CFM Real-Time Centimeter Level GPS Positioning System
A real-time cm-level GPS positioning system has been developed at the Center for Mapping (CFM) to support the construction industry. Precisely determined three-dimensional positions and ultrasonic devices used on the blades of earth-moving equipment are linked with the controls of the construction machine, thus automating all earth-moving operations. Three-dimensional, high-accuracy, real-time GPS positions can also be used effectively within the construction industry to determine a balanced site, to estimate the amount of soil moved, and to monitor the job progress daily at several sites. It is highly economical by reducing engineering work for each construction project, and providing the state highway departments with a faster and more efficient tool to inspect the contractor's work. The ability to determine the position of an excavator or loader within 1-2 cm accuracy in real-time will eventually lead to automated operations of earth-moving equipment. The ultimate goal is to feed the three-dimensional positions directly to the vehicle's hydraulic controls, and to automate the earth-cutting and moving operations in a way similar to an automated navigation system in an aircraft, that feeds course information into the aircraft's controls.
CFM Real-Time Centimeter Level GPS Positioning System has been developed as a joint effort of CFM and George J. Igel & Co., Inc., a Columbus-based construction company. CFM also partnered with AGTEK, a company based in Livermore, California. CFM developed a positioning module of the total system; AGTEK provided the Graphical User Interface(GUI). The GUI consists of graphics mode touch screen that includes several menu items to support the real-time, cm-level positioning survey. This screen is also capable of emulating a keyboard as a part of the user interface. Moreover, error detection algorithms have been implemented to intercept erroneous data transmitted over the radio link.
The positioning module of the prototype system comprises the base and the rover Turbo-Rogue dual-frequency GPS receivers, connected via radio link, and the positioning software. Each hardware component is enclosed in a waterproof and dustproof shell for continuous use in outdoor exposed environments. The prototype system is easy and fast to set up and operate. The real-time GPS software uses dual frequency code and phase observations and On-The-Fly ambiguity resolution. The system can determine the correct integer ambiguities within a few epochs when only five satellites are available. When satellite lock is lost, the software returns to the correct solution within a few epochs after satellite signal reacquisition.
Performance evaluations of the prototype system show that the accuracy of the CFM real-time cm-level GPS positioning system ranges between mm-level to a few cm-level, depending on the satellite geometry, length of the baseline and the multipath environment.
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