Future Global Navigation Satellite System Research Articles

Impacts of inter-satellite links on the ECOM model performance for BDS-3 MEO satellites

Inter-satellite link (ISL) plays an essential role in current and future Global Navigation Satellite System (GNSS). In this study, we investigate the impact of ISL observations on precise orbit determination for BeiDou-3 Navigation Satellite System (BDS-3) Medium Earth Orbit (MEO) satellites based on different Extended CODE Orbit Models (ECOM). Thanks to the better observation geometry of the Ka-band ISL data compared to the L-band data for BDS-3 MEO satellites, the ISL solution substantially reduces Orbit Boundary Discontinuity (OBD) errors, except for C30, which suffers from unstable Ka-band hardware delay. From the external quality analysis, ISL significantly enhances the reliability of the orbit of MEO satellites manufactured by the China Academy of Space Technology (CAST). The standard deviation (STD) of the satellite laser ranging (SLR) residuals is approximately 2.5 cm, and the root mean square (RMS) is reduced by 10–23% compared to L-band solutions. Besides, the Sun-elongation angle dependent systematic error in SLR residuals nearly vanishes based on the reduced 5-parameter ECOM (ECOM1) or extended 7-parameter ECOM (ECOM2) with ISL data. This is because the ISL reduces the correlation between state parameters and solar radiation pressure (SRP) parameters as well as those among SRP parameters, leading to a more accurate estimation of both orbit and SRP perturbations, particularly those along B direction. This confirms that the deficiency of the SRP models for BDS-3 CAST satellites can be compensated by using better observation geometry from ISL data. On the other hand, for the satellite manufactured by Shanghai Engineering Center for Microsatellites (SECM), the ISL allows for a more accurate estimation of the Bc1 parameter in the ECOM1 model. This only reduces linear systematic error, possibly because the impact generated by the satellite bus cannot be entirely absorbed by the B-direction parameters.

Hard SyDR: A Benchmarking Environment for Global Navigation Satellite System Algorithms.

A Global Navigation Satellite System (GNSS) is widely used today for both positioning and timing purposes. Many distinct receiver chips are available as Application-Specific Integrated Circuit (ASIC)s off-the-shelf, each tailored to the requirements of various applications. These chips deliver good performance and low energy consumption but offer customers little-to-no transparency about their internal features. This prevents modification, research in GNSS processing chain enhancement (e.g., application of Approximate Computing (AxC) techniques), and design space exploration to find the optimal receiver for a use case. In this paper, we review the GNSS processing chain using SyDR, our open-source GNSS Software-Defined Radio (SDR) designed for algorithm benchmarking, and highlight the limitations of a software-only environment. In return, we propose an evolution to our system, called Hard SyDR to become closer to the hardware layer and access new Key Performance Indicator (KPI)s, such as power/energy consumption and resource utilization. We use High-Level Synthesis (HLS) and the PYNQ platform to ease our development process and provide an overview of their advantages/limitations in our project. Finally, we evaluate the foreseen developments, including how this work can serve as the foundation for an exploration of AxC techniques in future low-power GNSS receivers.

A comparison analysis on forward error correction technology: a future perspective for GNSS

PurposeThe main aim of this study is to elaborately examine the error correction technology for global navigation satellite system (GNSS) navigation messages and to draw a conceptual decision support framework related to the modernization of the GNSS and other systems.Design/methodology/approachThe extensive simulation model developed in Matrix Laboratory (MATLAB) is used to evaluate the performance of forward error correction (FEC) codes such as Hamming, Bose–Chaudhuri–Hocquenghem, convolutional, turbo, low-density parity check (LDPC) and polar codes under different levels of noise.FindingsThe performance and robustness of the aforementioned algorithms are compared based on the bit length, complexity and execution time of the GNSS navigation message. In terms of bit error rate, LDPC coding exhibits more ability in the robustness of the navigation message, while polar code gives better results according to the execution time.Practical implicationsIn view of future new GNSS signals and message design, the findings of this paper may provide significant insight into navigation message modernization and design as an important part of GNSS modernization.Originality/valueTo the best of the authors’ knowledge, this is the first study that conducts a direct comparison of various FEC algorithms on GNSS navigation message performance against noise, taking into consideration turbo and newly developed polar codes.

Geosynchronous satellites expanding a future GNSS satellite constellation: A precise orbit determination study

The integration of geosynchronous orbit (GSO) satellites in Global Navigation Satellite Systems (GNSS) is mostly discussed to enable a regional enhancement for tracking. But how do GSO satellites affect the orbit determination of the rest of the constellation? How accurately can these orbits be determined in a future GNSS tracking scenario with optical links? In this simulation study we analyze the benefit of GSO satellites as an expansion of a MEO (Medium Earth Orbit) satellite constellation – we selected the Galileo satellite constellation – for MEO Precise Orbit Determination (POD). We address not only the impact on POD of MEO satellites but also the possibility to precisely determine the GSO satellites – geostationary orbits (GEO) and inclined geosynchronous orbits (IGSO) – in such an expanded MEO constellation. In addition to GNSS microwave observations, we analyze the influence of different optical links between the participating entities: Optical two-way Inter-Satellite Links (OISL) and ground-space oriented Optical Two-Way Links (OTWL). These optical measurements together with the GNSS microwave observations give a remarkable benefit for the POD capability. In the case of GNSS and OTWL, we simulate the measurements with regard to a network of 16 ground stations. We pay great attention to the simulation of systematic effects of all measurement techniques. We discuss the influence on the systematic errors as well as the formal orbit uncertainties. A MEO constellation expanded with GSO satellites as well as the use of optical links together with GNSS observations not only improves the MEO satellite orbits but also the GSOs to a great extent.

Some New Constructions of Girth-8 QC-LDPC Codes for Future GNSS

In this letter, we propose a new construction of girth-8 Quasi-Cyclic Low-Density Parity-Check codes (QC-LDPC) with various lengths for the global navigation satellite systems (GNSS). This scheme combines two steps. The first is the construction of a family of regular girth-8 QC-LDPC codes of various lengths and rates with two designed sequences. The second is the performance improvement of those from the first construction of half-rate cases using a proposed weight matrix so that the result becomes type-II QC-LDPC codes. This results in some final codes with short lengths of 600, 1200, and 1800, especially for future GNSS. We performed a simulation and confirmed that the proposed QC-LDPC codes of lengths 600 and 1200 have an additional coding gain about 0.3 dB at frame error rate 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> over the LDPC codes used in the Global Positioning System.

Some new RS-coded orthogonal modulation schemes for future GNSS

Considering the availability of the global navigation satellite system (GNSS) signals and the increase in its data rate in the future, this paper proposes some new Reed-Solomon coded (RS-coded) orthogonal modulation schemes using the Hadamard codes, as an alternative to the RS-coded code-shift-keying (CSK) modulation in the Quasi-Zenith Satellite System L-band experimental (QZSS-LEX) signals. The simulation result of the proposed schemes shows that, though the frame size and hence the data rate is increased with much reduced complexity in the receiver, the bit error rate performance of RS-coded orthogonal modulation is slightly better than or comparable to the QZSS-LEX.

Optical clock technologies for global navigation satellite systems

Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1 s and 10,000 s. We introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.

Reference system origin and scale realization within the future GNSS constellation \u201cKepler\u201d

Currently, Global Navigation Satellite Systems (GNSS) do not contribute to the realization of origin and scale of combined global terrestrial reference frame (TRF) solutions due to present system design limitations. The future Galileo-like medium Earth orbit (MEO) constellation, called “Kepler”, proposed by the German Aerospace Center DLR, is characterized by a low Earth orbit (LEO) segment and the innovative key features of optical inter-satellite links (ISL) delivering highly precise range measurements and of optical frequency references enabling a perfect time synchronization within the complete constellation. In this study, the potential improvements of the Kepler constellation on the TRF origin and scale are assessed by simulations. The fully developed Kepler system allows significant improvements of the geocenter estimates (realized TRF origin in long-term). In particular, we find improvements by factors of 43 for the Z and of 8 for the X and Y component w. r. t. a contemporary MEO-only constellation. Furthermore, the Kepler constellation increases the reliability due to a complete de-correlation of the geocenter coordinates and the orbit parameters related to the solar radiation pressure modeling (SRP). However, biases in SRP modeling cause biased geocenter estimates and the ISL of Kepler can only partly compensate this effect. The realized scale enabling all Kepler features improves by 34% w. r. t. MEO-only. The dependency of the estimated satellite antenna phase center offsets (PCOs) upon the underlying TRF impedes a scale realization by GNSS. In order to realize the network scale with 1 mm accuracy, the PCOs have to be known within 2 cm for the MEO and 4 mm for the LEO satellites. Independently, the scale can be realized by estimating the MEO PCOs and by simultaneously fixing the LEO PCOs. This requires very accurate LEO PCOs; the simulations suggest them to be smaller than 1 mm in order to keep scale changes below 1 mm.

EFFECT OF SPATIAL CORRELATION ON THE PERFORMANCES OF MODERNIZED GPS AND GALILEO IN RELATIVE POSITIONING

In the context of processing GNSS (Global Navigation Satellite System) data, it is known that the estimation of the ionospheric delays decreases the strength of the observation model and makes significant the time required to fix the ambiguities namely in case of long baselines. However, considering the double-differenced (DD) ionospheric delays as stochastic quantities, the redundancy in this case increases and leads to the reduction of time of fixing the ambiguities. The approach developed in the present paper makes two considerations: 1) the DD ionospheric delays are assumed as stochastic quantities and, 2) the spatial correlation of errors is accounted for based on a simple model of correlation. A simulation is made and aims to study the effect of these two mentioned considerations on the performances of the three multifrequency GNSSs; modernized GPS, Galileo and BDS which are not yet in full capability. For each GNSS, dual-frequency combinations of frequencies as well as triple-frequency combination are investigated in the simulation. The performances studied include: the time to fix the ambiguities with high success rate and the precision of coordinates in static relative positioning with varying baseline length. A method is developed to derive what we call the spatial correlation model which approximately gives the covariance between the individual errors belonging to two stations. Furthermore, the stochastic models that follow from accounting and neglecting the spatial correlation are developed. The LAMBDA (Least-squares Ambiguity Decorrelation Adjustment) method is implemented for ambiguity decorrelation. The results show that the time to fix the ambiguities caused by accounting the spatial correlation is less than the time of fix without the spatial correlation. Also, a slight superiority of Galileo in terms of performances is seen compared to the other GNSS. For all the dualfrequency combinations investigated, when processing a baseline length of 500 km with accounted spatial correlation, the time needed to successfully fix the ambiguities lies between 5 and 9 min, whereas it becomes only between 2.5 and 3 min for all the triple-frequency combinations, this is with a sampling time of 5 s. In addition, for all different combinations, the coordinates precision is less than 8 mm even for 500 km. We think that these high performances result from: 1) the precise codes of future GNSS signals, 2) the high redundancy in the observations equation and, 3) taking into account the spatial correlation in the definition of the stochastic model.

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.

Global precision analysis of carrier phase relative positioning in BeiDou navigation satellite system and United States global positioning system

In this paper, we derived a high-efficiency formula for calculating the precision of carrier phase relative positioning, analyzed the various factors that affect the positioning accuracy using the carrier phase, and proposed the concept of using a frequency dilution of precision to describe the quantitative effect of different frequency combinations on the positioning precision. To this end, we computed and plotted the global spatial distribution map of the relative positioning dilution of precision for single-day solution, half-hour solution, and single-epoch solution of the global positioning system (GPS), regional Beidou navigation satellite system (BDS2), future global Beidou navigation satellite system (BDS3), and their fusion systems. Using processing software with autonomous intellectual property rights (GCN and VENUS/ARSNet), we solved the measurement data and examined the positioning precision of the single-day solution and single-epoch solution of GPS and BDS2. The analysis demonstrated that the B1/B2 frequency positioning precision of BDS2 was better than that of L1/L2 frequency positioning of GPS, but the positioning precision of the BDS2 is worse than that of GPS over most of the service region of the BDS2. Further, the positioning precision of BDS3 is better than that of GPS in the Asia-Pacific region, while it is the opposite in other regions. Based on these conclusions, we put forth some optimization recommendations regarding the signal frequency of the navigation system and GPS measurement standards to serve as references for optimizing the system performance and formulating standards.

Initial analysis of the BDS satellite autonomous integrity monitoring capability

The integrity of Global Navigation Satellite System (GNSS) has the capability of sending a timely alarm to users when the GNSS is out of use, and it is one of the significant parameters of GNSS. Compared with ground-based integrity monitoring, integrity monitoring within the satellite constellation itself could lessen the effects of possible failures in the signal propagation path and ground systems and shorten the alarm time, which would quickly alert users of the system failure. Developing satellite autonomous integrity monitoring (SAIM) has been one of the main objectives of the upgrade plan for future GNSS such as GPS III, Galileo and the third generation of Beidou satellite navigation system (BDS3). To test the new technology, BDS3 experimental satellites are equipped with SAIM payload which is independently developed by China and it is the first satellite navigation system that applies SAIM method to monitor the navigation signals integrity. SAIM monitors signal quality and clock and frequency stability. We briefly introduce the payload design of SAIM on BDS3 experimental satellites, and evaluate in detail the monitoring measurements stability for two new civil signals of BDS3 open service, B1C_pilot signal, and B2a_pilot signal. By artificially setting signal faults, we analyze the time-to-alert performance of SAIM under its two kinds of alarming modes. The results show that the onboard observation stabilities of signal power, pseudorange, code?carrier bias, and code?carrier divergence are better than 0.12 dB, 0.1?ns, 0.14?ns, and 3.5?mm/s, respectively, which were reliable at 1-year interval. The onboard satellite clock step monitoring precision is approximately 0.5?ns. Once the abnormal signal is detected, the BDS SAIM may provide a warning within 6?s. With the characteristics of overcoming the ground monitoring network limitation and fast alarming ability, SAIM is expected to be an effective method to enhance the BDS3 system integrity monitoring capability.

Initial orbit determination of BDS-3 satellites based on new code signals

For the two newly launched satellites (PRN number 27 and 28) of the future global BeiDou navigation satellite system (BDS-3), there is no available broadcast ephemeris data and other initial orbit information, but the initial orbit is the fundamental of the comprehensive analysis of the satellites and their signals. Precise orbit determination (POD) also requires determination of a priori initial value with a certain precision in order to avoid problems such as filter divergence during POD. Compared with the Newton iteration method, which relies on the initial value, this study utilizes the Bancroft algorithm to directly solve the nonlinear equations with the advantage of numerical stability. The initial orbits of these two satellites are calculated based on new code signals, and their results are analyzed and discussed. The experimental results show that, with the exception of very few epochs, when the new code signal is utilized, the median and robust variance factor of the observed residuals computed using pseudo-range observations and the solved initial orbits are less than 4 and 2 m, respectively. It also shows that this solution can be used for rapid initial orbit recovery after maneuvers of the new BeiDou satellites.

Fault Exclusion in Multi-Constellation Global Navigation Satellite Systems

This paper comprehensively investigates the fault exclusion problem in multi-constellation Global Navigation Satellite Systems (GNSS). In future GNSS, the heightened likelihood of fault detection events will cause more interruptions in the continuity of the navigation operation. The main contribution of this paper is to establish the theoretical basis to quantify the contributions of fault events on continuity risk, therefore allowing us to assess the desired exclusion function performance based on specific continuity requirements. Accordingly, a new real-time exclusion algorithm is developed, for which the upper bounds on integrity risks are rigorously derived. Using the new method, performance is comprehensively investigated for two important civil aircraft navigation operations using various numbers of constellations. We show that high service availability can be achieved for both operations.

Soil Moisture Sensing Using Spaceborne GNSS Reflections: Comparison of CYGNSS Reflectivity to SMAP Soil Moisture

AbstractThis paper quantifies the relationship between forward scattered L‐band Global Navigation Satellite System (GNSS) signals, recorded by the Cyclone Global Navigation Satellite System (CYGNSS) constellation and Soil Moisture Active Passive (SMAP) soil moisture (SM). Although designed for tropical ocean surface wind sensing, the CYGNSS receivers also record GNSS reflections over land. The CYGNSS observations of reflection power are compared to SMAP SM between March 2017 and February 2018. A strong, positive linear relationship exists between changes in CYGNSS reflectivity and changes in SMAP SM, but not between the absolute magnitudes of the two observations. The sensitivity of CYGNSS reflectivity to SM varies spatially and can be used to convert reflectivity to estimates of SM. The unbiased root‐mean‐square difference between daily averaged CYGNSS‐derived SM and SMAP SM is 0.045 cm3/cm3 and is similarly low between CYGNSS and in situ SM. These results show that CYGNSS, and future GNSS reflection missions, could provide global SM observations.

Parity Space Projection Line Based Fault Detection Method for Advanced Receiver Autonomous Integrity Monitoring

The advanced receiver autonomous integrity monitoring (ARAIM) is an augmentation of future multi-constellation global navigation satellite system for fulfilling stringent navigation integrity specified in safety-critical applications. Until recently, the most extensively used ARAIM algorithm is the baseline algorithm using the multiple hypothesis solution separation (MHSS) test statistics. However, the MHSS tests are redundant, which bring loose threshold for each test statistic, due to duplicate allocations of continuity budget to numerous tests. The loose thresholds reduce the ARAIM performances on fault detection, effective monitoring threshold, and integrity. This paper proposes a new projection line based MHSS (PLB-MHSS) fault detection method in parity space. Specifically, we first develop projection line representations for the MHSS test statistics and the position error distribution. The PLB-MHSS method is then proposed using the largest projection of parity vector onto the projection lines as the test statistic. Finally, a corresponding integrity risk bounding method is developed. Without redundantly allocating the continuity budget, the PLB-MHSS method obtains tight fault detecting threshold. Simulation results show that, compared with the traditional MHSS method, the PLB-MHSS method is more sensitive on fault detection, and decreases about 9% of EMT and more than 13% of vertical protection level on average.

Accuracy assessment of Precise Point Positioning with multi-constellation GNSS data under ionospheric scintillation effects

GPS and GLONASS are currently the Global Navigation Satellite Systems (GNSS) with full operational capacity. The integration of GPS, GLONASS and future GNSS constellations can provide better accuracy and more reliability in geodetic positioning, in particular for kinematic Precise Point Positioning (PPP), where the satellite geometry is considered a limiting factor to achieve centimeter accuracy. The satellite geometry can change suddenly in kinematic positioning in urban areas or under conditions of strong atmospheric effects such as for instance ionospheric scintillation that may degrade satellite signal quality, causing cycle slips and even loss of lock. Scintillation is caused by small scale irregularities in the ionosphere and is characterized by rapid changes in amplitude and phase of the signal, which are more severe in equatorial and high latitudes geomagnetic regions. In this work, geodetic positioning through the PPP method was evaluated with integrated GPS and GLONASS data collected in the equatorial region under varied scintillation conditions. The GNSS data were processed in kinematic PPP mode and the analyses show accuracy improvements of up to 60% under conditions of strong scintillation when using multi-constellation data instead of GPS data alone. The concepts and analyses related to the ionospheric scintillation effects, the mathematical model involved in PPP with GPS and GLONASS data integration as well as accuracy assessment with data collected under ionospheric scintillation effects are presented.

A ranging code based on the improved Logistic map for future GNSS signals: code design and performance evaluation

Ranging code is the core component of the signal transmission scheme in any global navigation satellite system (GNSS); its performance directly influences on the technical indexes of positioning accuracy, compatibility, interoperability, anti-interference, security, synchronization realization, and so on. Therefore, research on ranging codes could provide theoretical support for the improvement of the performance of ranging codes and extension of their design methods to future satellite navigation signal structures. In order to improve the balance in classical chaotic sequences, a novel ranging code is proposed in this paper and constructed by a series of the improved Logistic-map chaotic sequences with different initial values through weighted optimization, summation, and quantization. Then a comprehensive performance evaluation method based on the Welch bound including three main indexes has been introduced, namely the performance of acquisition, tracking, and robustness against interfering narrowband signals. Finally, the three indexes are combined in a cost function by weighting to evaluate the proposed code, coarse/acquisition (C/A), Gold, Weil, and Random as well as the conventional chaotic codes, and the corresponding weighted coefficients can be adjusted flexibly according to the user groups or application types. Theoretical analysis and simulation results over an additive white Gaussian noise (AWGN) channel show that the proposed ranging code cannot only demonstrate excellent performance in acquisition and anti-narrowband interference while maintaining high quality in tracking performance as the C/A code but also significantly improve balance performance and strengthen reliability and security.

Time domain raised cosine-binary coded symbol modulation for satellite navigation

The number of satellite navigation signals in space grows dramatically as the number of global and regional navigation satellites constant increases. This phenomenon further aggravates an already crowded radio spectrum in-band and increases out-of-band (OOB) emissions. One feasible solution to the issue of signal compatibility is to design a spectrum-efficient modulation signal that has better navigation performance and backward compatibility with adjacent signals and services in operation. In this study, a time domain raised cosine (TDRC) pulse is introduced as an alternative waveform. A binary coded symbol (BCS) modulation family based on TDRC pulse called TDRC-BCS is also proposed as a candidate for future global navigation satellite system (GNSS). An extensive study on the multipath model analysis approach is also provided. The proposed modulation and existing modulations are then comprehensively evaluated. Theoretical analysis and simulation results show that TDRC-BCS signals offer superior navigation performance compared with existing modulations in terms of code tracking accuracy, multipath mitigation capacity, while maintaining comparable or better anti-jamming performance. These signals can also attain higher spectral efficiency and better backward compatibility with the existing GNSS signals. The proposed modulation scheme provides additional degrees of freedom for GNSS signal design.

SNR and Standard Deviation of cGNSS-R and iGNSS-R Scatterometric Measurements

This work addresses the accuracy of the Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) scatterometric measurements considering the presence of both coherent and incoherent scattered components, for both conventional GNSS-R (cGNSS-R) and interferometric GNSS-R (iGNSS-R) techniques. The coherent component is present for some type of surfaces, and it has been neglected until now because it vanishes for the sea surface scattering case. Taking into account the presence of both scattering components, the estimated Signal-to-Noise Ratio (SNR) for both techniques is computed based on the detectability criterion, as it is done in conventional GNSS applications. The non-coherent averaging operation is considered from a general point of view, taking into account that thermal noise contributions can be reduced by an extra factor of 0.88 dB when using partially overlapped or partially correlated samples. After the SNRs are derived, the received waveform’s peak variability is computed, which determines the system’s capability to measure geophysical parameters. This theoretical derivations are applied to the United Kingdom (UK) TechDemoSat-1 (UK TDS-1) and to the future GNSS REflectometry, Radio Occultation and Scatterometry on board the International Space Station (ISS) (GEROS-ISS) scenarios, in order to estimate the expected scatterometric performance of both missions.