Will the 1-bit GNSS receiver prevail?

In the recent years, the threat of radio frequency interference (RFI) to Global Navigation Satellite Systems (GNSS) receivers has been increasing due to an escalating number of RFI related GNSS issues, and enhanced complexity of jammers and spoofers. This poses significant integrity threats to various systems that depend on GNSS measurements for navigation and guidance. One of the most versatile methods to detect such instances of spoofing is the use of Automatic Gain Control (AGC) measurements provided by GNSS receivers. AGC is indicative of the change in thermal noise levels observed by the frontend of the receiver, thus it can be used to detect presence of RFI. If AGC is coupled with the carrier to noise ratio density (??/????) values of visible GNSS satellites, it is possible to gain insight into which specific constellations or satellites may have been spoofed. Although AGC has been available in some commercial GNSS receivers and customized frontends for some time, the introduction of raw GNSS measurements in Android 7 has recently allowed it to be accessible in some smartphones as well. This study looks at AGC measurements from multiple smartphone models which have different GNSS chipsets, assesses their behavior under RFI, points out the current limitations, and proposes future improvements that would assist in its usage as a GNSS RFI indicator. Furthermore, the paper discusses the future of AGC measurements that would assist in broadening their applications to all GNSS receivers.

This study evaluates the suitability of compact, lowcost Global Navigation Satellite System (GNSS) receiver module in estimating the effects of tropospheric delay on accuracy of GNSS position solution at GNSS laboratory, the University of Burdwan, West Bengal, India with geographical coordinate of 23.2545°N, 87.8467°E, −11.402 m, using compact, low-cost uBlox F9P GNSS module and high cost Leica GR 50 geodetic receiver in March 2023. GNSS observation data were logged by the two receivers for 24 hours at 1Hz dual-frequency and converted into Rinex observation files in GPS-only, GLONASS-only, and GPS+GLONASS hybrid modes. The converted Rinex observation data were uploaded to the Natural Resources of Canada (NRCan) online service to obtain the user fixed values of latitude, longitude and attitude, and the wet and dry tropospheric delay files via email. The converted Rinex observation files were post-processed using GNSS positioning analysis tool that estimate the GNSS positioning accuracy parameters namely Distance Root Mean Square (2DRMS), Circle of Error Probable (CEP), Spherical Error Probable (SEP), and Mean Radian Spherical Error (MRSE) from the users defined known fixed values of latitude, longitude and altitude. The total delay obtained from the two GNSS receivers were compared with their positioning accuracy parameters. The results showed better position solution on observation days with the least total zenith delay in all the constellations considered. The Leica GR 50 shows an average total zenith delay of 2.476 m in GPS mode, 2.486 m in GLONASS mode and 2.4680 m in GPS+GLONASS hybrid mode while the uBlox F9P shows 2.477 m in GPS mode, 2.494 m in GLONASS mode and 2.4684 m in GPS+GLONASS hybrid mode of operation. The GPS+GLONASS hybrid mode of operation reduces the total zenith delay and enhanced the position solution. Hence, there is no distinct difference between the two GNSS receivers, suggesting that compact, low-cost, GNSS receiver modules can be used for tropospheric study.

With the ever-increasing number of small satellites launched into space, on-board orbit determination is becoming more and more important. Precise satellite positions are needed for constellation maintenance, formation flying, communication optimization, collision avoidance and Earth observation. In addition, accurate time information is required for the functioning of the instruments on board. Low-cost off-the-shelf Global Navigation Satellite System (GNSS) receivers and antennas, adapted to the small size and weight of the satellite, allow precise on-board orbit determination and time synchronization for even the smallest satellites.We analysed more than one year of on-board navigation solutions (NAVSOL) from 8 Astrocast CubeSats, which are equipped with our multi-GNSS single-frequency payload. Based on the residuals from a dynamic orbit fitting, we found that the quality of the NAVSOL from the Astrocast satellites varies strongly both over time and between satellites, with an RMS ranging from a few meters to several tens of meters.In this study, we focus on three main effects that influence the quality of the NAVSOL from the Astrocast satellites: (1) The orbit-fit residuals show a positive radial offset in the range of about 6 m to 15 m, which is caused by ionospheric refraction. Our data show that GNSS signals at negative elevations are very strongly affected by the ionospheric refraction and thus account for the major part of the radial offset. (2) Discrepancies between the NAVSOL positions and velocities, and simulations with a GNSS signal generator show a once-per-revolution periodicity in the out-of-plane component, with amplitudes of 2 m and 5 cm/s in position and velocity, respectively. This effect is probably due to an inappropriate dynamic model used in the real-time navigation algorithm of the GNSS receiver. (3) Depending on the location of the GNSS antenna on board the satellite, the noise level varies by a multiple. This is assumed to be due to the antenna orientations, interference of the GNSS signals with electromagnetic waves coming from the satellite platform, signal attenuation during the transmission from the antenna to the receiver and differences in the soldering or tuning of the individual GNSS antennas.These findings help us refine hardware configurations and firmware settings in preparation for future Astrocast satellites or other nanosatellite missions. By eliminating or minimizing the mentioned effects, we expect to consistently achieve an accuracy on the level of a few meters for the real-time navigation solution of commercial off-the-shelf single-frequency GNSS receivers in space.

In a digital matched filter (DMF) the degradation of signal quality depends on the pre-sampling filter bandwidth and on the number of analogue to digital) converter (ADC) bits. Digital matched filters are used in the acquisition of Global navigation satellite systems (GNSS) signals. Automatic gain control (AGC) in the GNSS receivers is used to determine the maximum level of quantization by the ADC. In (Chang, 1982), the quantization degradation for different pre sampling bandwidths and adc bits is analysed in the presence of Additive Gaussian white noise (AGWN). Bandwidths of more than once (or twice) the data bit rate contribute only about 0.4 dB (or (0.2 dB) to the overall degradation of the DMF. Also 2 bit uniform step quantization with optimum ADC threshold setting is shown to recover most of the digital implementation degradation. In this paper by focusing on pre-sampling filter bandwidth equal to the data bit rate and 2 bit ADC, the effect of quantization on the received GNSS signal quality in the presence of continuous wave (CW) radio frequency interference (RFI) is analysed. It is shown that a one bit ADC can have much higher degradation than the 3.5 dB in the AGWN case. It is also shown that the effect of AGC on the quantization degradation has to do with both power and frequency of CW RFI. In the presence of CW RFI, the optimal value for ratio of maximum threshold of ADC to the effective noise RMS is shown to be about 1.35 which is higher than the case when RFI does not exist (0.8).

Global Navigation Satellite Systems (GNSS) rely on one-way signal travel time measurements from satellites to receivers. To ensure accurate ranging, GNSS must estimate and compensate for clock offsets on at least one end of the transmitter-receiver system. Consequently, clock offsets are highly correlated with GNSS-derived parameters such as station coordinates, tropospheric delay estimates, satellite radial orbit parameters, and apparent geocenter coordinates. Improper handling of clock offsets can lead to their absorption into these parameters, degrading geodetic product accuracy. Traditionally, clocks have been handled within individual techniques without being specially addressed. However, some research efforts and experimental trials have explored using VLBI to synchronize clocks for GNSS and refining clock models to reduce correlations with other estimated parameters in GNSS, thereby improving overall precision.With recent advancements, high-precision optical clocks and fiber-optic time transfer technologies provide new possibilities for enhancing clock synchronization in GNSS. Optical fiber offers a stable, interference-free medium for time and frequency transfer, eliminating atmospheric effects such as ionospheric and tropospheric delays that impact GNSS-based synchronization. Moreover, fiber-optic time transfer is inherently two-way, removing the need for external clock offset estimation. Thus, fiber-optic clock synchronization represents the most precise timing technique currently available. Establishing a common clock by connecting multiple GNSS receivers via optical fiber could significantly reduce the number of estimated clock parameters in GNSS solutions, leading to improved geodetic measurement precision and a more accurate geodetic reference frame.Despite its potential, the approach of implementing a common clock with optical fiber for GNSS receivers remains largely untested. Although optical fiber links have been successfully used in clock synchronization, their integration into GNSS receiver clocking is still in an experimental stage. Notable existing efforts include: (1) the Geodetic Observatory Wettzell, where multiple receivers are connected to a single clock via optical fiber, representing a local baseline setup, and (2) the GFZ experiment, where a receiver is linked to the ultra-stable clock of PTB, representing a regional baseline approach.This study characterizes existing fiber-optic connections among GNSS receivers to identify practical challenges and evaluate their impact on GNSS parameter estimation. We process data from receivers sharing a common clock using Bernese GNSS Software. Initially, PPP will be performed to compare GNSS-derived clock parameters across receivers sharing a common clock, verifying the effectiveness of fiber-optic clock synchronization. Following this validation, we will leverage GNSS-derived clock parameters to evaluate inter-receiver clock differences measured via optical fiber. Subsequently, we will implement a strategy in which only one receiver’s clock parameters are estimated, using fiber-measured clock differences as a priori constraints for other receivers in the PPP solution. This approach reduces the number of estimated clock parameters and serves as a preliminary test of the feasibility of a GNSS common clock framework.Future developments will focus on refining our analysis framework to enable simultaneous estimation of GNSS receivers operating under a common clock. This research contributes to the broader goal of integrating ultra-stable clocks into global GNSS networks, ultimately enhancing the stability of geodetic reference frames and improving GNSS-derived geodetic products.

Despite the significant advances in signal processing methods used nowadays, Global Navigation Satellite Systems (GNSS) receivers still experience substantial challenges, such as signal jamming, which remains a crucial source of degradation of the receiver performance. The presence of jamming signal influences the acquisition and tracking modules inside the receiver leading to loss-of-lock of the GNSS satellite signals. Consequently, GNSS receivers cannot provide reliable position, velocity and time services. The aim of this paper is to comprehensively explore the effects of linear chirp jamming on commercial receivers under high-dynamic scenarios. Moreover, the paper investigates the advantages of using combined GPS/GLONASS receivers under jamming conditions in comparison to using GPS-only receivers. In this paper, a SPIRENT GSS6700 Multi-GNSS Simulator controlled by Spirent SimGEN™ software is used to provide realistic controlled simulation scenarios. The linear chirp jamming signals are created using an Agilent interference signal generator (ISG) unit. Both commercial NovAtel ProPak-G2 Plus and NovAtel OEMV receivers are used to conduct these tests. The results show different behaviors of the various receivers in response to the applied jamming signals. The Carrier-to-Noise (C/N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ), the Dilution of Precision (DOP), and the navigation solution accuracy are used as measures to assess the performance of the receivers under study. Results show that the NovAtel OEMV receiver outperforms the NovAtel ProPak-G2 Plus receiver. Moreover, it is revealed that multi-constellation receivers achieved higher resistance for signal jamming effects than GPS only receivers.

With the continuing spread of distributed measurement and control systems (e.g. Cyber-Physical Systems, Industry 4.0) there is an increased need to implement synchronized sampling and actuation over the components of a distributed system. To achieve this goal, the clocks of the system must be synchronized, including the ones doing sampling and actuation. In most cases local synchronization alone is not enough; It is also needed to keep in sync with a global reference time in order, for example, to be able to implement timestamps based on global time and simultaneous sampling across system components that are spatially distributed (operate on different sites). This global reference time mostly means one which is provided by a GNSS (Global Navigation Satellite System) service, such as GPS. In these systems, it is the master clock which is responsible for attaining (e.g. via a GNSS receiver) and keeping the global reference time. It can then pass it on in the local communication network, usually using IEEE 1588 (Precision Time Protocol). It follows, that thus, the characteristics of such systems are heavily limited by the precision, accuracy and reliability of the used GNSS receiver. Our paper thus is about the examination of potentially promising GNSS receivers for which we developed a custom measurement system. This includes the design of a test PCB, the writing of a PC data collecting program, the formulation of MATLAB scripts which process the collected data, and the validation of the data collecting system via GNSS receivers simulated by TI evaluation boards. We also conducted measurements with the system on the Quectel L86 GNSS receiver, the results of which are presented in the paper.

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

The tracking performance of a global navigation satellite system (GNSS) receiver is altered by signal distortions of different kinds, ranging from transmitter and receiver imperfections to various forms of channel impairments. Accurate modeling of the receiver performance in the presence of signal distortions allows to improve GNSS-based positioning, tighten error bounds, and thus enhance the integrity and the availability of the GNSS services. We present a model for the code-tracking loop performance of a GNSS receiver when the GNSS signal undergoes an arbitrary cascade of nonlinear and linear distortions. The impact of the distortions on the pseudorange estimation is analyzed by deriving closed-form expressions for the bias and variance of the tracking error. These analytical expressions are based on the well-known open-loop linearized delay-locked loop model. The validation is then performed using a GNSS software receiver that evaluates the tracking error numerically.

Sports watches come equipped with Global Navigation Satellite System (GNSS) receivers capable of tracking GPS, GLONASS, and Galileo, and thus, can be considered low-cost GNSS receivers. In our research, we conducted three distinct experiments to evaluate the accuracy of 1) positioning, 2) distance determination, and 3) height determination based on sports watches using different GNSS combinations: GPS, GPS + GLONASS, and GPS + Galileo. For positioning, a professional GNSS receiver served as the reference. To determine height, we used a laser rangefinder. The assessment of distance measurement utilized a full-size athletics track. A noteworthy outcome of this study is that the increased cost of sports watches does not consistently correlate with higher GNSS positioning accuracy. The most accurate 2D results are achieved by the Polar M430 for the GPS case with the mean positioning error of 1.74 m, and the Garmin Fenix 6 PRO for GPS + GLONASS case with the mean error of 1.43 m.

The impact of L‐band equatorial ionospheric scintillation on Global Navigation Satellite Systems (GNSS) receivers is investigated in this paper using both real and synthetic scintillation data. To this end, various low‐latitude data sets, recorded during the most recent solar maximum, are exploited in post‐processing to develop and verify realistic simulation tools and evaluate GNSS receiver performance. A scintillation simulation model is implemented based on the phase screen formulation of Dr. Charles Rino (1979, 1982, and 2011) which allows oblique signal propagation in an anisotropic propagation medium with multiple irregularity layers (or phase screens) for multiple GNSS frequencies. The observed real scintillation parameters are used to drive GNSS signal simulations. The subsequent simulated GNSS signal time series are verified through comparison with real data for different signal tracking states including the most severe and challenging tracking scenarios. Using both real and synthetic data sets, the impact of scintillation on observation quality and receiver performance is evaluated in terms of probability of loss of phase and frequency lock, as well as the correlation of disturbed L‐band signals transmitted by GNSS satellites on the same transionsopheric path.

Due to the low signal power, the Global Navigation Satellite System (GNSS) signal is vulnerable to interference and even cannot be captured or tracked in harsh environments. As an alternative, the Low Earth Orbit (LEO) satellite has been widely used in the navigation field due to the advantages of low cost and strong signals. It is becoming a significant component of the new combined navigation system with GNSS. The combination of an LEO Doppler signal and GNSS observables can improve the positioning accuracy and high-precision positioning convergence time of the GNSS receiver. However, the GNSS signal receiving capability cannot be improved from this data fusion level. We propose a novel assisted structure where GNSS signal acquisition and Doppler tracking are assisted by LEO Doppler positioning. The receiver uses the LEO signal to achieve Doppler positioning firstly. Then, the coarse position with the GNSS navigation messages received from LEO, as well as the estimated clock information, is used to assist in the acquisition and tracking of GNSS. In this way, the GNSS receiver’s sensitivity can get the benefit from this integrated system. The paper presents the structure of the assisted receiver and analyzes the assisted GNSS signal acquisition and carrier tracking performance in detail. Simulation experiments of this assisted structure are carried out to verify its superiority of acquisition and tracking sensitivity in comparison with standalone GNSS receivers. Theoretical analysis and experimental results show that the proposed acquisition method can achieve 90% detection probability at a carrier-to-noise ratio (C/N0) of 15 dB-Hz, which is about 8 dB higher than the conventional acquisition method without assistance; the proposed tracking method can track weak signals of 5 dB-Hz, which is about 4 dB higher than the conventional method. Therefore, this novel LEO-assisted receiver has significantly improved weak signal acquisition and tracking sensitivity.

The landscape of Global Navigation Satellite Systems (GNSS) signals is undergoing significant change. The parallel development of new, more robust and versatile signals along with two new GNSS systems with global reach is having a dramatic impact on the development of GNSS receivers. Receiver manufacturers are continually developing and implementing unique signal acquisition and tracking algorithms, advanced integrity monitoring algorithms, advanced multipath mitigation algorithms and a host of other enhancements in an effort to improve the performance of GNSS receivers and make their products stand out in a crowded field. The objective of this research is to develop a methodology for comparing the pseudorandom noise (PRN) code tracking performance of various receivers. While the overall performance of a receiver depends on many more factors, effective PRN code tracking is fundamental to the operation of a GNSS receiver. The methodology developed in this research requires three fundamental measurements from the receiver under test – code pseudorange, carrier pseudorange and carrier-to-noise-density ratio (C/No). These measurements are generated by all GNSS receivers but may not always be available to the user. Therefore, this methodology can only be performed on receivers that output these values. This methodology might be useful as a tool to evaluate the suitability of various receivers in certain environments or applications (e.g., weak signal environments, high dynamic environments, unfriendly radio frequency (RF) interference environments, etc.) and also as a means of comparing the relative performance of receivers in those environments or applications. Expectations for the research were that the data collection and analysis techniques would generate code loop tracking jitter plots that closely tracked with theoretical curves. In addition, it was expected that different receivers would yield slightly different jitter plots giving them a unique “signature.” Finally, it was expected that some receivers might incorporate scalar code tracking loops and others might incorporate vector tracking loops. The methodology was developed and refined using three receivers from three popular manufacturers, each with a unique set of capabilities. Raw data was collected from each receiver and processed using purpose built algorithms to estimate the code loop tracking jitter for each signal for each receiver. The code loop tracking observations for each of the three receivers tested were compared with theoretical curves generated using closed form equations published in previous papers. In most cases, the data compared favorably. However, there were some significant outliers that are worthy of additional research and analysis. In addition, the jitter plots for one receiver tested indicated that it used scalar code tracking loops and the jitter plots for the other two receivers indicated that they use vector tracking loops. It appears to be possible to determine if a receiver uses scalar tracking loops or vector tracking loops based on the jitter estimates generated in this research. This research indicated that the methodology might be a useful tool for 2 characterizing GNSS receivers and, eventually evaluating their performance in various environments. However, more data collection and analysis is required to further validate and refine the methodology. Also, more analysis of the signals that yielded “non-conforming” jitter plots should be examined in more detail.

Ionospheric scintillation is a challenging issue for the Global Navigation Satellite System (GNSS). Data collected by the globally distributed GNSS receivers provide abundant information about the ionosphere. S4 is one of the most important parameters of the scintillation, which can be measured by the GNSS receivers. We established a simplified probability model for S4 measured by the GNSS receiver. This model fully considers the correlation of the signal intensity and the ambient noise introduced by the receiver. A factor that reveals the correlation feature of scintillated intensity was proposed. Based on this model, the Cramer–Rao bound (CRB) and the minimum-variance unbiased estimator for S4 were deduced and analyzed. The CRB shows that the uncertainty of S4 increases as the scintillation becomes severe and the decorrelation time becomes longer. Then an approximate probability model was established to describe the statistics of the common estimator of S4. Simulation tests were carried out to validate the proposed model. Based on the approximate model, the statistics of the common estimator was analyzed. We found that, apart from ambient noise, the variation of signal intensity leads to a minus bias for S4 measurements, which seems to have been neglected in the past. A method to correct this bias was proposed. We also found that the increase in the carrier-to-noise ratio decreases the bias but helps little in reducing the variance of the measurements. Considering the accuracy of S4 measurements and the robustness of the tracking loop, we found that for weak scintillation, the value 0.02 s is an ideal coherent time. For moderate scintillation, a relatively ideal coherent time is 0.004 s and for severe scintillation, 0.001 s is an ideal coherent time. Based on this analysis, suggestions for GNSS receiver configurations were proposed.

Recently, smart cities functionality and management have captured notable consideration. Owing to the rapid development in the information and communication technologies (ICT), various applications and services are highly engaged in the cities' operation. Specifically, intelligent road services as traffic management, driver behavior assessment and crowdsensing based road condition monitoring contribute towards better operability. To sustain decent performance of these applications, accurate and continuous positioning is an essential concern. Generally, Global Navigation Satellite System (GNSS) receivers are vulnerable to partial or complete outages due to multipath or signal blockage. Consequently, inertial navigation systems integrated with GNSS receivers are affected by inertial sensors noises and biases. In this paper, we apply wavelet packet de-nosing to eliminate noises of the Micro-Electro-Mechanical Systems (MEMS) grade inertial sensors. Afterwards, we integrate the de-noised reduced inertial sensor system (RISS) with GNSS receivers in real road experiment to assess the system performance. In addition, we show the significance ofthe proposed integration over the conventional one during multiple GNSS outages under various driving scenarios.