Thinking Globally, Grading Locally: What Is Local Positioning System?

March 1, 2000
Gx Bug Web

In addition to GPS, you’ll encounter the term LPS-local positioning system-in descriptions of automated, three-dimensional grade control systems. Although LPs can be used in conjunction with GPS, the technologies are different. LPs uses a control that is local to the job site-such as a tracking laser-rather than satellite signals, and according to one manufacturer, LPs is more accurate than GPS, measuring elevations to within 3 mm in real time.

Topcon Laser Systems Inc. of Pleasanton, CA, manufactures the 3D-MC, an LPs-based three-dimensional grade control system. Several components make up the system: an onsite tripod-based tracking laser, a grader-mounted receiver, proprietary software, and a standard laser- or sonic-based grade control system installed on the grader. The tracking laser transmits and receives information to and from the receiver via laser beams; communication can be maintained even with the grader on which the receiver is mounted moving at up to 40 mph. The “intelligent” receiver, Topcon’s LS-2000, includes a 360º laser receiver, 360º reflective prisms for tracking, and electronics that allow the LS-2000 to read the control data it receives from the tracking laser. The LS-2000 can also operate as a normal laser sensor.

Control software resides on a Windows-based PC attached to the tracking laser. The engineering plan for a site can be uploaded to the PC, and as the tracking laser continuously monitors the position of the grader, it compares the grader’s position to the elevation and cross-slope information for each position from the engineering plan, then transmits the data to the receiver onboard the grader. The grader operator can choose simply to view the information on an in-cab LED steering indicator and control the grader manually or opt for complete automatic control so that the information is translated to the equipment’s hydraulics and grade elevation is automatically maintained.

Topcon maintains that GPS technology will become invaluable for surveying job sites and laying out projects, as well as for controlling rough grading and earthmoving operations and for tracking and managing large fleets. For fine grading, however, Topcon says LPs provides an elevation accuracy that GPS just can’t achieve: about 3 mm versus 30 mm or more for GPS. Bruce MacKenzie, a grading foreman with MMC Inc. in Las Vegas, NV, has used the 3D-MC system for nearly a year on a project for McCarran International Airport in Las Vegas. “The tolerances are so tight that we wouldn’t be able to do it without this system.”

In addition, because the tracking laser is located at the site and is completely under the control of the system operator, the occasional inability to access satellite data isn’t an issue. The 3D-MC also reduces the cut-off problem sometimes encountered with laser systems-caused when equipment passes in between the transmitter and the receiver, severing the communication link-by mounting the receiver atop a mast on the grader. MacKenzie says the problem rarely occurs. “The prism that goes on top of the mast is high enough that, unless you’re just right up next to something, you don’t have a problem with losing signal at all. We can also set the 3D-MC so that, if we do lose signal, it’ll go to a known position and we can just drive the blade over there and pick it up again.”