden & R~lff, 2001). A combination of a 

GPS-based system and one of these other technologies 

could, however, provide the redundancy needed for a highly 

reliable system. 

These preliminary tests at a surface mine showed that a 

GPS-based PWS has the potential to significantly reduce 

accidents that involve collisions or driving over an edge at 

surface mining operations. This is accomplished by providing operators with the location of objects, people, and 

vehicles that may be in the equipment blind areas. Future 

work will involve larger scale and longer term tests to 

prove this technology adequately. In addition, several 

improvements will be made to the proximity warning 

algorithms, such as the integration of dead reckoning 

methods and the ability to use pseudolite signals. The 

integration of additional sensor inputs for travel direction 

and other methods of monitoring blind spots will be needed 

to increase accuracy and reliability. The ability to protect 

workers on the ground will be the fmal element needed to 

complete this system. 

Acknowledgements 

The authors would like to acknowledge the assistance of 

Alain Samaha and Thom Morley, Trimble, Sunnyvale, CAY 

and Glenn MacGougan, University of Calgary, in the 

development of this system. In addition, the assistance of 

Jim Hanson, Phelps Dodge Morenci, Morenci, AZ, and 

Nathan Lowe, National Institute for Occupational Safety and 

Health, Spokane, WA, were critical to the planning and 
execution of the tests at the mine site. Finally, the authors mining equipment. Applied Occupational and Environmental Hygiene, 

16, 798-808. like to thank the Dodge 'Orenci for Shields, S., Flinn, J., & Obregon, A. (2000). Differential GPS applications supporting and hosting the demonstrations. at the Morenci Copper Mine. GPS World, 11(3), 34-38. 

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