Q&A on the world’s Fastest GNSS RTK Rover: Leica GS18T

Dr. Xiaoguang Luo, Stefan Schaufler and Bernhard Richter discuss the latest developments in the field of sensor fusion in GNSS and inertial measurement unit (IMU). The new Leica GS18 T GNSS RTK rover combines GNSS and IMU to automatically adjust pole tilt from plumb. Discover how this can increase productivity, extend RTK applicability and reduce human errors.

What are the current challenges of conventional RTK surveying?
The need of manually levelling the pole with a circular bubble and the phase centre position being reduced to the pole tip (by considering the antenna phase centre offset and the length of the pole), result in a number of disadvantages for the user:

• In terms of productivity, levelling the pole takes time, particularly in stakeout where it needs to be repeated iteratively.
• With respect to accuracy, holding the pole vertically is influenced by human errors and instrumental imperfections, such as a misadjusted bubble.
• Regarding applicability, it is not always possible to hold the pole vertically on a target point, for example, when measuring building corners.

In terms of solving the user’s problems: what are the major advantages of the Leica GS18 T?
There are several benefits of the new rover:

• Free from on-site calibrations
• Immune to magnetic disturbances
• Applicable at large tilt angles
• Heading-aided 3D visualisation

The new Leica GS18 T GNSS RTK rover combines GNSS and IMU to automatically adjust pole tilt from plumb, which increases productivity, extends RTK applicability and reduces human errors. It improves the overall user experience beyond comparison.

Figure 1 - Leica GS18 T GNSS RTK rover with Leica CS20 field controller.

How does the Leica GS18 T rover answer the growing demand for speed onsite?
The definition of being “the world’s fastest GNSSS RTK rover” is based on three pillars: IMU-based tilt compensation technique in combination with instantaneous RTK. This enables the highest productivity (accuracy & reliability – particularly in topographic surveys) and provides similar accuracy as measurements taken by levelling the pole manually. Due to tilt compensation, there is no need to level the pole, which increases productivity by an average of 20 per cent over conventional GNSS RTK surveying practices. In addition, the GS18 T utilises high-rate accelerations and angular velocities from MEMS IMU to determine the attitude of the pole in real time. Since these IMU measurements are not affected by magnetic fields, the GS18 T is immune to magnetic disturbances and does not require any time-consuming on-site calibrations. It works out of the box and is faster than magnetometer-based systems.

Figure 2 - Leica GS18 T as the fastest GNSS RTK rover with the IMU-based tilt compensation.

There is always the question of accessing difficult targets – such as building corners and obstructed points?
With the GS18 T this is not considered a challenge anymore. Due to the IMU-based tilt compensation, the targets that were previously not accessible with GNSS, can now directly be measured with RTK, even at large tilt angles of more than 30 degrees. With the benefits of advanced signal tracking, the GS18 T is especially suitable for RTK applications where the sky is partially visible, for example, operating close to tree lines, under foliage or in urban canyons. By applying the IMU-based tilt compensation of the GS18 T, there is no limit to the maximum tilt angle as long as a sufficient number of GNSS satellites are tracked to be able to provide high-precision RTK solutions. Large tilt angles are a problem of the past. The GS18 T is applicable to hidden point measurements (for instance hidden corners or points partly blocked by parked cars).

Would this then directly impact the safety aspect while measuring in a potentially dangerous survey environment?
Exactly – without having to focus on levelling the pole, the user can pay more attention to his own safety. The risks of passing vehicles and operating machines are vastly reduced. In addition, attitude information is used to help users orientate themselves in the field by automatically updating the 3D visualisation of the surroundings (depending on the sensor orientation).

Figure 3 - Using the Leica GS18 T to measure building corners and obstructed points that were previously not measurable in conventional RTK surveying with a vertical pole.

It seems that you have successfully integrated two navigation sources, GNSS and INS?
Integrated GNSS/INS navigation systems which have long existed in the aerospace industry are now available in surveying applications. This sums up the successful integration:

Figure 4 - Schematic and simplified illustration of the GNSS/INS integration implemented in the Leica GS18 T.

Consistency checks between GNSS and INS are carried out constantly to enable a robust system that can cope with extreme pole dynamics, such as hard shocks. Since no magnetometer measurements are involved in the computation of tilt-compensated positions, the GS18 T is immune to magnetic disturbances.

When directly comparing conventional RTK vs. tilt compensation RTK – have you performed tests to demonstrate the practical advantages?
To demonstrate the benefits of using tilt compensation, the GS18 T was benchmarked against Rover A under open sky and strong multipath conditions. In the open-sky test (Fig. 12), two known points P1 and P2 that are separated by 8 m were measured alternately in the instantaneous mode for 10 minutes. Using Rover A, the pole needs to be levelled precisely before taking an instantaneous measurement, which is not necessary for the GS18 T due to tilt compensation. The number of measured points within 10 minutes represents a simple indicator for productivity.

Figure 5 - RTK performance benchmarking under open sky by measuring two points alternately in the instantaneous mode for 10 minutes (Rover A vs. GS18 T, pole length: 1.800 m).

Figure 6 - RTK positioning test in a strong multipath environment (pole length: 1.800 m) (a) Survey marker near a building with metal facades, (b) Tilt compensation RTK measurement with the Leica GS18 T.

Table 1 summarises the results from the open-sky test with respect to productivity and accuracy:

Table 1 - Comparison of the number of measured points within a 10-minute period and the resulting rms errors between GS18 T and Rover A (open sky, pole length: 1.800 m, instantaneous measurement).

Without the need to level the pole, the GS18 T significantly reduces the time spent on a measurement, and thus increases the number of measured points by 33 per cent from 57 to 76 within a 10-minute period. In the tilt compensation case, despite the additional error from attitude determination, the 3D rms error is only 3 mm larger when compared to Rover A and amounts to 2.4 cm, which is acceptable for most topographic surveys.

Table 2 summarises the results regarding availability, accuracy and reliability:

Table 2 - Comparison of the availability, accuracy and reliability of RTK fixed positions between GS18 T and Rover A in a strong multipath environment (pole length: 1.800 m, instantaneous measurement).

Using the GS18 T with tilt compensation, the availability of RTK fixed solutions increases by 15 per cent when compared to conventional RTK using Rover A. The positioning accuracy is significantly improved, on average by 50 per cent. The reliability gives the percentage that the position error is less than three times the CQ, which is slightly enhanced by up to 6 per cent for the horizontal components. Please also keep in mind that such a strong multipath environment is considered as an extreme case and is far beyond the standard conditions relevant for accuracy and reliability specifications. In addition, points closer than 10 cm to a building cannot be measured with Rover A at all since in this case it is not possible to level the pole at the target point.

Cars, power lines and buildings with structural steel – every surveyor faces these and further local magnetic disturbances on a daily basis. Does the new Leica GS18 T offer a solution to this problem?
The answer is simple: apart from no need of on-site calibrations, one major advantage of the IMU-based tilt compensation over the magnetometer-based approach is the immunity to magnetic field disturbances. We have compared two rovers under magnetic disturbances. Looking at the rms errors summarised in Table 5, the 2D accuracy of GS18 T is approximately 2 cm better than that of Rover B, whereas the 1D accuracy is at a similar level:

Table 3 - Comparison of the rms errors between GS18 T and Rover B under magnetic disturbances (parking lot, pole length: 1.800 m, 1-s static measurement).

Looking at the rms errors summarised in Table 3, the 2D accuracy of GS18 T is approximately 2 cm better than that of Rover B, whereas the 1D accuracy is at a similar level.

By comparing the 2D errors in Fig. 7a, the GS18 T provides higher accuracy and consistency than Rover B. Moreover, the 2D CQ estimates agree with the 2D errors, reflecting the positioning accuracy in a realistic manner. Regarding the results from Rover B in Fig. 7b, the 2D CQ values are significantly larger than the 2D errors if magnetic disturbances are detected, indicating unreliable tilt-compensated solutions. In this case, the user needs to repeat the measurement or to switch to the conventional RTK mode, which decreases productivity. Under certain circumstances, for example, when measuring points at larger tilt angles, the user would not be notified by a magnetometer-based system that the displayed accuracy cannot be achieved.

Figure 7 - Comparison of the 2D position errors and CQ between GS18 T and Rover B under magnetic disturbances (parking lot, pole length: 1.800 m, 1-s static measurement).

How does including heading-aided 3D visualisation improve the overall user experience?
By incorporating the sensor heading into 3D visualisation, the user can easily orientate himself in the survey environment and quickly move toward the target points, improving user experience and productivity.

Figure 8 - Example of heading-aided 3D visualisation when staking points with the Leica GS18 T (open sky, pole length: 1.800 m)(a) Navigation view, (b) View towards west, (c) View towards south, (d) View towards east.

Fig. 8 illustrates how the heading information helps when staking points with the GS18 T in the navigation view. If the stakeout point is more than 0.5 m away, the view shows the surroundings in the heading direction and follows the sensor from above and behind (Fig. 17a). The 3D view and stake instructions update automatically according to the current position and sensor heading, which changes from westward over southward to eastward in this example.

In your own words, how would you summarise the overall GS18 T advantages in comparison to other RTK rovers?
From a user perspective: using tilt compensation, instantaneous measurement provides a similar accuracy level as static RTK measurement, along with a favourable time-saving effect.

In comparison to conventional RTK with a vertical pole, tilt-compensating RTK significantly increases productivity by up to 33 per cent and considerably improves the near-building positioning performance regarding availability and accuracy.

On a parking lot with magnetic disturbances, the IMU-based tilt compensation produces more accurate positions and more realistic CQ than the magnetometer-based approach.

The IMU-based tilt-compensating RTK is applicable at large tilt angles of more than 30 degrees, where a 3D positioning accuracy of 2 cm is still achievable.

By incorporating sensor heading into 3D visualisation of the surroundings, the user can easily orientate himself in the surveying environment, which improves productivity and user experience.

The attitude information of tilt-compensated RTK measurements is fully traceable, enabling quality assurance for users themselves and their clients.

To learn more about the Leica GS18 T, please visit: leica-geosystems.com/gs18t

For in-depth information on high-performance GNSS signal tracking, the challenges in tilt compensation RTK and advanced signal tracking technologies, please download the white paper.