Effect Of Ionospheric Delay Research Articles

Method for Assemblage of an Open Access Data Set for Research in Geomagnetic Effects on GPS/GNSS Ionospheric Delay in Sub-equatorial Regions

Space weather, geomagnetic, and ionospheric effects are major sources of Global Navigation Satellite System’s (GNSS) Positioning, Navigation, and Timing (PNT) service degradation. Researchers studying the problem often face an obstacle in missing large data sets comprising ionospheric effects variables and space weather ones for their studies. Here a formal assemblage method is proposed for data set assemblage for GNSS ionospheric effects assessment that utilises experimental sets of space weather/geomagnetic observations, and raw GNSS pseudorange observations polluted with uncorrected ionospheric delay effects to estimate Total Electron Content (TEC) as a target variable outlining the ionospheric effects on GNSS. The five Observations Quality Principles (OQP) are proposed for the observations to be used in statistical learning model development to comply to. The proposed assemblage method is demonstrated in a particular case study of an assembled data set for Darwin, NT, Australia, in sub-equatorial zone, using a tailored software developed in the R environment for scientific computing, and a freely available TEC estimation software. The assembled massive data obtained in the demonstration of the proposed method is provided in the open-access manner in support of the international scientific community.

Complete three-dimensional coseismic displacements due to the 2021 Maduo earthquake in Qinghai Province, China from Sentinel-1 and ALOS-2 SAR images

On 22nd May 2021 (local time), an earthquake of Ms7.4 struck Maduo county in Qinghai Province, China. This was the largest earthquake in China since the 2008 Wenchuan earthquake. In this study, ascending/descending Sentinel-1 and advanced land observation satellite-2 (ALOS-2) synthetic aperture radar (SAR) images were used to derive the three-dimensional (3-D) coseismic displacements of this earthquake. We used the differential interferometric SAR (InSAR, DInSAR), pixel offset-tracking (POT), multiple aperture InSAR (MAI), and burst overlap interferometry (BOI) methods to derive the displacement observations along the line-of-sight (LOS) and azimuth directions. To accurately mitigate the effect of ionospheric delay on the ALOS-2 DInSAR observations, a polynomial fitting method was proposed to optimize range-spectrum-split-derived ionospheric phases. In addition, the 3-D displacement field was obtained by a strain model and variance component estimation (SM-VCE) method based on the high-quality SAR displacement observations. Results indicated that a left-lateral fault slip with the largest horizontal displacement of up to 2.4 m dominated this earthquake, and the small-magnitude vertical displacement with an alternating uplift/subsidence pattern along the fault trace was more concentrated in the near-fault regions. Comparison with the global navigation satellite system data indicated that the SM-VCE method can significantly improve the accuracy of the displacements compared to the classical weighted least squares method, and the incorporation of the BOI displacements can substantially benefit the accuracy of north-south displacement. In addition to the displacements, three coseismic strain invariants calculated based on the strain model parameters were also investigated. It was found that the eastern and western parts of the faults suffered more significant strains compared with the epicenter region.

Evaluation of the Effect of Higher-Order Ionospheric Delay on GPS Precise Point Positioning Time Transfer

In high-precision GPS precise point positioning (PPP) time transfer, errors caused by the effect of ionosphere delay have to be corrected. Usually the ionosphere-free combinations of the pseudo code and the carrier phase is used in GPS PPP data processing, and it effectively eliminates the effect of the first-order ionospheric delay. This study quantitatively analyzes the errors caused by higher-order ionospheric (Ion2+) delays in precise PPP time transfer. Data of two 7-day test periods, including low and moderate ionospheric conditions, from 20 stations located in middle- and low-latitude, were analyzed. The difference in clock solution with and without the Ion2+ correction, including the receiver clock solution and time-link clock solution, was deeply analyzed and discussed. The difference sequence shows a constant bias plus some variations with a diurnal variation. For the difference of the receiver clock solutions, the mean standard deviation of the variations is 3.92 ps in low-latitude, which is much larger than that of 2.59 ps in mid-latitude due to the influence of the larger ionospheric electron density on the low-latitude. The maximum constant bias reached more than 15 ps and was negative at most stations in the northern hemisphere, while it was positive at most stations located in the southern hemisphere. The difference in the time-link solutions correlates not only with time and region, but also with the length of the time-links. The largest difference in the long time-link SYDN-PTBB, BJNM-SYDN, AMC2-SYDN, etc., reaches more than 25 ps, while that of the short time-link IENG-PTBB, BRUX-PTBB, etc., is less than 3.5 ps. Therefore, the Ion2+ correction is necessary for high-precision PPP time transfer over long time-links, especially time-links made by one station located in the northern hemisphere and another located in the south hemisphere; however, it could be ignored for short time-links.

Estimation of Ionospheric Delay Influence on the Efficiency of Precise Positioning of Multi-GNSS Observations

Currently, Global Navigation Satellite Systems (GNSS) are developing at a fairly rapid pace. Over the last years US GPS and Russian GLONASS were modernizing, whilst new systems like European Galileo and Chinese BDS are launched. The modernizations of the existing and the deployment of new GNSS made a whole range of new signals available to the users, and create a new concept  multi-GNSS. Ionospheric delay is one of the major error sources in multi-GNSS observations. At present, GNSS users usually eliminate the influence of ionospheric delay of the first order items by dual-frequency ionosphere-free combinations. But there is still residual ionospheric delay error of higher orders. In this paper we present four different processing scenarios to exclude the higher orders ionospheric delay effects on multi-GNSS Precise Point Positioning (PPP) performance, including: “only GPS” and “GPS+GLONASS+Galileo+BDS” – without/with eliminating ionospheric delay error of higher orders. Dataset collected from one GNSS station BOR1 (Borowiec, Poland) over almost two years provided by multi-GNSS experiment (MGEX) were used for dual-frequency PPP tests with one- and quadconstellation signals. For the second pair of scenarios were used a IONosphere map EXchange format (IONEX) that supports the exchange of 2- and 3-dimensional TEC maps given in a geographic grid. Numeric experiments show that, the results of different pairs of scenarios differ at the submillimeter level. The results also show that the multi-GNSS processing are better than those based on “only GPS”.

Performance of Selected Ionospheric Models in Multi-Global Navigation Satellite System Single-Frequency Positioning over China

Ionospheric delay as the major error source needs to be properly handled in multi-GNSS (Global Navigation Satellite System) single-frequency positioning and the different ionospheric models exhibit apparent performance difference. In this study, two single-frequency positioning solutions with different ionospheric corrections are utilized to comprehensively analyze the ionospheric delay effects on multi-frequency and multi-constellation positioning performance, including standard point positioning (SPP) and ionosphere-constrained precise point positioning (PPP). The four ionospheric models studied are the GPS broadcast ionospheric model (GPS-Klo), the BDS (BeiDou Navigation Satellite System) broadcast ionospheric model (BDS-Klo), the BDS ionospheric grid model (BDS-Grid) and the Global Ionosphere Maps (GIM) model. Datasets are collected from 10 stations over one month in 2019. The solar remained calm and the ionosphere was stable during the test period. The experimental results show that for single-frequency SPP, the GIM model achieves the best accuracy, and the positioning accuracy of the BDS-Klo and BDS-Grid model is much better than the solution with GPS-Klo model in the N and U components. For the single-frequency PPP performance, the average convergence time of the ionosphere-constrained PPP is much reduced compared with the traditional PPP approach, where the improvements are of 11.2%, 11.9%, 21.3% and 39.6% in the GPS-Klo-, BDS-Klo-, BDS-Grid- and GIM-constrained GPS + GLONASS + BDS single-frequency PPP solutions, respectively. Furthermore, the positioning accuracy of the BDS-Grid- and GIM-constrained PPP is generally the same as the ionosphere-free combined single-frequency PPP. Through the combination of GPS, GLONASS and BDS, the positioning accuracy and convergence performance for all single-system single-frequency SPP/PPP solutions can be effectively improved.

Snow Depth Estimation with GNSS-R Dual Receiver Observation

Two estimation methods using a dual GNSS (Global Navigation Satellite System) receiver system are proposed. The dual-frequency combination method combines the carrier phase observations of dual-frequency signals, whereas the single-frequency combination method combines the pseudorange and carrier phase observations of a single-frequency signal, both of which are geometry-free strictly combination and free of the effect of ionospheric delay. Theoretical models are established in the offline phase to describe the relationship between the spectral peak frequency of the combined sequence and the antenna height. A field experiment was conducted recently and the data processing results show that the root mean squared error (RMSE) of the dual-frequency combination method is 5.04 cm with GPS signals and 6.26 cm with BDS signals, which are slightly greater than the RMSE of 4.16 cm produced by the single-frequency combination method of L1 band with GPS signals. The results also demonstrate that the proposed two combination methods and the SNR method achieve similar performance. A dual receiver system enables the better use of GNSS signal carrier phase observations for snow depth estimation, achieving increased data utilization.

Stochastic modeling of between-receiver single-differenced ionospheric delays and its application to medium baseline RTK positioning

Global navigation satellite system (GNSS) carrier phase integer ambiguity resolution is one of the key issues and challenges for precise relative positioning. For baselines greater than 10 km, integer ambiguity resolution becomes difficult because the ionospheric delay effects on the single-differenced (SD) observations are significant. One way to deal with this difficulty is to weight the ionospheric delays, instead of treating them in a deterministic way, giving rise to the so-called ionosphere-weighted model. With this model relative positioning technology is enabled to work with much larger inter-station distances than its current status. Here we seek to gain insight into how to reasonably weight the SD ionospheric delays, normally introduced in the ionosphere-weighted model as pseudo observables, which consists of two sequential steps. The first step seeks to construct realistic stochastic models for the GNSS observables with the ionosphere-fixed model where the SD ionospheric delays are assumed to be absent. With the results so obtained in the second step we estimate the variance of the SD ionospheric delays with the ionosphere-float model, in which the SD ionospheric delays are assumed to be fully unknown. Numerical tests based on GPS data from the National Geodetic Survey CORS network demonstrate a clear improvement in medium baseline real time kinematic (RTK) positioning performance, as compared to the ionosphere-weighted model that employs the unrealistic stochastic models for the pseudo SD ionospheric observables.

Evaluation of Ionospheric Delay Effects on Multi-GNSS Positioning Performance

Ionospheric delay is a significant error source in multi-GNSS positioning. We present different processing strategies to fully exploit the ionospheric delay effects on multi-frequency and multi-GNSS positioning performance, including standard point positioning (SPP) and precise point positioning (PPP) scenarios. Datasets collected from 10 stations over thirty consecutive days provided by multi-GNSS experiment (MGEX) stations were used for single-frequency SPP/PPP and dual-frequency PPP tests with quad-constellation signals. The experimental results show that for single-frequency SPP, the Global Ionosphere Maps (GIMs) correction achieves the best accuracy, and the accuracy of the Neustrelitz TEC model (NTCM) solution is better than that of the broadcast ionospheric model (BIM) in the E and U components. Eliminating ionospheric parameters by observation combination is equivalent to estimating the parameters in PPP. Compared with the single-frequency uncombined (UC) approach, the average convergence time of PPP with the external ionospheric models is reduced. The improvement in BIM-, NTCM- and GIM-constrained quad-constellation L2 single-frequency PPP was 15.2%, 24.8% and 28.6%, respectively. The improvement in convergence time of dual-frequency PPP with ionospheric models was different for different constellations and the GLONASS-only solution showed the least improvement. The improvement in the convergence time of BIM-, NTCM- and GIM-constrained quad-constellation L1/L2 dual-frequency PPP was 5.2%, 6.2% and 8.5%, respectively, compared with the UC solution. The positioning accuracy of PPP is slightly better with the ionosphere constraint and the performance of the GIM-constrained PPP is the best. The combination of multi-GNSS can effectively improve the positioning performance.

Nequick2 Model Behaviour for Global Ionospheric Delay Mitigation During Solar Cycle-24

Abstract The ionospheric delay is the major current source of potential range delay for single-frequency GNSS users. Different ionospheric delay mitigation methods have been developed to mitigate the ionospheric delay effects for single-frequency users. The NeQuick is a quick-run ionospheric electron density model particularly designed for trans-ionospheric propagation applications developed at the Aeronomy and Radio propagation Laboratory of the Abdus Salam International Centre for Theoretical Physics (ICTP), Italy. NeQuick2 is the latest version of the NeQuick ionosphere electron density model. NeQuick model been used by the European Space Agency (ESA) European Geostationary Navigation Overlay Service (EGNOS) project for assessment analysis and has been adopted for single-frequency positioning applications in the frame work of the European satellite navigation system (Galileo). NeQuick2 model adopted modifications related to the modeling of the F1 layer peak electron density, height and thickness parameter. Also, a new formulation of the shape parameter k has been adopted. This paper presents a global study for the behavior of the modified NeQuick2 model. The zenith ionospheric range delay correction by the model has been assessed using the highly accurate IGS-Global Ionospheric Maps (IGS-GIMs) for two different-latitude stations (Aswan, Egypt) (low-latitude) (24.1o N) and (Helsinki, Finland) (high-latitude) (60.2o N). The study was carried out during current solar cycle-24 over three different months that each of them reflects a different state of solar activity. It can be concluded that NeQuick2 model globally presents overestimation for ionospheric delay for quiet and medium ionospheric activity states respectively, while the model presents underestimation for high activity state of the ionosphere layer.

Precise point positioning for ground-based navigation systems without accurate time synchronization

Despite the broad range of navigation, positioning, and timing services offered by the global navigation satellite system (GNSS), its signals are vulnerable to blockage and interference, leading to a degraded performance or even unavailability. In contrast to GNSS, ground-based navigation systems exhibit better layout flexibility and more efficient terrain adaptability in shaded areas, with their additional advantages of accurate station coordinates and the absence of ionospheric delay effects enabling precise point positioning. However, accurate inter-station time synchronization is thought to be necessary for the ground-based precise point positioning. Locata utilizes a proprietary TimeLoc technology for tight time synchronization, allowing the clock difference in the carrier phase measurement equation to be ignored. However, for other ground-based navigation systems, accurate time synchronization requires accurate calibration of equipment delays and thus significantly increases the difficulty of engineering implementation. Herein, we propose a new precise point positioning method that does not rely on accurate inter-station time synchronization. According to this method, the uncalibrated clock difference and the integer carrier phase ambiguity are combined into a new ambiguity parameter, with the ambiguity and position coordinates subsequently solved in two steps. Further, in order to overcome the limitations of the Locata on-the-fly algorithm and the conventional “known-point initialization” method, a new “dynamic key point initialization” method is proposed. Finally, a simulation test and an indoor field trial are conducted, demonstrating that precise point positioning can be achieved by sharing a single clock source without relying on accurate inter-station time synchronization. An atomic clock is not needed and an inexpensive crystal oscillator is sufficient to act as the shared clock. The results of indoor positioning tests reveal real-time centimeter-level accuracy.

Ionospheric effects on repeat-pass SAR interferometry

InSAR measurements can be significantly affected by the atmosphere when the radar signal propagates through the atmosphere since it varies with space and time. Great efforts have been made in recent years to better understand the properties of the tropospheric effects and to develop methods for mitigating these effects. By using the basic principles of InSAR, the quantitative analysis of ionospheric delay effects on topography and surface deformation have been introduced for the first time. The measurement errors can be related to the vertical ionospheric total electron content (vTEC). By using the ionospheric observations, the effects of temporal ionospheric variations on InSAR have been analyzed. The results indicate that the ionospheric variations with time, season, solar cycle and geomagnetic activities can compromise the effectiveness of InSAR for both the measurement of topography and surface determination. The repeat-pass SAR interferometry errors induced by ionosphere should be corrected by actual measurements.

Wavelets and triple difference as a mathematical method for filtering and mitigation of DGPS errors

The accuracy of GPS position is primarily dependent on the satellite position, signal delay, and various environment noises such as ionospheric delay effects, ephemeris errors, satellite clock errors, multi-path distortion, tropospheric delay effects, and numerical errors. To overcome most of these errors the Differential Global Positioning System (DGPS) is used. Differential Global Positioning System is one of the most popular and reliable techniques for improving GPS accuracy by minimizing correlated errors between reference stations and rovers. The primary motivation of this paper is to improve GPS measurement accuracies by classifying errors in the frequency domain and subsequently separating, mitigating and decreasing them.Wavelet spectral techniques can separate GPS signals into sub-bands where different errors can be separated and mitigated. The main goal of this paper was the development and implementation of DGPS error mitigation techniques using triple difference and wavelet. This paper studies, analyzes and provides new techniques that will help mitigate these errors in the frequency domain. The proposed technique applied to smooth noise for GPS receiver positioning data is based upon the analysis of wavelet transform (WT). The technique is applied using wavelet as a de-noising tool to tackle the high-frequency errors in the triple difference domain and to obtain a de-noised triple difference signal that can be used in a positioning calculation.

A new ionosphere-free ambiguity resolution method for long-range baseline with GNSS triple-frequency signals

New GNSS systems (i.e. GPS modernization, BeiDou, and Galileo) will provide multiple navigation signals for reliable navigation services. The triple or even multiple frequency signals are expected to bring great benefits to the ambiguity resolution (AR) over long-range baselines, which is always regarded as a huge challenge. Another issue in the long baseline AR is the unmodeled ionospheric delay, which is one of the major errors in ranging signals. A new triple-frequency, ionosphere-free technique for ambiguity resolution of long-range baseline is developed in this study. In this technique, the optimal observation combinations are chosen considering the effect of ionospheric delay. At the same time, using this technique, the double difference (DD) ionospheric delay is nullified in the ambiguity search process. The performance of the new technique is examined using the simulated GPS triple frequency data as well as the real BDS observation. Results show that the ambiguity can be fixed within 10min for GPS and BDS long-range baselines with this new technique.

New data processing strategy for single frequency GPS deformation monitoring

Although the application of the single frequency receiver in GPS deformation monitoring is limited mainly by the effect of the ionospheric delays, the relevant studies have never been stopped due to the much cheaper price of the single frequency receivers. In this paper, we introduce a new data processing strategy for the deformation monitoring network where the baselines between any two nearest stations are processed instead of the baselines formed only between the reference stations and monitoring stations in the traditional strategy. As a result, most of the baselines in the monitoring networks are very short so as to the ionospheric effects can be safely ignored. The results from the experiments show that the new strategy can eliminate the effect of ionospheric delay by processing the short baselines in the network mode. The accuracy and integrity of the deformation solutions can be improved by the presented strategy.

Carrier-phase two-way satellite frequency transfer over a very long baseline

In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Performance Evaluation of USTEC Product for Single-Frequency Precise Point Positioning

Geodetic-grade dual-frequency GPS receivers are typically used for precise point positioning (PPP). Unfortunately, these receiver systems are expensive and may not provide a cost-effective solution in many instances. The use of low-cost single-frequency GPS receivers, on the other hand, are limited by the effect of ionospheric delay. A number of mitigation techniques have been proposed to account for the effect of ionospheric delay for single-frequency GPS users. Unfortunately, however, those mitigation techniques are not suitable for PPP. More recently, the U.S. Total Electron Content (USTEC) product has been developed by the National Oceanic and Atmospheric Administration (NOAA), which describes the ionospheric total electron content in high resolution over most of North America. This paper investigates the performance of USTEC and studies its effect on single-frequency PPP solution. A performance comparison with two widely-used ionospheric mitigation models is also presented.

Enhancing the accuracy of GPS point positioning by converting the single frequency data to dual frequency data

The global positioning system (GPS) has been used to support a wide variety of applications, such as high-accuracy positioning and navigation. Differential GPS techniques can largely eliminate common-mode errors between the reference and the rover GPS stations resulting from ionospheric and tropospheric refraction and delays, satellite and receiver clock biases, and orbital errors [1]. The ionospheric delay in the propagation of global positioning system (GPS) signals is one of the main sources of error in GPS precise positioning and navigation. A dual-frequency GPS receiver can eliminate (to the first order) the ionospheric delay through a linear combination of the L1 and L2 observations [2]. The most significant effect of ionospheric delay appear in case of using single frequency data. In this paper the single frequency data of concerned station are converted to dual frequency data by employing dual frequency data from 11 regional GPS stations distributed around it. Total electron content (TEC) was calculated at every GPS station to produce the mathematical model of TEC which is a function of latitude ( Φ) and longitude ( λ ). By using this mathematical model the values of TEC and L2 can be predicted at the single frequency GPS station for each satellite, after that the comparison between predicted and observation values of TEC and L2 was performed. The estimation method and test results of the proposed method indicates that the difference between predicted and observation values is very small.

Availability Enhancements for CAT IIIB LAAS

: Enhancements are under consideration for the Federal Aviation Administration's (FAA) Category I (CAT I) Local Area Augmentation System (LAAS) to meet the more stringent performance requirements of CAT IIIb operations. This paper analyzes the relative improvement predicted for various alternatives designed to enhance LAAS CAT IIIb availability. These enhancements include increasing the number of ranging sources, using less-conservative error models, taking credit for redundant GPS navigation sensors in the aircraft, using a second frequency to overcome the effects of ionospheric delay, or using differential carrier-phase measurements in the position solution. Availability under each scenario is computed for a vertical alert limit (VAL) ranging from 5.3 m (current requirement) to 10.0 m. Results show that unless VAL is increased beyond about 5.3 m, adequate (0.999) availability cannot be achieved merely by extending the CAT I architecture or by using any one of the above methods alone, with the exception of using dual frequency in both the ground facility and the aircraft. On the other hand, if VAL can be increased to approximately 7.5 m, some other alternatives alone and most pairwise combinations of these alternatives will provide adequate availability.

Surface reflected signals from the Global Positioning System for ionospheric measurements: Experimental results at aircraft altitudes

Several spaceborne altimeters have been built and flown, and others are being developed, to provide measurements of ocean and ice sheet topography. Until the launch of TOPEX (Ocean Topography Experiment), altimeters were single-frequency systems that were incapable of removing the effects of ionospheric delay on the radar pulse. With the current state-of-the-art in satellite altimetry, the ionosphere causes the largest single error when using single-frequency altimeters. Ionospheric models provide the only recourse short of adding a second frequency to the altimeter. Unfortunately, measurements of the ionosphere are lacking over the oceans or ice sheets where they are most needed. A possible solution to the lack of data density may result from an expanded use of the Global Positioning System (GPS). This paper discusses how the reflection of the GPS signal from the ocean can be used to extend ionospheric measurements by simply adding a GPS receiver and downward-pointing antenna to satellites carrying single-frequency altimeters. The viability of this concept hinges upon the ability to acquire and code-track the reflected signal for an extended period of time over a variety of sea states. The theory of specularly and diffusely reflected radio frequency radiation from a rough surface is reviewed. Results of experiments to demonstrate tracking of a reflected signal are presented for three aircraft flights over the Chesapeake Bay and the Eastern Shore of Virginia. The experimental hardware consisted of two off-the shelf receivers configured such that one received the GPS signal in the conventional manner using a right-hand circularly polarized (RHCP) antenna on top of the fuselage and the other could receive the reflected signal using a left-hand circularly polarized (LHCP) antenna on the bottom of the fuselage. Three tests were performed on the data to verify that the signals received in the bottom antenna were sea surface reflections: pseudorange double differences were compared against predicted geometric range double differences, characteristics of a signal reflected from a random surface were observed in the carrier to noise ratio and predicted specular points were plotted which demonstrate reflection only from wet areas. These tests indicated tracking of reflected signals for extended periods of time at altitudes of up to 5500m and sporadic signal acquisition at higher altitudes.

On the effect of ionospheric delay on geodetic relative GPS positioning

On the effect of ionospheric delay on geodetic relative GPS positioning