Visible GPS Satellites Research Articles

Simultaneous monitoring of the limited area ionosphere with the use of GPS and ionosonde

The importance of this study lies in its contribution to the field of ionospheric science, particularly in the monitoring and prediction of ionospheric parameters. This study focuses on comparing two techniques for spatio-temporal kriging of critical ionospheric parameters, namely the F2 layer critical frequency (foF2) and Total Electron Content (TEC), under both quiet and disturbed ionospheric conditions. The first technique, Ordinary Kriging (OK), relies solely on foF2 data, while the second technique, Kriging with External Drift (KED), incorporates the dynamic relationship between foF2 and TEC data to improve the interpolation process. This study demonstrates how KED, which utilizes dynamic secondary information, offers advantages over OK. By simultaneously monitoring the ionosphere from a single ionosonde station and collecting data from visible GPS satellites, KED enhances the accuracy of spatial structural analysis, enabling more precise estimations of ionospheric characteristics across interpolation grid nodes. The research highlights that the non-uniform distribution of ionosonde stations, especially in regions with limited foF2 data, can benefit from KED's incorporation of TEC data. Additionally, the study reveals the potential for improving TEC maps by assimilating foF2 data located at the periphery of the observation area. Furthermore, the paper discusses the determination of semivariogram structures for both foF2 and TEC data, emphasizing the spatial correlations between data points. These findings contribute to a better understanding of ionospheric behavior and the potential for enhanced ionospheric modeling.

Performance Analysis of Robust Cooperative Positioning Based on GPS/UWB Integration for Connected Autonomous Vehicles

The accurate position is a key requirement for autonomous vehicles. Although Global Navigation Satellite Systems (GNSS) are widely used in many applications, their performance is often disturbed, particularly in urban areas. Therefore, many studies consider multi-sensor integration and cooperative positioning (CP) approaches to provide additional degrees of freedom to address the shortcomings of GNSS. However, few studies adopted real-world datasets and internode ranging outliers within CP is left untouched, leading to unexpected challenges in practical applications. To address this, we propose a Robust Cooperative Positioning (RCP) scheme that augments the GPS with the Ultra-Wideband (UWB) system. A field experiment is conducted to generate a real-world dataset to evaluate the RCP scheme. Moreover, the analysis of the collected dataset enables us to optimise a simple but effective Robust Kalman Filter (RKF) to mitigate the influence of outlier measurements and improve the robustness of the proposed solution. Finally, a simulated dataset is derived from the real-world data to comprehensively study the performance of the proposed RCP method in urban canyon scenarios. Our results demonstrate that the proposed solution can crucially improve positioning performance when the number of visible GPS satellite is limited and is robust against various adverse effects in such environments.

A Method for dSTEC Interpolation: Ionosphere Kernel Estimation Algorithm

Ionospheric structure is important for estimating ionospheric delay for user stations in Global Navigation Satellite System (GNSS). However, most existing parameter estimation methods suffer from challenges due to data inaccuracy and unavailability of limited and sparse scattered data at ground reference stations. The high variability of active low latitude or disturbed ionosphere leads to GNSS signal scintillation. It is critical to capture ionospheric random structure and estimate ionospheric parameter using data of disperse receivers to improve accuracy. This paper proposes a unifying method named Ionosphere Kernel Estimation Algorithm (IKEA) to retrieve the information of ionospheric spatial structure. The proposed model utilities the semi-parametric representation theorem to incorporate prior information and constraints. The multiple kernel technique is adopted firstly to include physical correlations. Additionally, a learning approach is deployed to determine model parameters. The IKEA model has been verified based on simulated and experimental data at active low latitudes from a network of ground GNSS reference stations from all visible GPS and GALILEO satellites. The IKEA model reduces approximately 19.5% and 24.2% of differential Slant Total Electron Content (dSTEC) in the root mean square error with respect to Inverse Distance Weighting (IDW) and the Kriging model during high ionospheric activities. The IKEA architecture has been demonstrated effective to make robust ionospheric estimation, which may be further extended for various GNSS applications and beyond.

Autonomous Predictions Realized in Navigation Receivers for the Motion of GLONASS and GPS Spacecraft

A new algorithm for autonomous long-term predictions realized in ground navigation receivers for ephemerides of GLONASS and GPS spacecraft is developed and analyzed. The algorithm can be applied to determinations of user’s coordinates, when it is impossible or difficult to receive the spacecraft ephemerides (for example, in conditions of a bad radio-visibility of the satellites). It is shown that the accuracy of the method exceeds similar characteristics of other known solutions. It is also shown that the coefficients, which take account of the solar pressure and are specific for each satellite, differ for different generations of spacecraft. Simultaneously with parameters of the satellite motion, the Earth’s polar coordinates can be determined with the accuracy of several arc milliseconds. Using the autonomous algorithm for predictions of spacecraft ephemerides significantly advances both the search for visible GLONASS and GPS satellites and the first determination of user’s coordinates after “cold” starts of receivers. This article is based on the report presented at the conference Astrometry Yesterday, Today, and Tomorrow, conducted at Sternberg Astronomical Institute, October 14–16, 2019.

Stochastic Evaluation of Pre‐Earthquake TEC Enhancements

AbstractHere we test the precursory enhancement in ionospheric total electron content (TEC) which has been reported by Heki (2011, https://doi.org/10.1029/2011GL047908) and numerous Global Navigation Satellite System (GNSS) TEC observational studies before the 2011 Mw9.0 Tohoku‐Oki and many great earthquakes. We verify the frequency of this TEC enhancement via analysis of a 2‐month vertical TEC (VTEC) time series that includes the Tohoku‐Oki earthquake using the procedure, based on Akaike's information criterion, and threshold of Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353). The averaged occurrence rate of the TEC enhancement is much larger than that reported by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) when all of the visible GPS satellites at a given station are taken into account. The frequency assessment by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) using only one satellite underestimates the frequency of the TEC enhancement. In fact, the TEC enhancement is sufficiently frequent to explain all the reported enhancements prior to the great earthquakes as a product of chance. We also analyze the spatial distribution of the preseismic TEC enhancement and coseismic TEC depletion for the Tohoku‐Oki earthquake with the data after reducing intertrace biases. We observe significant postseismic depletion that lasted at least 2 h after the earthquake and extended at least 500 km around the center of the large‐slip area. This means that evaluation of the enhancements using reference curves which was adopted by Heki (2011, https://doi.org/10.1029/2011GL047908) and even by the recent papers (e.g., He & Heki 2016, https://doi.org/10.1002/2016GL069863; 2017, https://doi.org/10.1002/2017JA024012; 2018, https://doi.org/10.1029/2017JA024871) is in danger of mistaking a large and long‐lasting postseismic TEC depletion for a preseismic enhancement.

Methodical questions and accuracy problems of GPS observations by the example of the geodynamic proving ground in Bishkek

The key questions concerning the modern methodical tasks and accuracy of GPS measurements of crustal motion spanning are discussed for a full cycle of the survey from the organization of the field operations to the interpretation of the final results. The presented data rely on the 20-year experience of the geophysicists of the Research Station of the Russian Academy of Sciences in Bishkek (RS RAS) in GPS monitoring at the Geodynamic Proving Ground in Bishkek (GPGB) and in a large part of Central Asia. The comparative characteristics of the constellations of visible GPS and GLONASS satellites are analyzed from the standpoint of their practical application for precise scientific observations of crustal motions. The studies of the contemporary movements of the Earth’s crust by the methods of satellite geodesy generally comprise three stages: (1) organization of the measurement networks and acquisition of the data; (2) data processing; and (3) interpretation of the results. Each stage is associated with its own block of the tasks and problems, and neither is guaranteed against uncertainties and errors which may affect the results, conclusions, and reconstructions.

GBAS Reference Receiver Clock Adjustment Effects on Continuity and Integrity Performance

Ground-based augmentation system (GBAS) reference stations provide pseudo-range corrections (PRCs) and range rate corrections (RRCs) for visible GPS satellites along with the standard deviations of the PRC errors (σpr_gnd) and B-values for the reference receivers. When generating PRCs, a GBAS should adjust the reference receiver clock bias to limit the PRC values within a certain range specified by international standards. Since σpr_gnd should reflect the PRC errors accurately for system integrity, it must consider the effect of the clock adjustment procedure. Nevertheless, this has hardly been researched, since it is well known that the clock adjustment does not impact on the user position solution. This paper notices that σpr_gnd is also used for GBAS integrity monitoring functions which are implemented in range domain, not position domain. Thus, the work presented here investigates continuity and integrity performance risk in range domain in case a GBAS estimates σpr_gnd ignoring the clock adjustment procedure and uses it for integrity monitors. The work presented here analyzes the risk by a simulation and proposes strategies for σpr_gnd establishment considering the clock adjustment effects to prevent system performance risk.

The response of the ionosphere to faint and bright solar flares as deduced from global GPS network data

Results derived from analysing the ionosphere response to faint and bright solar flares are presented. The analysis used technology of a global detection of ionospheric effects from solar flares as developed by the authors, on the basis of phase measurements of the Total Electron Content (TEC) in the ionosphere using an international GPS network. The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the TEC which is determined from GPS data, simultaneously for the entire set of visible GPS satellites at all stations used in the analysis. This technique is useful for identifying the ionospheric response to faint solar flares (of X-ray class C) when the variation amplitude of the TEC response to separate line-on-sight to GPS satellite is comparable to the level of background fluctuations. The dependence of the TEC variation response amplitude on the bright flares location on the Sun is investigated.

VirtuaLites: concepts and numerical testing results

The increasing use of GPS and data integration technologies in agricultural operations has led to large gains in operational efficiencies. This paper presents a new method to bridge periods when the number of visible GPS satellites falls below the minimum required to provide a navigation solution by utilising an external positioning system. This can be an additional dead reckoning INS system or any other system that can provide an approximate position solution for the vehicle which can be improved through the processing of data from the available GPS satellites.

Contribution of pseudolite observations to GPS precise surveys

Although GPS has been widely used for precision surveys, many survey environments, like in urban areas, deep open-pit mines and valleys, limit the number of visible GPS satellites, deteriorating the survey accuracy. In addition some engineering projects require higher positioning accuracy in a particular direction. A GPS-only system hardly satisfies the requirements. Pseudolite-augmented GPS survey technique can strengthen the positioning geometry. Pseudolite (PL) measurements, however, suffer from various sources of errors different from GPS observations. In order to employ them for precise surveys these sources of errors must be treated properly. This paper discusses of the major sources of errors in PL measurements and the methodologies for their mitigation. Some experimental tests are then presented.

Modelling and prediction of GPS availability with digital photogrammetry and LiDAR

This paper describes an automated method for predicting the number of satellites visible to a GPS receiver, at any point on the Earth's surface at any time. Intervisibility analysis between a GPS receiver and each potentially visible GPS satellite is performed using a number of different surface models and satellite orbit calculations. The developed software can work with various ephemeris data, and will compute satellite visibility in real time. Real‐time satellite availability prediction is very useful for mobile applications such as in‐car navigation systems, personal navigations systems and LBS. The implementation of the method is described and the results are reported.

Terrain imaging using a SAR system based on reflected GPS signals

This paper describes a 3D multi-static synthetic aperture radar (SAR) imaging system which utilises reflected GPS signals from moving objects on the Earth’s surface. The principle of bi-static radar is used to model the reflected GPS signals. The movement of a visible GPS satellite serves as a base for a synthetic aperture over an observation time period. As an example, a MATLAB simulation has been carried out in order to detect the movement of imaged objects under the assumption of one static GPS receiver with two targets which move with different speeds. The influence of the visible satellite’s position and velocity on the spatial resolution of such a SAR system is discussed. Simulation results show that by measuring the cross-correlation of the reflected GPS signal from the terrain and objects on it, the detection of the objects can enjoy a good spatial resolution for the case of moving objects and a moving GPS receiver. Furthermore, the spatial resolution is also related to the selection of visible GPS satellites with respect to their azimuths, elevations and velocities. This system has the following useful features: (a) no dedicated signal transmitter is required; (b) the GPS signal frequency is reused; (c) GPS operates round-the-clock and its signals cover the entire Earth’s surface; (d) low power consumption; and (e) known GPS signal structure.

Ionospheric effects of the solar flares as deduced from global GPS network data

Abstract Results derived from analysing the ionospheric response to faint and bright solar flares are presented. The analysis used novel technology of a global detection of ionospheric effects from solar flares as developed by the authors (Afraimovich, 2000a; Afraimovich, 2000b), on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the TEC which is determined from GPS data, simultaneously for the entire set of visible GPS satellites at all stations used in the analysis. This technique is useful for identifying the ionospheric response to faint solar flares (of X-ray class C) when the variation amplitude of the TEC response to separate line-on-sight (LOS) is comparable to the level of background fluctuations. The dependence of the TEC variation response amplitude on the flare location on the Sun is investigated.