Benefits of Using GPS Guidance on Excavators and Shovels
By Chris Seymour, Automated Positioning Systems

In order to be an effective production tool, a GPS-based bucket position system must provide information about the position of the bucket that not only is accurate,
but must arrive rapidly enough to allow the operator to use the information while production digging.

TGPS-based control and guidance systems for mining and earthmoving equipment have been in use for a number of years. These systems have proven themselves in rugged applications as a way of cutting costs, increasing accuracy, reducing contamination and improving site safety.
High-precision GPS-based guidance systems on mining excavators and shovels offer a number of key benefits to mine operators. These include:
• Accurate selective mining of mineralized horizons.
• Accurately finding the low wall batter line in coal stripping operations, thereby reducing both over-dig and lost coal.
• Accurate representation of hazardous areas, such as loaded blast areas or areas underlain by old underground workings.
•More even benches, reducing truck cycle times as well as wear and tear on trucks. Such benches require less dozer and grader time.
• More accurately cut ramps, again improving tire life and truck cycle times.
• Selective mining of coal, ensuring high value seams are fully and cleanly recovered.
• Automatic generation of as-built plans.
• Less need for surveyors in the field and far fewer people on the ground around heavy machinery.

Logical Choice
Introducing GPS-based excavator/ shovel guidance systems can be regarded as a logical progression for many mines, most of which now use GPS for surveying. A single surveyor can accomplish in a few hours what once took a team of people days of tedious field and office work.
GPS guidance systems are increasingly being used directly on mining machinery— most notably dozers. Even the most basic GPS guidance systems allow the operator to complete complex earthmoving designs without the need for field staking, while more advanced systems allow for accurate real time productivity monitoring and the automatic generation of “as-builts” in the form of Digital Terrain Maps (DTMs).
In addition to dozers, GPS is now also being applied to other mining equipment including drills, trucks, graders, loaders, and dredges. The use of high precision GPS to determine the position of the bucket teeth of shovels and excavators is one of the “final frontiers”—perhaps surprising, given the fact that there are so many benefits in GPS-based guidance for what is the primary loading tool for most mines.
Most people would be familiar with the U.S.-based Navstar GPS network, which supplies signals to the great majority of GPS receivers, from basic consumer- level units to the most sophisticated military and industrial systems. The Navstar system maintains at least 24 satellites. To achieve the required levels of accuracy, GPS-based guidance systems need to be able to receive a signal from at least five satellites to achieve what is known as “lock.” However, due to the orbits of the Navstar satellites, at various times in the day there will be insufficient satellites visible to achieve lock and the signal will drop out. This problem is exacerbated in deep-pit applications, where mine walls limit the amount of sky that is visible.
Dropouts can range from a few minutes to several hours during the course of a day. GPS guidance systems can somewhat overcome this problem by also tapping into the Russian-based Glonass system, which currently has a network of 11 satellites out of a planned constellation of 18. This gives a total of 35 satellites available, speeding the acquisition of satellite lock and substantially reducing signal dropout. [Ed. Note: Proprietary systems, such as Novariant’s Terralite XPS, also can overcome satellite access problems by using a network of ground-based transmitter stations that broadcast machine position information to compatible GPS receivers mounted on mobile equipment.]
To achieve maximum accuracy, a GPS antenna mounted on the machine and a base station mounted at a fixed point are required. Any number of machines with GPS guidance systems onboard can access the base station. Using a base station, GPS guidance systems have an accuracy of about 1 centimeter.
Accuracy is lost in translating the positions to the bucket, but even on large mining excavators and shovels, it is possible to achieve accuracy within 50 mm. Since the bucket teeth are at least 250 mm long and wear back, this level of accuracy is satisfactory. However, accuracy also depends to some extent on how well the shovel or excavator is maintained. Slack bearings will introduce additional errors. On smaller machines, tighter tolerances can be obtained down to 25 mm.
Information about the position of the bucket must not only be accurate, but must be delivered rapidly enough to allow the operator to use the information while production digging. Latency is the time between the bucket reaching a certain position and the representation on the screen showing the position. To achieve satisfactorily low levels of latency, the GPS receivers and sensors must have a rapid response, the computer must run at a high enough speed and the software must use efficient algorithms. With GPS receivers operating at 5 Hz, sensors running at 20 Hz and a computer running at 700 MHz, latency is around 300 milliseconds—a satisfactory but still noticeable lag.

Studies show the most effective position for excavator-mounted GPS antennas is at the rear of the machinery house (left). To be useful to the operator,
the system’s display must be simple to use, with a touch screen for easy selection and checking of options while remaining visible under varying
ambient light conditions (right).