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High-precision Low Earth Orbit (LEO) satellite clocks are essential for LEO-augmented Positioning, Navigation, and Timing (PNT) services. Nowadays, high-precision LEO satellite clocks can be determined in real-time using a Kalman filter either onboard or on the ground, as long as the GNSS observations collected onboard LEO satellites can be transmitted to the ground in real-time. While various real-time and high-precision GNSS products are available nowadays in the latter case, their continuity and latencies in engineering reality are not as perfect as expected and will lead to unignorable impacts on the precision of the real-time LEO satellite clocks. In this study, based on real observations of Sentinel-3B, the impacts of different latencies and continuity of the real-time GNSS products on LEO real-time clocks are determined and discussed for two scenarios, namely the "epoch estimation" and "arc estimation" scenarios. The former case refers to the traditional filter-based processing epoch-by-epoch, and the latter case connects LEO satellite clocks from different rounds of filter-based processing under a certain arc length. The two scenarios lead to the "end-loss" and "mid-gap" situations. Latencies of the real-time GNSS products are discussed for the cases of orbit-only latency, clock-only latency, and combined forms, and different handling methods for the missing GNSS satellite clocks are discussed and compared. Results show that the real-time LEO satellite clock precision is very sensitive to the precision of real-time GNSS satellite clocks, and prediction of the latter becomes essential in case of their latencies. For the "end-loss" situation, with a latency of 30 to 120 s for the GNSS real-time clocks, the LEO satellite clock precision is reduced from about 0.2 to 0.28–0.57 ns. Waiting for the GNSS products in case of their short latencies and predicting the LEO satellite clocks instead could be a better option. For "arc-estimation", when the gap of GNSS real-time products increases from 5 to 60 min, the real-time LEO clock precision decreases from 0.26 to 0.32 ns. [ABSTRACT FROM AUTHOR]

Smartphone global navigation satellite system measurements in suburban environments suffer from severe multipath and noise, posing a challenge in dealing with the potential outliers. Using the range errors derived from smartphones, the characteristics of range errors can be further utilized to optimize the stochastic model and improve outlier diagnosis in user environments. Compared to existing algorithms, a new adaptive phase/code precision ratio scheme is proposed. Furthermore, a scale factor is utilized to deal with the over-optimization issue in the fitted weighting scheme functions, and adaptive a posteriori residual rejection method which considers the number of rejected measurements is introduced. The positioning improvements brought by the proposed algorithm are validated step by step. The proposed adaptive phase/code precision ratio can reduce the 68th and 95th horizontal positioning errors by 4 cm and 3 cm, respectively, compared to a ratio of 100. The percentage of horizontal positioning errors within 1 m can be improved by 7% (from 50 to 58%). Compared to the conventional carrier-to-noise (C/N0)-based weighting scheme, the proposed weighting scheme can reduce the 68th and 95th percentile horizontal positioning errors by 14 cm and 15 cm, representing improvements of 11% and 8%, respectively. Applying the new residual rejection method, the percentage of positioning errors within 1 m improves from 42 to 54% compared to C/N0-based weighting scheme. The results demonstrate great potential for achieving overall sub-meter level horizontal positioning performance with smartphones in suburban environments. [ABSTRACT FROM AUTHOR]

High received signal noise and limited multipath suppression capabilities cannot be neglected when it comes to smartphone-grade GNSS receivers and antennas and, along with frequent carrier-phase measurement discontinuities and losses, pose a challenge for advanced GNSS positioning techniques. To effectively utilize all satellite measurements in the absence of phase measurements, we proposed a pseudorange-only measurement enhanced PPP method with single- and dual-frequency combinations. In other words, the enhanced PPP utilizes the satellites with pseudorange-only observations that are typically excluded in traditional PPP processing with precise corrections. Validated with ten vehicle tests under different driving environments, the results show that application of the enhanced PPP approach surpasses the traditional PPP strategy for smartphone tracking through diverse obstruction and multipath profiles, and significant improvements of 64%, 23%, and 46% can be observed in the 95th percentile positioning error, 68th percentile positioning error, and overall root-mean-square (RMS) statistics, respectively. In addition, a new adaptive post-fit residual threshold is introduced to optimize the measurement quality control scheme. The results show that the solutions can be further improved with a 95th percentile positioning error of 1.8 m and overall RMS with 1.4 m for the horizontal component. These combined improvements further increase the utility of PPP processing in smartphone-based mobile positioning for mass-market applications. [ABSTRACT FROM AUTHOR]

Terrestrial radio navigation systems are an important data source for increasing the integrity of position, navigation, and timing information and for strengthening the immunity to the spoofing and jamming of satellite-based systems. A possible solution for maritime use is the medium frequency R-Mode. Here, the first real results were presented in the Baltic Sea within the R-Mode Baltic project. For the development of receivers and future signal evolution of R-Mode, there is a great need to provide simulated signals to the receiver hardware. For our laboratory work, we developed different hardware in the loop test systems which enable the simulation of each of the three components and the entire medium frequency R-Mode signal. With this setup, we are able to conduct reproducible tests of the R-Mode receiver's ranging and positioning performance without the necessity of field tests. Furthermore, the impact of R-Mode signal modifications can be initially analyzed without the need of an implementation in the real-world test bed. Firstly, we describe the usage of arbitrary wave generators that can be used to replay received or simulated signals. Due to their wide distribution in electronic laboratories, there are cost-efficient ways to build up test capabilities. For this work, we tested the Tektronix AFG 3022 and the Rigol DG1032. For further tests, we utilize software-defined radios that are capable of streaming continuous signals. We utilize the ETTUS N210 to directly output the simulated signals. Additionally, we test the LimeSDR with an external down-converter. To generate these signals, we utilize software packages that were created to support the development of the digital signal processing. This approach allows us to test our receiver with a continuous integration from pure software to hardware in the loop test. A comprehensive summary gives an overview of the pros and cons for the different suggested systems. [ABSTRACT FROM AUTHOR]

Graph optimization (GO) can correlate more historical information to increase the resistance against the GNSS outliers. Therefore, GO has the potential to obtain a higher accuracy and robust position in urban canyon. Here, we develop POSGO (POSition based on Graph Optimization), an open-source software designed for single-point positioning and relative positioning with multi-GNSS pseudorange in a GO framework. It is coded in C/C+ + language and recommended to run in the Linux environment. It can be easily extended to process the carrier phase and fuse the data from multiple sensors by adding corresponding graph factors. To assess the performance of the current version, data from a kinematic vehicle experiment in urban area are processed. The results indicate GO has better accuracy and robustness than classic least squares or Kalman filters, particularly in areas with severe occlusions. [ABSTRACT FROM AUTHOR]

Real−time single−frequency precise point positioning (PPP) is a promising low−cost technique for achieving high−precision navigation with sub−meter or centimeter−level accuracy. However, its effectiveness depends heavily on the availability and quality of the real−time ionospheric state estimations required for correcting the delay in global navigation satellite system (GNSS) signals. In this study, the dynamic mode decomposition (DMD) model is used with global ionospheric vertical total electron content (vTEC) RMS maps to construct 24 h global ionospheric vTEC RMS map forecasts. These forecasts are assimilated with C1P forecast products, and L1 single−frequency positioning solutions are compared with different ionospheric correction models. The study examines the impact of assimilating predicted RMS data and evaluates the presented approach's practicality in utilizing the IGRG product. The results show that the IGSG RMS prediction−based model improves positioning accuracy up to five hours ahead and achieves comparable results to other models, making it a promising technique for obtaining high−precision navigation. [ABSTRACT FROM AUTHOR]

As the Global Navigation Satellite System (GNSS) is widely used in all walks of life, the signal structure of satellite navigation is open, and the vulnerability to spoofing attacks is also becoming increasingly prominent, which will seriously affect the credibility of navigation, positioning, and timing (PNT) services. Satellite navigation signal authentication technology is an emerging technical means of improving civil signal anti-spoofing on the satellite navigation system side, and it is also an important development direction and research focus of the GNSS. China plans to carry out the design and development of the next-generation Beidou navigation satellite system (BDS), and one of its core goals is to provide more secure and credible PNT services. This paper first expounds on the principles and technical architecture of satellite navigation signal authentication, then clarifies the development history of satellite navigation signal authentication, and finally proposes the BDS authentication service system architecture. It will provide technical support for the construction and development of the follow-up Beidou authentication service. [ABSTRACT FROM AUTHOR]

In an urbanized environment, a superior baseband algorithm fundamentally enhances global navigation satellite system (GNSS) receivers by resisting interferences such as reflection leading to multipath signals and abnormal dynamics caused by user's motion or non-line-of-sight (NLOS) signals. A carrier aiding algorithm is frequently adopted to remove most dynamic errors on local code signals. Depending on this idea, all-channel spatial information based on the user's velocity is also exploited by existing vector tracking techniques to aid code tracking. As known, a carrier phase is one order of magnitude less noisy and more capable of mitigating multipath than a code phase. The traditional carrier frequency aiding improves the precision of the code phase estimation by suppressing random noise, but it is powerless in eliminating the biased error. To solve this issue, a real-time kinematic (RTK) solution, which is the product of the carrier phase, is proposed to remove the absolute biased error in the vector delay/frequency lock loop (VDFLL). Thus, the GNSS baseband can be optimized with both the carrier frequency and the absolute carrier phase aiding. Two stationary real-world experiments are conducted in open-sky and light-urban situations to verify the proposed GNSS software-defined radio (SDR) using collected GPS L1 C/A intermediate frequency (IF) data. The testing results demonstrate that the proposed SDR improves the pseudorange quality and positioning accuracy by a maximum of 48.5% and 42.5%, respectively. [ABSTRACT FROM AUTHOR]

The ARMAS-SOM framework fuses collaborative all-source sensor information in a resilient manner with fault detection, exclusion, and integrity solutions recognizable to a GNSS user. This framework uses a multifilter residual monitoring approach for fault detection and exclusion, which is augmented with an additional “observability” extended Kalman filter sublayer for resilience. We monitor the a posteriori state covariances in this sublayer to provide intrinsic awareness when navigation state observability assumptions required for integrity are in danger. The framework leverages this to selectively augment with offboard information and preserve resilience. By maintaining split parallel collaborative and proprioceptive instances and employing the “stingy collaboration” technique, we are able to maximize efficient use of network resources, limit the propagation of unknown corruption to a single donor, and maintain consistent collaborative navigation without fear of double-counting in a scalable processing footprint. Last, we preserve the ability to return to autonomy and are able to use the same intrinsic awareness to notify the user when it is safe to do so. [ABSTRACT FROM AUTHOR]

The mega-launch of low Earth orbit satellites (LEOs) represents a critical opportunity to integrate navigation and communication (NavCom), but first, challenges related to signal design must be overcome. This article proposes a novel signal scheme named CE-OFDM-PM. Via research on the in-band or adjacent band, it was found that the proposed signal scheme was suitable for S-band and had a wide normalized power spectrum density (PSD), high peak-to-side lobe ratio (PSR), and multiple peaks in autocorrelation. In an analysis of the simulation performance evaluation in navigation and communication, it is found that the proposed signal scheme has the potential for high accuracy, a code tracking accuracy of up to 0.85 m, a small mutual influence between the proposed signal scheme and other schemes, excellent anti-interference properties, and a better performance at both short and long distances in terms of its anti-multipath capability. Furthermore, the proposed signal scheme shows the ability to communicate between satellites and the ground and is outstanding in terms of its bit error rate (BER), CNR, and energy per bit to noise power spectral density ratio (Eb/N0). From the technical, theoretical, and application perspectives, our proposed signal scheme has potential as an alternative scheme in future BDS, PNTs, and even 5G/B5G. [ABSTRACT FROM AUTHOR]