AccuGrade GPS (CD700) General Information Caterpillar

General Information
2.1. 3D Guidance
3.2. The Global Positioning System (GPS)
4.3. Low accuracy corrected GPS (SBAS)
5.3. GPS Site Calibration
6.3. Converting The Site Calibration To A GPS Configuration File
7.1. Guidance Levels and Guidance Information

The AccuGrade System is designed for earthmoving equipment in the construction industry. The AccuGrade System uses the Global Positioning System (GPS) in order to provide operators with accurate 3D guidance.

3D Guidance

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 2 times each day at very high altitudes. The radius of the orbit is approximately 26600 km (16528 miles). 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 spares in orbit. The three orbital planes are separated by 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 that is based on radio signals that are received from several different satellites. The satellites have highly reliable clocks, so the timing of these satellite signals is very accurate. 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 location of the receiver 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 that are available from GPS.

Note: AVSpare is not responsible for the operation of any satellite-based positioning system or the availability of positioning signals.

Listed below are the five basic levels of position accuracy that are available.

"Autonomous" - 10 m (30 ft) to 15 m (50 ft)

"SBAS" - 1 (3.3 ft) to 2 (6.6 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 levels of accuracy for GPS.

Illustration 1g01374212

GPS levels of accuracy

(1) "Autonomous"

(2) "DGPS"

(3) "RTK Fixed"

(4) "RTK Float"

(5) "SBAS"

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.

Two GPS receivers are required in order to produce RTK positions. One GPS receiver is known as the GPS base station. The GPS base station has a fixed position. The other GPS receiver is known as the rover. This receiver is mounted on the machine.

The base station communicates to the rover through a wireless data link using a data radio. Illustration 2 shows this setup.

Illustration 2g01525721

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.

Note: Poor placement of the reference station can negatively affect the position accuracy of the rover.

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 CR900 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.

Low accuracy corrected GPS (SBAS)

If low accuracy GPS positions are adequate for the required site work, you can use Satellite-Based Augmentation System (SBAS) error corrections. SBAS corrected systems do not require additional site infrastructure.

Satellite-Based Augmentation Systems are free-to-air GPS correction services. SBAS networks consist of ground stations that are set at known positions around the world, and geo-stationary satellites that maintain a fixed position above the earth. The ground stations receive GPS signals from all GPS satellites in view. The GPS data is then sent to a master control site, which then transmits GPS corrections to the geostationary satellites. These satellites broadcast the information to all SBAS enabled GPS receivers.

The following SBAS networks are currently available:

WAAS - Wide Area Augmentation System (United States)

EGNOS - European Geostationary Navigation Overlay Service (Europe)

MSAS - Multi-Functional Satellite Augmentation System (Japan)

GPS receivers use a GPS receiver configuration file (.cfg) to convert from WGS84 coordinates to the site coordinate system. High accuracy (RTK) GPS systems use a configuration file generated from the site calibration. Inaccuracies in the entered position of the base station for the site calibration will result in a shift of the site coordinate system away from the local circuit coordinate system. When this configuration file is used by a GPS receiver using RTK, the shift is not exposed, as the relative positions of points on the site are still accurately known, and the accuracy of RTK positioning is unaffected. However, if the same configuration file is used by a GPS receiver using SBAS corrections, the shift in the site coordinate system away from the local circuit is exposed, and results in additional positioning errors.

To avoid additional errors, do one of the following:

  • Make sure the base station antenna position used for the RTK site calibration is accurate to within about 1 m (3.3 ft). This accuracy can be achieved via traditional surveying methods or long autonomous GPS occupations. Speak to your site surveyor for more information.

  • Use a configuration file generated by the AccuGrade Office software using the local circuit coordinate system, instead of the configuration file produced by the site calibration.

GPS Site Calibration

Note: A GPS site calibration that is poor in quality will result in low quality guidance information from the AccuGrade Grade Control System.

A GPS site calibration is a mathematical relationship between a grid coordinate system (northing, easting, and elevation) of a project and the GPS coordinate system (latitude, longitude, and ellipsoidal height, also known as WGS84 coordinates ).

This relationship is determined by using GPS to measure points with known grid coordinates, and then calculating the calibration parameters.

A surveying or civil engineering company should be used in order to establish the control points around a site. Make sure that you locate the control points so that they will not be destroyed during construction. Five or more control points located at, or near, the corners and center of the project will normally give good results.

Note: If you employ a GPS surveyor to create the GPS site calibration, make sure that you clearly specify the requirements. AVSpare recommends that you give them a copy of this section of the manual.

Some survey software can do both a full site calibration and a single point calibration.

A single point calibration cannot provide any guarantee of accuracy as the parameters that define a site calibration will be default values that may have no resemblance to the real values obtained through a full calibration. A single point calibration should only be employed on small sites with no existing control. AVSpare recommends that you carry out a full site calibration to guarantee the required accuracy all over the site.

When doing a site calibration, pay attention to the following points:

  • A minimum of five 3D local grid coordinates (north, east, elevation) and five observed GPS coordinates (latitude, longitude, height) to provide enough redundancy.

  • The set of GPS coordinates must be independently obtained from the set of grid coordinates.

  • The selected calibration points should be around the perimeter of the site. Do not work outside of the area enclosed by the calibration points, as the calibration is not valid beyond this perimeter.

  • When defining the acceptable accuracy limits, the calibration tolerances should not be larger than the accuracy tolerance for the site.

  • When measuring points, use a bipod on a staff or a tripod to maintain stability.

  • Check the accuracy of the calibration by visiting other control points that were not used in the calibration.

Converting The Site Calibration To A GPS Configuration File

When the site calibration is complete, the site surveyor uses a surveying software package to convert the site calibration data to a calibration file, for example Site.dc.

Once the site calibration file is generated, use AccuGrade Office to convert the calibration to a GPS configuration file which can then be copied to the data card. For information on how to do this, refer to the AccuGrade Office help file.

The site calibration files are stored in the display rather than the GPS receiver. For this reason, each time you change site, make sure you update the site calibration files.

To ensure the correct files for each design are in use, do one of the following:

  • Make sure there is a valid configuration (.cfg) file for the design in each design folder. When you load the design the associated configuration file is sent to the receiver.

  • Use the "GPS receiver configuration" item in the Setup Menu. This sends the site calibration to the GPS receiver and updates the parameters that are held in the display.

When a GPS receiver configuration (.cfg) file is sent to a GPS receiver, the system resets the receiver to factory defaults before applying new settings.

Guidance Levels and Guidance Information

The AccuGrade System can provide the following two levels of guidance:

2D 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 - Maintains a constant elevation.

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.