GNSS Applications Research Articles

Parallel Computation of Multi-GNSS and Multi-Frequency Inter-Frequency Clock Biases and Observable-Specific Biases

With the widespread application of GNSS, the delicate handling of biases among different systems and different frequencies is of critical importance, wherein the inter-frequency clock biases (IFCBs) and observable-specific signal biases (OSBs) should be carefully corrected. Usually, a serial approach is used to calculate these products. To accelerate the computation speed and reduce the time delay, a multicore parallel estimation strategy for IFCBs, code, and phase OSBs by utilizing task parallel library (TPL) is proposed, the parallel computations, including precise point positioning (PPP), IFCBs, and OSBs estimation, being carried out on the basis of data parallelisms and task-based asynchronous programming. Three weeks of observables from the multi-GNSS experiment campaign (MGEX) network is utilized. The result shows that the IFCB errors of GPS Block IIF and GLONASS M+ satellites are nonnegligible, in which the GLONASS M+ satellite R21 shows the largest IFCB of more than 0.60 m, while those of other systems and frequencies are marginal, and the code OSBs present excellent stability with a standard deviation (STD) of 0.10 ns for GPS and approximately 0.20 ns for other satellite systems. Besides, the phase OSBs of all systems show the stability of better than 0.10 ns, wherein the Galileo satellites show the best performance of 0.01 ns. Compared with the single-core serial computing method, the acceleration rates for IFCBs and OSBs estimation are 3.10, 5.53, 9.66, and 17.04 times higher using four, eight, sixteen, and thirty-two physical cores, respectively, through multi-core parallelized execution.

Extending the GPS IIIA antenna calibration for precise orbit determination of low Earth orbit satellites

High-precision applications of GNSS require accurate calibrations to correct for phase variations of the transmitting antennas. Calibrations distributed by the International GNSS Service (IGS), based upon observations from the global network of ground stations, are often used as the source of the transmitter calibrations as they are inherently linked to a specific definition of the International Terrestrial Reference Frame. The IGS provides antenna phase variations as a function of boresight angle for each basic block of navigation satellite. To support scientific missions operating in low Earth orbit, the antenna calibrations must be extended beyond the 14-degrees limit that can be observed by GNSS receivers on the Earth. Extended antenna calibrations to accommodate low Earth orbiting satellites have already been derived for the GPS Block II satellites. This paper derives an extension solution for the new GPS IIIA antenna calibration, based on a year of observations from the Sentinel-6 Michael Freilich altimetry mission and validated using Jason-3. These solutions are inherently consistent with the IGS-provided models of the Block II antenna calibrations. We use the new model for the GPS IIIA transmitter antenna calibration extensions to evaluate the impact on precise orbit solutions of both Sentinel-6 MF and Jason-3.

Monitoring the dynamic response of a pedestrian bridge by using low-cost GNSS receivers

The development of low-cost GNSS receivers with carrier-phase measurement capacity has led to low-budget GNSS applications of higher accuracy and precision. Recent studies have mainly been carried out with those low-cost receivers for landslide monitoring and achieved promising results. In this study, the performance of two closely-spaced high-rate low-cost GNSS receivers was assessed against the robotic total station (RTS) and geodetic GNSS receiver in monitoring the dynamic response of a major pedestrian suspension bridge at the mid-span. Potential accuracy improvement by the combination of two low-cost GNSS time-series was also examined. It was proved that multi-GNSS solution is required to resolve potential outliers and offsets of the low-cost GNSS time-series, due to cycle slip induced errors. The analysis of the low-cost GNSS time-series showed that the low-cost GNSS receivers can estimate (i) the main dominant frequencies of the bridge with the same accuracy as the geodetic-grade GNSS receiver and (ii) the amplitude of the bridge response with difference of ∼3 mm with respect the geodetic GNSS receiver due to higher noise level. This study revealed the prospect of utilising low-cost GNSS sensors in monitoring dynamic displacement with frequency of 1–3 Hz, corresponding to relatively rigid structures (e.g., short span bridges, etc.).

Analysis of CYGNSS Soil Moisture Retrieval Based on ANN over a Selected Region in Ethiopia

Soil moisture (SM) plays a vital role in agriculture, ecosystem functioning, water conservation, weather predictions and climate models. High spatial and temporal frequency data of soil moisture is crucial for agricultural and other important applications. Recent advancements have brought attention to the possibility of using GNSS reflectometry (GNSS-R) for applications on land such as snow sensing, soil moisture retrieval, sea surface monitoring and other applications in addition to positioning, navigation, and timing applications of GNSS. Cyclone Global Navigation Satellite System (CYGNSS) is designed to improve hurricane forecasting by studying the interaction between the ocean and the atmosphere within tropical cyclones. However recent studies show the opportunity of this system for high spatio-temporal soil moisture retrieval. This study presents a machine learning-based approach to get SM at a selected region in Ethiopia using CYGNSS data and analysis of the result. Artificial Neural Network (ANN) model is developed and used to predict soil moisture. The Soil Moisture Active Passive (SMAP) global soil moisture data have been used as reference data in the ML algorithm. The proposed approach has achieved a good correlation between predicted values of soil moisture and reference values from SMAP.

A Survey for GNSS Application of Ionospheric Information Extraction, Modeling, and Forecasting Techniques

lonospheric information plays a signifcant role in modern communication and navigation systems, This article provides a comprehensive survey of the application, modelingmethods,and results related to ionospheric infor-mation, The article frst introduces prerequisiteknowledge of the ionosphere,and then describesthe methods and techniques used in the extrac-tion of ionospheric information, the generationof ionospheric Vertical Total Electron Content(VTEC) maps, the modeling and interpolation ofthe ionosphere, and the forecasting of ionosphericinformation, The article also provides illustrativeexamples and fgures to demonstrate the effectiveness of the presented methods. Our surveyprovides insights and guidance for researchers andpractitioners interested in developing ionosphericmodeling and forecasting methods for GNSS applications.

Simulation Analysis of LEO Constellation Augmented GNSS (LeGNSS) Zenith Troposphere Delay and Gradients Estimation

Compared with current GNSS, which consists of medium- and high-altitude orbit satellites, Low Earth Orbit (LEO) satellites move faster and can greatly improve the observing geometry. In this contribution, LEO constellation augmented GNSS (LeGNSS) troposphere estimation is investigated, and the impacts of relevant factors are analyzed in detail. When the temporal resolutions of zenith troposphere delay (ZTD) and horizontal gradients are 1 h and 2 h, while standard deviations (STDs) of phase and pseudorange observations at the zenith direction are 0.005 m and 0.5 m, the accuracies of ZTD, north gradient, and east gradient augmented by LEO constellation improve by 15.7%, 29.6%, and 16.4%, respectively, compared with GNSS solution. The results of troposphere estimation under obstructed environment and using low-cost dual frequency receivers also become more robust after adding LEO observations. Most importantly, analysis results during periods with rapid varying troposphere parameters suggest that the contribution of LEO constellation becomes even bigger with increasing temporal resolutions of ZTD and horizontal gradients, indicating that LeGNSS can be used to extract tropospheric parameters with improved accuracies at high temporal resolution. Thus, the capability in rapidly capturing severe weather events can be improved by LeGNSS observations, and the performance can become even better with increasing number of LEO satellites. All these results suggest that LeGNSS might be an important tool in improving the performance of troposphere estimation, and would promote GNSS application in water vapor monitoring.

Assessment of the accuracy of low-cost multi-GNSS receivers in monitoring dynamic response of structures

The monitoring of bridges is a crucial operation for their structural health examination and maintenance. GNSS technology is one of the methods which are applied with the main advantage that the direct measurement of the bridge displacement is conducted in an independent global coordinate system. However, the high cost of the GNSS stations, which are consisted of dual-frequency receivers and geodetic GNSS antennas, is the main reason of the limited application of GNSS for bridge monitoring. In this study, we assessed the performance of low-cost multi-GNSS receivers in monitoring dynamic motion, similar to that of bridge response. The performance of the low-cost GNSS receivers was assessed based on controlled experiments of horizontal and vertical motion. For the horizontal motion, controlled experiments of circular motion of various predefined radius between 5 and 50 cm were executed where the low-cost GNSS receivers were assessed against dual-frequency geodetic receivers. For the vertical motion, manually controlled experiments of vertical oscillations of amplitude 8 and 15 mm were executed where the low-cost GNSS receivers were assessed against the Robotic Total Station (RTS). Finally, a low-cost monitoring system formed by two closely spaced low-cost GNSS receivers was applied in dynamic displacement monitoring of the Wilford Suspension Bridge. The analysis of the low-cost GNSS data revealed the beneficial contribution of (i) the multi-constellation on the accuracy and precision of the GNSS solution and (ii) the combination of closely spaced low-cost GNSS receivers, to limit potential cycle slips and the low-cost GNSS noise level and reach accuracy and precision similar to that of geodetic-grade GNSS receivers. This was confirmed in the bridge monitoring application, where the main modal frequency and the response amplitude of the bridge were identified successfully by the low-cost GNSS receivers’ data analysis.

An improved global grid model for calibrating zenith tropospheric delay for GNSS applications

An improved global grid model for calibrating zenith tropospheric delay for GNSS applications

Ionospheric scintillation characteristics and their effects over GNSS signal and position and navigation systems in the Brazilian region

The ionosphere is an ionized layer extending from about 50 km to 1,000 km of altitude. When an electromagnetic signal cross this layer it suffers a delay in its group velocity and an advance in its phase velocity. The ionosphere is very dynamics and after the sunset its F region equatorial bottomside is lifted up by the intensified eastward electric field, giving origin to an steep plasma gradient. This configurates an unstable condition with higher density plasma standing over lower density one. This instability pushes rarified plasma upward giving origin to large regions named Equatorial Plasma Bubbles (EPB) that rise at equatorial regions and map to low latitudes along the magnetic field lines and can reach continental extension. Through cascating process small scale irregularities (cm to km) are generated inside the EPBs. Due to refractive effects amplitude and phase scintillations are generated in the signal crossing these irregularities. Trans-ionospheric signals used in telecommunication links and in the GNSS applications are severely affected during ionospheric scintillations. In this work we will present the ionospheric scintillation morphology over Brazilian longitudinal sector, its effects over positioning and navigation systems and the existing methodology to mitigate them.

AN APPRAISAL OF THE ECMWF REANALYSIS5 (ERA5) MODEL IN ESTIMATING AND MONITORING ATMOSPHERIC WATER VAPOUR VARIABILITY OVER NIGERIA

This study research the performance of the ERA5 reanalysis model in estimating and monitoring the variability of atmospheric water vapour content over Nigeria. The ERA5 is a fifth-generation reanalysis model recently released by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA5 model comes with excitingly high spatial and temporal resolution when compared to earlier models like the ERA-Interim and ERA-40. However, like the previous models, the ERA5 comes with numerous modelling uncertainties arising from data fusion methods and observation schemes, which often affects its performance at the different regions of the Earth. In this study, ERA5 precipitable water vapour (PWV) was validated with GNSS PWV from permanent GNSS stations in Nigeria NIGNET for the period of 2012–2013. The performance of ERA5 was investigated at sub-daily, diurnal, and seasonal scales in relation to KöppenGeiger climate classification using standard statistical metrics (namely, coefficient of correlation (r), Root mean square error (RMSE), Reliability index (RI), Mean absolute errors (MAE) and mean bias). The r, RI, RMSE, MAE and mean bias values at sub-daily, diurnal and seasonal scales were computed as, (0.8670, 0.882, 0.979), (3.697 mm, 3.400 mm, 7.014 mm), (1.015, 1.019, 1.008), (2.769 mm, 2.706 mm, 1.939 mm) and (0.826 mm, 2.033 mm, 1.739 mm), respectively. The results indicate the strongest performance of ERA5 at seasonal scale with more than 95% agreement. The pattern of variability of ERA5 within the different climate zones of Nigeria showed good consistency with GNSS PWV and Köppen-Geiger climate classification. The study recommended the use of ERA5 in the retrieval of historic PWV records and near real-time GNSS applications.

Comparison concept and quality metrics for GNSS antenna calibrations

Precise values for absolute receiver antenna phase centre corrections (PCC) are one prerequisite for high-quality GNSS applications. Currently, antenna calibrations are performed by different institutes using a robot in the field or in an anechoic chamber. The differences between the antenna patterns are significant and require a sound comparison concept and a detailed study to quantify the impact on geodetic parameters, such as station coordinates, zenith wet delays (ZWDs) or receiver clock estimates. Furthermore, a discussion on acceptable pattern uncertainties is needed. Therefore, a comparison strategy for receiver antenna calibration values is presented using a set of individually and absolutely calibrated Leica AR25 antennas from the European Permanent Network (EPN), both from the robot (Geo++ company) and from the chamber approach (University of Bonn). Newly developed scalar metrics and their benefits are highlighted and discussed in relation to further structural analysis. With our metrics, properties of 25 patterns pairs (robot/chamber) could be used to successfully assign seven individual groups. The impact of PCC on the estimated parameters depends on the PCC structure, its sampling by the satellite distribution and the applied processing parameters. A regional sub-network of the EPN is analysed using the double difference (DD) and the precise point positioning (PPP) methods. For DD, depending on the antenna category differences in the estimated parameters between 1 and 12 mm are identified also affecting the ZWDs. For PPP, the consistency of the observables, i.e. potential differences in the reference point of carrier phase and code observations, additionally affects the distribution among the different parameters and residuals.

A novel dual-loop ground-radiation antenna for circularly polarized GNSS applications

In this investigation, a circularly polarized (CP) dual-loop ground-radiation antenna has been proposed for GPS applications using a transverse loop and a longitudinal loop. The transverse loop is designed within an 8 mm × 4.5 mm (0.042λ × 0.024λ) clearance, installed at the horizontal edge of the ground plane for excitation of the ground mode along the y-axis, whereas a 1 mm × 10 mm (0.005λ × 0.053λ) longitudinal loop is etched at the vertical edge of the ground plane for excitation of the ground mode along the x-axis. In this way, the magnitudes and phases of the orthogonal components of the radiated electric field have been controlled independently, satisfying the CP conditions. The proposed antenna produced right-hand circular polarization towards the + z-axis, which has been verified using rectangular and square-shaped ground planes. Both simulation and measurement were conducted, and the measured axial ratio bandwidth with reference to 3 dB was 20 MHz (1.27%) while the impedance bandwidth with reference to −6 dB was 141 MHz (8.95%).

Evaluation of earth rotation parameters from modernized GNSS navigation messages

Modernized navigation messages of global navigation satellite systems like GPS CNAV include earth rotation parameters (ERPs), namely the pole coordinates and UT1-UTC (∆UT1) as well as their rates. Broadcast ERPs are primarily needed for space-borne GNSS applications that require transformations between earth-fixed and inertial reference frames like navigation in earth orbit as well as to the moon. Based on a global tracking network of 23 stations, broadcast ERP values are obtained for the global systems GPS and BeiDou as well as the regional QZSS and IRNSS. Subsequent data sets at daily intervals show polar motion discontinuities of 0.4 to 0.7 mas for GPS, QZSS, and IRNSS, whereas BDS is worse by a factor of about two. Discontinuities in ∆UT1 range from 0.17 to 0.45 ms. External comparison with the C04 series of the International Earth Rotation and Reference Systems Service results in polar motion RMS differences of 0.3 to 1.0 mas and ∆UT1 differences of about 0.13 ms for GPS, QZSS, and IRNSS. Due to less frequent update intervals, BDS performs worse by a factor of 2 – 4. In view of the current GNSS-based positioning errors at geostationary or even lunar distances, the accuracy of GPS, QZSS, and IRNSS broadcast ERPs is sufficient to support autonomous spacecraft navigation without the need for external data.

Electrochemical genosensor based on gold nanostars for the detection of Escherichia coli O157:H7 DNA.

Escherichia coli O157:H7 (E. coli O157:H7) is an enterohemorrhagic E. coli (EHEC), which has been issued as a major threat to public health worldwide due to fatal contamination of water and food. Thus, its rapid and accurate detection has tremendous importance in environmental monitoring and human health. In this regard, we report a simple and sensitive electrochemical DNA biosensor by targeting Z3276 as a genetic marker in river water. The surface of the designed gold electrode was functionalized with gold nanostars and an aminated specific sensing probe of E. coli O157:H7 to fabricate the genosensor. Cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques were applied for electrochemical characterization and detection. The synthesized gold nanostars were characterized using different characterization techniques. The fabricated DNA-based sensor exhibited a high selective ability for one, two, and three-base mismatched sequences. Regeneration, stability, selectivity, and kinetics of the bioassay were investigated. Under optimal conditions, the fabricated genosensor exhibited a linear response range of 10-5 to 10-17 μM in the standard sample and 7.3 to 1 × 10-17 μM in water samples with a low limit of quantification of 0.01 zM in water samples. The detection strategy based on silver plated gold nanostars and DNA hybridization improved the sensitivity and specificity of the assay for E. coli O157:H7 detection in real water samples without filtration. The detection assay has the advantages of high selectivity, sensitivity, low amounts of reagents, short analysis time, commercialization, and potential application for the determination of other pathogenic bacteria.

Multilevel-teaching/training practice on GNSS principle and application for undergraduate educations: A case study in China

This work discusses a Multilevel-Teaching Practice on GNSS Principle and Application for Undergraduates of Surveying and Mapping in China, underlining the fast development of remote sensing technology. We explore i) the multidisciplinary cross-combination transformation of the traditional GNSS education of remote sensing; ii) we combine theoretical knowledge and practical results in the field of navigation and positioning; iii) we highlight the cutting-edge, intersecting and comprehensive teaching content; innovate the construction of a practical teaching platform for GNSS theory, practice and innovation ability training. A multi-level theoretical and practical curriculum system has been established to strengthen the foundation and professional skills, and cultivate scientific and technological innovation. We propose a teaching method of “Hybrid Online/Offline Teaching”, which makes use of the advantages of online/offline teaching form, effectively improving the quality of undergraduate teaching. In terms of practice and innovation ability training, we establish an open, diversified, research-based practice innovation-education platform based on virtual simulated experiments. The implementation of this teaching approach has achieved remarkable results on undergraduate training for GNSS education.

A Circularly Polarized Meander Loop Antenna Design for GNSS Application

A circularly polarized (CP) meander loop antenna design for GNSS applications is presented. The antenna size is 52.5 × 52.5 × 0.76 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and it is placed 10 mm above a 100 × 100 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> metal plane. The proposed antenna utilizes a meandered loop antenna and has applied a perturbed element into the loop to excite two equal electric fields with near 90° phase difference, thereby radiating a good CP radiation. In addition, a triangular patch is loaded to the two diagonal corners of the loop are for improving the CP performances, as well as maintaining its CP radiation even under the interference of a large metal surface beneath it. The simulated 10 dB impedance bandwidth of proposed antenna was 2.2% (1.557–1.593 GHz), and the measured one was 2.4% (1.553–1.591 GHz). The simulated and measured 3 dB AR bandwidths were 0.8% (1.57–1.583 GHz) and 1.02% (1.567–1.583 GHz), respectively. Last, the proposed antenna is suitable for GNSS application that has large metal surface/ground, such as the roof of a vehicle.

Compact wideband multi-arm 3-D wire antenna for GNSS applications

In this paper, a novel multi-arm crooked wire antenna designed with a multi-objective genetic algorithm (MOGA) is presented. The proposed topology offers a small size, lightweight, and a wideband −10 dB impedance matching. Its fully metallic nature preserves a high radiation efficiency. The multi-arm topology is discussed, and the impact of the number of arms and segments is studied. A proof of concept is optimized, fabricated, and tested. It occupies a volume less than 0.11 λ× 0.13 λ× 0.14 λ, where λ is the wavelength at the lowest operating frequency and covers the entire Global Navigation Satellite System services frequencies with a bandwidth of 503.5 MHz which represents 35.56%, without any matching network. The antenna’s fully metallic structure results in a radiation efficiency higher than 98% and a gain of 8.66 dBi. Measurement results agree with simulations and are elaborated around the frequencies of the lower (1164–1300 MHz) and upper (1559–1610 MHz) bands of the L-band spectrum. A realization-friendly fabrication technique is proposed to realize the 3-D multi-arm wire antennas with high accuracy. The rigid fully metallic structure makes the antenna very suitable to operate in harsh environments, as in aerospace and military applications where substrate-based antennas are unqualified.

Real-time single-frequency precise positioning with Galileo satellites

AbstractFollowing substantial progress achieved recently, the Galileo constellation provides a considerable satellite resource for the GNSS applications. In this regard, the performance assessment of real-time single-frequency precise positioning with Galileo satellites is the main objective of this research. For this purpose, several experimental tests were conducted in this study with two single-frequency positioning models, namely single-frequency code-based positioning and code-phase combination. The results show that Galileo presents an adequate number of visible satellites sufficient for single-frequency positioning. Also, the study demonstrates that, in comparison to GPS observations, Galileo observations have a significantly lower noise level. For the single-frequency code-based positioning, Galileo presents a better positioning accuracy than GPS by 25⋅8% on average. When compared with GPS, a 9⋅4% better positioning accuracy is acquired from Galileo for the single-frequency code-phase combination, with its average convergence time shorter than GPS by a ratio of 24⋅4%.

Multi-frequency quadrifilar helix antennas for cm-accurate GNSS positioning

Abstract For a few years now, GNSS multi-frequency quadrifilar helix antennas (QHA) are available to be used for precise GNSS applications. We performed test measurements with two types of multi-frequency QHA and compared them with a geodetic patch antenna. Although code and carrier phase noise and high-frequent multipath was determined to be larger as compared to the geodetic antenna, the fast-static horizontal coordinate accuracies are on the same level and demonstrate cm-accuracy capability. One of the QHA types exhibited an increased susceptibility to near-field multipath effects which resulted in a degraded accuracy of the vertical coordinate component.

GNSS and RPAS Integration Techniques for Studying Landslide Dynamics: Application to the Areas of Victoria and Colinas Lojanas, (Loja, Ecuador)

This research tests the application of GNSS and RPAS techniques to the spatiotemporal analysis of landslide dynamics. Our method began by establishing non-permanent GNSS networks on the slope surfaces to perform periodic measurements by differential GNSS. Similarly, RPAS flights were made to acquire high-resolution images, which were oriented and georeferenced using ground control points and structure-from-motion algorithms to ultimately obtain digital surface models and orthophotos. Based on GNSS measurements, the direction and velocity of displacements were accurately calculated, and orthophotos and DSMs were used to calculate horizontal and vertical displacements in a set of significant points throughout the study area, reaching accuracies higher than 0.035 m in the GNSS data and 0.10 m in the RPAS data. These values were within the accuracy required for such studies. Based on the field observations and the results from the photogrammetric studies, the two studied landslides were classified as very slow flows. These techniques are the basis for establishing early warning systems in areas of natural hazards based on the calculation of displacement speeds of the surface of slopes.