GPS/GNSS-based 3D grade control systems have made a major impact on how heavy construction is done, but they still have limitations when it comes to achieving very tight vertical accuracies. This is true regardless of which brand you use.
While multi-constellation (GPS, GLONASS, Galileo and more recently Beidou) grade control systems are able to work in more places and under more adverse conditions, these systems are still subject to various orbital errors and environmental factors, which can degrade the vertical accuracy.
Many contractors use GNSS-based 3D grade control exclusively for earthmoving tasks and even finishing. However, the manufacturers of these systems will confirm they cannot guarantee that the vertical accuracy will be good enough for finishing.
The various GNSS-based 3D machine systems will provide a horizontal accuracy that is adequate for any earthmoving and grading task. The challenge is to find a way to improve the vertical portion of your position.
Combining GNSS and lasers to improve vertical accuracy
To improve vertical accuracy, it is possible to combine the GNSS position with a terrestrial vertical reference, such as a rotating laser.
Such a system will use the GNSS signal to position the grader horizontally and the laser signal to get a very accurate vertical position, within a few millimetres.
Why use a conventional, construction-grade laser?
With a conventional laser, you have the benefit of being able to use the 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.
What this means: If you have a laser receiver on the grader which is physically 1m long, then you must keep your grader working within ±10cm from the height at which the laser is set up. Once benched, the laser will have a working window of around 20cm that it must stay within to remain accurate.
If your grader travels further than 20cm vertically, you will need to re-bench to maintain accuracy.
Why use a purpose-designed, fanned-beam laser?
A fanned-beam laser will also let you use a ‘normal’ sized laser receiver on your grader, which is far less susceptible to failure from mechanical shock and vibration.
Choosing the right laser for your project
No matter which solution you choose, a combined GNSS antenna and laser receiver 3D grade control system is incredibly versatile.
For most of your earthmoving tasks GNSS alone will be accurate enough, but when you need additional vertical accuracy a simple way to achieve this is to combine your GNSS 3D grade control system with a conventional, construction-grade laser or a purpose-designed, fanned-beam laser.
Which of these two options you choose will be dependent on the details of your project.
The benefits of using lasers to improve vertical accuracy
The main benefit of using laser guidance to improve vertical accuracy 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.
If you have five graders with GNSS systems already installed, you may only have to purchase two additional laser add-ons, since they can easily be shared between all the graders.
One thing to be aware of with 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;
- One from the laser receiver on the grader to the rotating laser on the job site.
However, the benefits of the combined GNSS/laser system far outweigh the additional planning that will be necessary to ensure disruption stemming from people, other equipment, vehicles or trucks breaking the line of sight will be kept to a minimum.
Using robotic total stations to improve vertical accuracy
This system does not make use of satellite signals for positioning and therefore works indoors or in places where there is a severe obstruction of the sky, such as directly underneath an overpass or underground.
A robotic total station automatically tracks a target mounted on a mast on the blade of your grader, placed where you would otherwise mount the GNSS antenna.
Because the robotic total station always knows where it is in the local co-ordinate system, it will also know where the target (your grader) is at all times – simply by locking onto the target and following it.
The robotic total station feeds the 3D position of the target to the control box in your grader many times per second via a radio link.
These positions are then used by the on-board computer to determine cut/fill for where the blade is at a given moment, in the same manner as a 3D position derived from GNSS would be used.
These systems are highly accurate and have been field-proven for more than a decade.
Although outnumbered by GNSS systems in the field, the ability to work underground and indoors will ensure that these systems will be around for a long time to come.
However, as with a combined GNSS/laser system, you must maintain clear line of sight between your grader and the robotic total station tracking it.
But, unlike a combined GNSS/laser system, the robotic total station can only track a single target at a time, so a dedicated robotic total station will be required for each machine.
Choosing the right system to improve your vertical accuracy
The various positioning technologies available for 3D grade control systems each solve a particular problem and are particularly well suited for a specific application.
It is unlikely that a single positioning technology will solve all the earthmoving and grading tasks faced by your company, so it is important that you stay educated about the various solutions available.
For many contractors, the ease with which various sensor technologies can be switched around is crucial, so speak to your positioning system sales representatives to ensure you have the maximum flexibility from whatever solution you choose.
Something else to look for is a consistent operator interface between different positioning technology, so the operator screen remains unchanged and familiar no matter which type of positioning is currently being used.
That means shorter learning curves and the ability for operators to very quickly switch from one type of system to another.