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

Dr. Xiaoguang Luo, Stefan SchauflerandBernhard Richter讨论GNSS和惯性测量单元（IMU）中传感器融合领域的最新发展。新的Leica GS18 T GNSS RTK Rover结合了GNSS和IMU，以自动从Plumb中调节极点倾斜。发现这如何提高生产率，扩展RTK的适用性并减少人体错误。

What are the current challenges of conventional RTK surveying?
需要手动水准极圆cular 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:

• 没有现场校准
• Immune to magnetic disturbances
• Applicable at large tilt angles
• Heading-aided 3D visualisation

新的Leica GS18 T GNSS RTK Rover结合了GNSS和IMU，以自动从Plumb中调节极点倾斜，从而提高生产率，扩展RTK的适用性并减少人体错误。它改善了整体用户体验无法比较。

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倾斜补偿techniquein combination withinstantaneous 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 ofadvanced signal tracking，GS18 T特别适合RTK应用，例如，在叶子下或城市峡谷中，在靠近树线附近运行的天空。通过应用IMU-based tilt compensationof 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 anglesare 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?
确切地说 - 不必专注于平整杆子，用户可以更加关注自己的安全。通过车辆和操作机的风险大大降低了。此外，态度信息用于通过自动更新周围环境的3D可视化（取决于传感器的方向）来帮助用户在现场取代自己。

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:

图4 -和简化示意图说明of the GNSS/INS integration implemented in the Leica GS18 T.

不断进行GNS和INS之间的一致性检查，以实现可以应对极端动力学（例如硬冲击）的强大系统。由于倾斜度计算位置的计算没有磁力计测量值，因此GS18 T不受磁干扰的影响。

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.

表1总结了开放基测试的生产率和准确性的结果：

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).

使用倾斜补偿的GS18 T，与使用RoverA的常规RTK相比，RTK固定溶液的可用性增加了15％，平均提高了定位精度，平均提高了50％。可靠性给出了一个百分比，即位置误差小于CQ的三倍，水平组件的最高可增强6％。还请记住，这种强大的多路径环境被认为是极端情况，并且远远超出了与准确性和可靠性规格相关的标准条件。此外，无法用漫游者A对建筑物的距离更接近10厘米，因为在这种情况下，不可能在目标点处将极点平衡。

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:

表3-在磁性干扰（停车场，极长：1.800 m，1 -S静态测量）下，GS18 T和RM之间的RMS误差的比较。

查看表3中总结的RMS误差，GS18 T的2D精度比Rover B好约2 cm，而1D精度在相似的水平上。

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.

用您自己的话来说，与其他RTK Rovers相比，您将如何总结GS18 T的总体优势？
From a user perspective: using倾斜补偿, instantaneous measurement provides a similar accuracy level as static RTK measurement, along with a favourabletime-saving effect.

In comparison to conventional RTK with a vertical pole, tilt-compensating RTK significantlyincreases productivityby up to33 per centand considerablyimproves the near-building positioning performanceregarding availability and accuracy.

On a parking lot withmagnetic disturbances, the IMU-based tilt compensation producesmore accurate positions and more realistic CQ而不是基于磁力计的方法。

The IMU-based tilt-compensating RTK is applicable atlarge tilt anglesof more than30 degrees，3D positioning accuracy of 2 cm仍然可以实现。

Byincorporating传感器进入3D可视化of the surroundings, the user can easily orientate himself in the surveying environment, which improves productivity and user experience.

Theattitude information倾斜补偿的RTK测量是完全可追溯, enabling quality assurance for users themselves and their clients.

To learn more about theLeica GS18 T, please visit:Leica-Geosystems.com/GS18T

高性能GNSS深入的信息signal tracking, the challenges in tilt compensation RTK and advanced signal tracking technologies, please download the white paper.