The idea behind the AccuGrade System is very simple. Computers and design software produce accurate 3D models of project designs. In the past, these models stayed in the office with the engineer. The engineer gave information to the surveyor. The surveyor then put stakes in the ground in order to guide the machine operators.
With the AccuGrade System, the 3D model comes into the field as a design surface. By adding GPS, you can accurately determine the position of the blade on your machine in both horizontal direction and the vertical direction on the design surface. The cut or the fill that is to be applied to the design surface can then be directly computed on the machine in order to provide you with guidance.
The Global Positioning System (GPS)
GPS is a system of satellites that orbit the earth twice a day at very high altitudes. The orbit radius is approximately 26600 km (16528 miles). GPS was set up by the government of the United States. GPS provides precise timing and positioning anywhere on earth, 24 hours per day with no charge to users.
Newer GPS receivers will be able to also receive data from the Global Navigation Satellite System (GLONASS). GLONASS consists of 21 satellites in 3 orbital planes, with 3 on-orbit spares. The three orbital planes are separated 120 degrees, and the satellites within the same orbit plane by 45 degrees. Each satellite completes an orbit in approximately 11 hours 15 minutes. This allows more GPS satellites to used to for the solution.
In very general terms, a GPS receiver computes a position based on radio signals received from several different satellites. The satellites have highly reliable clocks, so the timing of these satellite signals is known very accurately. The GPS receiver calculates the relative distance to each of the satellites. This calculation is based on the travel time of the signal and the speed of light (speed of the signal). The receiver then uses these distances in order to calculate the receiver's location on earth.
As a broadcast only radio system, GPS supports an unlimited number of users. The broadcast frequencies penetrate clouds, rain, and snow. GPS can also accurately guide operations in fog or dust as well as at night. There are different levels of precision and accuracy available from GPS. These levels depend on the GPS receivers and the methods that you use.
The four basic levels of position accuracy (1 sigma) that are available in real time (that is, instantaneously) from GPS are as follows:
- Autonomous, 10 m (30 ft) to 15 m (50 ft)
- DGPS, 0.3 m (1.0 ft) to 1.0 m (3.0 ft)
- RTK (Float), 0.2 m (0.7 ft) to 1.0 m (3.0 ft)
- RTK (Fixed), .02 m (.07 ft) to .03 m (.10 ft)
Illustration 1 shows the GPS accuracy levels.
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| Illustration 1 | g01015622 |
GPS accuracy levels (1) Autonomous (2) DGPS (3) RTK Fixed (4) RTK Float | |
The method that is used for machine grade control in construction is the same as that used by surveyors for stakeout in construction. The method is called Real-Time Kinematic (RTK), GPS.
In order to produce RTK positions two GPS receivers are required. One receiver is known as the GPS base station and is fixed in one place. The other GPS receiver is known as the rover. This receiver can be stationary or mobile.
The base station communicates to the rover ('s) through a wireless data link using a data radio. Illustration 2 shows this setup.
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| Illustration 2 | g01173793 |
GPS and the AccuGrade® System (1) GPS satellites (2) GPS base station and data radio (3) Repeater radio (optional) (4) Machine equipped with the AccuGrade System | |
Both GPS receivers make observations (measurements) of the GPS signals at the same time. The base station broadcasts the observed information together with the location and other information across the data radio link to the rover. The rover then combines the data from the base station together with its own data in order to compute a very accurate position relative to the base station.
A single base station can support an unlimited number of rovers, provided the rovers are within about 20 km (12.4 miles) of the base station. Normally, the range of the data radio link is the more limiting factor. You can use radio repeaters in order to extend the radio coverage.
In areas where the 900 MHz band is available, the TC900C data radio is recommended. In areas where 900 MHz is not available, the TC450 radio is recommended.
Both of the radios have a rugged housing, and brackets that are specifically designed for heavy machinery and construction environments.
Guidance Levels and Guidance Information
The AccuGrade System can provide the following two levels of guidance:
1D Guidance - 1D guidance provides cutting edge guidance relative to an external reference such as a string line or gravity, and is independent of the location of the machine.
3D Guidance - 3D guidance uses the 3D location of the machine to provide cutting edge guidance relative to an internal 3D digital design. 3D guidance is useful for constructing complex designs, such as highways and curved banks.
In addition, each level of guidance can provide different types of guidance information. The AccuGrade System can provide the following types of cutting edge guidance information:
Cross Slope - Cross slope guidance reports the slope of the cutting edge, projected in the direction of travel of the machine, relative to a preset value.
Lift Plus Cross Slope - Lift plus cross slope guidance reports the height of one end of the cutting edge relative to an existing reference surface, such as a string line or curb, and the cross slope of the cutting edge, relative to a preset value.
One-Point 3D - One-point 3D guidance reports the height of the cutting edge relative to the elevation of a design surface directly below a single point on the blade, and the horizontal offset of one tip of the cutting edge relative to a design feature. The height is calculated by assuming the cutting edge is at right angles to the direction of travel of the machine.
Two-Point 3D - Two-point 3D guidance reports the height of the cutting edge relative to the elevations of a design surface directly below two separate points on the blade, and the horizontal distance of one tip of the cutting edge from a selected design feature.