Precise Positioning into Urban Environments: A Low-cost Single-Frequency PPP System

Hongzhou Yang, Fei Liu and Yang Gao

Abstract:	With strong demands on precise positioning for mass-market applications from self-driving cars, UAV and smartphones, PPP systems and products based on low-cost GNSS chipsets and modules targeted for consumer markets are being developed and released to the markets. To date, most systems and products are operated in open-sky environments while only few were tested in sub-urban environments. It is time to investigate and develop low-cost PPP system and products for precise positioning in urban environments. Although with various challenges, this presentation will describe a low-cost single-frequency PPP system that has been successfully demonstrate 50 cm accuracy in open-sky and sub-urban environments and further enhanced to support precise positioning in urban environments. Featuring multi-constellations, global availability, fast convergence and continuous navigation solutions, Instant PPP (IP3) is developed as an ideal precise positioning solution for mass market applications. The availability and accuracy of IP3 in urban environments such as downtown area will be discussed and presented. In order to obtain high-accuracy and high-availability positioning solutions in urban environments, the IP3 library is uniquely designed and enhanced in several aspects. For instance, the instant receiver velocity based on the Doppler observations and the coordinate changes calculated from the carrier phase differences between two consecutive epochs are integrated for the one-step prediction of the receiver positions in the Kalman filter. Meanwhile, the weight of carrier phase, pseudo-range and Doppler observations are smartly tuned as a function of Signal to Noise Ratio (SNR) respectively. Additionally, the quality control part is designed to be adaptive to different scenarios, such as open-sky or urban environments. The receiver clock drifts for different constellations are specifically modelled in the velocity estimation to increase the degrees of freedom, which further enhances the solution availability in the extreme challenging situations. To evaluate the availability and accuracy of IP3 library in urban environments, several real-time vehicle-based field tests have been carried out with the IP3 evaluation kit in Calgary, Canada. The IP3 evaluation kit consists of a u-blox M8T module, an ARM based processor and a patch antenna. The raw GNSS data from GPS, GLOANSS and Galileo were logged and processed in real-time, with the real-time corrections streaming over Wi-Fi, which is connected to the cellphone hotspot. A navigation-grade IMU (KVH1750) and a high-end NovAtel Propak6 GNSS receiver were used as the reference system in the tests. Meanwhile, the 1 Hz GNSS raw observations from a nearby base station (UCAL) was streamed and stored for the generation of the accurate RTK/INS reference solutions, which were processed in post-mission by commercial software. The testing results indicate the IP3 library can provide 50 cm accuracy in the sub-urban areas with 100% solution availability. When comes to the urban environment such as downtown area with abundant high buildings, the positioning Root-Mean-Square error (RMS) of IP3 degrades to meter level while the solution availability remains 100%. In the typical urban environments, the GNSS observations are all affected at different levels. For example, the availability of pseudo-range and Doppler observations are much higher than the carrier phase observations in challenging urban areas, nevertheless they are more vulnerable to the multipath effects. According to the tests, the SNR can largely reflect the change of the environments and thus was used in the weighting process for different types of observations. Apart from the positioning results, the accuracy and availability of the velocity solution are also evaluated in the urban environments. The RMS of IP3 velocity solution in the sub-urban area is at centimeter level while degrades to decimeter level in urban area such as downtown. Additionally, the standard deviations of the positioning and velocity solutions outputted by IP3 library match well with the true errors, hence the IP3 library can be conveniently and easily integrated with other navigation systems such as inertial navigation system (INS). More field tests will be carried out in different urban areas and the results will be presented lately. IP3 can provide precise positioning solutions with low-cost GNSS receivers even in urban environments. By novel integration of low-cost GNSS and PPP technology, IP3 is an ideal precise positioning solution for mass-market applications and can be easily integrated with other navigation systems such INS.
Published in:	Proceedings of the 33rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2020) September 21 - 25, 2020
Pages:	1779 - 1793
Cite this article:	Yang, Hongzhou, Liu, Fei, Gao, Yang, "Precise Positioning into Urban Environments: A Low-cost Single-Frequency PPP System," Proceedings of the 33rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2020), September 2020, pp. 1779-1793. https://doi.org/10.33012/2020.17584
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