Autonomous Vehicles have become the focus of research around the world.Meanwhile, the continuous development of various emerging technologies in recent years has paved the way for the realization of fully autonomous driving.Furthermore, high-precision navigation and positioning systems are crucial to self-driving cars.For navigation and positioning technology of autonomous vehicles, Global Navigation Satellite System (GNSS)/Inertial Navigation System (INS) integrated navigation is of great significance to realizing high-precision navigation and positioning of autonomous vehicles.This paper researches and summarizes the development and current situation of GNSS, INS, and GNSS/INS integrated navigation technology.GNSS/INS integrated navigation algorithms are divided into four different models of combination methods: Ultra tightly Coupled, Tightly Coupled, Semi-tightly Coupled, and Loosely Coupled.Besides, the advantages and disadvantages of these four integrated navigation algorithms are compared and discussed.The characteristics of different GNSS/INS coupling models are summarized in the last chapter, and the potential research direction of GNSS/INS in the future is proposed. 

Research on GNSS INS & GNSS/INS Integrated Navigation Method for Autonomous Vehicles: A Survey

Conventional ambiguity resolution (AR) strategy of directly fixing all raw frequency ambiguities for uncombined precise point positioning (PPP) generally requires a long initial time to ensure fixing reliability. The rich signals from multi-frequency GNSS can provide new opportunities for rapid PPP AR. In this paper, based on a unified multi-frequency uncombined PPP model, the raw frequency ambiguities are linearly transformed to the between-satellite-single-differenced extra-wide-lane (EWL), wide-lane (WL), and narrow-lane (NL) ambiguities, and a cascading ambiguity resolution (CAR) method of fixing EWL/WL/NL ambiguities sequentially is proposed. Meanwhile, a partial ambiguity fixing (PAF) strategy with ambiguity subset adaptively selected based on the successively increased elevations is also adopted in each step to improve the fixing rate. Further, experiments with globally distributed stations are carried out to verify the algorithm. With the constraints of EWL/WL AR, the precision of NL ambiguity and its variance-covariance matrix can be effectively optimized, so the time to first fix (TTFF) is significantly shortened, and the ratio value is also improved to varying degrees. As for BDS-only solution, the average TTFF is shortened from 23.7 min to 10.9 min, with an improvement over 50%; for GPS/BDS, GPS/Galileo, and GPS/Galileo/BDS combined solutions, the TTFF is shortened from 14.3 min, 9.4 min, and 6.7 min to 8.1 min, 2.6 min, and 1.8 min, which are, respectively, shortened by 43.4%, 72.3%, and 73.1%. In general, the proposed CAR strategy can shorten the TTFF of multi-GNSS multi-frequency PPP to about 2 min. The performance of EWL/WL/NL ambiguity-fixed solutions is also analyzed. The NL solution is generally at centimeter-level accuracy over the entire period; however, limited by the atmosphere errors during the convergence stage, the EWL/WL solutions can only obtain decimeter-level accuracy, and the difference between them and NL solution gradually decreases with the continuous improvement of atmosphere accuracy. 

/pdf/multi-gnss-fast-precise-point-positioning-with-multi-2h7h7lfi.pdf

Multi-GNSS Fast Precise Point Positioning with Multi-Frequency Uncombined Model and Cascading Ambiguity Resolution

Conventional ambiguity resolution (AR) strategy of directly ﬁxing all raw frequency ambiguities for uncombined precise point positioning (PPP) generally requires a long initial time to ensure ﬁxing reliability. The rich signals from multi-frequency GNSS can provide new opportunities for rapid PPP AR. In this paper, based on a uniﬁed multi-frequency uncombined PPP model, the raw frequency ambiguities are linearly transformed to the between-satellite-single-diﬀerenced extra-wide-lane (EWL), wide-lane (WL), and narrow-lane (NL) ambiguities, and a cascading ambiguity resolution (CAR) method of ﬁxing EWL/WL/NL ambiguities sequentially is proposed. Meanwhile, a partial ambiguity ﬁxing (PAF) strategy with ambiguity subset adaptively selected based on the successively increased elevations is also adopted in each step to improve the ﬁxing rate. Further, experiments with globally distributed stations are carried out to verify the algorithm. With the constraints of EWL/WL AR, the precision of NL ambiguity and its variance-covariance matrix can be eﬀectively optimized, so the time to ﬁrst ﬁx (TTFF) is signiﬁcantly shortened, and the ratio value is also improved to varying degrees. As for BDS-only solution, the average TTFF is shortened from 23.7min to 10.9 min, with an improvement over 50%; for GPS/BDS, GPS/Galileo, and GPS/Galileo/BDS combined solutions, the TTFF is shortened from 14.3min, 9.4 min, and 6.7min to 8.1 min, 2.6min, and 1.8min, which are, respectively, shortened by 43.4%, 72.3%, and 73.1%. In general, the proposed CAR strategy can shorten the TTFF of multi-GNSS multi-frequency PPP to about 2 min. The performance of EWL/WL/NL ambiguity-ﬁxed solutions is also analyzed. The NL solution is generally at centimeter-level accuracy over the entire period; however, limited by the atmosphere errors during the convergence stage, the EWL/WL solutions can only obtain decimeter-level accuracy, and the diﬀerence between them and NL solution gradually decreases with the continuous improvement of atmosphere accuracy.

In order to take full advantages of the BDS-3 penta-frequency signals in the long-baseline RTK positioning, a long-baseline RTK positioning method based on the BDS-3 penta-frequency ionospheric-reduced (IR) combinations is proposed. First, the low-noise and weak-ionospheric delay characteristics of the multi-frequency combinations of BDS-3 is analyzed. Second, the multi-frequency extra-wide-lane (EWL)/ wide-lane (WL) combinations with long-wavelengths are constructed. Third, the fixed IR EWL combinations are used to constrain the IR WL, of which the ambiguities can be obtained in a single epoch. There is no need to consider the influence of ionospheric parameters in the third step. Compared with the estimated ionospheric model, the proposed method reduces the number of parameters by half, so it is suitable for the use of multi-frequency and multi-system real-time RTK. The results using real data show that stepwise fixed model of the IR EWL/WL combinations can realize long-baseline instantaneous decimeter-level positioning. 

Long-Baseline Single-Epoch RTK Positioning Method Based on BDS-3 Penta-Frequency Ionosphere-Reduced Combinations

At present, the Beidou-3 global satellite navigation system (BDS-3) has been fully completed, which can provide the broadcast of four-frequency signals (BIC, B1I, B3I and B2a). Compared with triple-frequency or dual-frequency signals, four-frequency signals can form more extra-wide-lane/wide-lane (EWL/WL) combinations, which helps to the rapid resolution of ambiguity and the improvement of positioning performance. However, most of the current research on four frequency signals focuses on baseline solutions, and research in dynamic environments is not yet available. Therefore, the single epoch positioning performance of the BDS-3 four-frequency EWL/WL observation in the dynamic environment is studied in this paper. First, the BDS-3 four-frequency linear combination observation equation is given, the optimal linear combination is selected through certain criteria, and four linear combination schemes are analyzed. Then, the stepwise ambiguity fixing method of EWL/WL is used to obtain stable WL observation for positioning. Finally, the influence of the four linear combination schemes on the single-epoch dynamic positioning performance of the BDS-3 four-frequency EWL/WL observation are analyzed through vehicular experiment. The experimental results show that the dynamic positioning accuracy of schemes 1 and 3 are basically the same in the horizontal and vertical directions, both reaching the decimeter level. Therefore, the EWL/WL combination of scheme 1 and scheme 3 is better for dynamic positioning. 

Performance Analysis of BDS-3 Four-Frequency EWL/WL Single Epoch Dynamic Positioning

Quad-frequency signals have thus far been available for all satellites of BeiDou-3 and Galileo systems. The major benefit of quad-frequency signals is that more extra-wide-lane (EWL) combinations can be formed with quad-frequency than with triple- or dual-frequency, of which the ambiguities can be fixed instantaneously in medium and long baselines. In this paper, the long-baseline positioning algorithm based on optimal triple-frequency EWL/wide-lane (WL) combinations of BeiDou-3 and Galileo is proposed. First, the theoretical precision of multi-frequency combinations of BeiDou-3 and Galileo is studied, and EWL/WL combinations with a small noise amplitude factor and a small ionospheric scalar factor are selected. Then, geometry-free methods are used to estimate the a priori precision of EWL/second EWL/WL signals for different combination schemes. Second, the double-differenced (DD) geometry-based function models of quad-frequency configurations and three different triple-frequency configurations are given, and the DD ionospheric delays are estimated as unknown parameters. In the end, the real BeiDou-3 and Galileo data are used to evaluate the positioning preference. The results show that, when using fixed EWL observations to constrain WL ambiguities, the proposed triple-frequency EWL/WL signals composed of (B1I,B3I,B2a) of BeiDou-3 and (E1,E5a,E6) of Galileo can achieve the same precision as the quad-frequency signals. Therefore, the method proposed in this article can realize long-baseline instantaneous decimeter-level positioning while reducing the dimension of matrix and improving calculation efficiency.

Assessment of Quad-Frequency Long-Baseline Positioning with BeiDou-3 and Galileo Observations

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Assessment of Quad-Frequency Long-Baseline Positioning with BeiDou-3 and Galileo Observations