Open Cut » Overburden Removal
The global positioning system (GPS) is a satellite based system which can be used for positioning and/or surveying to 2cm accuracy horizontally and 3cm vertically anywhere on the earth's surface. There are 21 satellites currently in orbit with three spares.
GPS can be used in open cut mining operations as long as the satellites can be viewed in a direct line of sight. Tests undertaken at two mines, under worst case conditions, show that GPS works well in most situations.
Close to highwalls (within 3-4 metres) depending on the positioning of the satellites at any given time, the GPS system dropped back to two dimensional fixing, with only three satellites visible. GPS needs four satellites for a three dimensional fix.
One area of concern is the effect called multipath and this is where the satellite signal bounces off high walls and/or metal structures and gives the GPS receiver a false position fix. The aerial must be carefully mounted to ensure minimal multipath. This is very important when considering mining equipment and the type of structures involved.
There are many areas where GPS technology can improve the productivity of coal mines. Surveying will benefit greatly from real time kinematic GPS. Both accuracy and time utilisation will improve with the aid of this equipment.
Positioning of drill rigs and monitoring truck fleets will also improve mine productivity.
At this time, GPS technology comes in four levels of accuracy and cost:
100m GPS $1,000 each
2-5m DGPS $20,000 first unit
<1m DGPS $25,000 first unit (extra units $7,000 each)
2cm RTK $55,000 first unit
The price and accuracy of GPS is starting to fall and within two years it will be a very cost effective positioning and surveying system. This opens up many opportunities in the open cut mining industry, particularly with surveying, machine positioning and control.
Like the Transit before, the GPS system is owned and maintained by the United States government. Development of the system began in the seventies and at that stage was predominantly used for US military based tasks.
Virtually all of the development of GPS equipment was under contract from the US Department of Defence. Many of the contractors to the government also had commercial involvement in similar technologies, especially receivers.
This meant that by the early eighties, commercial interest in GPS began to blossom. At this point the system was only partially implemented and so did not provide 100% coverage. As the USDOD experimented, the system would be switched off or malfunction regularly.
Transit systems having set the pace for commercial and even personal leisure use of satellite navigation, GPS development expanded rapidly. By the mid eighties, an ever increasing range of commercial GPS receivers were becoming available.
These were still limited by the partial implementation of the system. Placement of satellites into orbit continued with a later generation of satellites. By 1990, although not fully implemented, the system was sufficiently reliable to make general commercial and even personal use viable.
Although the application of GPS was always an area of theoretical interest to universities, the late eighties and early nineties also saw an explosion of papers, dissertations and proceedings describing the techniques for highly accurate positioning based on GPS.
This was largely motivated by the potential applications in surveying as partial availability of the system was relatively more acceptable in such applications.
The system has now been completed and is in full commercial use.
The positioning accuracy of GPS is dependent of the accuracy of the pseudo-range measurements. For time to relate to distance, the characteristics of the transmission medium must be known. This is because the speed of ratio waves is dependent on the nature of the media through which they travel.
This means that as the density of the atmosphere changes so does the speed of the GPS signals. This affects the pseudo-range measurements. To this end a GPS receiver will model the ionospheric and tropospheric delays of the GPS signals.
Using this model it is only possible to approximately halve the errors introduced by the atmosphere. Thus one of the advantages of using a GPS receiver that uses both the L1 and L2 GPS signals is that the atmospheric conditions can be better measured and so eliminated from the calculations.
The use of differential corrections, especially over a short baseline where the distance between the base station and the remote is only a few tens of kilometres, also facilitates the removal of much of the atmospheric distortion to the pseudo-range measurements.
As the baseline length increases the correlation between the atmospheric errors at the two sites decreases introducing increased errors of the order of 1m per 100km.
One issue often brought up when discussing the availability of GPS and the use of systems reliant on it is control. It has been reported that because GPS is controlled by the military, it can be turned off at any time or have its services severely curtailed.
As the majority of US GPS usage involves commercial GPS equipment, any alteration to the GPS system that made commercial equipment inoperable would affect the US military most of all. Furthermore, even military GPS equipment must use the C/A code to access the satellites.
In addition, GPS has now been approved by the US FAA for use in commercial aircraft. Although there are other considerations as well, these two factors alone are sufficient reason to understand that GPS is an available positioning system, not prone to the whims of the US military.
Claims to the contrary do not stand up to the demonstrated performance over the last five years. The US DOD continue to purchase large quantities of commercial equipment.
Because GPS user equipment does not transmit, there is no limit to the number of GPS receiver that can be used on the system. There are no licensing fees for the receivers and no requirement to register their ownership. There are no restrictions on the use of GPS for positioning, and the received almanac and ephemeris information can be stored and retransmitted if required.
Although the US DOD maintain a close watch on the behaviour of the satellites, occasionally one will have a mishap and provide bad information for a couple of orbits. It is the user's responsibility to handle these problems.
The US DOD make no claims as to the behaviour of the systems at any instant, however there are many publications and information sources that keep an independent watch on the performance of GPS and publish the results into the public arena.