iRTK Inertially Aided RTK for Robust and Precise Positioning

Enhancing centimeter-level precision in Global Navigation Satellite System (GNSS) positioning holds immense potential for advancing various emerging technologies. These technologies encompass autonomous driving vehicles, augmented reality applications, and unmanned aerial vehicles (UAVs). However, GNSS’s capability to deliver this high level of accuracy, primarily through real-time kinematic (RTK) positioning, is currently constrained by the requirement of an unobstructed line-of-sight to GNSS satellites. In urban environments, where these technologies are most needed, GNSS signals often encounter obstructions and reflections. Consequently, the availability of RTK solutions in urban areas typically falls below 50-70%, depending on the nature of the surrounding buildings. This limitation severely restricts the viability of RTK for safety-critical applications, such as autonomous UAVs for traffic monitoring in cities.

Better RTK availability in urban areas

The objective of this project was to significantly enhance RTK availability in urban areas to exceed 99%. This was tried to achieve by harnessing GPS+Galileo L1/E1 and L5/E5a signals, combined with inertial aiding. The project employed a cost-effective microelectromechanical (MEMS) gyro and accelerometer, referred to as an inertial measurement unit (IMU), to detect cycle-slips in GNSS carrier phase measurements. Cycle-slips are the primary cause of low RTK availability in urban settings. To ensure continuous GNSS signal tracking, even in the presence of obstructions, a GPS/Galileo RTK module was developed and integrated with an ultra-tightly coupled GNSS/IMU receiver. To mitigate multipath interference, a synthetic antenna aperture approach was adopted. Furthermore, the Kalman filter was extended based on IMU calibration campaigns and a parameter sensitivity analysis, enabling the use of more cost-effective IMUs or seamless bridging of longer GNSS signal outages, such as those occurring in underpasses.

IMU-based reference system

To validate the project’s findings, a ring-laser IMU-based reference system was integrated into a measurement vehicle, ensuring ground truth accuracy within approximately 1 cm. This measurement vehicle is also carry competing solutions for comparative analysis. A fisheye camera aids in identifying GNSS signal degradations, and the gathered data will be leveraged to optimize the developed algorithms.

Enhancement of GNSS receiver algorithms

The primary benefit of this project lied in the enhancement of GNSS receiver algorithms, which will subsequently be offered for sale or licensing. The market for these receivers is addressed in a technology-oriented manner, similar to competitors operating in this high-end technology domain. Additionally, scientific outcomes in the field of RTK and integrated navigation was published to contribute to the advancement of navigation technologies. Finally, the project aimed to further bolster expertise in conducting experiments and developing small-scale series products.