Global Navigation Satellite System Service Research Articles

Construction of global IGS-3D electron density (Ne) model by deep learning

In this study, we construct a global IGS-3D Ne model that generates global 3-D electron density (Ne) from International Global Navigation Satellite Systems (GNSS) Service (IGS) total electron content (TEC) data through deep learning. As a first step towards this, we make a model to generate a vertical electron density profile from a TEC value using Multi-Layer Perceptron (MLP). In this process, we use the vertical electron density profiles and the corresponding TEC values of the IRI-2016 model from 2001 to 2008 for training, 2009 and 2014 for validation, and 2010 to 2013 for a test. The next step is to generate global IGS electron density profiles using the global IGS TECs as input data for the model, which is called the global IGS-3D Ne model. We evaluate the IGS-3D Ne model by comparing the electron density profiles from the incoherent scatter radars (ISRs) at three stations with the IGS-3D Ne model from 2010 to 2013. The evaluation shows that the electron density profiles from the IGS-3D Ne model are closer to the ISR data than those of the IRI model, especially at high latitudes. The IGS-3D Ne model shows that the averaged root mean square error (RMSE) values between IGS and ISR electron density profiles are 0.37 log(m−3), 0.22 log(m−3), and 0.34 log(m−3) for all test datasets at Jicamarca, Millstone Hill, and EISCAT stations, respectively. These results suggest that our method has sufficient potential to enhance the ability to predict global electron density profiles.

Quasi‐Diurnal Lunar Tide O1 in Ionospheric Total Electron Content at Solar Minimum

AbstractFor the first time, characteristics of the geographical and seasonal distribution of the quasi‐diurnal lunar O1 tide were derived from a time series of ionospheric total electron content (TEC) maps provided by International Global Navigation Satellite System Service (IGS). The data analysis is focused on solar minimum in 2008 and 2009 where disturbing influences of geomagnetic and solar activity were minimal. We found that the magnitude of the O1 tide is as strong as the “dominant” semidiurnal lunar M2 tide. Relative amplitudes of 10% and larger are observed in some regions for the O1 component in TEC. The O1 component is particularly strong in northern hemispheric winter over the west coast of South America. There, two maxima occur which are northward and southward of the magnetic equator in the Equatorial Ionization Anomaly (EIA) crest regions. Following Yamazaki et al. (2017, https://doi.org/10.1002/2017ja024601), it might be assumed that a longitudinal anomaly of ionospheric conductivities in the Peruvian sector leads to a stronger modulation of the equatorial electrojet by the lunar tides. Electrodynamic lifting of plasma and transport to the EIA crests may explain the variations of the O1 component in TEC. Contrary to many studies, we find the O1 component (period 25.82 hr) more important than the M1 component (period 24.84 hr, a lunar day). We show that the geographical distribution of the O1 component is totally different from that of the M1 component which is smaller. The seasonal variation of O1 shows maximal amplitudes in northern hemispheric winter and minimal amplitudes in southern hemispheric winter.

Extension of the BDS IGSO and MEO satellite antenna patterns with FengYun-3C and LuTan-1 onboard data

The International Global Navigation Satellite System (GNSS) Service (IGS) has recommended calibrating the extended satellite antenna Phase Center Variation (PCV) model for all navigation satellites to support low-Earth-Orbit (LEO) satellite applications. Owing to the absence of ground-calibrated PCVs for the BeiDou Navigation Satellite System (BDS), tracking data from global ground stations were used for in-orbit estimations. However, limited by the observed maximum nadir angle, PCVs based on ground data cannot cover the entire range of nadir angles observed using by LEO satellites. In this study, the FengYun-3C and LuTan-1 missions were used to extend the PCVs of the BDS Inclined GeoSynchronous orbit (IGSO) and medium-Earth-orbit (MEO) satellites. The PCVs corresponding to the ionosphere-free linear combinations of B1I/B2I, B1I/B3I, and B1C/B2a were calibrated in-orbit by extending the maximum nadir angle from 9° to 10° for the IGSO and from 13° to 15° for the MEO. There was good consistency among the estimated PCVs of satellites of the same type. In addition, these estimated PCVs were close to the corresponding satellite block-specific values estimated based on ground tracking data. The impact of the extended PCVs of the BDS satellite antenna was investigated by comparing them with the unextended PCVs model in the receiver antenna PCVs pattern estimation and precise orbit determination of the FengYun-3C and LuTan-1 satellites. The results showed that the extended BDS satellite PCVs effectively reduced the unmodeled error of the unextended BDS satellite PCVs in the low-elevation region of the LEO receiver antenna PCV patterns. In addition, compared with the introduction of phase center offset alone, the introduction of ground-based PCVs can improve the overlapping orbit differences (OODs) of LEO satellites, and the extended BDS satellite PCVs can further improve the OODs. Taking B1C/B2a as an example, the improvement ratio of OODs in the 3D direction of Lutan-1A increased from 5.2 % to 8.0 %, and that of Lutan-1B can rise from 1.7 % to 4.2 %.

Method and Validation of Real‐Time Global Ionosphere Modeling Constraint by Multi‐Source GNSS/LEO Data

AbstractThis study applies the zero‐differenced integer ambiguity method, named PPP‐Fixed, to extract real‐time ionospheric data and eliminate the latencies of rapid/final Global Ionosphere Maps (GIMs). The PPP‐Fixed method is also used to derive ionospheric data for post‐processed GIM generation, named SGG Post‐GIM, combined with low earth orbit satellite data. The obtained hardware delays are applied to revise real‐time ionospheric data. Meanwhile, the estimated multi‐source ionospheric model is regarded as historical data to estimate an ionospheric prediction model for constraint using the semi‐parameter model. Then, the Kalman filter is employed to estimate the parameters to generate real‐time GIM. Finally, the accuracy of estimated real‐time GIM, named SGG RT‐GIM, and SGG Post‐GIM is assessed. During the experimental period, the mean differences of SGG Post‐GIM and SGG RT‐GIM relative to GIMs provided by the international Global Navigation Satellite System service, named IGSG, are −0.46 and −0.57 Total Electron Content Unit (TECU), respectively. The corresponding Root Mean Square (RMS) values are 1.64 and 3.08 TECU. Over the test period, the mean positioning errors of the single‐frequency precise point positioning corrected by IGSG, SGG Post‐GIM, SGG RT‐GIM, and Klobuchar model are 0.14, 0.19, 0.21, and 0.25 m in the horizontal direction, respectively, while the corresponding errors are 0.36, 0.33, 0.38, and 0.64 m in the up direction. Further, the mean biases of experimental days for the self‐consistency assessment are 0.06, −0.01, and −0.07 TECU for IGSG, SGG Post‐GIM, and SGG RT‐GIM, respectively. The corresponding RMS values are 1.19, 1.15, and 1.57 TECU.

Article: View on Regulations Concerning Communication, Navigation and Surveillance (CNS) Service in Civil Aviation

This article presents the overall regulatory environment related to the provision of Communication, Navigation and Surveillance (CNS) services in civil aviation. Through the evolution of international standards established by the Chicago Convention and through the development of CNS services, insight was given into the importance ofCNSservices in ensuring safe and efficient air traffic.The functions of the CNS system are presented and explained, as well as the key contribution of international organizations such as ICAO and International Telecommination Union (ITU) in the development of these services. Through research on the complexity of CNS services, the evolution in the past decades, the text explains the legal aspects related to civil aviation, but also international telecommunications and space activities that are related to civil aviation. The legal framework that regulates space activities, that is, the use of satellites in air traffic management, is especially covered. airspace (CNS/ATM). ICAO initiatives are additionally addressed, particularly the integration of satellite technology through Future Air Navigation Systems (FANS) and the legal significance of resolutions such as A32-19 on Global Navigation Satellite System (GNSS) services from the 1998 Rio Conference. The narrative further examines the implementation of CNS/ATM concepts, legal considerations in air-ground communications, harmonization efforts, and the consolidation of resolutions for effective global implementation. The text concludes by highlighting ICAO’s ongoing commitment to align technological advancements with legal frameworks, promoting the seamless utilization of CNS/ATM systems worldwide. The significance of regional and national regulations, with a focus on European CNS regulations, is underscored for enhancing air traffic safety and fostering the development of CNS services. CNS Services, Chicago Convention, ICAO international standards, Annex 10, ICAO resolutions, CNS/ATM, ITU, FANS, GNSS, regional and national regulations

A comparative analysis of the performance of various GNSS positioning concepts dedicated to precision agriculture

Abstract Automated guidance systems for precision agriculture rely on Global Navigation Satellite Systems (GNSS) and correction services for high accuracy and precision in field operations. This study evaluates the performance of selected GNSS positioning services for precision agriculture in a field experiment. We use three correction services: SF1, SF3, and RTK, which apply varying positioning concepts, i.e., Wide Area Differential GNSS, Precise Point Positioning, and Real-Time Kinematics, respectively. The tractor is autonomously steered along multiple predefined paths located in open-sky areas as well as near the heavy tree cover. The reference route of the vehicle is determined by classical surveying. Tractor trajectories, a SF1 and SF3 corrections, are shifted from predefined straight paths, unlike in the case for RTK. Offsets of up to several decimeters are service- and area-specific, indicating an issue with the stability of the reference frame. Additionally, the varying performance of the correction services implies that environmental conditions limit the precision and accuracy of GNSS positioning in precision agriculture. The pass-to-pass analysis reveals that SF1 improves the declared accuracy, while SF3 is less reliable in obstructed areas. RTK remains a stable source for determining position. Under favorable conditions, the pass-to-pass accuracy at 95% confidence level is better than 11.5 cm, 8.5 cm, and 4.5 cm for SF1, SF3, and RTK, respectively. In the worst-case scenario, the corresponding accuracies are: 25.5 cm, 65.5 cm, and 22.5 cm.

A rapid ray tracing method to evaluate the performances of ERA5 and MERRA2 in retrieving global tropospheric delay

Atmospheric reanalysis plays an important role in retrieving the atmospheric tropospheric delays with ray tracing for space geodetic techniques. In order to represent the complex weather and climate conditions better, the spatiotemporal resolutions of the newly developed atmospheric reanalysis products are improved significantly. The increased spatiotemporal resolution provides a great opportunity to improve the accuracy of the tropospheric delays derived from ray tracing, but it remains a challenge due to the highly increased computation costs. In this paper, we develop a rapid ray tracing method with refined height interval determination to accommodate the increased spatiotemporal resolution of the atmospheric reanalysis products. The accuracy of this method was validated by the 2010 International Association of Geodesy Working Group 4.3.3 ray tracing Comparison Campaign reference results. Zenith and slant delays were obtained by tracing 342 global International Global Navigation Satellite System Service (IGS) stations. Compared to the traditional method, this reduced memory footprint by 16.1%, global refractivity field construction time by 13.6%, and per ray trace time by 22.5% while maintaining accuracy. Based on this methodology, ray tracing using state-of-the-art fifth-generation European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA5) and second Modern-Era Retrospective Analysis for Research and Applications (MERRA2) at 342 IGS stations assessed tropospheric delay performance in 2021. Results showed significant ERA5 and MERRA2 slant delay and mapping factor differences up to the decimeter level, especially for the wet component. Additionally, using IGS zenith total delay (ZTD) as a reference, ERA5 ZTD bias and root mean square error (RMSE) were 2.3 and 11.9 mm, versus that of 1.8 and 16.2 mm for MERRA2 ZTD. At extreme weather-affected AIRA stations over August 5–9, 2021, ERA5 ZTD mean and RMSE differences were −3.0 and 19.8 mm, and −5.3 and 21.7 mm for MERRA2 ZTD. Tropospheric delays and models derived from ERA5 can support space geodetic applications given improved performance and temporal resolution.

Ionospheric Response to Solar EUV Radiation Variations Using GOLD Observations and the CTIPe Model

AbstractTo understand the global response of thermospheric‐ionospheric (TI) parameters to variations in solar irradiance measurements from the Global‐Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC) have been used. Various parameters such as GOLD O/N2, O2, and the nighttime peak electron density (Nmax) have been compared with the CTIPe model simulations. The GOLD observed Nmax shows a number of significant features including a winter anomaly and an equatorial ionization anomaly. The comparison with solar proxies showed that the GOLD QEUV correlates very well with the EUV observations compared to the F10.7 index. The study also examined the relationship between the solar proxies and Nmax on different time scales and found that Nmax responded significantly to QEUV at both medium‐ and long‐term timescales. Furthermore, a low correlation between Nmax in the equatorial region and solar proxies was found. A delayed ionospheric TEC response against solar flux variations within the 27‐day solar rotation was investigated. This ionospheric delay of TEC with respect to solar flux was observed to be less than 1 day, which was reproduced in model simulations. The current study has shown that the GOLD observations can be used to investigate the delayed ionospheric response and to gain a better understanding of the influence of solar activity on the TI system.

Investigating the Performance of IGS Real-Time Global Ionospheric Maps under Different Solar Conditions

In recent years, real-time global ionospheric map (RT-GIM) products have been actively developed by the international global navigation satellite system (GNSS) service (IGS) and its ionosphere associate analysis centers (IAACs) along with the increase of RT-GNSS multi-frequency and multi-constellation observations. In this study, the accuracy and consistency of three RT-GIM products from the Chinese Academy of Sciences (CAS), Wuhan University (WHU), and IGS are evaluated and analyzed utilizing three validation methods, namely, comparison with JASON-3 vertical total electron content (VTEC), the difference of slant total electron content (dSTEC), and IGS combined final GIM (IGSG) data. The test period was from 1 January 2019 to 31 December 2022, including the different solar activities. First, the comparison with JASON-3 data illustrates that the quality of the three RT-GIM products over oceans is in great consistency with that of the IGSG during different levels of solar activity and the daily mean bias (MEAN) values in low and high solar activities are approximately 5 and 10 TECU, respectively. The root mean square (RMS) values under low and high solar activities can be up to 7 and 12 TECU. Furthermore, the dSTEC validation results present that the MEAN values of RT-GIM products from different IAACs at high- and mid-latitude stations are about 0.5 TECU, which is smaller than those at low-latitude stations at about 1 TECU over continental regions. The standard deviation (STD) and RMS values for various RT-GIM products are within 3 and 4 TECU at low latitudes, respectively. In terms of the comparison with IGSG, the result shows that IGS combined RT-GIM (IRTG) presents better consistency than CAS RT-GIM (CRTG) and WHU RT-GIM (WRTG) in 2021 and 2022, with average annual STD and RMS values of 2.56 and 2.78 TECU, respectively. The daily biases of the RT-GIM products relative to IGSG can reach 4 TECU in high solar activities and the daily STD and RMS values are mainly within the 5 to 6 TECU range, respectively.

Longitudinal, Latitudinal, and Local Time Variations of the 14.5‐Day Periodic Oscillation in the Ionosphere During 2014–2015 SSW

AbstractBased on global total electron content (TEC) maps from the International Global Navigation Satellite Systems Service (IGS), the ionospheric perturbations during the 2014–2015 sudden stratospheric warming (SSW) event were investigated. The analyses revealed both phase‐shifted semidiurnal perturbations and 14.5‐day periodic signal with zonal wavenumber 0 in the IGS TEC, indicating an enhanced lunar tidal impact on the ionosphere during the 2014–2015 SSW period. The 14.5‐day periodic oscillation in the IGS TEC presents an obvious latitudinal variation, with the maximum appearing in Equatorial Ionization Anomaly crest regions, which is due to the modulation of the equatorial ionospheric fountain effect by wind perturbations modulated by the lunar tide. Additionally, the 14.5‐day periodic oscillation presents a longitudinal pattern with three peaks in the northern hemisphere and four peaks in the southern hemisphere. Local time dependence of the 14.5‐day periodic perturbation is also significant.

Medium-short-term prediction of polar motion combining the differencing between series with the differencing within series

SUMMARY The accuracy of polar motion forecasting has been the focus of attention in the fields of satellite navigation and deep space exploration. However, the traditional or differential methods for forecasting X or Y series based on LS and AR models are straightforward and monolithic, and cannot reduce the range of forecast errors. Therefore, this study proposes a new method (called the between-within, B–W method) that combines the X, Y and Y–X series forecasts of the traditional and differential methods in pairs according to the mathematical relationship of Y–X. This approach is one way to obtain the minimum range of forecast errors by making full use of the advantages of each method in the combination. A total of 262-hindcast experiments were conducted during 2010–2020 with strictly simulated time delays. For forecasts of 1–180 d at the x-pole, the average improvement is 10.7 per cent over Bulletin-A. For the y-pole at 1–90 d an average improvement of 11.7 per cent over Bulletin-A is achieved. In addition, further incorporation of the last 1 d IGS (International Global Navigation Satellite System Service) Ultra-rapid (IGU) data can effectively improve the MAE at 1–10 d. The 2016–2018 performance of the B–W method at the x-pole may be related to the amplitude and phase of the Chandler wobble, and the 2013–2016 performance at the y-pole may be related to El Niño climate change events. In terms of overall stability, the B–W method is superior to the IERS Bulletin-A in the medium-short-term and has potential practical application.

Ionospheric 14.5 Day Periodic Oscillation during the 2019 Antarctic SSW Event

The International Global Navigation Satellite Systems Service (IGS) ionospheric total electron content (TEC) data are used to study the periodic perturbation in the ionosphere during the 2019 Antarctic sudden stratospheric warming (SSW) event, a rare Southern Hemisphere minor SSW event in the last 40 years. A 14.5 day periodic signal with a zonal wavenumber of 0 is observed in the mesosphere and the lower thermosphere (MLT) region and the ionosphere during this SSW period, which could be related to the lunar tide. The 14.5 day periodic disturbance in the IGS TEC exhibits local time dependence and latitudinal variation, with the maximum amplitude appearing between 1000 and 1600 LT in the equatorial ionization anomaly (EIA) crest regions. Additionally, the 14.5 day periodic oscillation shows an obvious longitudinal variability, with the weakest amplitude appearing in the longitudinal region of 30° W–60° E.

Earth rotation parameters from BDS, GPS, and Galileo data: An accuracy analysis

The Earth Rotation Parameters (ERPs) are estimated with the BeiDou Navigation Satellite System (BDS), GPS, and Galileo observations from approximately 100 selected International Global Navigation Satellite System (GNSS) Service (IGS) tracking stations around the world. The results are compared with the IGS final products (IGSF) and the International Earth Rotation and Reference Systems Service (IERS) series 14C04 at 12 UTC. The accuracy of the ERPs derived from the GPS-only reaches the IGS accuracy level. The standard deviations of the differences in the X-pole, Y-pole, and Length of Day (LOD), compared with the IGSF, are 23 μas, 20 μas, and 7 μs/day, respectively. Meanwhile, the standard deviations of the differences in the X-pole, Y-pole, and LOD, in contrast with IERS 14C04, are 28 μas, 25 μas, and 9 μs/day, respectively. Similarly, the ERPs are estimated using BDS-only and Galileo-only observations from approximately 100 selected IGS stations. For ERPs derived from BDS Medium Earth Orbit (MEO-only) satellites, the standard deviations of the differences in the X-pole, Y-pole, and LOD, compared with the IGSF, are 76 μas, 53 μas, and 19 μs/day, respectively, and 75 μas, 55 μas, and 19 μs/day, respectively, compared with IERS 14C04. Likewise, for the ERPs derived from Galileo-only observations, the standard deviations of the differences in the X-pole, Y-pole, and LOD, compared with the IGSF, are 48 μas, 31 μas, and 12 μs/day, respectively, and 48 μas, 30 μas, and 14 μs/day, respectively, in contrast with IERS 14C04. Despite the three orbital planes, tracking station distribution, and unoptimized models for the BDS MEO satellites, these results are close to the IGS accuracy level. The impact of spurious system-specific signals on these solutions is investigated using spectral analysis, a notable spurious signal appears for the 3-plane BDS MEO and Galileo constellations. The pronounced peaks at the frequencies of 2.97 cycles per year (cpy) for Polar Motion and Polar Motion rate, 1.98 cpy for LOD are significantly larger than those from GPS.

Long-Term Performance Evaluation of BeiDou PPP-B2b Products and Its Application in Time Service

Precise Point Positioning (PPP) is an official service of the BeiDou Global Navigation Satellite System (BDS-3) through the PPP-B2b signal. In this paper, we mainly focus on the long-term performance evaluation of BDS-3 PPP-B2b products and their application in time service. Since the PPP-B2b product is only available in and around China area, the arcs of PPP-B2b products are about several hours. We propose to evaluate the time datum stability by using all available satellites. Then, 557 day PPP-B2b products are collected for this experiment. The results show that there are large jumps in the GPS satellite clock time datum series. However, the BDS-3 satellite clock datum stability is almost at the same level with current Space State Representation (SSR) corrections from the International Global navigation satellite system Service (IGS). The difference between PPP-B2b GPS and BDS-3 satellite clock time datum will be absorbed into the Inter System Bias (ISB) parameter. Thus, it should be specially noted that the ISB parameter cannot be estimated as constant values if users use PPP-B2b products. In addition, the accuracy of the BDS-3 satellite clock is significantly better than that of the GPS for both the Root Mean Square Error (RMSE) and standard deviation (STD). The average Signal in Space Range Errors (SISREs) is 0.22 ns and 0.13 ns for GPS and BDS-3, respectively. The one-way timing experiment shows BDS-3 timing stability is 2.9 × 10−14@104 s. In addition, 10 baselines from 13 km to 4494 km are formed for time synchronization evaluation by using PPP-B2b products. The average RMSEs of time synchronization is from 0.46 ns to 1.58 ns and from 0.66 ns to 1.19 ns for GPS and BDS-3, respectively. As for STD, the average values are from 0.27 ns to 0.74 ns and from 0.27 ns to 0.47 ns for GPS and BDS-3, respectively. Overall, the results show that the time datum stability, accuracy, and service performance of BDS-3 PPP-B2b products has been stable over the past two years.

The Design and Maintenance of Low-Orbit Navigation Constellation for Traffic Control in a Smart City

The traffic control issue in the smart city scenario gives rise to the higher requirements of Global Navigation Satellite System (GNSS) services, especially in terms of navigation accuracy, together with coverage continuity, and multiplicity. The dense urban environment leads to higher elevation angles for navigation in such areas, which requires a lower altitude of the constellation, as well as a larger number of satellites. In the existing literature, the design and maintenance of the Low Earth Orbit (LEO) navigation constellation that fulfills the requirements of the smart city are not provided. Hence, based on the requirements and constraints of the smart city scenario, this article studies the relation between orbital height, user elevation angle, and coverage. It designs the configuration of an LEO navigation constellation that not only achieves global sensing coverage, but also provides a continuous lane-level navigation service with multiple coverages for the key area. In addition, considering the atmospheric drag in low orbits and the constraint of satellite power and attitude control, a method is proposed by rotating solar panels to change the effective frontal area of the satellite to achieve relative configuration maintenance of the LEO constellation. The results show that the LEO navigation constellation has a 0 s revisit time in five chosen smart cities, and each city has more than four-times coverage every second; the Geographic Dilution of Precision (GDOP) values of five cities are smaller than 0.47. The average navigation accuracy of five cities is 2.01. With the conduction of the one-year station-keeping simulation, the phase deviation of two satellites is less than 0.6° and it gradually converges to 0.1°, where the semi-major axis deviation is less than 80 m. With our proposed method, the active station-keeping control is not needed in one year, and the fuel consumption can be reduced. Finally, the continuity of the navigation service can be assured.

POINT POSITIONING PERFORMANCE OF TRIMBLE-RTX IN DIFFERENT SATELLITE COMBINATIONS

Today, new global positioning algorithms, techniques and technologies are emerging thanks to services such as International Global Navigation Satellite Systems (GNSS) Service (IGS) that offer orbit and clock correction data of satellite systems to all GNSS users. These solutions are developed in order to eliminate the disadvantages of current positioning techniques such as requiring more than one GNSS receiver and not exceeding a certain distance between the reference station and the rover. One of these innovative technology products is Trimble CenterPoint RTX Post-Processing (Trimble-RTX) technology. In this study, the point positioning performance of the Trimble-RTX service was investigated. The 31-day RINEX data obtained from 2 IGS stations are used in 4 different satellite combinations (GPS (G), GPS+GLONASS (G+R), GPS+GALILEO (G+E), GPS+GLONASS+GALILEO (G+R+E)) processed with Trimble-RTX. By comparing the obtained coordinates with the coordinates of the stations published by IGS, the accuracy and precision of the coordinates were evaluated for each satellite combination. As a result of the evaluation, it was seen that there were generally no significant differences between the results obtained from 4 different satellite combinations at the stations.

Observable-specific phase biases of Wuhan multi-GNSS experiment analysis center’s rapid satellite products

Precise Point Positioning (PPP) with Ambiguity Resolution (AR) is an important high-precision positioning technique that is gaining popularity in geodetic and geophysical applications. The implementation of PPP-AR requires precise products such as orbits, clocks, code, and phase biases. As one of the analysis centers of the International Global Navigation Satellite System (GNSS) Service (IGS), the Wuhan University Multi-GNSS experiment (WUM) Analysis Center (AC) has provided multi-GNSS Observable-Specific Bias (OSB) products with the associated orbit and clock products. In this article, we first introduce the models and generation strategies of WUM rapid phase clock/bias products and orbit-related products (with a latency of less than 16 h). Then, we assess the performance of these products by comparing them with those of other ACs and by testing the PPP-AR positioning precision, using data from Day of the Year (DOY) 047 to DOY 078 in 2022. It is found that the peak-to-peak value of phase OSBs is within 2 ns, and their fluctuations are caused by the clock day boundary discontinuities. The associated Global Positioning System (GPS) orbits have the best consistency with European Space Agency (ESA) products, and those of other systems rank in the medium place. GLObal NAvigation Satellite System (GLONASS) clocks show slightly inconsistency with other ACs’ due to the antenna thrust power adopted, while the phase clocks of other GNSSs show no distortion compared with legacy clocks. With well-estimated phase products for Precise Orbit Determination (POD), the intrinsic precision is improved by 14%, 17%, and 24% for GPS, Galileo navigation satellite system (Galileo), and BeiDou-3 Navigation Satellite System (BDS-3), respectively. The root mean square of PPP-AR using our products in static mode with respect to IGS weekly solutions can reach 0.16 cm, 0.16 cm, and 0.44 cm in the east, north, and up directions, respectively. The multi-GNSS wide-lane ambiguity fixing rates are all above 90%, while the narrow-lane fixing rates above 80%. In conclusion, the phase OSB products at WUM have good precision and performance, which will benefit multi-GNSS PPP-AR and POD.

An artificial neural network model in predicting VTEC over central Anatolia in Turkey

This research investigates the capability of artificial neural networks to predict vertical total electron content (VTEC) over central Anatolia in Turkey. The VTEC dataset was derived from the 19 permanent Global Positioning System (GPS) stations belonging to the Turkish National Permanent GPS Network-Active (TUSAGA-Aktif) and International Global Navigation Satellite System Service (IGS) networks. The study area is located at 32.6°E-37.5°E and 36.0°N-42.0°N. Considering the factors inducing VTEC variations in the ionosphere, an artificial neural network (NN) with seven input neurons in a multi-layer perceptron model is proposed. The KURU and ANMU GPS stations from the TUSAGA-Aktif network are selected to implement the proposed neural network model. Based on the root mean square error (RMSE) results from 50 simulation tests, the hidden layer in the NN model is designed with 41 neurons since the lowest RMSE is achieved in this attempt. According to the correlation coefficients, absolute and relative errors, the NN VTEC provides better predictions for hourly and quarterly GPS VTEC. In addition, this paper demonstrates that the NN VTEC model shows better performance than the global IRI2016 model. Regarding the spatial contribution of the GPS network to TEC prediction, the KURU station performs better than ANMU station in fitting with the proposed NN model in the station-based comparison.

An investigation of a new artificial neural network-based TEC model using ground-based GPS and COSMIC-2 measurements over low latitudes

Mapping the ionosphere over low-latitude regions with a high accuracy is a challenging task due to the complex nature of the ionospheric structure. In this study, the artificial neural network (ANN) technique was used to investigate the determination of the total electron content (TEC) in low latitude regions using measurements from 187 ground-based International global navigation satellite system (GNSS) Service (IGS) stations and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-2) mission in the period from 1 October 2019 to 30 September 2021. To reduce the systematic discrepancies between the space-borne and ground-based observational systems, the ANN technique was also utilized to calibrate the COSMIC-2 derived TECs to the ground level. The RMSE of the calibrated COSMIC-2 TECs was 1.78 TECU. This, in comparison with the RMSE value of 4.10 TECU for the uncalibrated COSMIC-2 TECs, demonstrated 57% improvement. It is found that the performance of the new model was strongly associated with the value of TEC and low-latitude ionospheric structure, such as the equatorial ionization anomaly (EIA). Compared with the IRI-2016 model, the Technical University of Catalonia (UPC) rapid global ionospheric maps (GIMs) (UQRG), the Center for Orbit Determination in Europe (CODE) final GIMs (CODG), and the Chinese Academy of Sciences (CAS) final GIMs (CASG), the new model outperformed the IRI-2016 model and was competitive with the GIMs. All results suggest that the new model developed using both ground-based and space-borne measurements together with the ANN technique can model the ionospheric structure, such as the EIA. Therefore, the new model developed may become a good reference for the studies of the structure and variation of the ionosphere over low latitudes.

Variation of receiver code biases under the influence of the receiver type and antenna configuration in the IGS network

Receiver code biases (RCBs) are known to be time delays within the receiver caused by their hardware imperfections. To better understand the characteristics of RCBs, the un-combined (UC) and ionosphere-free (IF) precise point positioning functional models are adapted and re-parameterized to estimate the variation of RCBs as a time-variant parameter. In this study, we analytically studied the temporal variations of RCBs; although there exists a benchmark difference between the UC and IF models, their estimates are in accordance with each other. Additionally, this contribution assesses the inter-day stability of RCBs with weekly observations from 165 globally distributed international global navigation satellite system service stations equipped the receivers of three mainly types. The inter-day stability results of RCB revealed that the RCBs of POL2 and OUS2 have better stability over consecutive 7 d and the single differenced (SD) RCBs can reach 0.2 m in the best case. The results show that 74.83% of the stations are equipped with Trimble receivers under the condition that the mean SD RCB values are between −0.5 and 0.5 m, while 85.57% of the stations are equipped with Septentrio receivers and the stations equipped with Javad can reach 84.35% under this condition. The RCB estimates are also relatively stable for the case in which the receiver hardware device stays unchanged. The relationship between RCBs, receiver type, and antenna configuration is found using six groups of receivers. A strong correlation exists between RCBs, receiver type, and antenna configuration, which is more obvious among Septentrio receivers. The results show that the Pearson correlation coefficients were all higher than 0.9, and the standard deviation of between-receiver RCBs was smaller than 0.327 m when equipped with Septentrio receivers. We concluded that there is a strong relationship between the receiver-related pseudorange biases and the receiver and antenna setup.