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GNSS Data Processing and Navigation in Challenging Environments

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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Remote Sensors".

Deadline for manuscript submissions: closed (31 December 2020) | Viewed by 36252

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Special Issue Editors

Prof. Dr. Nobuaki Kubo

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Guest Editor

Department of Maritime Systems Engineering, Tokyo University of Marine Science and Technology, Tokyo 135-8533, Japan
Interests: multipath mitigation; software GNSS receiver; RTK/PPP; PPP-RTK; GNSS applications (car, ship, pedestrian)
Special Issues, Collections and Topics in MDPI journals

Dr. Taro Suzuki

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Guest Editor

Future Robotics Technology Center, Chiba Institute of Technology, Japan
Interests: robotics navigation and multipath mitigation in GNSS

Dr. Yize Zhang

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Guest Editor

Faculty of Marine Technology, Tokyo University of Marine Science and Technology, 2-1-6 Etchujima, Koto-ku, Tokyo, Japan
Interests: RTK/PPP algorithm and data processing

Special Issue Information

Dear Colleague:

Global navigation satellite systems (GNSS) are the core technology for many kinds of applications in terms of positioning and navigation. With the advent of low-cost multi-GNSS and multi-frequency receivers, the use of GNSS will become more popular—especially in precise positioning such as DGNSS, RTK, and PPP. However, GNSS also has unstable accuracy in challenging environments and is very vulnerable to interference. To cope with these issues, many researchers are currently developing and improving signal processing, data processing, multi-GNSS/frequency capability use, and sensor or feature aiding. It is very important for us to improve the accuracy, availability, and reliability of GNSS for future critical applications such as autonomous driving, unmanned monitoring of construction, and surveys in difficult environments as well as unmanned aerial and ground vehicles.

The aim of this Special Issue is to collect high-quality research articles and review papers on advances in GNSS data processing and navigation in challenging environments. Developments that improve the performance and efficiency of GNSS data processing for typical urban conditions (strong multipath, NLOS, etc.), machine learning and neural network approaches to GNSS, and sensor fusion using IMU, speed, camera and Lidar etc. 3D map aiding are also welcome.

Dr. Nobuaki Kubo
Dr. Taro Suzuki
Dr. Yize Zhang
Guest Editors

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Keywords

New algorithms in multipath mitigation
Improved GNSS data processing as well as signal processing
Advances in precise point positioning (PPP)
Advances in real-time kinematic (RTK) positioning
Position, navigation, and timing (PNT) in challenging environments
Sensor fusion for high-precision positioning
3D map and other features aiding

Published Papers (12 papers)

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19 pages, 2304 KiB

Open AccessArticle

NLOS Multipath Classification of GNSS Signal Correlation Output Using Machine Learning

by Taro Suzuki and Yoshiharu Amano

Sensors 2021, 21(7), 2503; https://0-doi-org.brum.beds.ac.uk/10.3390/s21072503 - 03 Apr 2021

Cited by 39 | Viewed by 5811

Abstract

This paper proposes a method for detecting non-line-of-sight (NLOS) multipath, which causes large positioning errors in a global navigation satellite system (GNSS). We use GNSS signal correlation output, which is the most primitive GNSS signal processing output, to detect NLOS multipath based on machine learning. The shape of the multi-correlator outputs is distorted due to the NLOS multipath. The features of the shape of the multi-correlator are used to discriminate the NLOS multipath. We implement two supervised learning methods, a support vector machine (SVM) and a neural network (NN), and compare their performance. In addition, we also propose an automated method of collecting training data for LOS and NLOS signals of machine learning. The evaluation of the proposed NLOS detection method in an urban environment confirmed that NN was better than SVM, and 97.7% of NLOS signals were correctly discriminated. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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23 pages, 4782 KiB

Open AccessArticle

Influence of Precise Products on the Day-Boundary Discontinuities in GNSS Carrier Phase Time Transfer

by Xiangbo Zhang, Ji Guo, Yonghui Hu, Baoqi Sun, Jianfeng Wu, Dangli Zhao and Zaimin He

Sensors 2021, 21(4), 1156; https://0-doi-org.brum.beds.ac.uk/10.3390/s21041156 - 06 Feb 2021

Cited by 3 | Viewed by 2031

Abstract

Global navigation satellite system (GNSS) precise point positioning (PPP) has been widely used for high-precision time and frequency transfer. However, the day-boundary discontinuities at the boundary epochs of adjacent days or batches are the most significant obstacle preventing PPP from continuous time transfer. The day-boundary discontinuities in station estimates and time comparisons are mainly caused by the code-pseudorange noise during the analysis of observation data in daily batches, where the absolute clock offset is determined by the average code measurements. However, some discontinuities with amplitudes even more than 0.15 ns may still appear in station clock estimates and time comparisons, although several methods had been proposed to remove such discontinuities. The residual small amplitude of the day-boundary discontinuities in some PPP station clock estimates and time comparisons through new GNSSs like Galileo seems larger, especially using precise clock products with large discontinuities. To further understand the origin of the day-boundary discontinuities, the influence of GNSS precise products on the day-boundary discontinuities in PPP station clock estimates and time comparisons is investigated in this paper. Ten whole days of Multi-GNSS Experiment (MGEX) from modified Julian date (MJD) 59028 to 59037 are used as the observation data. For a comparative analysis, the station clock estimates are compared with global positioning system (GPS) and Galileo observations through PPP and network solutions, separately. The experimental results show that the daily discontinuities in current combined GPS final and rapid clock products are less than 0.1 ns, and their influence on the origin of day-boundary discontinuities in PPP station clock estimates and time comparison are statistically negligible. However, the daily discontinuities in individual Analysis Centers (ACs) GPS products are more extensive, and their influence on the origin of the day-boundary discontinuities in GPS PPP station clock estimates cannot be ignored. The day-boundary discontinuities demonstrate random walk noise characteristics and deteriorate the station clocks’ long-term frequency stability, especially at an average time of more than one day. Although Galileo clock daily discontinuities are different from those of GPS, their influence on the day-boundary discontinuities in station clock estimates is nearly similar to the GPS PPP. The influence of daily discontinuities of Galileo clocks on PPP time comparison is similar to GPS and is not particularly critical to time comparison. However, combined and weighted MGEX products should be developed or Galileo IPPP should be used for remote comparison of high-stability clocks. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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18 pages, 14707 KiB

Open AccessArticle

Improvement of Reliability Determination Performance of Real Time Kinematic Solutions Using Height Trajectory

by Aoki Takanose, Yoshiki Atsumi, Kanamu Takikawa and Junichi Meguro

Sensors 2021, 21(2), 657; https://0-doi-org.brum.beds.ac.uk/10.3390/s21020657 - 19 Jan 2021

Cited by 3 | Viewed by 2360

Abstract

Autonomous driving support systems and self-driving cars require the determination of reliable vehicle positions with high accuracy. The real time kinematic (RTK) algorithm with global navigation satellite system (GNSS) is generally employed to obtain highly accurate position information. Because RTK can estimate the fix solution, which is a centimeter-level positioning solution, it is also used as an indicator of the position reliability. However, in urban areas, the degradation of the GNSS signal environment poses a challenge. Multipath noise caused by surrounding tall buildings degrades the positioning accuracy. This leads to large errors in the fix solution, which is used as a measure of reliability. We propose a novel position reliability estimation method by considering two factors; one is that GNSS errors are more likely to occur in the height than in the plane direction; the other is that the height variation of the actual vehicle travel path is small compared to the amount of movement in the horizontal directions. Based on these considerations, we proposed a method to detect a reliable fix solution by estimating the height variation during driving. To verify the effectiveness of the proposed method, an evaluation test was conducted in an urban area of Tokyo. According to the evaluation test, a reliability judgment rate of 99% was achieved in an urban environment, and a plane accuracy of less than 0.3 m in RMS was achieved. The results indicate that the accuracy of the proposed method is higher than that of the conventional fix solution, demonstratingits effectiveness. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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21 pages, 10403 KiB

Open AccessArticle

Intrinsic Identification and Mitigation of Multipath for Enhanced GNSS Positioning

by Qianxia Li, Linyuan Xia, Ting On Chan, Jingchao Xia, Jijun Geng, Hongyu Zhu and Yuezhen Cai

Sensors 2021, 21(1), 188; https://0-doi-org.brum.beds.ac.uk/10.3390/s21010188 - 30 Dec 2020

Cited by 8 | Viewed by 2257

Abstract

In global navigation satellite system (GNSS)-based positioning and applications, multipath is by far the most obstinate impact. To overcome paradoxical issues faced by current processing approaches for multipath, this paper employs an intrinsic method to identify and mitigate multipath based on empirical mode decomposition (EMD) and Hilbert–Huang transform (HHT). Frequency spectrum and power spectrum are comprehensively employed to identify and extract multipath from complex data series composed by combined GNSS observations. To systematically inspect the multipath from both code range and carrier phase, typical kinds of combinations of the GNSS observations including the code minus phase (CMP), differential correction (DC), and double differential (DD) carrier phase are selected for the suggested intrinsic approach to recognize and mitigate multipath under typical positioning modes. Compared with other current processing algorithms, the proposed methodology can deal with multipath under normal positioning modes without recourse to the conditions that satellite orbits are accurately repeated and surrounding environments of observing sites remain intact. The method can adaptively extract and eliminate multipath from solely the GNSS observations using intrinsic decomposition mechanism. From theoretical discussions and validating tests, it is found that both code and carrier phase multipath can be identified and distinguished from ionospheric delay and other impacts using the EMD based techniques. The resultant positioning accuracy is therefore improved to an obvious extent after the removal of the multipath. Overall, the proposed method can form an extensive and sound technical frame to enhance localization accuracy under typical GNSS positioning modes and harsh multipath environments. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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23 pages, 20181 KiB

Open AccessArticle

Consistency of the Empirical Distributions of Navigation Positioning System Errors with Theoretical Distributions—Comparative Analysis of the DGPS and EGNOS Systems in the Years 2006 and 2014

by Mariusz Specht

Sensors 2021, 21(1), 31; https://0-doi-org.brum.beds.ac.uk/10.3390/s21010031 - 23 Dec 2020

Cited by 14 | Viewed by 2429

Abstract

Positioning systems are used to determine position coordinates in navigation (air, land and marine). The accuracy of an object’s position is described by the position error and a statistical analysis can determine its measures, which usually include: Root Mean Square (RMS), twice the Distance Root Mean Square (2DRMS), Circular Error Probable (CEP) and Spherical Probable Error (SEP). It is commonly assumed in navigation that position errors are random and that their distribution are consistent with the normal distribution. This assumption is based on the popularity of the Gauss distribution in science, the simplicity of calculating RMS values for 68% and 95% probabilities, as well as the intuitive perception of randomness in the statistics which this distribution reflects. It should be noted, however, that the necessary conditions for a random variable to be normally distributed include the independence of measurements and identical conditions of their realisation, which is not the case in the iterative method of determining successive positions, the filtration of coordinates or the dependence of the position error on meteorological conditions. In the preface to this publication, examples are provided which indicate that position errors in some navigation systems may not be consistent with the normal distribution. The subsequent section describes basic statistical tests for assessing the fit between the empirical and theoretical distributions (Anderson-Darling, chi-square and Kolmogorov-Smirnov). Next, statistical tests of the position error distributions of very long Differential Global Positioning System (DGPS) and European Geostationary Navigation Overlay Service (EGNOS) campaigns from different years (2006 and 2014) were performed with the number of measurements per session being 900’000 fixes. In addition, the paper discusses selected statistical distributions that fit the empirical measurement results better than the normal distribution. Research has shown that normal distribution is not the optimal statistical distribution to describe position errors of navigation systems. The distributions that describe navigation positioning system errors more accurately include: beta, gamma, logistic and lognormal distributions. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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14 pages, 6366 KiB

Open AccessArticle

An SVM Based Weight Scheme for Improving Kinematic GNSS Positioning Accuracy with Low-Cost GNSS Receiver in Urban Environments

by Zhitao Lyu and Yang Gao

Sensors 2020, 20(24), 7265; https://0-doi-org.brum.beds.ac.uk/10.3390/s20247265 - 18 Dec 2020

Cited by 26 | Viewed by 2565

Abstract

High-precision positioning with low-cost global navigation satellite systems (GNSS) in urban environments remains a significant challenge due to the significant multipath effects, non-line-of-sight (NLOS) errors, as well as poor satellite visibility and geometry. A GNSS system is typically implemented with a least-square (LS) or a Kalman-filter (KF) estimator, and a proper weight scheme is vital for achieving reliable navigation solutions. The traditional weight schemes are based on the signal-in-space ranging errors (SISRE), elevation and C/N0 values, which would be less effective in urban environments since the observation quality cannot be fully manifested by those values. In this paper, we propose a new multi-feature support vector machine (SVM) signal classifier-based weight scheme for GNSS measurements to improve the kinematic GNSS positioning accuracy in urban environments. The proposed new weight scheme is based on the identification of important features in GNSS data in urban environments and intelligent classification of line-of-sight (LOS) and NLOS signals. To validate the performance of the newly proposed weight scheme, we have implemented it into a real-time single-frequency precise point positioning (SFPPP) system. The dynamic vehicle-based tests with a low-cost single-frequency u-blox M8T GNSS receiver demonstrate that the positioning accuracy using the new weight scheme outperforms the traditional C/N0 based weight model by 65.4% and 85.0% in the horizontal and up direction, and most position error spikes at overcrossing and short tunnels can be eliminated by the new weight scheme compared to the traditional method. It also surpasses the built-in satellite-based augmentation systems (SBAS) solutions of the u-blox M8T and is even better than the built-in real-time-kinematic (RTK) solutions of multi-frequency receivers like the u-blox F9P and Trimble BD982. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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26 pages, 6418 KiB

Open AccessArticle

Statistical Distribution Analysis of Navigation Positioning System Errors—Issue of the Empirical Sample Size

by Mariusz Specht

Sensors 2020, 20(24), 7144; https://0-doi-org.brum.beds.ac.uk/10.3390/s20247144 - 13 Dec 2020

Cited by 19 | Viewed by 3977

Abstract

Positioning systems are used to determine position coordinates in navigation (air, land, and marine). Statistical analysis of their accuracy assumes that the position errors (latitude—δ_φ and longitude—δ_λ) are random and that their distributions are consistent with the normal distribution. However, in practice, these errors do not appear in a random way, since the position determination in navigation systems is done with an iterative method. It causes so-called “Position Random Walk”, similar to the term “Random Walk” known from statistics. It results in the empirical distribution of δ_φ and δ_λ being inconsistent with the normal distribution, even for samples of up to several thousand measurements. This phenomenon results in a significant overestimation of the accuracy of position determination calculated from such a short series of measurements, causing these tests to lose their representativeness. This paper attempts to determine the length of a measurement session (number of measurements) that is representative of the positioning system. This will be a measurement session of such a length that the position error statistics (δ_φ and δ_λ) represented by the standard deviation values are close to the real values and the calculated mean values (

\bar{φ}

and

\bar{λ}

) are also close to the real values. Special attention will also be paid to the selection of an appropriate (statistically reliable) number of measurements to be tested statistically to verify the hypothesis that the δ_φ and δ_λ distributions are consistent with the normal distribution. Empirical measurement data are taken from different positioning systems: Global Positioning System (GPS) (168′286 fixes), Differential Global Positioning System (DGPS) (864′000 fixes), European Geostationary Navigation Overlay Service (EGNOS) (928′492 fixes), and Decca Navigator system (4052 fixes). The analyses showed that all researched positioning systems (GPS, DGPS, EGNOS and Decca Navigator) are characterized by the Position Random Walk (PRW), which resulted in that the empirical distribution of δ_φ and δ_λ being inconsistent with the normal distribution. The size of the PRW depends on the nominal accuracy of position determination by the system. It was found that measurement sessions consisting of 1000 fixes (for the GPS system) overestimate the accuracy analysis results by 109.1% and cannot be considered representative. Furthermore, when analyzing the results of long measurement campaigns (GPS and DGPS), it was found that the representative length of the measurement session differs for each positioning system and should be determined for each of them individually. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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17 pages, 10803 KiB

Open AccessArticle

Design of Entry Detection Method for Top-Bounded Spaces Using GPS SNR and Spatial Characteristics for Seamless Positioning in Logistics Facilities

by Kenichi Tabata, Madoka Nakajima and Naohiko Kohtake

Sensors 2020, 20(23), 6864; https://0-doi-org.brum.beds.ac.uk/10.3390/s20236864 - 30 Nov 2020

Cited by 1 | Viewed by 1979

Abstract

With the widespread use of indoor positioning technology, various services based on this technology are beginning to be offered to consumers and industrial applications. In the case of logistics facilities, in addition to indoor and outdoor spaces, there are top-bounded spaces (TBSs): elongated areas that are covered with roofs or eaves on the upper parts of buildings. The sides of such spaces are open, and workers and forklifts work in these areas. Only a few studies have been conducted on positioning methods for this unusual environment, and the way by which Signal-to-Noise Ratio (SNR) of Global Positioning System (GPS) changes with the stay in TBSs is unclear. Therefore, we conducted preliminary experiments and confirmed that TBS dwellings are difficult to stably detect with existing methods due to the combination of satellites with variable and unchanged SNRs. In this study, we designed a simple processing flow for selecting satellites with high probabilities of changing SNRs by using the spatial characteristics of TBSs as parameters (height, depth, and side opening orientation). We propose a method to detect the stay in TBSs using the SNR change rates of the selected satellites. As a result of evaluation experiments with three TBSs, we successfully detected the stay in TBSs with about 30% higher probability than those of an existing method. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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17 pages, 11899 KiB

Open AccessArticle

Real-Time Detection of Orbital Maneuvers Using Epoch-Differenced Carrier Phase Observations and Broadcast Ephemeris Data: A Case Study of the BDS Dataset

by Rui Tu, Rui Zhang, Lihong Fan, Junqiang Han, Pengfei Zhang and Xiaochun Lu

Sensors 2020, 20(16), 4584; https://0-doi-org.brum.beds.ac.uk/10.3390/s20164584 - 15 Aug 2020

Cited by 4 | Viewed by 2265

Abstract

The orbital maneuvers of the global navigation satellite system (GNSSs) have a significant influence on the performance of the precise positioning, navigation, and timing (PNT) services. Because the Chinese BeiDou Navigation Satellite System (BDS) has three types of satellites in the geostationary orbit (GEO), inclined geosynchronous orbit (IGSO), and medium earth orbit (MEO) maneuvers occur more frequently. Thus, it is essential to determine an effective approach for the detection of orbital maneuvers. This study proposes a method for the detection of orbital maneuvers using epoch-differenced carrier phase observations and broadcast ephemeris data. When using the epoch-differenced velocity estimation as a basic data solution model, the time discrimination and satellite identification factors are defined and used for the real-time detection of the beginning and the pseudorandom noise code (PRN) of satellites. The datasets from four GNSS stations (WUH1, BJF1, POHN, CUT0) from the year 2016 were collected and analyzed. The validations showed that the beginning, the PRN of the orbital maneuver of the satellite can be precisely detected in real time for all GEO, IGSO, and MEO satellites, and the detected results also showed good consistency, with the beginning time at a difference of 1–2 min across different stations. The proposed approach was observed to be more sensitive, and the detected beginning time was about 30 min earlier than the single point positioning approach when the high-precision carrier phase observation was used. Thus, orbital maneuvering can be accurately detected by the proposed method. It not only improves the utilization of the collected data but also improves the performance of PNT services. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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25 pages, 9229 KiB

Open AccessArticle

GNSS Multipath Detection Using Continuous Time-Series C/N₀

by Nobuaki Kubo, Kaito Kobayashi and Rei Furukawa

Sensors 2020, 20(14), 4059; https://0-doi-org.brum.beds.ac.uk/10.3390/s20144059 - 21 Jul 2020

Cited by 34 | Viewed by 4578

Abstract

The reduction of multipath errors is a significant challenge in the Global Navigation Satellite System (GNSS), especially when receiving non-line-of-sight (NLOS) signals. However, selecting line-of-sight (LOS) satellites correctly is still a difficult task in dense urban areas, even with the latest GNSS receivers. This study demonstrates a new method of utilization of C/N₀ of the GNSS to detect NLOS signals. The elevation-dependent threshold of the C/N₀ setting may be effective in mitigating multipath errors. However, the C/N₀ fluctuation affected by NLOS signals is quite large. If the C/N₀ is over the threshold, the satellite is used for positioning even if it is still affected by the NLOS signal, which causes the positioning error to jump easily. To overcome this issue, we focused on the value of continuous time-series C/N₀ for a certain period. If the C/N₀ of the satellite was less than the determined threshold, the satellite was not used for positioning for a certain period, even if the C/N₀ recovered over the threshold. Three static tests were conducted at challenging locations near high-rise buildings in Tokyo. The results proved that our method could substantially mitigate multipath errors in differential GNSS by appropriately removing the NLOS signals. Therefore, the performance of real-time kinematic GNSS was significantly improved. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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15 pages, 2962 KiB

Open AccessTechnical Note

Independent Component Extraction from the Incomplete Coordinate Time Series of Regional GNSS Networks

by Tengfei Feng, Yunzhong Shen and Fengwei Wang

Sensors 2021, 21(5), 1569; https://0-doi-org.brum.beds.ac.uk/10.3390/s21051569 - 24 Feb 2021

Cited by 6 | Viewed by 1638

Abstract

Independent component analysis (ICA) is one of the most effective approaches in extracting independent signals from a global navigation satellite system (GNSS) regional station network. However, ICA requires the involved time series to be complete, thereby the missing data of incomplete time series should be interpolated beforehand. In this contribution, a modified ICA is proposed, by which the missing data are first recovered based on the reversible property between the original time series and decomposed principal components, then the complete time series are further processed with FastICA. To evaluate the performance of the modified ICA for extracting independent components, 24 regional GNSS network stations located in North China from 2011 to 2019 were selected. After the trend, annual and semiannual terms were removed from the GNSS time series, the first two independent components captured 17.42, 18.44 and 17.38% of the total energy for the North, East and Up coordinate components, more than those derived by the iterative ICA that accounted for 16.21%, 17.72% and 16.93%, respectively. Therefore, modified ICA can extract more independent signals than iterative ICA. Subsequently, selecting the 7 stations with less missing data from the network, we repeatedly process the time series after randomly deleting parts of the data and compute the root mean square error (RMSE) from the differences of reconstructed signals before and after deleting data. All RMSEs of modified ICA are smaller than those of iterative ICA, indicating that modified ICA can extract more exact signals than iterative ICA. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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17 pages, 21714 KiB

Open AccessLetter

Skymask Matching Aided Positioning Using Sky-Pointing Fisheye Camera and 3D City Models in Urban Canyons

by Max Jwo Lem Lee, Shang Lee, Hoi-Fung Ng and Li-Ta Hsu

Sensors 2020, 20(17), 4728; https://0-doi-org.brum.beds.ac.uk/10.3390/s20174728 - 21 Aug 2020

Cited by 9 | Viewed by 3213

Abstract

3D-mapping-aided (3DMA) global navigation satellite system (GNSS) positioning that improves positioning performance in dense urban areas has been under development in recent years, but it still faces many challenges. This paper details a new algorithm that explores the potential of using building boundaries for positioning and heading estimation. Rather than applying complex simulations to analyze and correct signal reflections by buildings, the approach utilizes a convolutional neural network to differentiate between the sky and building in a sky-pointing fisheye image. A new skymask matching algorithm is then proposed to match the segmented fisheye images with skymasks generated from a 3D building model. Each matched skymask holds a latitude, longitude coordinate and heading angle to determine the precise location of the fisheye image. The results are then compared with the smartphone GNSS and advanced 3DMA GNSS positioning methods. The proposed method provides degree-level heading accuracy, and improved positioning accuracy similar to other advanced 3DMA GNSS positioning methods in a rich urban environment. Full article

(This article belongs to the Special Issue GNSS Data Processing and Navigation in Challenging Environments)

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