Title: Trends, Innovations and Enhancements for Low-Cost PPP
Author(s): D. Calle, E. Carbonell, P. Navarro, I. Rodríguez, P. Roldán, G. Tobías
Published in: Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2017)
September 25 - 29, 2017
Oregon Convention Center
Portland, Oregon
Pages: 139 - 170
Cite this article: Calle, D., Carbonell, E., Navarro, P., Rodríguez, I., Roldán, P., Tobías, G., "Trends, Innovations and Enhancements for Low-Cost PPP," Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2017), Portland, Oregon, September 2017, pp. 139-170.
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Abstract: Low-cost Precise Point Positioning (PPP) is one of the main techniques identified by the industry which may trigger an improvement of the end-users positioning accuracy to the decimetre-level. GNSS users have been limited to single-frequency receivers, since the possibility of moving to multi-frequency devices was a-priori discarded mainly due to budgetary constraints. However, mass-market receiver manufacturers have been working to address this limitation and current trends indicate that low-cost users may have dual-frequency off-the-shelf receivers in the market in the upcoming years. This new context will allow the application of existing dual-frequency PPP techniques for this user segment, as well as enabling a significant improvement in the achievable performances. When talking about PPP techniques, the main drawback derived from the use of single-frequency receivers is the need to estimate the ionospheric delays together with phase ambiguities. Furthermore, the use of the noisy code-phase measurements provided by low-cost receivers and antennas complicates the separation of these two contributions in the estimation process. This situation leads to long convergence times which are critical for potential mass-market users and applications. As it is widely known, dual-frequency receivers allow to simplify this process by cancelling the ionospheric term by means of the ionosphere-free combination. Nevertheless, the use of frequency combinations inherently increases the measurements noise by approximately a factor of 3, which can be highly detrimental in the case of using mass-market receivers. This issue can be tackled by employing a per frequency processing approach, which implies the need of an adequate modelling ionospheric delays. Therefore measurements noise is not penalized as much as when performing linear combinations, resulting in better observability on the ionosphere and ambiguity contributions. In addition, the robustness and availability of the solution is also benefited by this technique since tracking loses of one of the frequencies, which are common in harsh environments, do not lead to satellite rejection if other frequency is still available. Additionally, the aforementioned per frequency processing can be complemented with the use of assisted regional information provided through an aiding channel; this option allows achieving even better performances. magicFAST is a GMV service which provides high-quality prediction of regional corrections allowing PPP users to add this valuable information in the estimation filter. This feature has opened the door to faster convergence thanks to code-noise reduction and quicker phase ambiguity estimation. In addition, most end-user applications relying on low-cost chipsets for localization require having an accurate and reliable positioning solution in harsh environmental conditions and within a reduced timeline. These expectations need to be flown down to the low-cost PPP paradigm, and the industry has to provide the means to ensure the availability and quality of the solution in cluttered environments, such as urban canyons, dense foliage areas, etc, where GNSS signal tracking is degraded. In this line, GMV is also working in the integration of GNSS-based PPP with inertial sensors measurements, not only regarding the robustness of the solution in challenging environments, but also with the aim to provide a degree of dead reckoning solution when GNSS signals are completely lost. The outcome of this effort is a tightly coupled GNSS+IMU strap-down navigation filter which benefits from the strength derived of the combination of both information sources and improves the solution reliability. These new combined capabilities available in the magicPPP platform combined with the new feature of raw GNSS measurements provision available in Android 7, makes of these devices an ideal framework to demonstrate the potential performance of the system. Throughout this paper, current trends and future receiver evolutions relevant for Low-Cost PPP will be analysed as well as their applicability to smartdevices. The performance enhancement and robustness improvement introduced by the use of per-frequency processing techniques, regional corrections and inertial units will be presented together with a deep analysis of the impact of these techniques on high accuracy positioning users. The solutions herein introduced will be described, focusing on the algorithms developed and the innovations accomplished. In addition, this paper will present the results obtained during a dedicated experimentation campaign conducted using GMV’s magicPPP service and low-cost GNSS receivers from smartphones in different user conditions.