The Triple-frequency Multi-system RTK Engine for Challenging Environments

J. Van Hees, F. Boon, P. Jacobs, F. Kleijer, J. Viana, B. Durinck, A. Simsky

Abstract: Developers of RTK algorithms face multiple challenges. One of them is the simultaneous presence of about 50 satellites from four GNSS systems (GPS, GLONASS, Galileo, BeiDou). Another is the use of the third frequency, which shall be introduced in all GNSS systems. The users expect a state-of-the-art RTK algorithm to work in harsh environments including forests and urban canyons, as well as survive the periods of high ionosphere activity or even scintillation. In this presentation we introduce a new RTK engine which is designed to address all these challenges. The working of the new RTK engine is based on thoroughly developed noise models, which reflect diverse geographic and environmental conditions in which RTK rovers operate. The outline of the new engine was already presented at this conference (Van Meerbergen, G., Simsky, A., Boon, F., A novel decision directed Kalman filter to improve RTK performance. ION GNSS 2010, Portland, OR, 22-24 September 2010). It is based on the real-time estimation of various biases by the Kalman Filter, which is used to compensate the errors of code and phase measurements. It also includes a flexible software design, which simplifies the inclusion of new GNSS systems. In this presentation we focus on the actual features and capabilities, on the results of testing and performance analysis of the new engine. One of the essential novelties of the new engine is the adaptive approach to error modeling. The multipath environment and the magnitude of ionosphere delays are continuously assessed in separate pre-processing filters. The resulting indicators are used to fine-tune the noise models and to optimize the performance of the RTK engine. This adaptive approach has proven its efficiency for real-time modeling of multipath errors. It significantly improves the time-to-fix and the reliability of RTK in urban surveys and near buildings. Ionosphere disturbances present a serious challenge for RTK operations, especially in ionospherically active regions, which occupy a great portion of the globe. The new engine is capable of estimating differential ionosphere delays in real time, taking into account the baseline, geographical position and time of day and also the pre-processing assessment of ionospheric activity. The presentation will contain performance examples for various geographic regions. Owing to the real-time modeling of ionosphere delays, the accuracy of our RTK positional solution is almost insensitive to the baseline length up to the values of 40 km. A customary linear dependence of positional errors vs baseline, the slope of which is typically about 3 ppm, now looks like an almost horizontal line with a slope of about 0.3 ppm. The standard deviation of positional errors stays on the level of 1-2 cm even for baselines of about 50 km. Significant experience with scintillation was accumulated in the framework of CIGALA and CALIBRA projects, where the network of Septentrio’s PolaRxS receivers was set up in Brazil. These data helped us to introduce algorithmic countermeasures to prevent the RTK engine from using phase measurements affected by scintillation. These countermeasures include analysis of phase and Doppler residuals and also the detection of scintillation events within the tracking loops (e.g., by monitoring of the S4 parameter). The use of Galileo measurements for RTK was first demonstrated within the framework of the Galileo Test User Receiver project. Now Galileo and BeiDou are becoming a regular feature of Septentrio RTK rovers in addition to GPS and GLONASS. The increase in RTK availability due to the use of Galileo and BeiDou was demonstrated even with the current partial deployment of these constellations (Viana, J., Sleewaegen, J.-M., Boon, F., Bollard, E., RTK: Adding BeiDou. IEEE Worksop on Asia-Pacific Satellite Navigation and Positioning, Brisbane, Australia, February 2014). In the future we expect RTK to operate even in urban canyon conditions, where it was traditionally regarded as non-functional. The new engine benefits from the extensive array of problem detection, RAIM and satellite selection schemes, which help to remove or de-weight the satellites with unreliable measurements, to optimize the sub-constellation to be used for fixing, and to avoid wrong fixes. The use of three frequencies for RTK will also be addressed in the presentation. Tomorrow’s RTK engines will benefit from the use of the third frequency for all GNSS systems, which shall increase availability of phase measurements, accelerate fixing and help to introduce additional RAIM procedures to verify the reliability of fixing.
Published in: Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014)
September 8 - 12, 2014
Tampa Convention Center
Tampa, Florida
Pages: 69 - 99
Cite this article: Van Hees, J., Boon, F., Jacobs, P., Kleijer, F., Viana, J., Durinck, B., Simsky, A., "The Triple-frequency Multi-system RTK Engine for Challenging Environments," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 69-99.
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