Navigation Message Research Articles

Optimizing the co‐design of message structure and channel coding to reduce the TTD for a Galileo 2nd generation signal

<h3>Abstract</h3> Recently, the joint design of the GNSS message structure and the associated channel-coding scheme have been investigated as a means to reduce the Time-To-First-Fix (TTFF) and particularly the time to retrieve the Clock and Ephemerides Data (CED). In this context, a new method to co-design the navigation message and the channel-coding scheme structure is proposed in this paper. This new co-design enables us to reduce the time to retrieve the CED while enhancing error-correction capabilities under degraded channel conditions. In order to fulfill such requirements, some structured coding schemes are designed, which provide both maximum distance separable (MDS) and full diversity properties under a non-ergodic channel assumption.

Alternative navigation systems use for satellite positioning in Russia

The article provides a comparative analysis of the joint application and accuracy of determining the locations by GLONASS, Galileo, BeiDou satellites, obtained from the navigation messages and available forecasting algorithms.Nowadays the high-precision satellite definitions are based not only on the two-frequency measurement mode use, but also on the information inclusion from various existing satellite systems in processing [1,2]. Practical experiments performed in 2016 at latitude B=47° and presented in [3], confirmed the advantages of the GPS and GLONASS satellite systems’ joint use. To date, alternative GNSSs have appeared - the European “Galileo” and the Chinese “BeiDou”, the capabilities’ studies of which are not given due attention to. Therefore, we will try to compare the location determination accuracy by GLONASS, Galileo, BeiDou satellites received by the navigation messages, by comparing them with the exact data provided by the International GNSS Service (IGS), as well as with the available GNSS prediction algorithms available on the satellite observation equipment manufacturer Trimble site.

Differential Timing Method Based on Modified Traceability Model

The common view time transfer and two-way time and frequency transfer methods are currently the main means for achieving time synchronisation at nanosecond level. However, these methods have some limitations in real time and cost, which limit their wide applications in many fields, such as time synchronisation among base stations of the upcoming 5G network. In order to meet the requirements of nanosecond time synchronisation, a low-cost differential timing method is proposed in this paper by changing the manner of generation of traceability model parameters in GNSS navigation messages. The time deviation between GNSS system time and the timing laboratory that maintains Coordinated Universal Time (UTC) kept by timing laboratory named k (UTC(k)) is monitored by receiving the GNSS signal in space with monitoring receivers. The new traceability model parameters are generated with the monitored time deviations and then broadcast to users through the GNSS navigation message. The precision of the one-way timing method can be improved from tens of nanoseconds to the order of several nanoseconds with the proposed method. In addition, there are obvious advantages to carry out this method on the geostationary satellites in the BeiDou navigation satellite (BDS) constellation. The proposed method is verified on an experimental platform based on the UTC(NTSC) time frequency signal and the geostationary satellites in the BDS-3 constellation.

Behavior of Broadcast Ionospheric-Delay Models from GPS, Beidou, and Galileo Systems

Abstract The GNSS observations suffer from different types of errors that could affect the achieved positioning accuracy based on the receiver type used. Single-frequency receivers are widely used worldwide because of its low cost. The ionospheric delay considers the most challenging error for single-frequency GNSS observations. All satellite navigation systems, except GLONASS, are advising their users to correct for the ionospheric delay using a certain model. Those models’ coefficients are sent to users in the system’s navigation message. These models are different in their accuracy and behavior based on its foundation theory as well as the updating rate of their coefficients. The GPS uses Klobuchar model for mitigating the ionospheric delay. BeiDou system (BDS-2) adopts a slightly modified Klobuchar model that resembles GPS ICA (Ionospheric Correction Algorithm) with eight correction parameters but is formulated in a geographic coordinate system with different coefficients in origin and updating rate. Galileo system uses a different model (NeQuick model). This article investigates the behavior of the three models in correcting the ionospheric delay for three stations at different latitudes during 3 months of different states of ionospheric activity, comparing with International GNSS Service-Global Ionospheric Maps (IGS-GIMs). It is advised from this research’s outputs to use the GPS model for mitigating the ionospheric delay in low-latitude regions during the state of low-and medium-activity ionosphere. It is advised to use the BeiDou model for mitigating the ionospheric delay in mid-latitude regions during different states of ionospheric activity. It is advised to use the Galileo model for mitigating the ionospheric delay in high-latitude regions during different states of ionospheric activity. Also, the Galileo model is recommended for mitigating the ionospheric delay for low-latitude regions during the state of high-activity ionosphere.

Navigation Message Authentication Based on One-Way Hash Chain to Mitigate Spoofing Attacks for GPS L1

Global Navigation Satellite Systems (GNSS) is vulnerable to significant threats such as spoofing attacks. Data and signal authentication can be used as an effective approach to mitigate such security threats. This paper proposes a Navigation Message Authentication (NMA) method based on Timed Efficient Stream Loss-tolerant Authentication (TESLA), which is using one-way encryption chains. The contributions of this paper are two techniques for implementing TESLA using reserved bits for GPS L1 data. The TESLA algorithm is applied to the extracted navigation message of the GPS L1C/A. Due to the flexibility of the GPS L1C message structure, the protocol is also implemented on Civil Navigation signals (CNAV-2) of GPS L1C. In comparison with Elliptic Curve Digital Signature Algorithm (ECDSA), the Authentication Rate (AR) increases to two minutes for each authentication and performance in term of the Authentication Error Rate (AER) is optimized, by using the TESLA method.

Calculation of the Ephemeris and Clock Corrections of GLONASS and GPS Navigation Space Vehicles in Ultra-Rapid Regime on the Basis of Measurement Data

An algorithm and program for calculation in ultra-rapid regime of the ephemeris and time information of GLONASS and GPS navigation space vehicles are developed. The results presented here were obtained at the Main Metrological Center of the State Time and Frequency Service and Determination of the Parameters of the Earth’s Rotation at the All-Russia Research Institute of Physicotechnical and Radio Measurements (VNIIFTRI). Calculations of the ephemeris and time information are performed on the basis of the data of code and phase measurements at two carrier frequencies. The measurement data are presented in the form of hourly RINEX files of observations and files of navigation messages from tracking stations in the network of the IGS international service of global navigation satellite systems (International GNSS Service). Results of test calculations of the ephemeris and clock corrections of GLONASS and GPS navigation satellites in ultra-rapid regime are presented. An analysis of the precision of the results as compared to a posteriori (final) data from other data processing and analysis centers is presented.

Galileo model of group delay accuracy for advanced RAIM users

Group delays (GDs) are errors affecting Global Navigation Satellite Systems (GNSS) due to non-idealities of the satellite or the receiver hardware and can compromise safety critical applications (eg, advanced receiver autonomous integrity monitoring [ARAIM]) when integrity requirements are at major or even hazardous safety level. The paper shows that GPS uses L1 and L2 signals in the generation of the navigation message, and as a consequence, an L1-L5 dual-frequency user must include the GD contribution in the integrity assessment. For Galileo, whose navigation message refers to the aviation frequencies (E1 and E5), the GDs can be modeled separately from the Signal In Space Accuracy (SISA), because they affect only single-frequency users. This model permits a reduction of User Ranging Accuracy (URA) values with respect to those of GPS, thus improving availability for dual-frequency users. Characterizing the GD of Galileo navigation messages, the paper describes a method to include the GD error model in ARAIM and its potential advantages.

The Design and Implementation of GPS CA Code Simulation Experiment

The research on GPS C/A code has certain practical significance. This paper chiefly explain the principles of C/A code generation. On the basis of studying the principle of GPS pseudo-random sequence C/A code generation, the 1.023MHz C/A code is simulated to modulate and transmit 50Hz navigation message information at the frequency of L1 carrier signal. Based on the GUI interface of MATLAB, an executable program is designed. By clicking the button, the time domain diagram and spectrum diagram of navigation message signal, C/A code, L1 carrier signal, spread spectrum, modulation and noise added signal are generated, and the spectrum analysis of GPS L1 satellite signal is completed on MATLAB.

SIS accuracy and service performance of the BDS-3 basic system

On December 27, 2018, the basic system of the third-generation BeiDou navigation satellite system (BDS-3) completed the deployment of its constellation of 18 MEO networking satellites as well as the construction of the operation control system (OCS) and began to provide basic navigation services to users worldwide. Compared with BDS-2, BDS-3 aims to offer users better navigation signals and higher precision with a series of new technologies. For example, the spaceborne atomic clock of BDS-3 is upgraded for higher performance, the Ka-band inter-satellite link is adopted for inter-satellite ranging and communication, and new B1C and B2a signals are broadcast in addition to B1I and B3I signals (compatible with BDS-2). In addition, a 9-parameter model based on a spherical harmonic function is employed for ionospheric delay corrections. Using the observation data from 18 satellites of the basic system, this paper conducts a comprehensive evaluation of the pseudorange measurement characteristics, signal-in-space (SIS) accuracy of navigation messages and global service capability of BDS-3. The results indicate that the pseudorange measurement multipath effect and observation noise of BDS-3 satellites are better than those of BDS-2; additionally, with the support of inter-satellite links, the user range error (URE) of the BDS-3 satellite broadcast ephemeris is better than 10 cm, the precision of the broadcast clock parameter is better than 1.5 ns, and the SIS accuracy is better than 0.6 m overall. Different from the traditional Klobuchar model, the BeiDou global broadcast ionospheric delay correction model (BDGIM) can provide ionospheric delay corrections better than 70% for worldwide single-frequency users. The service capability evaluation of the basic system consists mainly of the accuracy improvement of the B1I and B3I signals according to BDS-2 as well as the global positioning accuracy of the new signals. These results prove that the BDS-3 basic system has achieved the design goal; that is, both the horizontal and the vertical global positioning accuracies are better than 10 m (95%). In the future, 6 MEO satellites as well as 3 GEO satellites and 3 IGSO satellites for regional enhancement purposes will be deployed for full operation; consequently, BDS-3 will definitely provide a higher SIS accuracy and better service capability.

BD-II NMA&amp;SSI: An Scheme of Anti-Spoofing and Open BeiDou II D2 Navigation Message Authentication

Due to openness and lack of authentication, like other members of the Global Navigation Satellite System (GNSS) Club, BeiDou-II civilian navigation signals are vulnerable to all kinds of spoofing attacks. As a result, both the positioning and timing functions of BeiDou-II civil satellite navigation are likely to be controlled by the spoofer. In this paper, an anti-spoofing scheme of BeiDou-II Navigation Message Authentication & Spread Spectrum Information (BD-II NMA&SSI) is proposed by using SM cryptographic algorithms and spread spectrum information to resist spoofing attacks. The SM cryptographic algorithms are used for generating authentication information to detect spoofing attacks. The spread spectrum information is to protect the authentication information in the D2 navigation messages from modification. Experimental results show that the scheme guarantees the authenticity of BD-II satellite navigation messages, meets the requirement of anti-spoofing and takes a rather less effect on the satellite navigation system.

Performance Analysis of Long-Time Coherent Integral Acquisition Algorithm for Weak GNSS Signals

Autonomous navigation places an extremely high demand on the acquisition sensitivity of the global navigation satellite system (GNSS) receivers. To increase the efficiency and sensitivity of the acquisition algorithm of GNSS weak signal, we improve the acquisition algorithm named Improved Acquisition Algorithm with Doppler Frequency Compensation (IAADFC) proposed in our previous study. The bottleneck restricting the performance improvement of the IAADFC algorithm is that it is difficult to estimate Doppler frequency and navigation message bit flip estimation in a very weak signal environment. We provide a detailed analysis and comparison among the proposed IAADFC algorithm, non coherent integration (NCI) algorithm. Simulation results demonstrate that our IAADFC acquisition algorithm can effectively realize the acquisition of GNSS signal with a singal-to noise ratio that down to 15 dB•Hz which is the current most advanced level of signal acquisition.

A New Ionospheric Model for Single Frequency GNSS User Applications Using Klobuchar Model Driven by Auto Regressive Moving Average (SAKARMA) Method Over Indian Region

The single-frequency users of Global Navigation Satellite System (GNSS) require an effective mathematical model that mitigate the dominant errors due to ionospheric delays. Klobuchar model approximately reduces ionospheric effect up to 50% through its coefficients in the navigation message, which is not sufficient for the GNSS single frequency users at critical applications. Hence, a new model, for Single Frequency GNSS User Applications using Klobuchar model driven by Auto Regressive Moving Average Method (SAKARMA) is proposed to forecast and enhance the precision of ionospheric delay estimations for GNSS users. The hourly VTEC maps are obtained from the Assimilated Indian Regional Vertical Total Electron Content (AIRAVAT) by the process of data assimilation using the Kalman filter exclusively for the Indian region (longitude: 65°E to 100°E; latitude: 5°N to 40°N) using 26 GPS TEC stations over Indian region. The accuracy of the SAKARMA model is investigated using the AIRAVAT maps for various Indian geographic regions during both geomagnetic quiet and disturbed conditions of September month in 2016 year. Furthermore, in order to test SAKARMA model, a dual-frequency Navigation with Indian Constellation (NavIC) receiver located at the KL Education Foundation, Guntur, India (geographic: 16.37°N, 80.37°E; geomagnetic: 7.44°N, 153.75°E) is used to collect the observations during 2 - 12 September 2017. Furthermore, SAKARMA model is also validated with Klobuchar model, Klobuchar-style coefficients provided by the Center for Orbit Determination in Europe (CODE) (CODKlob) Model, BeiDou System (BDS2) Model and NeQuick 2 Model over a low latitude NavIC station in forecasting the ionospheric delays. The experimental results of SAKARMA for NavIC have revealed that the MAPE for proposed SAKARMA model is 9-17% (accuracy: 83-91%), while 34-53% (accuracy: 47-66%) for the Klobuchar model. Thus, the results illustrate that the proposed SAKARMA model is capable of predicting the ionospheric delays for single frequency GNSS/NavIC users.

Design of an outdoor position certification authority

We present the design of an outdoor position certification authority. Such an authority aims at certifying the geolocalisation of a mobile device equipped with a global navigation satellite system receiver. Such a receiver is capable of acquiring radio signals (low-level data) and navigation messages (high-level data) in outdoor environments coming from different constellations of global/regional satellite navigation systems and satellite-based augmentation systems. To date, this information is unreliable from a security point of view because it can be easily forged by malicious attackers through specialised spoofing techniques. An outdoor position certification authority defines a client/server architecture through which a user can certify his position by sending the geolocalisation information needed to verify it to one or more remote servers. There are several scenarios for which this service can be very useful and, with the advent of the internet of things age, devices that might require such a service will grow in number.

Estimation of precise orbits and clock corrections of GLONASS and GPS navigation satellites in ultra-rapid regime based on observation data

The article presents the results on development of an algorithm and a program for calculation in ultra-rapid regime of ephemeris-time information for GLONASS and GPS navigation satellites, achieved by now in the Main metrological center of the state service of time and frequency and determination of earth rotation parameters of VNIIFTRI. Calculation of ephemeris-time information is carried out by data of code and phase measurements at two carrier frequencies. The measurement data are presented in the form of hourly RINEX observation files and navigation message files from tracking stations involved in the IGS (International GNSS Service) network. Results of test calculations of ephemeris and clock corrections of GLONASS and GPS navigation satellites are presented. The analysis of the accuracy of the obtained results compared to the aposteriori data of other analysis centers is given.

SCER Spoofing Attacks on the Galileo Open Service and Machine Learning Techniques for End-User Protection

Spoofing attacks pose a clear cybersecurity risk for all systems relying on Global Navigation Satellite Systems (GNSS) for time synchronization or positioning. Secure Code Estimation and Replay (SCER) spoofing attacks are the most challenging type of spoofing attacks, as these may be problematic even for future GNSS protection systems, like Navigation Message Authentication (NMA) or Spreading Code Authentication (SCA). This is one of the reasons that make the development of complementary protection techniques, like the one proposed in this work, necessary. In the first part of the paper, the spoofing SCER attacks are analyzed in detail for GPS and, particularly, for Galileo. The role of the Galileo Pseudorandom Noise (PRN) intra-satellite non-orthogonality distortion term in hindering the attacks is discussed and a detailed comparison between GPS and Galileo expected quality curves for the SCER attack is provided. A complementary detection method for end-user receivers (assuming NMA is used) against SCER attacks is proposed, based on the application of machine learning and a proposed set of features extracted from the receiver search space, assuming the attacker was not able to null the satellite signal.

Chirp-rate quasi-orthogonality based DSSS-CDMA system for underwater acoustic channel

Underwater network node positioning is a key supporting technology for underwater networks. Generally, nodes at known locations (anchor nodes) are used to transmit location information to the node to be located, and the node to be located performs location calculation according to the arrival time of the received information, which requires multiple access communication between the anchor nodes and the node to be located. In order to reduce the multiple access interference (MAI) and distinguish the information of different anchor nodes, it is necessary to study the underwater multiple access method. The GPS positioning system uses direct sequence spread spectrum code division multiple access(DSSS-CDMA) technology, and the positioning satellite transmits a signal formed by the BPSK modulation by transmitting a pseudo-random sequence (PN sequence) to the binary code sequence corresponding to the navigation message to the user equipment to be located. The underwater acoustic (UWA) channel has a complex multipath structure and Doppler effect, which causes a large interference to the communication system. Compared with narrowband communication, spread spectrum communication has a strong anti-interference ability, and can maintain the reliability of the communication system in the UWA channel. Based on this, this paper proposes a quasi-orthogonal Chirp-rate based DSSS-CDMA method under UWA channel, and carries out simulation analysis and experimental verification.

Computing GPS satellite velocity and acceleration from the broadcast navigation message

We present an extension to the Global Positioning System (GPS) broadcast navigation message user equations for computing GPS space vehicle (SV) velocity and acceleration. Although similar extensions have been published, the extension presented herein includes a distinct kinematic method for computing SV acceleration, which significantly reduces the complexity of the equations and improves the mean magnitude results by approximately one order of magnitude by including oblate Earth perturbation effects. Additionally, detailed analyses and validation results using multiple days of precise ephemeris data and multiple broadcast navigation messages are presented. Improvements in the equations for computing SV position are also included, removing ambiguity and redundancy in the existing user equations. The recommended changes make the user equations more complete and more suitable for implementation in a wide variety of programming languages employed by GPS users. Furthermore, relativistic SV clock error rate computation is enabled by the recommended equations. A complete, stand-alone table of the equations in the format and notation of the GPS interface specification is provided, along with benchmark test cases to simplify implementation and verification.

Accuracy analysis of GPS broadcast ephemeris in the 2036th GPS week

Satellite orbit error is one of main sources in Global Positioning System (GPS) receiver positioning error. Using broadcast ephemeris in navigation message, the receiver can calculate spatial position of GPS satellites, then combine pseudorange measurements and achieve positioning solution. In this work, taking broadcast ephemeris in the 2036th GPS week as an example, orbital position of the satellite with the pseudo-random-noise (PRN) code 4 is calculated. Furthermore, using GPS precision ephemeris as a reference, orbital error of the broadcast ephemeris for the PRN 4 satellite is investigated. The results show that the maximum difference of the satellite in three directions is 36 meters, 57 meters and 39 meters, respectively. The three-dimension distance deviating from precision ephemeris is at most 61 meters. The maximum deviation occurs only during certain time periods in the whole GPS week.

Performance of EGNSS-Based Timing in Various Threat Conditions

Today’s society is highly reliant on time and frequency synchronization, e.g., in communications systems and financial networks. Precise timing is more and more derived from satellite navigation receivers that are unfortunately very susceptible to various signal threats. We studied the performance of global navigation satellite system (GNSS) timing under different operating conditions and tested the effectiveness of different techniques that improve timing receiver robustness. These features were tested under various threat scenarios related to specific vulnerabilities in GNSS-based timing, such as interference and navigation message errors, and their efficiency was analyzed against the corresponding scenarios. We found that interference or meaconing-type spoofing can threaten GNSS timing, but it can be detected by means of automatic gain control (AGC) and carrier-to-noise ratio-based methods. GNSS interruptions due to interference can be bridged by a local oscillator holdover technique based on a Kalman filter whose parameters are based on a GNSS time solution. Navigation message errors are mitigated by the European Geostationary Navigation Overlay Service (EGNOS), and constellation-wide timing errors can be detected by the use of a dual-constellation [global positioning system (GPS)-Galileo] cross-check. Dual-frequency operation for timing, in addition to mitigating first-order ionospheric effects, was found to be more robust to interference with the option to fall back to single frequency.

The method of cycle-slip detection and repair GNSS meaturements by using receiver with high stability frequency oscillator

Abstract A method of phase GNSS measurement control is described. The method is based on comparison of geometric range increments which are calculated using measurement data and approximate receiver coordinates and satellite navigation message position taking into account troposphere and ionosphere correction increments. Applicability of moving average for phase GNSS measurement control to detect the cycle slips and outliers is demonstrated. Test results of cycle slip detecting in phase measurements made by the receiver with high stability frequency oscillator are provided.