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A Global Navigation Satellite System (GNSS) receiver usually needs a sufficient number of full pseudorange measurements to obtain a position solution. However, it is time-consuming to acquire full pseudorange information from only the satellite broadcast signals due to the navigation data features of GNSS. In order to realize fast positioning during a cold or warm start in a GNSS receiver, the existing approaches require an initial estimation of position and time or require a number of computational steps to recover the full pseudorange information from fractional pseudoranges and then compute the position solution. The BeiDou Navigation Satellite System (BDS) has a unique constellation distribution and a fast navigation data rate for geostationary earth orbit (GEO) satellites. Taking advantage of these features, we propose a fast positioning technique for BDS receivers. It simultaneously processes the full and fractional pseudorange measurements from the BDS GEOs and non-GEOs, respectively, which is faster than processing all full measurements. This method resolves the position solution and recovers the full pseudoranges for non-GEOs simultaneously within 1 s theoretically and does not need an estimate of the initial position. Simulation and real data experiments confirm that the proposed technique completes fast positioning without a priori position and time estimation, and the positioning accuracy is identical with the conventional single-point positioning approach using full pseudorange measurements from all available satellites.

This article presents a two-step stochastic hybrid estimation (TS-SHE) algorithm for robust and accurate carrier phase tracking of global navigation satellite system (GNSS) signals in urban environments with significant power degradation and fluctuation. The proposed algorithm adaptively combines a bank of parallel Kalman filters (KFs) with different dynamic state models to cope with the nonstationarity of GNSS signals. Different from conventional approaches, including the phase lock loop (PLL), the KF, and the interacting multiple-model (IMM) method, which strongly rely on the a priori fixed signal model to ensure good tracking performance, we develop a novel stochastic filter transition strategy utilizing the information from reliable signal condition evaluation to overcome the performance degradation caused by the model mismatch in urban environments. Therein, for the first time, we use the prior information of signal fading conditions for more accurate combined weighting of all component filters. Specifically, to obtain the prior information, GNSS data are sequentially buffered and analyzed in the first step. Then, in the second step, optimized filter weights are generated, and the overall combined carrier phase is estimated. We analyze the theoretical performance of the new algorithm and show the effects of different design parameters on the performance. The primary advantages of the proposed algorithm include: 1) the ability to rapidly recover carrier phase observation when blocked signals are reacquired and 2) high accuracy in carrier phase estimation of GNSS signals corrupted by strong multipath fading. Simulation and real data experiment results show the enhanced robustness and improved accuracy of the proposed TS-SHE algorithm compared with conventional carrier phase tracking methods.

Ionospheric dispersion causes severe distortions and ranging performance degradations of wideband global navigation satellite system (GNSS) signals. To compensate the non-linear and asymmetrical group delay of the dispersive ionosphere, an all-pass filter in the form of cascaded biquad sections was proposed in our previous study. On this basis, this paper develops an optimal delay-equalization all-pass filter by using the trust-region algorithm to construct the desired group delay and improve the fitting accuracy. The simulation results of the AltBOC(15,10) signal performance demonstrate that the proposed design is more accurate for distortion compensation and thus more effective for ranging bias mitigation than the previous approach.

By forging global navigation satellite system (GNSS) signals similar to authentic ones, a spoofer can make receivers track forged signals (spoofing signals) and generate wrong position, velocity and time results. Receiver autonomous integrity monitoring (RAIM) can be extended to the field of spoofing detection and exclusion (SDE). However, it is well known that when there are six or more signals and only one spoofing signal among them, RAIM can effectively exclude the spoofing signal. In this study, based on maximum likelihood estimation (MLE) theory and the idea of the traverse, one-dimensional traversal MLE-RAIM (TMRAIM) is proposed, which can exclude multiple spoofing signals. Theoretically, the influence of spoofing biases will be reflected in pseudorange residuals, and then affect the probability distribution of the parity vector. Through MLE deduction, the authors can find corresponding spoofing signals which are relevant with the maximum probability of the parity vector only once under the supposed number of spoofing signals. By just traversing the number of spoofing signals, TMRAIM can run effectively on real-time GNSS receivers with low complexity. The SDE ability and time complexity are analysed in detail and two field experiments are constructed. Experimental results demonstrate the method is feasible and effective for anti-spoofing applications.

In this article, we propose a space-time double difference observation model in order to locate the Global Navigation Satellite System (GNSS) meaconer. At present, there are a few studies on the localization of the GNSS spoofer. Most of the methods require multiple receivers with known positions and synchronized clocks, which can be implemented by tracking authentic GNSS signals or other approaches. Furthermore, the positioning accuracy depends on the geometry of the receivers and the spoofer. In this article, we discuss the feasibility of locating the meaconer based on a single receiver and analyze its limiting conditions. In addition, we propose a new method to locate the meaconer by one receiver without knowing its position and without synchronizing its clock. The receiver here can be either stationary or mobile during the accumulation of observations, and the position dilution of precision is independent of the receiver location. Furthermore, the method only needs observations of spoofing signals to locate the meaconer. Therefore, even when authentic signals cannot be tracked by the receiver, the method still works. After spoofing signals are identified, we can locate the meaconer by the satellite transmit time and ephemeris of the tracked spoofing signals when the meaconer replays GNSS signals of more than one satellite with different or the same replay delays. Experiments verify that when the meaconer replays GNSS signals of four satellites, the estimate accuracy with the proposed method can be 200, 50, and 25 m for 2, 5, and 10 min of observations, respectively.

Diversity reception of multipath Global Navigation Satellte System (GNSS) signals offers a new insight into carrier phase-based high-precision positioning. The focus of this paper is to demonstrate the fading independence between space and frequency diversity GNSS signals. In harsh urban environments, multipath components arrive to the mobile receiver antenna with different phases and Doppler shifts, therefore giving rise to the discontinuity of code and Doppler observations and large tracking errors. In this paper, an empirical model of fading GNSS signals is constructed, including power fluctuations and spread metrics. Based on this model, real BeiDou Navigation Satellite System (BDS) signals from two GNSS dual-frequency antennas are characterized, at both information and signal level. The block processing algorithm is utilized for signal investigation. Results show that: (1) a high proportion of asynchronous loss-of-lock (around 16%) is experienced by observations of diversity signals; and (2) power fluctuations of fading signals are uncorrelated in frequency separated branches unconditionally, yet for space diversity signals the independency exists in dynamic fading channels only. The results above corroborate the significant potential gain of diversity reception, and could be further implemented in researches of diversity combined GNSS parameter estimation in dense fading conditions.

In many harsh environments where a global navigation satellite system (GNSS) is absent or satellite signals are blocked, pseudolite positioning systems (PLPSs) can provide stand-alone positioning service with high accuracy and flexible deployment of base stations. Nevertheless, since distances from users to pseudolites in PLPS are much closer than those to satellites in GNSS, PLPSs suffer severely from nonlinearity problems because linearization applied in traditional linearization-based positioning algorithms leads to errors in the mathematical model that impact the convergence of iterations and possibly significantly deviate positioning solutions from actual positions. To enhance the convergence performance of positioning algorithms under severe nonlinearity situations, we propose a projected cancellation (PC) technique that linearizes pseudoranges at the reference and user sites on a virtual site. Single-differenced pseudoranges effectively eliminate the nonlinear error terms. Therefore, the observation model is transformed into an approximately linear version, and the traditional positioning algorithms are more suitable to be employed. Based on this, we develop a fusion positioning algorithm that adaptively combines the projected cancellation technique with different linear positioning algorithms according to the severity of nonlinearity in order to achieve a positioning service with more rapid convergence and a higher convergence success rate. Both simulated and experimental results under various cases are obtained and compared with those of traditional algorithms. Numerical results show that the proposed algorithm improves the overall positioning accuracy by approximately 30% under severe nonlinearity situations without additional time cost compared with the existing algorithms. Moreover, it provides a significant reduction of about 48% in time cost, yielding equivalent accuracy as the Levenberg–Marquardt algorithm in normal applications fields. Real-world experiment results validate the applicability and superiority of the new method to solve the nonlinearity problem in practical applications.

Global Navigation Satellite System (GNSS) spoofing attacks and defensive countermeasures are developing rapidly. Aiming at misleading the estimation of a fake solution toward the victim receiver, spoofing attacks are likely to cause severe consequences. To this end, the deception is achieved by manipulating the information of pseudorange in most cases. Hence, the difference of pseudorange between spoofing and authentic signals can be an effective separation basis. In this paper, a new approach based on relative positioning is proposed to separate GNSS spoofing from authentic signals, which is capable of substituting the detection and classification investigated in conventional works. Monte Carlo simulations and outdoor experiments illustrate this point in the end.

With the wide use of mobile navigation, the number of GNSS portable terminals such as smartphones without any ability of spoofing defense is growing rapidly, which induces a critical threat to our daily lives. To cope with the challenge of spoofing attacks, in this paper, we are going to develop a prototype of an online spoofer localization system, using raw GNSS measurements captured from Android smartphones in the spoofing area. We assume that each smartphone receives both authentic signals and spoofing signals simultaneously. With spoofing signals discriminated, a normal PVT solution can be recovered with authentic signals and then TOA measurements of spoofing signals can be captured from the smartphone and form a log file. We select a combination of smartphones with a minimum number of users and a maximum positioning accuracy. Each smartphone selected for spoofer localization uploads its logged data file to the server. Synchronization of observations and iterative least square (LS) algorithm will be carried out online to solve the position of the spoofer. Some difficulties and challenges, such as smartphones geometry and time synchronization are discussed in this paper. A fundamental field experiment is conducted to verify the feasibility of our proposed scheme.

Time Synchronization Attack (TSA) is a typical kind of GNSS spoofing, which can deceive receivers with fake GNSS signals. Different from other kinds of spoofing, TSA only aims at timing receivers whose positions are always unchanging and easy to acquire. So that TSA signals may be more difficult to defend as more sophisticated spoofing is realizable when attackers get the static victim position. Therefore, specialized anti-TSA methods are necessary to maintain the safety of timing receivers. And among different kinds of TSA detections, those that can be applied in a single receiver and do not require any additional facilities are widely accepted for being easy to popularize. Hence, in this paper, we come up with such a TSA detection and discrimination method by proposing a tailored Signal Quality Monitor (SQM) method, the Multi-Channel Joint SQM. This method can detect the abnormal synchronicity of distortions of different correlation peaks in different receiving loops. And this abnormal synchronicity is proved to be inevitable for TSA as long as it wants to meet the basic constraints, providing correct positions and avoiding sudden timing jumps. Thus, by jointly monitoring different correlation peaks, the proposed method can greatly improve the detection performance against TSA and provide further spoofing discrimination ability. Besides, as a kind of SQM, the proposed method can work as soon as the spoofing begins to seize receiving channels, thereby alerting the victim promptly. And in this paper, these aforementioned anti-TSA abilities are verified on a self-designed real-world test-bed. From experiment results in five different TSA conditions, the proposed method is believed to provide reliable detection and discrimination when the spoofing power advantage is lower than 8dB, while higher spoofing power is too conspicuous for stealthy TSA. And the proposed method can be more practical by combining it with a simple signal power meter. Moreover, further experiments show that the proposed method outperforms off-the-shelf SQM methods in TSA conditions.

Pose information is of great importance for control and navigation. For outdoor applications, Global Navigation Satellite System (GNSS) is a preferable choice because of its ability to supply global consistent positioning results. GNSS has been utilized in a mass of applications such as aviation and autonomous driving. On the other hand, a GNSS receiver is prone to interferences like multipath or spoofing attack. Therefore, a GNSS measurement should pass the authentication before incorporating into the pose estimation. Recently graph optimization based SLAM (Simultaneous Localization and Mapping) system that is tightly coupled with GNSS measurements has been developed to achieve a better positioning performance. However, little existing GNSS fault detection method can be directly adapted to such scheme. In this article we propose an innovation based GNSS fault detector. The proposed detector exploits the prior information derived from the marginalization process of the graph optimization, and can detect abnormalities such as drifts or jumps in GNSS positioning results. Moreover, the theoretically performance of the detector is evaluated in terms of the protection level, and several experiments over public datasets are conducted for performance validation.

This study presents a Rao-Blackwellised particle filter (RBPF)-based encoder/inertial navigation system (INS)/global navigation satellite system (GNSS) integration method for improving the navigation performance of an autonomous land vehicle (ALV) with wheel slipping. In contrast to traditional integration methods, the proposed integration method introduces an overall wheel slip consideration for the ALV, which greatly improves the accuracy of the velocity estimation, especially when the inertial sensor is low cost. Additionally, the proposed integrated system uses double-difference pseudorange measurements instead of single point positioning results provided by low-cost GNSS receivers, which greatly improves the accuracy of the position estimation. To verify the navigation performance of the proposed integrated system, comparisons between the states estimated by the proposed system, the EKF-based integrated system and the joint wheel-slip and motion-estimation system are provided. The results of the experiment show that the proposed integrated system has the highest accuracy in both the position estimation and the velocity estimation among the three compared systems, and can improve the navigation performance during GNSS signals outages.

As an effective deceptive interference technique for military navigation signals, meaconing can be divided into two main types: those that replay directly and those that replay after signal separation. The latter can add different delays to each satellite signal and mislead the victim receiver with respect to any designated position, thus has better controllability and concealment capability. A previous study showed there to be two main spatial processing techniques for separating military signals, whereby either multiple large-caliber antennas or antenna arrays are used to form multiple beams that align with all visible satellites. To ensure sufficient spatial resolution, the main lobe width of the antenna or beam must be sufficiently narrow, which requires the use of a large antenna aperture or a large number of array elements. In this paper, we propose a convenient and effective signal separation method, which is based on an antenna array with fewer elements. While the beam of the array is pointing to a specified satellite, the other satellite signals are regarded as interference and their power is suppressed to a level below the receiver's sensitivity. With this method, the number of array elements depends only on the number of visible satellites, thus greatly reducing the hardware cost and required processing capacity.

Spoofing attacks for Global navigation satellite system (GNSS) would cause the abnormal changes of receiver measurements. Monitoring the abnormal changes is the main pattern of anti-spoofing techniques. Anomalies of Doppler shifts are important for GNSS spoofing detection. Most researchers focus on the consistency of Doppler such as the consistency of carrier Doppler and code rate. But the method is powerless in some cases. Doppler shift measurement is proportional to change rate of pseudorange, and the relationship would be broken in the case of spoofing attack. As both Doppler measurements and pseudoranges are related to velocity of receiver, two approaches of computing velocity can be used to check the consistency of Doppler shifts and pseudoranges measurements. An efficient spoofing detection method based on the consistency check of velocities is proposed in the paper. Principle of the method is given in detail and its performance evaluations are provided as well. Simulation results demonstrate the effectiveness of the solution.

A coordinate system is essential for the positioning and navigation services of the ground-based positioning system. The coordinate establishment requires accurate and sufficient position information of base stations. In global navigation satellite system (GNSS)-denied environments, it is challenging to determine the locations of base stations using GNSS services with GNSS signals being interfered and blocked. In this case, manual surveying constraints rapid deployment of the ground-based positioning system. By receiving ranging signals from anchors whose absolute location is known in advance, the locations of all base stations can be determined using iterative trilateration, which however causes cascading error accumulation for large networks. In this paper, using ranging signals emitted by base stations to obtain mutual distance measurements, we propose a coordinate establishment algorithm to address this problem. Introducing the Euclidean distance matrix (EDM), the problem is formulated as an EDM least squares optimization problem with the anchor information incorporated. The optimal solution is obtained via the proposed algorithm based on the shortest path algorithm and the nonlinear conjugate gradient method. Simulations and experimental results reveal that the proposed algorithm outperforms existing methods in the aspects of positioning accuracy and computational complexity in most cases.

Micro-electro-mechanical system (MEMS)-based inertial measurement unit (IMU) together with single-frequency Global Navigation Satellite System (GNSS) receiver provides a cost-efficient solution for outdoor navigation. An accurate initial alignment is necessary to the GNSS/IMU integrated navigation system to guarantee a good performance. Existing in-motion alignment methods mostly rely on the velocity derived from the GNSS receiver, which is hard to acquire when the receiver suffers from temporary signal blockage or deliberate interference. To mitigate the problem, we propose a carrier Doppler-based initial alignment method, which extends the availability of attitude estimation to cases where only three satellites are available. The proposed method is composed of two steps: the first step estimates a coarse initial attitude without any prior knowledge, and the second step dynamically refines the attitude estimation. Field experiment results show that the proposed method can provide attitude estimation with enough accuracy when only three satellites are available and has equivalent performance compared with the traditional optimization-based-alignment (OBA) method under general cases. The implementation of the proposed method is available on https://github.com/Wiggrim/D2R .

Global navigation satellite systems (GNSSs) usually adopt constant-envelope multiplexing (CEM) techniques to deal with the nonlinear characteristic of high-power amplifiers (HPAs) on satellites. However, almost all the existing navigation signal CEM techniques assume that the bandwidth of the transmitter is large enough when compared to that of navigation signals, which often contradicts reality, especially for the multiplexing of wideband navigation signals such as the signals in the Galileo E5 and BDS-3 B2 bands. Consequently, the constant-envelope characteristic deteriorates severely when the composite signal passes through the filter, which leads to nonlinear distortion and low amplification efficiency. Even if the nonlinear distortion can be mitigated by using amplifier linearization techniques, the actual transmitted signal power still suffers from a high peak-to-average power ratio (PAPR) of the composite signal. In order to address this problem, we present a new band-limited, dual-frequency multiplexing technique. The main premise of this method is to consider the impact of the filter on the navigation signals at first and then introduce the band-limited auxiliary signals to reduce the peak power of the composite signal as much as possible, thereby increasing the actual transmission power of the navigation signals. Simulation results demonstrate that the proposed method has lower PAPR and higher amplification efficiency than existing methods. In addition, implementation of the proposed multiplexing technique is still based on a look-up table, which has little change compared with previous CEM methods, and the complexity of signal generation is affordable.

Spoofing interference is an emerging threat to Global Navigation Satellite System (GNSS) receivers. Recent studies have shown that for GNSS receivers with vector tracking structure, both the overlap of correlation peaks and the inconsistency of received signals caused by spoofing interference can affect the coherent integration results. Therefore, signal quality monitoring (SQM) technology has a broader application range for spoofing detection in the vector tracking structure. However, since the inconsistency of received signals can influence all signal channels, the commonly used SQM methods cannot be used for spoofing identification. This article proposes the corresponding SQM spoofing identification methods for two spoofing scenarios with and without overlapping correlation peaks. Specifically, for a signal channel in the first spoofing scenario, the influence degree of the inconsistency of received signals on the coherent integration results of the early (E), prompt (P), and late (E) correlators is the same. By contrast, the overlap of correlation peaks does not satisfy this relationship. According to this difference, the corresponding method can identify the overlap of correlation peaks in the signal channel. For the second spoofing scenario, the corresponding method uses the pseudo-code phase differences estimated from the coherent integration results to distinguish the spoofing signals from the authentic signals. Furthermore, a series of simulations are conducted to prove the feasibility of the proposed two spoofing identification methods.

Spoofing attacks are severe threat to Global navigation satellite system (GNSS). Using the propagation characteristic of spoofing signals, a lot of methods have been put forward to detect and discriminate spoofing. In this paper, we propose a double difference method using Doppler shift measurements of independent receivers, which does not require time synchronization and can be easily integrated on commercial off-the-shelf (COTS) receivers. We derive the theoretical model of Doppler frequency double difference (DFDD) in detail, and analyse the detection performance. Numerical simulations and field experiments are conducted to verify the feasibility of the proposed method.

With the continuous construction and improvement of global navigation satellite systems (GNSSs), the number of navigation signals broadcast by GNSSs has increased significantly, and the spectrum resources are becoming increasingly scarce. In order to solve the contradiction between limited spectrum resources and the growing demand for positioning, navigation and timing services, some scholars have proposed a multicarrier modulation scheme which multiplexes narrow-band signals in several adjacent spectrum slots into a composite wide-band signal. On the one hand, multicarrier modulation signals can make full use of the spectrum resources, and on the other hand such signals can provide diversified processing strategies. Based on the idea of cognitive receivers and the inherent characteristics of multicarrier signals, we propose two different unambiguous tracking techniques named energy aggregation method and multi-dimensional loop tracking method respectively. In the first method, the energy of different signal components is aggregated, which can improve the positioning performance while maintaining the unimodal characteristic of the correlation function. In the second method, the potential high-precision ranging performance of high-frequency subcarriers in the multicarrier signal is further exploited. By introducing additional subcarrier loop, the one-dimensional correlation function is extended to two-dimensional. In the dimension of subcarrier, the correlation function is sharper and still has the characteristic of multiple correlation peaks, but in the dimension of pseudo-random noise (PRN) code, the correlation function presents the unimodal characteristic, so it can assist the subcarrier loop to eliminate ambiguity. The proposed tracking methods can be embedded in the architecture of cognitive receivers, where GNSS receivers can flexibly configure the receiving parameters according to the internal and external environment, and select proper receiving strategy. For example, in scenarios where the requirement in positioning precision is moderate, the system could select single-component receiving and processing strategy. In scenarios with high requirement for positioning performance, the system will choose multicomponent receiving mode. Simulation results show that in the above two methods, multicomponent wideband reception can improve the ranging accuracy successfully. Besides, the ranging accuracy of the second method is much better than that of the first method.

The integrity of GNSS characterizes the ability to alert users in time when reliable services could not be provided as expected. BeiDou Satellite-Based Augmentation System (BDSBAS) is an important component of the generalized BeiDou Navigation Satellite System (BDS), and would provide Dual-Frequency Multi-Constellation (DFMC) augmented services for life-safety applications. Thus, users could be better protected against integrity risks with the first-order ionospheric error, which is the largest ranging uncertainty, removed and observation redundancy provided. The potential evil waveform (EWF) existing in iono-free combined signals is an important threat affecting the integrity performance. It is urgent to particularly and systematically design the signal quality monitors for DFMC SBAS to monitor the potential imperfections or failures on navigation signals. For BDSBAS, a hybrid Signal Quality Monitoring (SQM) algorithm based on both chip domain observables (CDO) and multi-correlator observables (MCO) is proposed in this paper on the basis of modulation characteristics of B1C and B2a signals, thus improves detection performances without significantly raising complexity in implementation. Configurations and procedures of the proposed hybrid SQM algorithm are expounded and detailed evaluation schemes are designed in this paper. In addition, the proposed algorithms are simulated and evaluated by applying the Threat Model and Threat Spaces of both signals lately adopted by ICAO. The results show that the DF civil signals of BDS could be well guaranteed to meet the integrity monitoring requirements of DFMC SBAS by this hybrid SQM algorithm.

GNSS array antenna have been widely used for single-tone, narrow-band, wide-band interference mitigation in satellite navigation application, however, this induces biases to code phase and carrier phase measurement, which will restrict its usage in high-precision application. Biases could be compensated based on calibration of array pattern in Microwave Anechoic Chamber, which is complicated and could not handle platform influence. Towards this issue, this paper proposed a self-supervised calibration method of array antenna phase center for high-precision GNSS application cooperated with inertial technology. Firstly, high-precision GNSS receiver is designed for amplitude and carrier phase measurement of multi-antennas simultaneously. Secondly, we use a single axis servo turntable to speed up the calibration process by rotation scheme optimization, satellite direction vectors are projected to array coordinate system based on platform attitude measured by an inertial north finder. Finally, data smoothing and local interpolation are utilized for measurement pattern fitting. Open-sky test is carried out, thus validating the effectiveness of this method.

Signal fading is the main disturbing phenomenon for Global Navigation Satellite System (GNSS) receivers in urban environment, which induces frequent loss of carrier lock in GNSS phase tracking loops. By taking advantage of the prevalent configuration of multi-frequency reception in emerging GNSS receivers, this paper proposes a Frequency Diversity combined GNSS carrier Tracking Loop (FDTL) for robust and centimeter-level positioning under harsh propagation conditions. Specifically, a bank of correlators is used to estimate the signal parameters of interest with a Maximum Likelihood Estimation (MLE) discriminator to avoid the need for a separate acquisition which normally requires the most processing time in GNSS receivers. The Doppler relationship between multifrequency signals offers an inherent constraint which is used to improve accuracy of MLE measurements. In the loop tuning process, a frequency diversity combined Adaptive Kalman Filter (AKF) is implemented for system dynamic propagation and measurement update. The primary advantages of the proposed FDTL algorithm include: 1) significantly improved carrier tracking robustness compared with traditional Phase Lock Loops (PLLs); 2) enhanced sensitivity obtained from the diversity combination. The effectiveness of the algorithm is demonstrated by simulated and real GNSS fading signals.

For high-precision positioning applications in large-scale urban environment, since the performance of traditional stand-alone GNSS-based RTK degrades greatly due to the signal blockage and multipath effect, multi-sensor integration systems become the most popular solutions most of which require to build LiDAR maps before positioning. However, not all the applications are suited to this scheme. For the applications that cannot build prior maps, if only odometry-based sensors are used, the issue of accumulative error will arise in large-scale area. Therefore, we still hope to use RTK in multi-sensor integration to maintain the global accuracies of positioning, as well as mapping, and as a matter of course, the aim of this paper is to improve RTK performance in complicated environments by sensor integration. With this purpose, we propose a feature-based RTK/LiDAR/INS integrated system which is designed to utilize the measurements of the repeatedly observed LiDAR features to support RTK, specifically, to improve RTK fix rate. In detail, we adopt tight coupling scheme, after initializing the LiDAR feature map with RTK, we can introduce the measurements of the LiDAR feature with previously obtained global parameters to RTK to estimate a more accurate search center for ambiguities in GNSS-challenged environments, and thus, to improve the success rate of RTK ambiguity resolution. Then, the RTK result derived at each epoch can be used to update the global parameters of the new features, which forms a mode of positioning while mapping. Both theoretical analyses and simulation experiment results prove that integrating with LiDAR feature measurements can improve the performance of RTK in urban environments.

Due to the open signal structure and low power of Global Navigation Satellite System (GNSS) civil signals, GNSS receivers are vulnerable to spoofing attacks that will make the victim receiver get the wrong position, velocity, and time (PVT) results. Many anti-spoofing methods are proposed but mainly focus on spoofing detection. It is relatively difficult to identify and exclude all spoofing signals, but also more meaningful for getting the precise positioning results. In the paper, we propose a spoofing exclusion method based on density clustering by using the known rough user position. The rough position is relatively easy to get, such as through cellular network, inertial measurement unit (IMU), Loran-C, etc. We detect spoofing by comparing the difference between the rough position and the positioning result of the victim receiver. If spoofing signals exist, pseudorange-distance biases are computed by the rough position, the satellite positions, and the measured pseudoranges. Further, the density clustering algorithm for pseudorange-distance biases is used to exclude spoofing signals and recover the precise positioning results of the receiver. The detailed analyses of the factors affecting spoofing exclusion are carried out, and the relevant experimental results prove the effectiveness of the proposed method.

The Global Navigation Satellite System (GNSS) serves people as an essential precise time synchronization source. However, to provide public services on a global scale, GNSS civil signals need to have open structures. And their power is much lower than thermal noises at the receiver end. These make spoofing a major threat and GNSS timing users are easy to be deceived by a kind of spoofing known as the Time Synchronization Attack (TSA). The real-world damage of TSA is proved by researchers and demands of TSA defense methods emerge. Typical anti-spoofing methods can bring some degrees of security, but they mainly focus on physical parameters that do not affect timing solutions. So attackers always have possibilities to cheat these methods. A more robust way is to directly monitor key timing results, like clock biases and clock drifts. Researchers have come up with several methods of this kind, but their requirements of complex computations and incessant mitigations may bring difficulties to actual usages. To complement these deficiencies, this paper proposes a TSA detection and discrimination method based on the abnormal similarity between signals from different satellites. Specifically, we calculate clock drifts from each satellite signal separately and monitor the correlation values of different clock drifts time-differenced sequences. As TSA signals need to avoid sudden changes in timing results and keep actual position solutions, spoofing is ought to maintain a same changing rate of pseudorange differences among all spoofing-authentic signal pairs. This additional similarity will greatly enlarge the correlation value, thus, TSA is revealed. After detected and discriminated by the proposed method, spoofing affects will be much easier to erase. We achieve this method by monitoring timing results only and requiring few complex computations, so it will be a sufficient complement to off-the-shelf real-world TSA defenses. Our motivations and method effects are verified on an open test-bed.

Compared with the traditional GPS L1 C/A signal, the existing and emerging new-generation wideband GNSS signals suffer more severe distortions due to the nonlinear characteristics of a high-power amplifier (HPA) in the satellite transmitter. HPA nonlinearity can lead to in-band signal distortion and out-of-band spectral regrowth, which degrade the ranging performance of a GNSS receiver. Researchers have reported power loss, correlation peak asymmetries, lock point bias as well as carrier tracking error caused by the HPA nonlinearity. However, few pre-distortion approaches that compensate for the HPA nonlinear effects have been presented in the GNSS field. We first take BDS-3 B2 asymmetric constant envelope binary offset carrier signal as an example to present the HPA nonlinear effects on wideband GNSS signals. Then, the design principle of a predistorter is investigated, and a Hammerstein predistorter is proposed to eliminate the nonlinear HPA with memory. Finally, we compare the constellation diagram, power spectrum, correlation function, code tracking bias with and without the Hammerstein predistorter. The comparison results demonstrate that a digital predistorter is a potential solution to compensate the HPA nonlinear distortions for wideband GNSS signals. This pre-distortion approach is of great significance that can effectively guarantee the nominal signal quality in space not only during the maintenance of the current GNSSs but also for the future emerging local or global navigation systems.

The optimized design of the spreading modulation technique is considered to be the major way to achieve both spectrum compatibility and performance improvement. To allow a variety of signals to share the limited frequency band of GNSS and further improve the signals’ ranging accuracy and anti-interference performance, several new spreading modulation techniques have been proposed in recent years. In this chapter, we will introduce a variety of typical spreading modulation techniques such as BPSK with rectangular chips (BPSK-R), binary offset carrier (BOC), binary coded symbol (BCS), composite BCS (CBCS), time-multiplexed BOC (TMBOC), composite BOC (CBOC), quadrature multiplexed BOC (QMBOC), as well as alternative BOC (AltBOC). The processing ambiguity threat of BOC and MBOC signals and possible solutions are also discussed. Considering that in the next-generation GNSS implementations, new requirements and new constraints will constantly emerge and new modulation methods will be needed, this chapter is not meant to be regarded as a review of the development of navigation signal modulation technology, or an interpretation of the signal formats being used by several major satellite navigation systems. Instead, we regard the spreading modulations used in satellite navigation signals as general techniques and focus our discussion on the principles of these modulation techniques and the design ideas behind their generation processes.

This chapter takes a close look at how we can enable future development by implementing excellent signal designs with higher adaptability and flexibility. This chapter can also be regarded as a comprehensive example of how to apply the theory and techniques provided in this book to future signal design. In this chapter, we will see that under the conventional idea of separately optimizing carrier frequency, spreading modulation, and multiplexing, a Gordian knot is emerging in future GNSS signal design. In order to get out of the cycle of contradictions among measurement accuracy, services variety, RF compatibility, as well as multiplexing efficiency, the concept of using multicarrier modulation and multiplexing joint design is proposed. Performance analysis with some typical case studies demonstrates that the proposed multicarrier constant-envelope composite (MCC) signal provides a promising solution for the next generation GNSS signal design.

Theoretical evaluation is an objective and convenient method to acknowledge the performance of a signal. In this context, since the obtained performance is largely associated with the processing techniques implemented by receivers, the theoretical evaluation of a signal should also comprehensively consider the receiving strategies. In order to provide better performance for a wide range of users, the performance under traditional matched receiving, as well as under the various other receiving strategies, should be considered and optimized, where the theoretical evaluation criteria are treated as the guidance and objective for the signal design process. With respect to signal receiving, theoretical evaluation is a convenient method to compare the performance under various receiving strategies, thus providing useful information for choosing appropriate implementations for receiver manufacturers. Accordingly, the theoretical evaluation of signal performance becomes an essential topic for both the signal design and receiving in GNSS. In this chapter, we will discuss the theory and methods of navigation signal performance analysis based on time and frequency-domain characteristics of the spreading modulation waveform. A set of evaluation methods that both supports the various local receiving strategies and decouples from unrelated factors is provided. Specifically, the mismatch of the code chip waveform of the local despreading signal is considered. These methods have provided direct guidance for the spreading modulation optimization and selection of the next-generation satellite navigation signals.

In order to utilize the frequent sign altering in the spreading codes to measure the distance accurately, and to achieve better performance of multiple access, multipath resistance, and anti-interference, direct sequence spread spectrum (DSSS) technology is used in satellite navigation signals. With different spreading chips, the time domain waveform and power spectrum shape of the signal will exhibit significant differences, which will affect many key performances. Therefore, the spreading chip waveform becomes the key factor in determining the intrinsic performance of the satellite navigation signal, and the spreading modulation design has become the core technology of the GNSS signal design. From this chapter, we will start our journey on the spreading modulation design of satellite navigation systems. In this chapter, we will discuss some basic concepts and properties of direct sequence spread-spectrum technology, illustrating the mathematical models of spreading modulated signals and their important characteristics in time and frequency domains. These characteristics will be used frequently in the following chapters.

In the absence of GNSS or when signals from satellites are blocked in harsh environments, a ground-based positioning system can be used to estimate the position of users and receivers. Nevertheless, ground-based systems suffer dramatic nonlinear error resulting from the linearization used in typical positioning algorithms. Robust positioning algorithms that are capable of handling strong nonlinearity cases are therefore of great value. In this paper, we propose an algorithm termed promoted iterative least-squares based on nonlinear-compensation (PILSBON) to effectively alleviate the influence of nonlinear effects. This algorithm is based on the accurate expression of the nonlinear error terms in the double-differenced pseudorange measurement model. In order to eliminate the effects of nonlinearity for targeted solutions, the PILSBON uses iterative numerical estimation to compensate for nonlinear error so that the nonlinear position determination is transformed into a linear model, which can then be estimated with a linear estimation algorithm. In this paper, we analyze the properties of the PILSBON and compare it to conventional solutions. Because of its specific strategy for nonlinearity, our results show that the PILSBON improves the overall accuracy by approximately 30% compared with conventional solutions according to statistic RMSE data from over 500 experiments and positions. Moreover, by deploying a practical ground-based positioning system with six transmitters on the rooftop of our laboratory building, we demonstrate that the PILSBON algorithm can be efficiently employed in real-world experiments.

This paper describes a robust integrated positioning method to provide ground vehicles in urban environments with accurate and reliable localization results. The localization problem is formulated as a maximum a posteriori probability estimation and solved using graph optimization instead of Bayesian filter. Graph optimization exploits the inherent sparsity of the observation process to satisfy the real-time requirement and only updates the incremental portion of the variables with each new incoming measurement. Unlike the Extended Kalman Filter (EKF) in a typical tightly coupled Global Navigation Satellite System/Inertial Navigation System (GNSS/INS) integrated system, optimization iterates the solution for the entire trajectory. Thus, previous INS measurements may provide redundant motion constraints for satellite fault detection. With the help of data redundancy, we add a new variable that presents reliability of GNSS measurement to the original state vector for adjusting the weight of corresponding pseudorange residual and exclude faulty measurements. The proposed method is demonstrated on datasets with artificial noise, simulating a moving vehicle equipped with GNSS receiver and inertial measurement unit. Compared with the solutions obtained by the EKF with innovation filtering, the new reliability factor can indicate the satellite faults effectively and provide successful positioning despite contaminated observations.

Global Navigation Satellite System (GNSS) safety issues of have been of concern for some time. Spoofing attacks have received much attention as they can be difficult to detect and have the potential to cause disruption at best and major damage in extremis. To mitigate such threats, a spoofing detection method based on the consistency check of Doppler positioning fixes and pseudorange positioning fixes is proposed. The primary contributions of this paper include establishing a Generalised Likelihood Ratio Test (GLRT)-based statistical detection model for the introduced spoofing detection method and efficiently improving the accuracy of the Doppler positioning method as well as the performance of the detection approach by a modified α-filter-based Doppler smoothing technique. Theoretical performance of the proposed detection model is analysed. Monte Carlo simulations were also conducted to verify the theoretical analysis. Moreover, grounded on the developed test statistic and the optimised threshold, a consistency check module was specifically realised in both software defined and real-time GNSS receivers. Additionally, a Doppler smoothing technique was applied to the receivers. Spoofing attack experiments on both software defined and real-time platforms validated the effectiveness of the statistical spoofing detection model.

The Spatial Only Processing Power Inversion (SOP-PI) algorithm is frequently used in Global Navigation Satellite System (GNSS) adaptive array receivers for interference mitigation because of its simplicity of implementation. This study investigates the effects of SOP-PI on receiver measurements for high-precision applications. Mathematical deductions show that if an array with a centro-symmetrical geometry is used, ideally, SOP-PI is naturally bias-free; however, this no longer stands when non-ideal factors, including array perturbations and finite-sample effect, are added. Simulations are performed herein to investigate how exactly the array perturbations affect the carrier phase biases, while diagonal loading and forward-backward averaging are proposed to counter the finite-sample effect. In conclusion, whether SOP-PI with a centro-symmetrical array geometry will satisfy the high precision demands mainly depends on the array perturbation degree of the element amplitude and the phase center.

A receiver-spoofer is one of the most covert global navigation satellite system (GNSS) spoofing attacks and can only be effectively detected by the combination of multiple anti-spoofing technologies. In this paper, an analysis of influencing parameters for receiver-spoofers indicates that the ratio of the spoofing signal amplitude versus the authentic signal amplitude (spoofing-signal ratio) is a key parameter for spoofing results. For a spoofer to ensure covertness, the goal is to maintain a low spoofing-signal ratio. The carrier phase difference and code phase difference between authentic signals and spoofing signals resulted in errors in the position estimation of the target receiver increase the lower limit of the spoofing-signal ratio required for successful spoofing. A spoofing signal alters the phase of local replicate code based on the original balance of a receiver phase discriminator to seize control. Based on this principle, the lower limit of the spoofing-signal ratio that corresponds to various phase discriminator spacings, carrier phase differences, and code phase differences is deduced in this paper. Two tests are designed for the simulation source and authentic navigation signals to verify the deduced formula. The lower limit of the spoofing-signal ratio obtained from these tests matches the calculated results, which proves the validity and effectiveness of the derived algorithm.

For Global navigation satellite system (GLASS), spoofing attack is extremely dangerous and destructive since it can lead GLASS receivers to generate misleading time and position information. Thus, reliable and timely spoofing detection and mitigation methods are of great importance. A new GLASS acquisition method with the capability of spoofing detection and mitigation is proposed in the paper. The proposed method can not only perform acquisition, but also detect and mitigate spoofing signals based on joint consistency detection of code and carrier Dopplers. We provide performance analysis and numerical simulation results to demonstrate the validity of the proposed method when there are one or even more spoofing signals. Successful acquisition probability of the authentic signal is compared to that achieved by traditional acquisition method. The proposed method can be easily implemented with only a little modification of current receiver acquisition module.

Global navigation satellite systems are now threatened by spoofing attacks. The double-antenna countermeasure is one of the most robust spoofing detection techniques. However, the existing double-antenna countermeasure is based on the assumption that the measurements are synchronized, which may result in failure if the assumption is not valid. In the paper, we propose an unsynchronized model for the pseudorange and carrier phase measurements and discuss why these measurements are not suitable for spoofing detection when the measurements are not synchronized. In addition, a new detection method based on double-antenna power measurements is proposed. This method can be used in the unsynchronized case, extending the application of the double-antenna-based spoofing countermeasure.

Signal multiplexing techniques are those that enable the efficient transmission of multiple signals through a single modulator, up converter, power amplifier chain, and antenna aperture, without mutual interference. In the construction of new-generation global navigation satellite systems (GNSSs), signal multiplexing has encountered many challenges as the number of signals to be multiplexed increases and signal elements become more diverse and complex. In the past 15 years, many novel and advanced constant envelope multiplexing (CEM) techniques have been proposed. However, increased requirements for adaptability and flexibility of signal multiplexing techniques for future evolutional GNSSs have resulted in the need for further improvement in generalized CEM design theory. The uniform CEM mathematical framework and more general CEM design approaches can offer a good starting point to meet these challenges. In this article, we provide a comprehensive tutorial on the important concepts, recent advances, representative applications, and the remaining challenges of GNSS signal multiplexing techniques. To provide readers with a global overview of multiplexing techniques in GNSS and to foster new research ideas, a mathematical cornerstone and intrinsic conceptual relationship of assorted multiplexing techniques along with the high adaptability and high flexibility challenges of CEM design will be two key discussions.

China's BeiDou Navigation Satellite System (BDS) has evolved from a demonstration system (BDS-1) to a regional navigation satellite system (BDS-2) and has finally entered the phase of a global navigation system (BDS-3). In 2018, the BDS-3 implementation achieved historic milestones. A baseline constellation with 18 MEO satellites and one GEO satellite has been established. The BDS has the ability to provide fundamental positioning, navigation, and timing (PNT) services to global users. The current BDS features some distinctive design and unique service capabilities. First, the BDS provides a regional message communication service and a global short message communication service with 1000 and 40 Chinese characters at a time, respectively. Second, the BDS also provides a global search and rescue (SAR) service, a satellite-based augmentation service (SBAS), as well as a regional precise point positioning (PPP) service. This special issue of the Institute of Navigation's journal, NAVIGATION, provides a collection of papers presenting research achievements in the BDS over the last 2 years. It covers essential fields necessary to ensure the successful operation of BDS, such as constellation design, operational and control system design and construction, frequency and signal design, updated coordinate reference system and time system, satellite orbit determination, ionospheric model, and software receiver. Papers in this special issue present the development status of the BDS from several perspectives and introduce the main performance and progress to readers, which is helpful for researchers in the field of satellite navigation and beneficial to GNSS terminal manufacturers as well. The publication of the issue also provides an opportunity to promote users' understanding and applications of the BDS. We hope that readers find useful information in this issue, and we look forward to sharing our continued success in future BDS development.

Since the widespread existence of interference and spoofing, the authentication of position, velocity, and timing (PVT) solution is necessary for a Global Navigation Satellite System (GNSS) receiver. Single-antenna spoofing attack that transmits all the counterfeit signals by a single antenna is a common one. In this case, all the counterfeit signals would experience a common propagation channel which causes the same fading and delay from the spoofer to a receiver. Inspired by the characteristic of such spoofing attack, we propose a PVT solution authentication method based on monitoring the combination of clock bias and drift for moving receivers in the paper. The basic principle of the method is based on the consistency check of clock bias and drift variations between local clock and GNSS timing solution. It can verify the authenticity of PVT solution using not only the clock bias but also the clock drift variations. This allows the Commercial off-the-shelf (COTS) GNSS receivers to obtain better performance.

In this paper, we introduce a flexible delay control method for the GNSS direct meaconing signal. Since GNSS has been playing an important role in human life, we need to pay attention to its security issues, especially spoofing attacks. In order to explore the corresponding anti-spoofing techniques, we should know more about spoofing methods so that we can study the characteristics of spoofing signals. As far as an encrypted GNSS signal is concerned, only the direct meaconer can generate the corresponding spoofing signal. In detail, the meaconer receives authentic signals from different directions and delays them with different delays, respectively. However, the delay control method has not yet been public, which restricts researchers to explore more efficient anti-spoofing methods. To this end, we introduce a flexible delay control method for meaconer based on the integer and fractional delay filters. In detail, the integer filter can delay the signal with an integer delay that is an integer multiple of sampling period, and the fractional filter can delay the signal by a fraction delay. We implement the fractional filter using the Farrow structure so that we only need to adjust one parameter in the filter if the replay delay changes. As a consequence, the delay control method is flexible and easy to implement on a hardware platform. We verify the introduced method by carrying out meaconing attacks against both the GNSS-SDR receiver and the GPS hardware receiver of our lab.

With the widely application of Global Navigation Satellite System (GNSS) nowadays, GNSS spoofing attack becomes an unavoidable problem. Various anti-spoofing methods are proposed in recent years, including methods operate in GNSS receivers and methods operate in integrated navigation systems, e.g. GNSS/INS (Inertial Navigation System) system. However, methods operate in GNSS receiver have difficulty in detecting sophisticated spoofing attack, and methods operate in integrated navigation system introduce the risk of misleading the integrated navigation system. In the paper we propose a spoofing detection method making use of raw GNSS measurements and raw IMU measurements. Through utilization of IMU measurements, the proposed method can achieve better performance than methods using GNSS measurements only. In addition, the proposed method operates ahead of the integrated navigation algorithms, which increases the overall robustness. A series of field experiments are conducted and experiment results verify the performance of the proposed method.

Adaptive array signal processing can effectively enhance the global navigation satellite system (GNSS) receiver’s performance in challenging environments with radio interference and multipath. However, it may introduce significant distortion on the crosscorrelation functions, leading to biases in code phase and carrier phase. For high accuracy GNSS applications, these biases must be mitigated. The most challenge for current anti-jamming solutions is that they are fairly sensitive to manifold errors, especially for code phase. In this paper, a novel algorithm referred as Polyphase Decomposition Based Space Frequency Adaptive Processing (PD-SFAP) is proposed to suppress interferences and reduce the measurement biases. Theoretical and numerical simulations verify the superiority of the proposed algorithm.

In this paper, we propose a GNSS spoofing detection and identification method based on clock drift monitoring using only one signal. At present, there are many anti-spoofing techniques based on time information monitoring, such as clock bias and drift monitoring. However, these techniques estimate the time information using observations of at least four signals. As a result, they can only detect spoofing but cannot identify it. Besides, in order to estimate the time information, they need to solve a group of nonlinear equations iteratively, which requires a heavy computational load. In view of this, we propose a new method to estimate the clock drift using only one signal. Therefore, the proposed method can not only detect the spoofing signal but also identify it, and it can work normally even when authentic signals cannot be received. Furthermore, the method estimates the drift by solving a linear equation without any iteration, which has a lighter computational load compared to the previous methods. Experiments show that the method can detect and identify the spoofing signal when the spoofer moves, manipulates the time information, or is driven by an ordinary clock, such as a crystal clock.

To realize robust high-precision positioning in outdoor diverse environments, the prevailing solution is to integrate multiple complementary sensors like GNSS RTK, INS and LiDAR. In the cases where there is no prior LiDAR map, the global accuracies of fusion positioning and mapping depend on the only global positioning technology, RTK, whose cm-level fix result availability, however, plunges greatly in GNSS-Difficult areas. To improve RTK performance, we propose a new RTK/INS/LiDAR fusion positioning algorithm base on LiDAR features in ambiguity domain, in which we tightly integrate the measurements of the repeated-observed LiDAR features with the ones of RTK. Both theoretical analysis and simulation experiment results prove that the addressed method can provide a higher RTK fix rate to maintain the global accuracy and stability of the fusion positioning in GNSS-Difficult areas.

In conventional global navigation satellite system (GNSS) receivers, usually full pseudorange measurements are required to complete a single point position fix. However, to obtain full pseudorange measurements takes longer time than for fractional pseudorange measurements. Considering such a fact, in order to shorten the time to first fix and improve the position accuracy during cold or warm start of a dual-constellation GNSS receiver, we propose a positioning algorithm using full and fractional pseudorange measurements from the two navigational constellations. This method uses four full pseudorange measurements from one constellation along with fractional ones from either or both constellations to obtain a potentially rapid position result with an identical accuracy to that of the conventional positioning method using full measurements. Tests with simulated and real Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) data demonstrate that the proposed method can generate correct single point position solutions and the position error is identical with the result from the conventional approach using the full pseudorange measurements.

Global Navigation Satellite System (GNSS) is vulnerable to spoofing attack because of its low power and open civil signal structure. Loran-C system is independent of GNSS and can be used for anti-spoofing. In order to detect, identify the spoofing signals and exclude its influence on position, velocity, and time (PVT) results, the paper proposes the GNSS anti-spoofing method assisted by Loran-C system. When the number of Loran-C signals is relatively less, the difference observations of Loran-C are used to identify whether the GNSS positioning result is right or wrong, and the factors affecting the detection performance are studied. When the Loran-C receiver can provide independent positioning information, all spoofing signals are identified by the clustering algorithm after the consistency detection of GNSS pseudorange observations, Loran-C positioning information and satellite positions. Then if the number of remained GNSS signals after spoofing exclusion is less than 4, the GNSS signals and Loran-C signals are combined to improve the positioning accuracy and the Dilution of Precision (DOP) is used to measure the positioning accuracy. Finally, the feasibility of the proposed method is verified by simulation experiments.

As an important supplement and backup to the Global Navigation Satellite System (GNSS), the ground-based positioning system is desirable for low cost and high flexibility. In GNSS denied environments, the spatial reference cannot be determined via GNSS services. A large number of systems obtain the coordinates of base stations by artificial measurement, but this can be limited by terrain or other conditions and is difficult to achieve rapid deployment. The spatial reference establishment methods affect its deployment flexibility, while the accuracy of the coordinates of base stations has a crucial influence on the positioning performance. Some methods estimate the coordinates of base stations via code measurements, but cannot establish high-precision spatial references. Existing methods based on carrier measurements rely on the known calibration points of a receiver, which still need artificial measurement. To overcome the shortcomings of existing methods in accuracy or autonomy, this research proposes a carrier measurements based spatial reference establishment method, which can precisely estimate the coordinates of base stations without known calibration points. A theoretical model of the coordinate estimation problem is established for ground-based positioning systems, and the proposed algorithm obtains the maximum likelihood estimation based on the observations of a mobile receiver. The simulation results show that the proposed method can achieve centimeter-level accuracy of coordinate estimation without dependence on calibration points. Compared with existing methods, the proposed method has advantages in estimation accuracy and autonomy, and can be applied in the rapid deployment of ground-based positioning systems, thereby improving the flexibility and practicability.

The multipath fading independency between different Global Navigation Satellite System (GNSS) frequency bands and space-separated antennas can be used to improve robustness of high-precision GNSS receivers in harsh urban environments. This research focuses on a new sensitive and robust fading monitoring architecture for GNSS diversity branches, and implementation of the method on an Adaptive Diversity reception-based Tracking Loop (ADTL). Firstly, fluctuations in the amplitude and phase of multipath fading signals are characterized. Performance of different design parameters in the fading monitoring method is analyzed in details. Next, diversity combining strategies including the non-linear Selective Combining (SC) and the Maximum-Ratio Combining (MRC) are utilized in ADTL for different fading conditions. Experimental results with Monte-Carlo simulation and real GNSS fading signals demonstrated that: (1) the proposed novel method can achieve robust and sensitive fading monitoring with response delay less than 20 ms; (2) Compared with traditional Phase Lock Loops (PLLs), the ADTL algorithm significantly reduced the navigation bit error rate due to half-cycle sudden-phase change caused by phase losing of lock.