Computers are used in order to create three-dimensional (3D) models of the project designs. The 3D model is converted into a design surface for use at the site by the operator of the compactor.
The AccuGrade System uses the data from the satellites in order to compute the position of the compactor drum relative to the design surface. The compaction that is applied and the exact location of the drum can be directly displayed and recorded on the compactor. This method provides information for the site that is related to productivity and compaction in real time.
The GPS site calibration is required at the start of any new project or work area. The GPS site calibration is used to transform the "GPS-derived" coordinates into "local plane" coordinates.
A design is also normally required. The design can be in the following formats:
- DTM (TIN or Grid)
- Road
- Level
- Sloping surface
The Display has a color screen. The brightness of the screen can be adjusted for different operating conditions. Information on the screen is available in the following views:
- Plan
- Cross section
- Profile (only if a design is loaded)
- Text
The operator can move between these views at any time. The Display uses the GPS positions and the machine dimensions in order to compute the position of the drum.
Illustration 1 shows the process map for setting up a GPS System. The information that pertains to the base station and the radio network do not apply if a Satellite-based Augmentation System (SBAS) is used for machine guidance.
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| Illustration 1 | g01375262 |
If the design does not change, the AccuGrade System does not need any other inputs. The site manager and the operator can respond quickly if a design is updated. There is no need to wait for a surveyor. The AccuGrade System can be expanded for additional machines. Because the base station can support any number of machines, adding machine control through GPS to the other machines is simply a matter of installing the appropriate hardware.
Note: It is possible to use the AccuGrade System without a design file for limited applications. When working without a design file, the system has two functions:
- Display current compaction value
- Log compaction and position data to the data card
Without a design file, the system is unable to display the position of the machine on the site. The System cannot provide any guidance information to the operator.
Due to these limitations, the instructions in the Operation and Maintenance Manual direct the user to include a design file on the data card or to create a simple design file directly from the AccuGrade Display.
Overview of 3D Machine Guidance
If a site plan or a background map is loaded into the AccuGrade System, then the System can display the compactor's location on the site. The System can also measure the distance of the compactor from various features of the design. An example of a feature is an Avoidance Zone. This information can be reported to the operator.
If a design file is loaded into the AccuGrade System, then the System can display the compactor's location on the design surface. The System can also display the compactor's horizontal position and the vertical position relative to the design surface.
Horizontal Guidance
The AccuGrade System can display the compactor's horizontal position relative to a selected alignment. The system displays this information as right or left of the selected alignment. Guidance to the right or to the left is provided for the side of the drum that has focus.
Vertical Guidance
The AccuGrade System can display the compactor's vertical position relative to the design surface. The System computes the actual elevation at the center of the drum, the right edge of the drum, and the left edge of the drum. The system also uses the elevation of the design at points under the drum.
The selected points are based on the method that is used for vertical guidance. The elevations of the design at the right and left edges of the drum are calculated. The calculated elevations are compared to the actual elevations. The difference between the two is the cut or the fill at the right and left edges of the drum.
Design File
The design file that is used by the AccuGrade System is loaded into the Display from a data card.
The following items apply when the Display contains a design:
- The "Select Design" menu item displays the name of the loaded design.
- The "Vertical Offset" and the "Horizontal Offset" menu items are available in the "Setup Menu - Configuration".
Note: When the AccuGrade System is turned on, the System attempts to load a design. This action occurs only if a data card is inserted and the design is on the data card.
The following items apply when no design has been loaded in the Display:
- In the Text view screens, "N/A" appears as the value of the parameters that are used for guidance.
- The Cross section views display a "No design" message.
GPS Site Calibration
| NOTICE |
|---|
A poor quality GPS site calibration will result in poor quality guidance information from the AccuGrade System. |
Site Calibration Overview
A GPS site calibration is a mathematical relationship between a project's grid coordinate system (northing, easting, and elevation) and the GPS coordinate system (latitude, longitude, and ellipsoidal height). The GPS coordinates are also known as WGS84 coordinates.
This relationship is determined by using GPS to measure points with known grid coordinates (control points), and then calculating the transformation parameters.
A surveying firm or a civil engineering company should be employed to establish the control points around a site. Ensure that the control points are located in areas that protect the points from damage during construction. Five or more control points are required. The control points should be located as close to the corners of the project and to the center of the project as possible. These locations will normally provide good results.
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 (*.dc).
Once the *.dc file is generated, a tool such as AccuGrade Office is used to convert the calibration to a GPS configuration file (*.cfg). The *.cfg file is copied to the data card. Refer to Systems Operation, "Data Card".
The site calibration files are stored in the AccuGrade Display. The files are not stored in the GPS receiver. Ensure that the site calibration file is updated each time the site is changed.
Global Navigation Satellite Systems
Overview
Global Navigation Satellite Systems (GNSS) is a general term that refers to one or more specific satellite systems that are used for positioning. A GNSS is a network of satellites that transmit "ranging" signals to GPS receivers. These signals are used to calculate the longitude, the latitude, and the height of the GPS receiver. The calculations are used for positioning and for navigation on the earth. The Global Positioning System (GPS) is one example of a network of satellites within GNSS. Other examples include the Russian GLObal NAvigation Satellite System (GLONASS) and the Galileo system which is being developed in Europe.
The GPS was developed by the United States Department of Defense in the 1980s in order to provide navigation for military forces. The system was also designed to allow navigation for civilian use. A license is not required in order to use the signals. The signals are used for applications such as machine control and surveying.
Note: Within this manual, GPS and GNSS are used interchangeably.
GPS Network
A GPS configuration consists of a "constellation" of twenty-four primary satellites. The satellites are orbiting the earth two times per day at very high altitudes. The system sends three-dimensional information to receivers on the earth. There is no cost to a user with a receiver that can process the information.
The position of the receiver is determined by using the signals that are received from the satellites. The satellites are equipped with highly accurate clocks. The satellites continuously broadcast the signals to the earth. The signals include both the position of the satellite and the time of that position.
GPS receivers on the earth receive the information from the satellites. The information is used to calculate the position of the receiver. The receiver keeps an internal time. The internal time is compared with the time that is sent by the satellite. The difference between the two times is used for calculating the receiver's distance from the satellite.
The receiver must be receiving and processing signals simultaneously from at least four satellites or else the position cannot be determined. The satellites are arranged so that every point on the earth can be in contact with at least four satellites under normal conditions.
As a "broadcast only" system, GPS supports an unlimited number of users. The broadcast frequencies penetrate clouds, dust, fog and rain. GPS can accurately guide operations under any of these conditions. There are different levels of precision and accuracy that are available from the GPS. These levels depend on the GPS receiver and the type of method that is used for calculating the measurement.
Levels of GPS Position Accuracy
There are five different levels of precision and accuracy that are available from GPS. These levels depend on the GPS technology that you use.
Illustration 2 shows the way that the five levels relate to each other. This Illustration is not drawn to scale. The AccuGrade System uses either the SBAS method (2) or else the RTK Fixed method (5) for guidance.
Note: All measurements reflect a 1 Sigma precision.
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| Illustration 2 | g01372378 |
(1) Autonomous: 10 m (30 ft) to 15 m (50 ft) (2) SBAS: 1 m (3.3 ft) to 2 m (6.6 ft) (3) DGPS: 30 cm (12 in) to 2 m (6.6 ft) (4) RTK Float: 20 cm (8 in) to 100 cm (40 in) (5) RTK Fixed: 2 cm (0.8 inch) to 3 cm (1.2 inch) | |
(2) SBAS - SBAS is a subset of DGPS. SBAS is a "low precision" GPS measurement method. SBAS uses stations on the ground and satellites that remain in the same position above the earth for corrections. These satellites are known as geostationary satellites. SBAS does not require any additional infrastructure on the site. Additional positioning errors are inherent to this method.
(5) RTK Fixed (high precision) - RTK Fixed uses a base station and a second receiver. It is a "high precision" GPS measurement method. RTK Fixed uses very expensive components and advanced algorithms to perform the measurements. The result is very precise, repeatable calculations.
Real Time Kinematic Fixed GPS
Real Time Kinematic (RTK) Fixed GPS is a high precision GPS measurement method. This is the same method that is used by surveyors when the surveyor is staking out a construction site. This is the recommended method to use for any grade control that requires a high degree of precision.
In order to produce RTK positions, two GPS receivers are required. One receiver is known as the "rover". This receiver is located on the compactor. The second receiver is located at the base station. This receiver is in a known location. The exact coordinates of this receiver are calculated. The coordinates are recorded.
GPS Base Station
A base station is typically installed at a fixed location such as the roof of the office on the field site. Some sites use a mast with a permanently mounted GPS antenna. This configuration is shown in Illustration 3. Other sites are set up so that the GPS antenna can be easily removed from the mounting location.
A GPS base station consists of the following components:
- GPS receiver
- GPS antenna
- Radio
- Radio antenna
- Connecting cables
- Power supply
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| Illustration 3 | g01372898 |
Typical base station setup (7) Base station radio antenna (8) GPS antenna | |
The GPS base station installation is critical to the successful operation of GPS guidance on any construction site. The base station in illustration 3 shows a typical setup. The base station radio antenna (7) is mounted high. The GPS antenna (8) is securely braced to the building. This location provides maximum range for the signals. There is adequate distance between the data radio and the GPS antenna.
There are a few things about the setup that could be improved. The base station is set up on a metal building. The metal could cause mutipath reflections. Additionally, the nearby trees to the left of the building could block signals from satellites that are low in the horizon. These items are explained in the next few paragraphs.
For an effective installation, pay attention to the following items:
- Ensure that the antenna is clear of items that could obstruct the GPS signals.
- Trees
- Passing vehicles
- Parked vehicles
- Buildings
- Hills
- Trees
- Ensure that the antenna is clear of sources of electromagnetic interference.
- Overhead power lines
- Overhead phone lines
- Radio transmitters
- Radar installations
- Overhead power lines
- Ensure that the antenna is clear of any object that could cause a multipath signal. Multipath refers to a reflected GPS signal.
- Metal roofs
- Metal walls
- Passing vehicles
- Parked vehicles
- Large bodies of water (harbors, lakes, ponds, reservoirs)
- Railroad tracks
- Overhead line supports (utility poles)
- Metal roofs
- Ensure that the data radio is elevated several meters off the ground. A higher location will provide optimum coverage for the site.
- Ensure that there is at least 2 m (6.6 ft) between the GPS antenna and the data radio.
- Ensure that the GPS antenna is firmly braced.
- Ensure that the pole the GPS antenna is mounted to cannot move.
- Ensure that the GPS antenna is set to the correct antenna type. Refer to the operating manual of the GPS antenna.
Note: The GPS signal will be degraded if the antenna type is not set properly.
System Communications Overview Using RTK Fixed
In order to calculate an "initial position solution", a GPS receiver must be able to receive signals from at least five satellites. Once the GPS receiver has calculated an initial position, the GPS receiver must be able to receive observations continuously from at least four satellites.
In order to compute the position, the AccuGrade System uses only satellites that are observed simultaneously at both the base station and at the rover.
Note: The term rover refers to any antenna or radio that is mounted on the compactor or that is not fixed in a permanent location.
The measurements that are sent from the base station are calculated for individual satellites. These measurements are used by the rover in order to correct the rover's measurements for the same satellites.
The base station transmits the location of the base station and the measurements that are made by the station at that location. The information is sent to the machines that are working near the base station. This configuration permits the rover to generate corrections to the GPS system's measurements. The AccuGrade System uses this information in order to calculate the position of the rover.
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| Illustration 4 | g01374574 |
GPS and the AccuGradeTM System (6) GPS satellite constellation (7) Base station radio antenna (8) Base station GPS antenna (9) Rover receiver (10) Optional "Repeater" radio (11) Rover radio | |
A wireless link between the radios is used for communications. Refer to Illustration 4. The link consists of two radios that transmit data. One radio is located at the base station. This radio is known as the "base station" radio. The second radio is located on the compactor. This radio is known as the rover radio.
Both the base station receiver (8) and the rover receiver that is mounted on the compactor (9) perform simultaneous measurements of the signals from the satellite system (6) .
The base station uses the radio (7) in order to broadcast the information across the link to the rover radio (11) that is on the compactor.
One base station can support an unlimited number of receivers. The receivers must be within 20 km (12.4 miles) of the base station.
Note: 20 km (12.4 miles) is a nominal range. The range is based on ideal conditions.
A repeater radio (10) is used in order to increase the strength of the information from the radio at the base station. The information is sent from the base station. The repeater receives the information. The information is sent from the repeater to the radio on the compactor. This repeat broadcast provides coverage when the compactor is out of the range of the base station. A repeater is also used when the transmissions from the base station are obstructed by hills or by buildings.
Note: The TC900C radio is recommended in areas that have the 900 MHz band. The TC450C radio is recommended in areas that do not have the 900 MHz band.
The AccuGrade System combines the data from the rover receiver and the data that was received from the base station. This combined information is used in order to compute a very accurate position of the drum relative to the known coordinates of the base station.
The radio signals that are received by the GPS receivers are very low power. Although the signals pass freely through clouds and dust, the signals are easily disrupted by trees, or by structures such as bridges and overpasses. These disruptions may be caused by any of the following:
- The signal can be lost if the compactor moves out of the sight of one or more satellites.
- This signal can be lost if the compactor moves under an object that disrupts the signal.
- The signal can be lost if one or more satellites move below the horizon.
Because of the possibility of disruption, particular attention should be given to the quality of the information that is being received. The quality should be suitable for the type of work that is being performed.
Satellite-based Augmentation System
A Satellite-based augmentation system (SBAS) is a low precision GPS method that can be used for grade control. SBAS systems do not require additional infrastructure on the site. This method should only be used when the infrastructure for an RTK Fixed method is not available. Additional positioning errors are inherent to this method.
An SBAS network uses the following devices for error correction:
- Ground stations that are located at known positions around the Earth
- Geostationary satellites that maintain a fixed position above the Earth
The ground stations receive GPS signals from all GPS satellites that are in view of the ground station. The GPS data is sent to a "master control site". The site transmits GPS corrections to the geostationary satellites. The satellites broadcast the information to the earth using the same frequency as one of the GPS signals.
The information can be used by any GPS receiver that is capable of decoding the information. Some SBAS systems are privately maintained. These systems require a subscription fee for using the corrections. Other SBAS systems are publicly supported. These systems can be used at no additional cost to the user.
The AccuGrade system uses signals from the following systems. All of these systems are available at no cost to the user:
WAAS - "Wide Area Augmentation System" is available in the United States
EGNOS - "European Geostationary Navigation Overlay Service" is available in Europe
MSAS - "Multi-Functional Satellite Augmentation System" is available in Japan
GPS receivers use a GPS receiver configuration file (*.cfg). The *.cfg file is used in order to convert from WGS84 coordinates to the site coordinate system.
Note: WGS84 is the GPS coordinate system. This system uses latitude, longitude and ellipsoidal height in order to determine position.
Effects of Position Inaccuracies
High accuracy RTK GPS systems use a configuration file that is generated from the site calibration. Inaccuracies in the position that is entered for the base station for the site calibration will result in a shift of the site coordinate system away from the local coordinate system.
When this configuration file is used by a GPS receiver that is using RTK positions, the shift is not exposed. This is because the relative positions of points on the site are still accurately known. The accuracy of RTK positioning is unaffected. However, if the same configuration file is used by a GPS receiver that is using SBAS corrections, the shift in the site coordinate system away from the local coordinate system is exposed. This exposure results in additional positioning errors.
In order to avoid additional errors, do one of the following:
- Make sure the base station antenna position that is used for the RTK site calibration is accurate to within 1 m (3.3 ft). This accuracy can be achieved using traditional surveying methods. The accuracy can also be achieved using long, autonomous GPS occupations. Speak to your site surveyor for more information.
- Use a *.cfg configuration file that is generated by AccuGrade Office using the local coordinate system. This file is used in place of the configuration file that is produced by the site calibration. Speak to your site surveyor for more information.
GPS Quality
The AccuGrade System will continually generate estimates of the "GPS position error".
The maximum value of the error that is acceptable for the application can be set. If the estimate of the error exceeds the limits that are defined for the selected mode, then a "Low Accuracy GPS" message appears.
The following modes are used in the AccuGrade System:
- Fine
- Medium
- Coarse
The accuracy of the GPS position is used in order to determine the following items:
- "Positional" accuracy of the compaction operation
- Accuracy of the productivity data
- Accuracy of map recording if map recording is enabled
The on-site manager sets the limit for each mode. The limit that is set will depend on the type of work that is being performed. The on-site manager can also deactivate the modes.
Three items that indicate the current accuracy can be displayed on the guidance views.
- H. GPS Err.
- V GPS Err.
- GPS Acc.
Table 1 indicates the color that is used to show whether the current error is within the selected limits of the GPS accuracy mode.
| Color | GPS Error Indication |
| Black | The error is less than 90% of the limit. |
| Blue | The error is approaching the defined limit. |
| Red | The error has exceeded the limit. |
GPS Status Warning Messages
During periods of low accuracy, the AccuGrade System provides warning messages. Table 2 summarizes these messages.
| Condition | Indication |
| The GPS position is not available. | A warning flashes repeatedly in the middle of the current guidance screen. No messages that are related to the GPS are displayed in the "Setup" menu. |
| In the Plan view, the symbol for the compactor disappears. | |
| In Cross section view, the last available cross section appears, but no drum is shown. | |
| In the Text view, fields that show information in real time indicate N/A. Examples include the "Speed" field and the "Heading" field. Fields that do not indicate guidance still appear, if those fields are available. An example is the "Satellites" field. | |
| Low accuracy GPS positions are available. | A warning flashes repeatedly in the middle of the current guidance screen. |
| In the Plan view, the position continues to be updated. | |
| In the Cross section view, the Display no longer shows the compactor drum. | |
| GPS mode is either: Invalid GPS configuration or Old Position |
Graphical guidance is no longer provided. |
| In Guidance views, both the compactor and the compactor drum are hidden. These items are no longer updated. |