An assessment of smartphone and low-cost multi-GNSS single-frequency RTK positioning for low, medium and high ionospheric disturbance periods

The emerging GNSSs make single-frequency (SF) RTK positioning possible. In this contribution two different types of low-cost (few hundred USDs) RTK receivers are analyzed, which can track L1 GPS, B1 BDS, E1 Galileo and L1 QZSS, or any combinations thereof, for a location in Dunedin, New Zealand. These SF RTK receivers can potentially give competitive ambiguity resolution and positioning performance to that of more expensive (thousands USDs) dual-frequency (DF) GPS receivers. A smartphone implementation of one of these SF receiver types is also evaluated. The least-squares variance component estimation (LS-VCE) procedure is first used to formulate a realistic stochastic model, which assures that our receivers at hand can achieve the best possible ambiguity resolution and RTK positioning performance. The best performing low-cost SF RTK receiver types are then assessed against DF GPS receivers and survey-grade antennas. Real data with ionospheric disturbances at low, medium and high levels are analyzed, while making use of the ionosphere-weighted model. It will be demonstrated that when the presence of the residual ionospheric delays increases, instantaneous RTK positioning is not possible for any of the receivers, and a multi-epoch model is necessary to use. It is finally shown that the low-cost SF RTK performance can remain competitive to that of more expensive DF GPS receivers even when the ionospheric disturbance level reaches a Kp-index of 7-, i.e. for a strong geomagnetic storm, for the baseline at hand

Low-cost, high-precision, single-frequency GPS–BDS RTK positioning

The integration of the Chinese BDS with other systems, such as the American GPS, makes precise RTK positioning possible with low-cost receivers. We investigate the performance of low-cost ublox receivers, which cost a few hundred USDs, while making use of L1 GPS + B1 BDS data in Dunedin, New Zealand. Comparisons will be made to L1 + L2 GPS and survey-grade receivers which cost several thousand USDs. The least-squares variance component estimation procedure is used to determine the code and phase variances and covariances of the receivers and thus formulate a realistic stochastic model. Otherwise, the ambiguity resolution and hence positioning performance would deteriorate. For the same reasons, the existence of receiver-induced time correlation is also investigated. The low-cost RTK performance is then evaluated by formal and empirical ambiguity success rates and positioning precisions. It will be shown that the code and phase precision of the low-cost receivers can be significantly improved by using survey-grade antennas, since they have better signal reception and multipath suppression abilities in comparison with low-cost patch antennas. It will also be demonstrated that the low-cost receivers can achieve competitive ambiguity resolution and positioning performance to survey-grade dual-frequency GPS receivers

Undifferenced and Uncombined GNSS Time Transfer and its Space Applications

This thesis presents a framework for developing a state-of-the-art undifferenced and uncombined (UDUC) time transfer technique for space applications. It addresses challenges in GNSS time transfer, such as multi-frequency signal modelling, satellite clock estimation, and hardware delay variations. The thesis introduces the UDUC POD method for GNSS time transfer in space and explores the feasibility of constructing a LEO-based space-time reference. This PhD dissertation is among the first to investigate the UDUC GNSS time transfer

An analysis of multi-GNSS observations tracked by recent Android smartphones and smartphone-only relative positioning results

In this study we assess the quality of multi-GNSS observations of recent Android smartphones. The results reveal a significant drop of smartphone carrier-to-noise density ratio (C/N0) with respect to geodetic receivers, and discernible differences among constellations and frequency bands. We show that the higher the elevation of the satellite, the larger discrepancy in C/N0 between the geodetic receivers and smartphones. Thus we show that a C/N0 weighting scheme may be superior to the elevation dependent one usually adopted for GNSS observations. We also discover that smartphone code pseudoranges are noisier by about one order of magnitude as compared to geodetic receivers, and that the code signals on L5 and E5a outperform those on L1 and E1, respectively. It is shown that smartphone phase observations are contaminated by the effects that can destroy the integer property and time-constancy of the ambiguities. There are long term drifts detected for GPS L5, Galileo E1, E5a and BDS B1 phase observations of Huawei P30. We highlight competitive phase noise characteristics for the Xiaomi Mi 8 when compared to the geodetic receivers. We also reveal a poor quality of other than GPS L1 phase signals for the Huawei P30 smartphones related to the unexpected drifts of the observations. Finally, the positioning experiment proves that it is feasible to obtain a precise cm-level solution of a smartphone to smartphone relative positioning with fixed integer ambiguities

Multi-GNSS integer ambiguity resolution enabled precise positioning

In this PhD thesis multi-Global Navigation Satellite System (GNSS) positioning results when combining the American Global Positioning System (GPS), Chinese BeiDou Navigation Satellite System (BDS), European Galileo and Japanese Quasi-Zenith Satellite System (QZSS) will be presented. The combined systems will be evaluated in comparison to the single-systems, for short (atmosphere-fixed) to long (atmosphere-present) baselines. It will be shown that the combined systems can provide for improved integer ambiguity resolution and positioning performance over the single-systems

Improving Reliability and Assessing Performance of Global Navigation Satellite System Precise Point Positioning Ambiguity Resolution

Conventional Precise Point Positioning (PPP) has always required a relatively long initialization period (few tens of minutes at least) for the carrier-phase ambiguities to converge to constant values and for the solution to reach its optimal precision. The classical PPP convergence period is primarily caused by the estimation of the carrier-phase ambiguity from the relatively noisy pseudoranges and the estimation of atmospheric delay. If the underlying integer nature of the ambiguity is known, it can be resolved, thereby reducing the convergence time of conventional PPP. To recover the underlying integer nature of the carrier-phase ambiguities, different strategies for mitigating the satellite and receiver dependent equipment delays have been developed, and products made publicly available to enable ambiguity resolution without any baseline restrictions. There has been limited research within the scope of interoperability of the products, combining the products to improve reliability and assessment of ambiguity resolution within the scope of being an integrity indicator. This study seeks to develop strategies to enable each of these and examine their feasibility. The advantage of interoperability of the different PPP ambiguity resolution (PPP-AR) products would be to permit the PPP user to transform independently generated PPP-AR products to obtain multiple fixed solutions of comparable precision and accuracy. The ability to provide multiple solutions would increase the reliability of the solution for, e.g., real-time processing: if there were an outage in the generation of the PPP-AR products, the user could instantly switch streams to a different provider. The satellite clock combinations routinely produced within the International GNSS Service (IGS) currently disregard that analysis centers (ACs) provide products which enable ambiguity resolution. Users have been expected to choose either an IGS product which is a combined product from multiple ACs or select an individual AC solution which provides products that enable PPP-AR. The goal of the novel research presented was to develop and test a robust satellite clock combination preserving the integer nature of the carrier-phase ambiguities at the user end. mm-level differences were noted, which was expected as the strength lies mainly in its reliability and stable median performance and the combined product is better than or equivalent to any single ACs product in the combination process. As have been shown in relative positioning and PPP-AR, ambiguity resolution is critical for enabling cm-level positioning. However, what if specifications where at the few dm-level, such as 10 cm and 20 cm horizontal what role does ambiguity resolution play? The role of ambiguity resolution relies primarily on what are the user specifications. If the user specifications are at the few cm-level, ambiguity resolution is an asset as it improves convergence and solution stability. Whereas, if the users specification is at the few dm-level, ambiguity resolution offers limited improvement over the float solution. If the user has the resources to perform ambiguity resolution, even when the specifications are at the few dm-level, it should be utilized

Cost-Effective GNSS Hardware for High-Accuracy Surveys and Its Prospects for Post-Processed Kinematic (PPK) and Precise Point Positioning (PPP) Strategies

This dissertation determines for the first time the vertical accuracy achievable with low-cost mass-market multi-frequency, multi-GNSS (LM3GNSS) receivers, and antennas in the context of Ellipsoid Reference Survey (ERS), usually employed in bathymetric operations aboard survey platforms. LM3GNSS receivers are relatively new in the market, and their emergence is driven by the automobile industry and several mass-market applications requiring location-based solutions at high accuracies. It is foreseeable that emerging hydrographic survey platforms such as autonomous surface vehicles, small unmanned aircraft, crowd-sourced bathymetric platforms, and offshore GNSS buoy will find LM3GNSS receivers attractive since they are power- and cost-effective (often less than $1,000 per unit). Previous studies have shown that some mass-market GNSS receivers\u27 positioning accuracy is at the sub-meter level in some positioning strategies, but the authors rarely discussed the vertical accuracy. In rare cases where attention is given to the vertical component, the experiment design did not address the dynamic antenna scenario typical of hydrographic survey operations and the positioning performance that meets the hydrographic survey community\u27s aspirations. The LM3GNSS receivers and low-cost antennas considered in this dissertation achieved vertical accuracies within 0.15 m at a 95% confidence level in simulated precise point positioning (PPP) and post-processed kinematic positioning strategies. This dissertation characterizes the signal strength, multipath, carrier-phase residuals, and code residuals in the measurement quality assessment of four LM3GNSS receivers and four low-cost antennas. The dissertation investigates the performances of the LM3GNSS receivers and low-cost antennas in different antenna-receiver pairings, relative to a high-grade GNSS receiver and antenna in simulated-kinematic and precise point positioning (PPP) strategies. This dissertation also shows that solutions with an uncalibrated antenna improve with a cloned ANTEX file making the results comparable to those achieved with high-end GNSS antenna. This dissertation also describes a GNSS processing tool (with graphic user interface), developed from scratch by the author, that implements, among others, orbit interpolation and geodetic computations as steps towards multipath computation and analysis. The dissertation concludes as follows: (1) The LM3GNSS hardware considered in this dissertation provides effective alternative positioning and navigation performance for emerging survey platforms such as ASV and sUAS. (2) LM3GNSS hardware can meet vertical positioning accuracy on the order of 0.15 m at a 95% confidence level in PPP strategy on less dynamic platforms. (3) LM3GNSS receivers can provide PPK solutions at medium (30 – 40 km) baselines with a vertical positioning accuracy better than 0.15m at a 95% confidence level. (4) LM3GNSS receivers in PPP strategy should meet IHO S-44 order-1 and order-2 in shallow waters. (5) Zephyr3 antenna, being a high-end GNSS antenna, may not always offer the best performance with the LM3GNSS receiver, especially in a dynamic environment. (6) Given the current tracking capabilities, the measurement quality, and positioning performances of LM3GNSS receivers relative to the geodetic grade receiver, it is foreseeable that the distinction between high-end GNSS and LM3GNSS receivers will most likely fade away as GNSS hardware technology advances. (7) Maximizing an LM3GNSS receiver in PPK strategy requires a multi-constellation-enabled reference station and high (i.e., 1 Hz) data tracking rate; otherwise, the PPK solutions will likely drift up to 20 cm

GLONASS FDMA data for RTK positioning: a five-system analysis

The use of the GLONASS legacy signals for real-time kinematic positioning is considered. Due to the FDMA multiplexing scheme, the conventional CDMA observation model has to be modified to restore the integer estimability of the ambiguities. This modification has a strong impact on positioning capabilities. In particular, the ambiguity resolution performance of this model is clearly weaker than for CDMA systems, so that fast and reliable full ambiguity resolution is usually not feasible for standalone GLONASS, and adding GLONASS data in a multi-GNSS approach can reduce the ambiguity resolution performance of the combined model. Partial ambiguity resolution was demonstrated to be a suitable tool to overcome this weakness (Teunissen in GPS Solut 23(4):100, 2019). We provide an exhaustive formal analysis of the positioning precision and ambiguity resolution capabilities for short, medium, and long baselines in a multi-GNSS environment with GPS, Galileo, BeiDou, QZSS, and GLONASS. Simulations are used to show that with a difference test-based partial ambiguity resolution method, adding GLONASS data improves the positioning performance in all considered cases. Real data from different baselines are used to verify these findings. When using all five available systems, instantaneous centimeter-level positioning is possible on an 88.5 km baseline with the ionosphere weighted model, and on average, only 3.27 epochs are required for a long baseline with the ionosphere float model, thereby enabling near instantaneous solutions

Multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution

Multi-frequency and multi-GNSS measurements from modernized satellites are properly integrated for PPP with ambiguity resolution to achieve the state-of-the-art fast and accurate positioning, which provides an important contribution to GNSS precise positioning and applications. The multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution, which is accomplished by a unified model based on the uncombined PPP, are thoroughly evaluated with special focus on Galileo and BDS