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Laser augmentation of GNSS position

Combining lasers and satellites offers high accuracy when you need it

                              augmentation in 3D grade control systems

Available on:

Trimble GCS900
Topcon Millimeter GPS
Caterpillar AccuGrade

Leica PowerGrade 3D

Carlson Carlson Grade

GNSS-based 3D grade control systems have made a major impact on how earthmoving is done, but they still have limitations when it comes to meeting tight vertical tolerances.

The GNSS system will give you a horizontal accuracy that is pretty good for most earthmoving and grading task so the challenge is to find a way to improve the vertical.

For this purpose, we can combine the GNSS position with a terrestrial vertical reference such as a rotating laser.

This system will use the GNSS signal to position the grader horizontally and the laser signal to get a very accurate vertical position – to within a few millimeters.

There are two ways this can be done:
1. By using a conventional, construction grade laser.
2. By using a purpose-designed, 'fanned-beam' laser.

With a conventional laser, you have the benefit of being able to use a laser you already own, but you will be limited to working within a vertical range that cannot be wider than the physical length of the laser receiver you put on the grader.

Trimble's GCS900 system with an SR300 is an example of such a set-up

The laser receiver on the grader is physically 1m long so the elevation of the moldboard must be kept within 50 cm from the height at which the laser is set up. (Assuming the laser is set up to hit the middle of the receiver at the start of operation).

Topcon Millimeter GPS and Trimble
                              GCS900 with SR300

Topcon's Millimeter GPS (left) and Trimble's GCS900 system

The other way to solve this is to use a fanned-beam laser such as the one use by Topcon's Millimeter GPS system.

Instead of sending out a single plane of light like a standard grade laser, this laser sends out a “fanned” beam that lets the operator work to high vertical precision within a 10m elevation range.

A fanned-beam laser allows for the use of a much smaller laser receiver on the top of the grader mast, but the drawback is that this special laser costs more than a grade laser and cannot be used for other construction leveling application.

A strong selling point for these systems is that an unlimited number of machines can run off of a single laser. This not only lowers the cost of achieving high vertical accuracy, it also reduces the potential for set-up errors, because all machines are working from the same laser.

One thing to be aware of with such a combined GNSS/laser system is that you are now faced with maintaining two lines of sight to your grader: One from the GNSS antenna to the satellites and one from the laser receiver on the grader to the rotating laser on the job site.

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