China Satellite Navigation Conference (CSNC 2024) Proceedings

With the emergence of new GNSS applications such as autonomous driving and UAV, it is necessary to consider integrity requirements of real-time kinematic (RTK), to ensure the security and reliability of positioning, on the basis of high-precision services. The performance of RTK depends on the data quality of the reference station, the distance between the user and the reference station, etc. GNSS service anomalies, environmental occlusions and interferences, reference station faults and other factors may lead to RTK fault and cannot meet the requirements of high-performance applications. In order to improve the RTK performance, this paper presents initial research on the processing of SBAS-RTK integrated positioning. First, RTK fault factors are analyzed and their effects are evaluated by simulation to summarize the main risk events. Next, SBAS and RTK differential data are integrated for data detection, weighted positioning and data check to eliminate the impact of various risk events and improve the reliability of RTK. Finally, based on the simulation tests, the performance of the algorithm is verified and evaluated. The results show that the SBAS-RTK integrated positioning can identify the faults of more than 3 m in the RTK differential data and effectively ensure the reliability of positioning.

In view of the public signal format of the Satellite-Based Augmentation System (SBAS), there is a risk of spoofing attack security threats. The navigation message authentication technology can improve the anti-spoofing ability of SBAS on the system side. At present, Europe and the United States have carried out research on SBAS message authentication technology and actively promoted the standardization of SBAS message authentication. In this paper, aiming at the construction and development of the Beidou Satellite Based Augmentation System (BDSBAS), the design of the TESLA authentication protocol based on the Chinese commercial cryptography standard is carried out. Simulation verification. The simulation results can provide theoretical support for the standardization of BDSBAS message authentication.

Aiming at the problem of few visible satellites and short visible time for LEO cooperative navigation constellation leading to the difficulty of positioning, this paper studies multi-epoch positioning method based on the idea of replacing time to space. Taking advantage of the characteristics of the LEO satellite, such as high angular velocity and high signal landing power, through limited time of navigation signal observation, the pseudo-range and Doppler frequency positioning algorithm based on least squares method is adopted to achieve reliable positioning of static or quasistatic users. On this basis, the paper analyzes the Cramer-Rao lower bound of the positioning error, and through simulation analysis based on the error ellipsoid theory, it is found that the single LEO satellite multi-epoch positioning error distribution shows strong directivity, and the size and distribution of the positioning error is related to the relative position of user and satellite track. Therefore, the paper suggests that the positioning error of single LEO satellite is not suitable to be divided into horizontal and vertical errors according to the error analysis method of GNSS because of the anisotropy of each direction, but should be described separately by the three directions of east, north and vertical. The paper can provide technical support for the system design and positioning application of future LEO navigation satellites.

Aiming at the problem of low precision positioning solutions for high dynamic users caused by Ka band LEO communication and navigation integrated satellites in point beam polling mode, a LEO satellite/inertial integrated navigation and positioning algorithm based on accumulated error estimation and correction is proposed. A extended Kalman filtering estimator is used to achieve the state error estimation at the output of inertial navigation. By the error correction, the rapid growth of nonlinear error is suppressed when low precision inertial sensors is used for integrated navigation, and the positioning accuracy for high dynamic users is improved. The simulation results show that in the condition of low precision inertial navigation the proposed algorithm can significantly improve the dynamic positioning performance. The three dimensional positioning and velocity measurement accuracy of high dynamic users can reach below 10 m (1σ) and 0.1 m/s (1σ) respectively.

Communication and navigation integration will be a staple of integrated PNT system. LEO satellites, served as an important part of the system, provide support for constructing the future PNT system and improving its service performance. Spoofing is a potential factor to affect reliability of GNSS services, LEO satellites can provide reliable information as independent navigation sources and show good anti-spoofing performance with dual-direction communication links and authentication function, which improve the anti-spoofing performance of the whole system. A LEO-aided GNSS spoofing monitoring algorithm is proposed, constructing a subset-separating detection model based on Chi-square distribution in velocity domain with LEO doppler measurements and GNSS pseudorange rates to detect and recognize spoofed navigation satellites by divided space into subsets. The simulation results show that the novel algorithm can detect and recognize fault navigation satellites effectively, and positioning accuracy after exclusion of spoofed satellites can be ensured. LEO can improve anti-spoofing performance of GNSS observably.

The reliability of satellites navigation system has always been the focus of international community, the risks of integrity and hostile interference are important factors affecting the reliability of airborne satellites navigation system. LEO satellites with advantages of running in low orbits, having rapid change of satellite geometry and strong anti-interference capability of satellite signal, can significantly enhance the credibility of BDS. But when most or even all satellites are spoofed, LEO satellites and BDS navigation system can hardly detect and recognize faults. Hence, we add inertial navigation system as an independent navigation source to the whole system and propose a credibility algorithm based on inertial navigation system, combining INS with all visible satellites to detect and recognize fault satellites in more directions, constructing chi-square test statistics based on the Kalman Filter innovation sequence on the basis of satisfying the need of integrity. The simulation results show that LEO/INS-aided system can detect and recognize fault satellites in more than one directions. Under the indexes of 90% integrity default detection rate and recognition rate, compared with single BDS system, the two indexes of the INS-aided system improve about 20% and 40% respectively. For hostile faults, the LEO satellites can significantly improve the anti-spoofing performance of integrated navigation system, the detection rate increases about 25% and the recognition rate increases more than 30%.

Multi-constellation global navigation satellite system (GNSS) will open the possibility to enhance navigation service performance due to more redundant satellites in view. However, there is a heightened likelihood of satellite fault occurrences, which can result in more mission interruptions and increase the continuity risk. In response, fault exclusion algorithms are indispensable to reducing the continuity risk and maintaining continuous navigation services, however, will bring about increased integrity risk resulting from incorrect exclusions. For that, we introduce a critical parameter in the advanced receiver autonomous integrity monitoring (ARAIM) algorithm to guarantee minimum integrity performance sacrifices while satisfying the continuity performance. In this algorithm, the parameter is used to allocate the continuity requirement between fault detection and exclusion (FDE) tests. An experiment involving 24 h data of GPS and BDS is conducted to demonstrate the performance of ARAIM based on the determined parameter. The results suggest that ARAIM based on the determined parameter can provide more availability of 88.70%, whereas ARAIM based on a given half allocation provides 83.48% availability.

In the urban environment, the positioning results of RTK/INS integrated navigation are often influenced by grosses, which degrades the positioning accuracy and damages the stability of the navigation system. To solve this dilemma, a loose coupled RTK/INS positioning method based on robust estimation is proposed. Firstly, during RTK positioning, for eliminating corrupted measurements caused by Non-Line-of-Sight (NLOS) signals and multipath, this paper researches the RTK positioning algorithm based on bifactor robust estimation. In this method, the ambiguity is fixed and solved back into the Double-Difference (DD) equation in least square model. The posterior residual vector is used for the bifactor robust processing to eliminate the influence of some abnormal observations. Then, the RTK positioning results are loosely integrated with Inertial Navigation System (INS), and the robust processing based on Huber equivalent weight function is implemented in the process of the Kalman filter update. Finally, the proposed algorithm is validated by vehicle data in a typical urban environment. The experimental results show that compared with the traditional least squares-based RTK, the accuracy of the bifactor RTK method is improved by 2.48 cm in the horizontal direction. Meanwhile, in contrast with the RTK/INS method based on the single robust processing, the accuracy of the proposed robust bifactor error method in the horizontal direction is increased by 3.85 cm. The navigation accuracy and reliability are significantly enhanced. In dynamic environment, this research has certain theoretical reference and practical value for vehicular and autonomous driving applications.

The application of PPP-RTK technology in life safety-related fields has strict requirements for accuracy and integrity, requiring the construction of a complete monitoring and control system for multiple risk sources. Aiming at the lack of authoritative indicator system for multiple risk sources of PPP-RTK integrity services, as well as the traditional average allocation and proportional allocation based on empirical data, which fails to fully consider the needs of various risk source integrity risk indicators, resulting in the difficulty of ensuring the availability of overall monitoring of risk sources, this paper proposes an optimal allocation method for multiple risk source integrity risk indicators for PPP-RTK integrity services. Based on the theory of multiple hypothetical solution separation, this method obtains multiple risk source integrity risk indicators by assigning and adjusting the risk indicators of the integrity of various risk sources to make the virtual protection level of the positioning domain equal. Simulation experiments show that compared with the traditional method, this method realizes the construction of multiple risk source index systems that match the requirements of risk source integrity indicators. At the same time, this paper tests the availability of risk source integrated monitoring by 1 times, 1.5 times and 2 standard deviations, and the results show that the availability of the proposed method is increased by 1.38%, 9.75% and 12.72%, respectively.

In order to realize the requirement of stable operation of Leo Navigation system, redundant backup of constellation system should be designed in constellation networking stage and constellation operation stage respectively. A certain number of satellites to be launched should be backed up on the ground ahead of time to ensure system reliability is maintained above a certain level. In this paper, by comparing several kinds of reliability calculation methods of constellation system and the strategy of constellation network supplement, the optimal scheme of accuracy and calculation resource consumption is selected synthetically. The author complete design the number of satellite backup and launch plan in the operation phase based on requirements of different levels of single satellite reliability and system reliability. In this paper, author apply Monte Carlo method multiple sampling to the life distribution of a single satellite and compare it with the real-time operating hours of satellites in orbit to obtain the current system reliability, which can be used to obtain the constellation system reliability at any time and get different network backup strategy. The analysis results show that this method can provide a quantitative basis for the design and optimization of the backup strategy of navigation constellation.

Broadcasting integrated signal of communication and navigation in low earth orbiting (LEO) broadband constellation to achieve the navigation augmentation is an existing trend. Considering the needs of communication services, the integrated signal generally has burst characteristics, and the receiver only receives the signal from the satellite in some specific time slots, and the observation has obvious asynchronous arrival characteristics. Such asynchronous arrival observations are quite different from the synchronous observations obtained by receiving the continuous navigation signals broadcast by the existing GNSS satellites, which will lead to many problems if using the traditional positioning method. Combining the characteristics of asynchronous observations, this paper proposes an asynchronous observation positioning algorithm based on factor graph optimization (FGO), which can use asynchronous observation in multiple epochs for joint estimation to reduce positioning error. Simulation shows that the positioning algorithm based on FGO can effectively improve the positioning accuracy compared with the traditional method.

Aiming at the problem of uplink signal interference in the satellite-earth link of GEO satellite system, this paper analyzes the satellite communication link interference model and interference evaluation methods, and the link interference model is constructed by considering various factors comprehensively. Then, the interference evaluation method based on the spectral separation coefficient is used to quantitatively evaluate the attenuation of carrier-to-noise ratio of the useful signals received by the receiving station from multiple dimensions. The results show that the interference effect increases with the increase of uplink interference signal power, and presents an exponential increase after reaching a certain value; Under the same interference signal power, the increase of the number of interference stations will lead to the decrease of the attenuation rate of carrier-to-noise ratio of useful signals and gradually moderate. The closer the interference station is deployed to the area near the transmitting station, the more obvious the interference effect will be. However, due to the power characteristics of the satellite transparent transponder, its position change in the nearby region has little influence on the interference effect.

Ionospheric delay error is one of the main errors of satellite navigation system. The unstable physical characteristics of ionosphere can severely affect the performance of satellite navigation. Therefore, civil aviation standards require that the impact of ionospheric anomalies (especially ionospheric storms) should be considered in integrity monitoring. To meet the safety requirement of satellite navigation service in civil aviation, it is urgent to analyze the performance of dual-frequency multi-constellation integrity monitoring under stormy condition. In this paper, the empirical model and temporally correlated stochastic error model are used to analyze the ionospheric errors in carrier smoothed code. The Gaussian overbound method is then applied to conservatively represent the error distribution of the temporally corelated stochastic error model. The worst-case overbounding result is used to revise Integrity Support Message (ISM) and evaluate the performance of Advanced Receiver Autonomous Integrity Monitoring (ARAIM) algorithm under stormy condition. The experimental results show that the occurrence of ionospheric storm is able to increase the nominal bias and error variance in ISM and degrade the performance of ARAIM. This study provides an effective method to refine the ISM in ARAIM during ionospheric storm period.

Performance of machine learning (ML) methods to identify scintillation events is analyzed for a variety of scenarios. It shows that spectrum embodies various features on scintillation variation. Different methods can be developed with ML to find out potential scintillation impacts, and satisfied results generally could be arrived with accuracy of 95%. It also point out that the descending trend existed over Fresnel frequency is essential to distinguish a potential scintillation. So selecting a proper frequency band to make spectral features more distinguishable plays an important role in ML realization. It further shows that precise GNSS routine observations with the sampling rate of 1 Hz can be served to recognize scintillation event if a sound spectrum range has been chosen. When a set of parameters on spectrum characteristics could be derived and used for ML training, better performance can even be expected.

High precision and low delay satellite orbits are the key requirements for many Earth observation missions today. This paper briefly introduces the development of precision orbit determination of low orbit satellites, and the opportunities brought by GNSS satellite-based enhancement service for real-time centimeter-level orbit determination of low orbit satellites, then analyzes the orbit accuracy and clock error accuracy of current satellite-based enhancement real-time products, and then introduces the mathematical model of precision orbit determination of low orbit satellites with simplified dynamics real-time filtering solution in detail. At last, the Sentinel-3A satellite GPS observation data for 10 days were used to simulate the in-orbit mode by POD4LEO software. The results show that, compared with the scientific orbit provided by the European Copernicus Center, the RMS of the computed orbit in the radial, tangential, normal and three-dimensional directions are 2.5 cm, 2.5 cm, 2.0 cm and 4.1 cm respectively, and the SISRE caused by the orbital error is about 3.0 cm. The effectiveness and feasibility of the real-time orbit determination method proposed in this paper and the developed software are verified, which has practical significance in the future low orbit navigation enhanced satellite or other low orbit satellite engineering applications.

With the completion of the technical revision of BeiDou System in international civil aviation standards, the accuracy, continuity, integrity of BeiDou System will become the focus of global civil aviation users. During the assessment of the BeiDou open service performance, it was found that in March 2022, the BDS- 3 MEO-3 (PRN21) satellite had a serious abnormal event. The event occurred in which PRN21 satellite had a frequency offset fault of the satellite clock and last-ed 20 h. The satellite clock error in the event produced a slope fault of 0.76 m/s, and the maximum satellite clock error reached 36616 m. According to the B1I signal message health identifier, the events are divided into 13 h of integrity events and 7 h of continuity events. In accordance with the performance evaluation methods stipulated in the international civil aviation standards, the accuracy, continuity and integrity of the BeiDou satellite space signal performance were evaluated in this quarter. The results show that the single satellite integrity risk of the system in the quarter caused by the event is 2.23 × (10(−4)), and the continuity risk of PRN21 satellite is 0.398%, both of which are far higher than the ones in the same quarter in 2021. Receiver Autonomous Integrity Monitoring (RAIM) was performed to monitor the faults, and the algorithm achieved fault detection and recognition when the satellite clock error reached 16m which meet the integrity requirements of enroute, terminal, NPA. Based on the analysis of this incident, reasonable suggestions are put forward for the aviation application of the BeiDou System.

With the rapid development of LEO communication constellations, the performance enhancement of existing medium and high orbit constellations has been realized by making full use of the characteristics of LEO communication satellites, such as high transmission rate, low orbit height and large constellation scale, which has attracted extensive attention and become a research hotspot. Based on the systematic analysis of the available resources of the LEO communication constellation, this paper presents a system architecture of navigation augmentation based on the LEO communication constellation. It analyzes the available constellation and frequency resources. The performance of improving the reception sensitivity of BeiDou user terminals and providing emergency backup positioning based on the LEO communication constellation is quantitatively evaluated through theoretical calculation and simulation analysis. The research results can provide a reference for designing and constructing an integrated positioning, navigation, and timing service system.

On July 31st, 2020, the construction of the BeiDou Satellite-Based Augmentation System (BDSBAS) was completed, which can provide two service signals, single frequency augmentation and dual frequency dual constellation augmentation, and is currently carrying out civil aviation testing and certification of single frequency augmentation services. When conducting service coverage area assessment, domestic scholars generally use the grid point evaluation method used by the US Wide Area Augmentation System (WAAS), which uses the premise that WAAS has passed civil aviation test certification and verified the reliability of pseudodistance integrity in the entire service area. However, BDSBAS has not yet completed the test certification, so this method is debatable for the evaluation of BDSBAS service coverage areas. Therefore, this paper proposes a service coverage area evaluation method and process applicable to BDSBAS, which realizes the accurate evaluation of ionospheric grid coverage area and positioning service coverage area through the national multi-station dense networking monitoring. In this paper, the observation data of the Chinese monitoring station of the Crustal Movement Observation Network of China (CMON) in August and O4ctober 2021 are evaluated, and the results show that the BDSBAS single frequency service area in northern China can achieve 99% coverage of APV-I level availability, while most BDSBAS single frequency service areas in southern China are below 99% availability. Compared with the grid point evaluation method, it is obvious that the availability of single frequency enhanced services in the southern region is poor due to ionospheric anomalies, which verifies the effectiveness and correctness of this method. This method can effectively support BDSBAS civil aviation testing and certification work in the future.

A long convergence time for Global Navigation Satellite System (GNSS) Precise Point Positioning (PPP) is a critical hindrance to its widespread application. Due to pseudolites (PLs) are very close to receivers, moving users can generate rapid geometric variations, reduce the correlation between parameters, and thus accelerate parameter convergence. However, considering the limited coverage of pseudolite system (PLS), the augmentation time is limited. It is therefore crucial to make the GNSS PPP parameters (especially the GNSS ambiguity parameters) converge within the limited augmentation time. Therefore, the potential of PLS should be explored as much as possible, and the gains brought by PLS should be effectively utilized. In order to use the fixed solution of PLS more safely and reliably, a dual-filter tightly coupled (DF-TC) model is proposed. When the number of PLs is sufficient and the augmentation time is long enough, the positioning performances of all the couple models are not significantly different. In the case of short-time augmentation, the DF-TC model exhibits the best augmentation performance, it can significantly reduce the augmentation time required by GNSS, which means that the required coverage of PLS can be smaller and will be more flexible and convenient for applications.

Signal in Space (SIS) performance, particularly SIS Range Error (SISRE), is the core index for evaluating the navigation, positioning, and timing accuracy and integrity monitoring of the Global Navigation Satellite System (GNSS). In this research, the SISRE performance and on-orbit status of BDS-3 satellites are monitored and evaluated comprehensively based on the historical data composed of the broadcast ephemeris and observation data from IGS and iGMAS tracking stations from July 31, 2020, to May 31, 2022. The performance evaluation results show that the average accuracy of BDS-3 SISRE is 0.25 m, which is much better than the system design index. Based on the independent monitoring results, from the annual average of satellite on-orbit adjustment from 2020 to 2022, the overall operation of the system is good and tends to be stable. It is worth noting that the MEO satellite had two unplanned outages of satellite clocks whose health information was not updated or lagged in updating. Therefore, it is suggested that the system extensively use monitoring methods such as the inter-satellite link (ISL) and Satellite Autonomous Integrity Monitoring (SAIM) in the future to strengthen the worldwide monitoring of BDS-3 satellite on-orbit operation status.

As an important feature of BeiDou Navigation Satellite System (BDS), BeiDou augmentation services includes ground-based augmentation service, satellite-based augmentation service and Precise Point Positioning (PPP) service, which are crucial to the promotion of high-precision “BeiDou +” applications. BDS PPP service broadcasts augmentation messages via three BDS Geostationary Earth Orbit (GEO) satellites (C59, C60, C61). Previous studies have evaluated the performance of BDS PPP service, mainly analyzing the accuracy of the recovered PPP-B2b precise products and assessing the static and simulate-kinematic positioning accuracy, but lack the epoch completeness rate analysis of raw augmentation messages. In this paper, we developed our own BDS-3 PPP-B2b augmentation message real-time acquisition and decoding software, which can automatically store, decode, and display augmentation messages in real time. Based on this software and a set of GNSS receiver and antenna, PPP-B2b augmentation messages from August 28th to September 3rd, 2022, were analyzed. The results show that: (1) in terms of the satellite system, the average epoch completeness rates of both BDS-3 and GPS are higher than 98.55%, and BDS-3 is slightly better than GPS; (2) in terms of message types, the minimum epoch completeness rates are: 99.98% for type 1 satellite mask, 99.85% for type 4 clock correction, 98.91% for type 3 DCB correction, and 96.66% for type 2 orbit correction; (3) in terms of two GEO satellites, the average epoch completeness rates broadcasted from C59 (100.00%/99.51%/99.51%/99.95% for BDS-3, 100.00%/98.55%/99.90% for GPS) and C60 (99.98%/99.51%/99.50%/99.94% for BDS-3, 99.98%/98.55%/99.88% for GPS) are almost the same.

The basic navigation and positioning service and precise single-point positioning service of contemporary global navigation satellite systems (GNSS) and its augmented systems mainly serve people and machines that are supervised or operated by people. Its system architecture and signal framework have undergone nearly 40 years of application and performance continuous improvement, and have encountered the bottleneck of the contradiction between accuracy and real-time performance that cannot be reconciled. It is urgent to research at the level of system architecture and signal framework to seek breakthroughs, and solve the contradiction between dm/cm-level positioning accuracy and second-level real-time performance from the system design source so that GNSS can step into the next generation, serve the machine independently, and provide real-time and accurate space-time information support for the intelligent era and society of “No Man, intelligence and IoT”. The paper first summarizes that the contemporary GNSS system adopts “sub-meter-level broadcast ephemeris + m/cm-level code pseudorange measurement signal” to realize instantaneous meter-level positioning, and further adopts “cm-level precision ephemeris enhancement + cm/mm-level carrier pseudorange measurement signal”. After 10 to 40 min of initialization and convergence, cm-level positioning is realized. Secondly, the current status of the space-time reference establishment, maintenance and synchronization system architecture, the current status of civil space signal framework, and the current status of civilian message structure are analyzed one by one, and the bottleneck of the contradiction between accuracy and real-time is summarized and analyzed. Then, it is proposed to use a new generation of code-based precision point positioning system (NextGen-Code-PPP) to realize instantaneous dm-level positioning, which is “cm-level broadcast ephemeris + cm-level code pseudorange measurement signal”, and further use “cm/mm-level carrier pseudorange measurement” to realize instantaneous cm-level positioning system-level solutions. Finally, the space-time reference system architecture and signal frame sources, such as the space “net” ground “net” space-time reference, unambiguous instantaneous cm-scale space meta signals, high-power fast message signals of UHF/VHF and other new frequency bands are analyzed, and a feasible conclusion is given.

In order to address the problem that the anti-jamming performance of GNSS receiver array is seriously degraded in high dynamic environment, a polarization-spatial joint null broadening beamforming algorithm based on matrix reconstruction is proposed in this paper. Firstly, the received signal model of polarization sensitive array (PSA) is established. Then, the Capon spectrum is reset by setting the virtual interference in the interference and polarization angles’ neighborhood. Secondly, the interference-plus-noise covariance matrix (IPNCM) is reconstructed by the Capon spectrum, which realizes the polarization-spatial joint null broadening. Finally, the conjugate gradient (CG) method is utilized to get the polarization adaptive weight vector, which avoids the high computational complexity caused by matrix inversion. The simulation results show that this algorithm can effectively broaden the null, which has good suppression effect on high dynamic interference, and have less computational complexity compared with similar algorithms.

To improving the PNT service performance with LEO navigation augmentation system has become a research hotspot in the field of satellite navigation. One of the main goals is to shorten the required convergence time of precise point positioning (PPP). In order to achieve the rapid precise point positioning, it is necessary to broadcast the dedicated LEO navigation augmentation signals. Different from the traditional GNSS signal, the LEO navigation augmentation signal needs to provide the capability of high precision continuous measurement, high data rate broadcasting at the same time. For this reason, this paper proposes a multiplexed code shift keying (MCSK) signal scheme to meet the need of LEO navigation augmentation signal, and studies the receiving method of MCSK signal. The MCSK signal adopts the code period time division multiplexing technology, which combines the code period of modulated low rate message and the code period of modulated high rate message into a single signal to achieve high-precision measurement, and high data rate broadcasting. With the help of simulation, the tracking performance and demodulation performance of MCSK signal are analyzed, and the effectiveness of MCSK signal is verified.

DME is a kind of important equipment for air route navigation and landing guidance. It has high transmitting power, a large number of installations, a wide range of signal coverage region, and also works at same frequency band with BeiDou-3/B2a signal. DME will interfere the weak satellite navigation signal. In order not to affect the signal capture, improve BeiDou receiver carrier tracking and code tracking accuracy, and reduce BER (Bit error rata), it is especially necessary to carry out the analysis and evaluation of DME to BeiDou-3/B2a signal interference and suppression measures research. Here, through the analysis of characteristics of DME and BeiDou-3/B2a signal, based on RTCA/DO-292 standard about radio signal on GPS/L5, Galileo/E5a interference impact analysis, proposed DME on BeiDou-3/B2a signal interference effective carrier-to-noise ratio degradation evaluation ideas and analysis methods. Based on the effective carrier-to-noise degradation algorithm model, DME to BeiDou-3/B2a signal interference evaluation was carried out for two hundred and five DME ground beacons co-located with VOR (Very-high-frequency Omnidirectional Range) for air route navigation in China. The interference evaluation method of DME to BeiDou-3/B2a in the paper provides technical guarantee for improving accuracy, availability, continuity and integrity of BeiDou navigation system, carrying out research on interference suppression measures and BeiDou system application.

GNSS civil signal interface standard is open, malicious attackers can generate legitimate navigation signals according to the interface control document, GNSS open service signals face spoofing interference attacks. Anti-spoofing methods mainly include navigation signal detection, navigation information encryption authentication etc. Common signal authentication techniques include two types of navigation message authentication and spreading code authentication. This paper proposes a spreading code authentication method based on CSK modulation, which firstly performs CSK modulation on the authentication message chunks, and then modulates on the original satellite navigation signal by phase hopping modulation. This method improves the authentication information rate while ensuring the constant envelope signal, and gives the corresponding authentication information demodulation scheme. The analysis shows that the method can change the authentication message rate and the assigned power ratio by controlling the CSK (U, N) parameters and setting the phase hopping angle to meet the authentication requirements of different strength signals.

Aiming at the influence of multipath effect on positioning accuracy, this paper proposes an anti-multipath positioning method based on MEDLL and WLS. Firstly, a multipath detection index MDCR was proposed based on MEDLL multipath detection method, and its indicator of multipath interference was compared with SAM, CNR, CDCM. Then, combining the correlation coefficients between these four indexes and the pseudo-distance multipath error and the fuzzy comprehensive evaluation method, a WLS localization algorithm based on multipath detection is proposed. Simulation results show that the positioning error of the proposed algorithm is better than that of the weighted positioning algorithm based on elevation Angle in both horizontal direction and 3D position under both static and dynamic multi-path conditions.

For the needs of navigation users for ubiquitous and reliable PNT (positioning navigation and timing) services, we can utilize the global coverage of communication satellite system and its differences with navigation constellation in orbit, frequency and intensity of signal and adopt the mode of integrated into the communication system to realize the navigation enhancement, supplement new space-based navigation nodes with high efficiency cast ratio. At the transmitting end of communication satellite signal, the idle time slot of the communication service signal broadcasting period is used and the communication service frame format is not changed. The downlink spectrum resources of the communication service are allocated as needed to broadcast signal that modulate a known fixed symbol in the data domain. The user terminal generates the same modulation symbol locally, coherently accumulates multiple sub-band signals broadcast by the same beam, and performs correlation processing with the received data, so as to realize high precision code phase alignment and complete the pseudorange measurement function. The performance of navigation enhancement signal based on a downlink signal data format of L-band mobile communication is simulated. The results show that the acquisition and tracking of the signal can be realized by using the auto-correlation characteristics of the scrambling code of the communication signal. Multiple sub-band coherent synthesis can achieve ranging and frequency measurement performance close to GNSS Open Service Performance signal and It also has higher capture margin. We take a communication technology test satellite as the verification platform, the broadcasting and acquisition of single sub-band and six sub-bands navigation enhancement signal were carried out, and the receiving processing and ranging performance analysis are carried out after the acquisition data are superimposed with Doppler frequency offset, pseudorange variation and noise. The results are consistent with the theoretical simulation, which verifies the engineering feasibility of the signal enhancement scheme proposed in this paper.

The distortion caused by the non-ideal characteristics of satellite navigation signal generation channel has increasingly become an important aspect that effects the further improvement of navigation signal quality, especially the com-plex linear and nonlinear distortion introduced by power amplifier. One way to solve this problem is to strictly control the characteristics and states of power amplifiers and other devices, but the cost is huge. The second method is using system level solutions such as predistortion to conduct accurate modelling and compensation through training and learning. This method consumes a lot of no-board resources and requires high algorithm reliability due to the introduction of more complex processing in the signal generation link, so its use is greatly restricted. To solve these problems, we can use time-domain high-speed pulse width modulation (HsPWM) method to generate navigation signals. The method has a more deterministic distortion, which can solve this problem to a certain extent. At the same time, 2-level generation in the time domain can eliminate DAC, frequency converter and input filter, and use digital power am-plifier. As the channel links are reduced and the familiarity rate is improved, it can be flexibly configured and continuously optimized and upgraded.

The Global Navigation Satellite System-based Synthetic Aperture Radar Interferometry (GNSS-based InBSAR) uses in-orbit satellites as transmitters, and the receiver is stationary on the ground, which uses repeat-pass interference to realize deformation inversion. Compared with the traditional InSAR system, GNSS-based InBSAR has obvious advantages such as short revisit time, wide coverage and low system cost. This paper proposes a fast configuration optimization algorithm for GNSS-based InBSAR to solve the problem that the traditional configuration optimization algorithm consumes a lot of computing resources and does not have the ability to deploy quickly in disaster areas. This method simulates the trajectory of actual satellites in a repeat-pass period by polynomial fitting, and quickly optimizes the resolution and three-dimensional deformation accuracy at each time in a repeat-pass period, to obtain the best experimental configuration. This method can greatly reduce storage resources and simulation calculation time, and achieve rapid configuration optimization selection. The effectiveness of the algorithm is verified by the experimental data.

Under high dynamic conditions, a robust carrier tracking approach is essential for global navigation satellite system (GNSS) receivers. In this paper, the powerful Kalman filter (KF) technique is adopted in GNSS carrier tracking. The correlation signals are used as system measurements to discard the discriminator restricted by linear region. Then, a linear measurement equation is established based on the error-state, so that the system model is linear. Hence the KF can be used instead of the nonlinear KF which requires more computational costs. Furthermore, to exploit the potential of the KF under different conditions, an adaptive KF (AKF) based on the variational Bayesian approach is proposed. The proposed filter has accurate and robust estimation performance, especially under high dynamic conditions. Simulation results verify the applicability of the proposed system model and the superiority of the proposed filter compared with the traditional KF and existing AKFs.

Due to the open structure and the low receiving power of the global navigation satellite system (GNSS) signal, the receiver is vulnerable to some malicious spoofing interference. The signal quality monitoring (SQM) algorithm is favored because it can simply and effectively detect the correlation peak distortion in the spoofing interference. In this paper, we focus on the SQM metrics for BOC (1,1) component of the B1C signal, firstly introduce some existing SQM metrics of BOC (1,1) signal, then the statistical characteristics and detection threshold of the metrics are analyzed. On this basis, the metrics are merit-based selected by the receiver operating characteristic (ROC) curve and the curve of detection rate in the simulation experiment, and the metrics are verified by the mathematical modeling on the basis of the experimental spoofing scenario, which proves that the metrics participating in the combination are complementary. Finally, the metrics are combined under the condition of a low correlation. The experimental results show that compared with the existing metrics, the metrics obtained by the merit-based selection and combination have better performance in spoofing detection.

GNSS is an essential source of information for daily life, providing positioning and timing data. However, due to the low power of satellite navigation information at the receiving end, the open signal structure, as well as with the development of spoofing technology, the problem of spoofing and jamming at the receiving end has become increasingly severe. The GNSS spoofing detection technology based on DOA offers robust detection performance, which can adapt to various scenarios. To address the issues of low angular resolution and poor detection performance in a low SNR environment, we propose a CNN-DOA-based spoofing detection method. Firstly, we use the Toeplitz matrix reconstruction algorithm to estimate the DOA of the coherent signal. Then, we use the DOA spectrum generated by random angle distribution and random SNR as samples, with the DOA value and authenticity of the signal used as labels for CNN network training. When the power ratio of the spoofing signal to the authentic signal is greater than 1 dB (SNR = 0 dB), the detection accuracy of the spoofing signal is nearly 90%. Compared to the PI algorithm and the ADBF algorithm, the proposed model has higher resolution and robustness.

To analyze the anti-spoofing performance of the adaptive nulling anti-jamming satellite navigation receiver in response to direct meaconing attrack, this paper derives the theoretical formula for the power inversion (PI) algorithm used in nulling anti-jamming receiver to suppress real satellite signal and meaconing signal in the case of limited number of snapshots. Through the two indicators of signal absolute power and carrier to noise ratio, analyzes the impact of repeated signal input power on the suppression effect. It is found that the PI algorithm has good suppression effect on direct forwarding, making the receiver not spoofed. Finally, using the antenna array software receiver for simulation, the conclusion is validated according to the capture code phase and carrier-to-noise ratio.

With the development of high-mobility satellite navigation systems such as LEO satellite navigation enhancement system, the ability of traditional fixed channel models to simulate channel characteristics has been widely challenged. This work focuses on the background theory, time-varying emulator design, and performance assessment application for the necessity of a channel simulator to enable the propagation modeling from LEO satellite to mobile receiver in the high-mobility satellite navigation scenario. The concept of satellite Doppler effect, separation of satellite Doppler shift, and local Doppler shift are discussed in this work, along with the sampling criteria for time-varying channel modeling. The time-varying simulator based on the model of the time-varying satellite navigation channel is provided, and it is used to assess the tracking loop performance of the navigation software receiver. The results show that the rapidly updating time-varying channel will subject the receiver to severe fluctuations and may possibly result in the tracking loop losing lock. In general, the rapidly time-varying channel scenario is more challenging to track and produce stable output than the general multipath channel scenario.

Short time burst spread spectrum signal has attracted extensive attention in the field of LEO satellite navigation because of its strong anti-interference and good concealment. However, due to the short duration of the short-time burst spread spectrum signal, the acquisition and tracking of the signal is more demanding. The closed-loop tracking algorithm widely used in the navigation field is no longer suitable, and the open-loop carrier synchronization algorithm is needed, which means that the received signal needs to be cached, frequency offset estimation and compensation, which brings great difficultly to the signal receiving and processing. In addition, the traditional L&R carrier frequency offset estimation algorithm is difficulty to balance the estimation range and accuracy. Aiming at the above problems, an improved L&R algorithm is proposed in this paper, which considers the estimation accuracy and greatly expands the estimation range. At the same time, a complete signal receiving and processing framework is designed, and the influence of different parameters on signal receiving preference is fully analyzed in the process of signal receiving, which provides guidance for receiver design and reference for short-time burst spread spectrum signal system design. Through theoretical analysis and simulation verification, the results show that the proposed method can effectively expand the estimation range of frequency offset without affecting the accuracy of frequency offset estimation.

GNSS NLOS signals in complex urban canyons can produce large ranging errors, and the detection and classification of LOS and NLOS signals is a key factor in improving the accuracy of vehicle navigation and positioning. Vision sensors can reflect scene occlusion information in real time, and machine learning is high speed and accurate in processing multiple types of features. In this paper, the advantages of both methods are combined, and the NLOS signals detected by INS and fisheye images are used as visual labels. Six feature values of signal-to-noise ratio and pseudorange consistency, altitude angle and azimuth angle of two frequency points are selected, and NLOS signals based on gradient boosted decision tree (GBDT) and random forest (RF) are designed respectively. The models were trained using urban in-vehicle dynamic data collected by low-cost UBLOX receivers, and compared with three single-learner models: logistic regression (LR), support vector machine (SVM) and naive Bayesian (NB). The experimental results show that the proposed model can be used for real-time dynamic scenes in complex urban environments, is not dependent on additional hardware devices and is suitable for general users. The LOS/NLOS signal classification performance of the GBDT and RF algorithms were comparable, with precision, recall and F1_score reaching over 86%, significantly outperforming the LR, SVM and NB algorithms. The feature importance ranking based on both GBDT and RF algorithms showed that the importance of altitude angle and azimuth angle was greater than the importance of pseudorange consistency and signal-to-noise ratio at both frequencies.

More and more researches have focused on many aspects of Signal Quality Monitoring (SQM) algorithms which have advanced significantly over these years. However, traditional methods could disclose only the partial aspects of signal quality, little has been done on the comprehensive analysis of different parameters that could together reflect the whole characteristics of signal quality. In this paper, we provided a new method for synthetic power spectrum deviation evaluating, and the analysis parameters related to signal power, waveform, frequency spectrum and correlation curve were further improved based on the traditional methods. The evaluating methods of repairing capability, which is unique for BDS-3, and signal polarization mode were proposed and suggested to be added to SQM algorithms. Based on this, we presented a comprehensive analysis of SQM algorithms. Finally, based on the latest data collected by the 40-m large-aperture antenna at the Haoping Radio Observation from May to September of 2020, the initial signal quality analysis of the new-generation BDS-3 navigation signals for all the thirty in-orbit satellites were given. Results showed that compared with the traditional signal quality assessment system, the satellite signal quality assessment system presented in this paper can fully reflect the merits and disadvantages of satellite navigation signals. Research results can further complement the existing signal quality evaluation system, and provide guide reference for BDS and other GNSS satellite in signal performance testing at various stages.

GNSS has been widely used in military, aviation and other fields due to its high precision and wide coverage. However, GNSS signal is so weak that it is susceptible to interference. The antenna arrays anti-jamming algorithms are effective methods to enhance the robustness of the GNSS receiver single point position. According to whether the steering vector of the GNSS signal is needed, the algorithms are generally divided into two categories, i.e. minimum variance distortion less response (MVDR) algorithm and power inversion (PI) algorithm. Both of those two algorithms can implement null steering in the interference direction to suppress interference. The former needs the steering vector of the GNSS signal and can implement beamforming steering in the GNSS signal direction while the latter does not. The cost of those two anti-jamming algorithms is the introduction of pseudo-code measurement error. In this study, the principle of pseudo-code measurement error introduced by those two algorithms is analyzed. Several experiments are implemented to evaluate the pseudo-code measurement errors introduced by those two algorithms when the GNSS signal parameters and the inference signal parameters vary. Results show that –5 to 5 m errors are introduced into the pseudo-code measurement for the PI algorithm when the GNSS and the interference signal parameters vary. For the MVDR algorithm, the pseudo-code measurement errors are smaller than those of PI algorithm. Receiver single point position (SPP) is tested using real BDS observations and simulated pseudo-code measurement errors introduced by those two algorithms. Results show that several meters level position errors are introduced into the receiver SPP.

Smartphones usually use poor performance of linear polarization antenna and low-cost positioning chip, which leads to the existence of a large number of pseudorange observation gross errors, carrier phase observation cycle slips and other issues, seriously affecting the positioning accuracy of smartphones. In order to evaluate the positioning performance of the smartphone precise point positioning model, this paper uses Huawei mate30 smartphone to carry out static and dynamic experiments, from the aspects of model parameters, positioning accuracy, and posterior residuals, the positioning performance of the uncombined model, the single-frequency constraints model, the dual-clock offsets model and the single and dual-frequency model are compared and analyzed. The results show that among the four models, the single and dual-frequency model is more suitable for processing smartphone GNSS observation data. In the plane and elevation directions, the positioning accuracy of the model is improved compared with other models, and the elevation direction is significantly improved. At the same time, the model also performs best in terms of posterior residuals. In static mode, the horizontal direction positioning accuracy of the model is 0.466 m, and the vertical direction positioning accuracy is 0.351 m. In dynamic mode, the horizontal direction positioning accuracy of the model is 1.214 m, and the vertical direction positioning accuracy is 1.453 m. The positioning accuracy in both static and dynamic scenarios can meet the vast majority of daily life needs to use smartphones.

In order to shorten the time to first fix (TTFF) of the satellite receiver, a fast positioning method using both carrier and pseudo code observation is proposed. First, use the Doppler information acquired by the carrier tracking loop to reduce the a priori time accuracy to 100 s. Then, use the code phase acquired by pseudo-code tracking loop, and use the five-state equation to solve the receiver position. In this way, positioning can be achieved before frame synchronization and bit synchronization. Numerical simulation and test results show that positioning can be achieved within 1s through Beidou/GPS dual system in the global or BeiDou alone in the Asia-Pacific region, when the a priori position accuracy is less than 10 km and the a priori time accuracy is less than 2 h.

The antenna group delay (GD) is an important component of the total GD of the satellite navigation and positioning system, and its measurement accuracy will directly affect the positioning accuracy of the system. The Radio Technical Commission for Aeronautics (RTCA) specified the GD measurement method and limit of the GNSS airborne active antenna explicitly in its technical standard DO-301. In this paper, an antenna GD calibration method based on space scan is proposed, which can eliminate the GD measurement error caused by the multiple reflections between antennas during the calibration process, and as a result the high precision GD calibration of the standard antenna is achieved with an extended uncertainty only 0.14 ns (k = 2). Then, the GD of a certain type of domestic Beidou airborne antenna is measured using the standard antenna whose GD is precisely calibrated using the proposed method. The measurement frequency range is 1561.098 MHz ± 2.5 MHz (B1I) and the results shows that the maximum deviation of the GD (Δτ) is 7.43 ns consistent with the requirement of the standard DO-301. The proposed method in this paper enhanced the GD calibration accuracy of the standard antenna, so using this kind of calibrated standard antenna to measure the GD of the Beidou airborne antenna, the accuracy and reliability of the measurement results can be guaranteed.

Binary offset carrier (BOC) navigation signals can alleviate the problem of spectrum resource constraints while improving navigation positioning accuracy and anti-interference capabilities, and have a broad development prospect in Beidou/5G indoor and outdoor seamless high reliability positioning. However, while BOC signals have many of the advantages mentioned above, they also have the problem of multimodality in their autocorrelation functions. Using traditional capture methods can cause ambiguity issues. Therefore, this article is based on the idea of traditional optimization methods such as Aspect(Autocorrelation Side Peak Cancellation Technique) and PCF(Pseudo Correlation Function), Aiming at the problems existing in existing algorithms, an autocorrelation function reconstruction algorithm based on auxiliary signal coordination is proposed to eliminate side peak ambiguity while maintaining a high correlation peak and improving the anti-interference ability of the signal. Theoretical and matlab simulation results show that the algorithm proposed in this paper improves the peak height by about twice compared to the traditional PCF algorithm when applied to higher-order BOC signals, with no significant increase in the main lobe width, and has better anti-interference performance compared to other current optimization algorithms.

The Popular of Global Navigation Satellite System (GNSS) is attributed to its good performance. However, the performance of a conventional GNSS receiver is severely degraded or even failed to provide services in complex environments. Direct Position Estimation (DPE) combines all the visible satellite signals for energy accumulation in the navigation domain first, and then the navigation solution is estimated directly from the accumulation results. Hence the navigation performance of GNSS can be effectively improved in complex environments by DPE. However, the signal accumulation of DPE requires calculating the correlation values of all the visible satellite signals at each candidate grid in the navigation domain, the computation load of which limits its application. To alleviate this problem, a GPU-based parallel implementation of the DPE receiver is designed, which decomposes the operations involved in the joint accumulation into three levels of parallel processing. The effectiveness of the designed parallel implementation of the DPE receiver is verified by experiment results. With the available hardware in our laboratory, the designed DPE implementation improves the efficiency to approximately 146 times on average over the conventional DPE implementation.

Satellite fault or errors in ephemeris calculation may affect satellite position, signal transmission time and other parameters, resulting in receiver positioning failure or incorrect positioning results. To cope with these abnormal situations, in addition to using RAIM method, GNSS receivers usually filter the signals according to the transmission times before performing the positioning calculation, so as to exclude these signals with notably abnormal transmission time. However, when there is spoofing interference, this way may cause all the authentic signals to be excluded, and only the spoofing signals are used for positioning calculation. When the spoofing signals satisfy the consistency, RAIM method cannot detect anomaly, which affects the safe application of satellite navigation system. Aiming at this problem, this article proposes an anti-spoofing method based on signal transmission time density clustering, and analyses the selection of the density clustering threshold. This method can not only deal with abnormal situations such as satellite fault or errors in ephemeris calculation, but also detect spoofing interference which deviates from the authentic signal transmission time to a certain extent, thus improving the anti-spoofing capability of GNSS receivers.

In recent years, Global Navigation Satellite System (GNSS) spoofing detection techniques have attracted wide attention. Compared with jamming, spoofing which always hides in the shadows has stronger concealment leading to more threat to the security of receivers. Carrier-to-noise ratio (C/N0) is one of the important measurements of receiver. Spoofing may cause abnormal changes in the C/N0, but the variations of the C/N0 are not obvious in most cases, and it is liable to cause false alarm in the presence of jamming. Based on antenna arrays, this paper theoretically derives the effective C/N0 model of array receivers as spoofing successfully or unsuccessfully captures tracking loop. And we demonstrate that the C/N0 single difference (CSD) between satellites can be the testing statistic for spoofing detection. Simulation results show that the proposed method can detect the abnormal C/N0 features of counterfeit signals after anti-jamming for the antenna array. This method does not require modifications to the hardware configuration of conventional receivers and can provide an effective defense for array receiver in the combination of jamming and spoofing attacks.

The carrier-to-noise ratio ( $$C/{N}_{0}$$ C / N 0 ) is widely recognized as a vital measurement in Global Navigation Satellite System (GNSS) as it serves as a common indicator of signal power. Its significance is apparent in numerous scenarios such as receiver tracking loop operation, spoofing detection, satellite signal quality monitoring, positioning and system state evaluation. Thus, research on estimating $$C/{N}_{0}$$ C / N 0 has gained importance in practical applications. Most existing $$C/{N}_{0}$$ C / N 0 estimation methods rely on the tracking process as a premise, leading to poor accuracy in specific scenarios where the signal is sporadic, intermittent or data is relatively short. Moreover, the signal tracking process limits the response speed and accuracy of existing methods, making them impractical for emergency scenarios requiring rapid estimation. To address these limitations and expand the applicability of $$C/{N}_{0}$$ C / N 0 estimation, this paper proposes a fast $$C/{N}_{0}$$ C / N 0 estimation method based on the acquisition correlation ratio (ACR). This approach estimates $$C/{N}_{0}$$ C / N 0 in the acquisition stage using only millisecond observation information. Simulation and experimental results indicate that the proposed method can significantly enhance the estimation accuracy of $$C/{N}_{0}$$ C / N 0 in short data scenarios compared to existing methods, achieving an estimation error of 0.52 dBHz with 50ms data.

GPS L2 civil (L2C) signal adopts time-division multiplexed CM and CL code for spread spectrum modulation, which makes the design of replica code one of the crucial factors that determine the implementation complexity and system performance in GPS L2C baseband signal processing. Firstly, this paper introduces several commonly used replica codes in the acquisition of L2C CM code. Through theoretical analysis, the performances of these methods are compared from the perspectives of sensitivity and complexity. Furthermore, simple CM and CL tracking methods are compared based on discriminator performances. Then, a GPS L2C software baseband signal processing module with multiple optional replica code shapes which can be implemented for acquisition and tracking schemes as required is developed in this paper. The module is utilized to evaluate the impact of the above replica code designs on acquisition and tracking performance in real-world experiments. The research results provide a test and evaluation platform for users to optimize the design of L2C receiver according to their requirements of complexity and signal processing performance.

The BeiDou Satellite Based Augmentation System (BDSBAS) achieved Initial Operating Capability (IOC) from 2020, which is embedded into BDS as a component. The ionospheric enhancement corrections provided by BDSBAS protects users from threats generated by ionospheric disturbances. This paper addresses the ionospheric algorithm applied in BDSBAS, which consist of the Grid Ionospheric Vertical Delay (GIVD), its corresponding error bound called Grid Ionospheric Vertical Error (GIVE) and the processors used in BDSBAS ionospheric solution. The performance of BDSBAS ionospheric enhancement service is analysed, focusing on service coverage, ionospheric delay correction accuracy and corresponding integrity risk. Results shows that the BDSBAS ionospheric enhancement service basically cover all airspaces over China and surrounding area, with a satellite cut-off angle of 15 degrees at the boundary. Compared to the rapid Global Ionospheric Map (GIM), the accuracy of BDSBAS GIVD is about 2.0–3.0 TEC unit (TECU) during Day of Year (DOY) 130–160, 2021. No misleading information appeared during the test period which indicates that the probability of hazardously misleading information of BDSBAS ionospheric correction is limited below 10–7 per approach.

This paper proposes an intelligent navigation algorithm based on long and short term memory network (LSTM) optimized by the fireworks algorithm (FWA) to address the issue of low positioning accuracy of pure inertial navigation systems under satellite blocking. The LSTM network is used to provide simulated satellite navigation positioning information, and the training, predicting, and validation modes are designed to evaluate the network's prediction accuracy and state under dynamic changes in the carrier movement environment and measurement conditions. The FWA is utilized to dynamically adjust the LSTM network parameters and maintain pseudo location availability in the shortest possible training time. In the presence of satellite rejection, the FWA-LSTM network is flexibly selected to correct the inertial navigation system and maintain network availability. Simulation results demonstrate that the FWA-LSTM method provides supplementary support for satellite navigation under complex conditions, enhancing the training efficiency and availability ratio of the navigation system.