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Multi-temporal Synthetic Aperture Radar Interferometry (MT-InSAR) is the only geodetic technique allowing to measure ground deformation down to mm/yr over continuous areas. Vegetation cover in equatorial regions favors the use of L-band SAR data to improve interferometric coherence. However, the electron content of ionosphere, affecting the propagation of the SAR signal, shows particularly strong spatio-temporal variations near the equator, while the dispersive nature of the ionosphere makes its effect stronger on low-frequencies, such as L-band signals. To tackle this problem, range split-spectrum method can be implemented to compensate the ionospheric phase contribution. Here, we apply this technique for time-series of ALOS-PALSAR data, and propose optimizations for low-coherence areas. To evaluate the efficiency of this method to retrieve subtle deformation rates in equatorial regions, we compute time-series using four ALOS-PALSAR datasets in contexts of low to medium coherence, showing slow deformation rates (mm/yr to cm/yr). The processed tracks are located in Ecuador, Trinidad and Sumatra, and feature 15 to 19 acquisitions including very high, dominating ionospheric noise, corresponding to equivalent displacements of up to 2 m. The correction method performs well and allows to reduce drastically the noise level due to ionosphere, with significant improvement compared with a simple plane fitting method. This is due to frequent highly non-linear patterns of perturbation, characterizing equatorial TEC distribution. We use semivariograms to quantify the uncertainty of the corrected time-series, highlighting its dependence on spatial distance. Thus, using ALOS-PALSAR-like archive, one can expect a detection threshold on the Line-of-Sight velocity ranging between 3 and 6 mm/yr, depending on the spatial wavelength of the signal to be observed. These values are consistent with the accuracy derived from the comparison of velocities between two tracks in their overlapping area. In the case studies that we processed, the time-series corrected from ionosphere allows to retrieve accurately fault creep and volcanic signal but it is still too noisy for retrieving tiny long-wavelength signals such as slow (mm/yr) interseismic strain accumulation.

The demand for real-time high-precision positioning for global navigation satellite system applications is difficult to satisfy. In this regard, a single-frequency receiver is found to play an important role in overcoming this challenge, especially in developing countries where economic factors are a major restriction. Hence, the development of built-in models, such as the Klobuchar model, is an important objective for single-frequency users to mitigate the effect of ionospheric delay errors in real-time applications. Accordingly, this study aims to devise a new approach to enhance the behavior of the Klobuchar model and increase its efficiency in resolving the aforementioned problem. The new approach seeks to enhance the behavior of the Klobuchar model without refining or increasing its coefficients. To eliminate the ionospheric delay disturbance, the proposed methodology applies normalization and filtration processes to the raw ionospheric delay probability distribution estimated by the unified least squares technique. A final assessment of the new method for enhancing the Klobuchar behavior in predicting the precise position of a single-frequency static receiver under different weather conditions around the globe is presented in this paper. [ABSTRACT FROM AUTHOR]

Several low-Earth orbit (LEO) satellites are equipped with dual-frequency altimeters, theoretically scanning the entire ionosphere in the nadir direction. These two frequencies enable the determination of ionospheric delay and, thus, total electron content (TEC) below the satellite orbit. This information helps in altimetric range determination but is limited to sea and ocean areas. Therefore, global and local ionospheric models are needed for ionospheric corrections over coastal regions and lands. At the same time, altimetry-derived TEC is an important source of validation data for global navigation satellite system (GNSS)-TEC models over the oceans, where the number of GNSS stations is limited. This study compares the application of a high-resolution regional GNSS-TEC model determined from Precise Point Positioning and modeled by least-squares collocation (PPPLSC), and global ionosphere maps (GIMs), in the determination of ionospheric corrections along coastal altimetry tracks. The ionospheric delay values from 5 models are then compared with altimetry-derived TEC from 3 satellites, in the region of southeastern Asia, during a time of moderate TEC values and solar conditions. The reason for the choice of area is that altimetric observations from coastal zones meet difficulties related to atmospheric corrections, e.g., ionospheric correction, which can be affected by the land in the altimeter footprint. For this reason, along with the rapid progress of inland satellite hydrology, we are encouraged to study the consistency of ionospheric delays in coastal regions. The study shows overall discrepancies of 30% of the entire ionospheric delay, which is 2-3 cm even in the case of 35 TEC unit (TECU = 1016 el/m2) values. For this reason, in the case of increased solar activity, the GIMs can have even less TEC consistency with the altimetry-derived TEC, resulting from different orbital altitudes, data gaps, and modeling techniques. The GIMs, modeled by low-order spherical harmonics, have particularly low resolution and do not represent well the equatorial ionization anomaly (EIA). [ABSTRACT FROM AUTHOR]

Interferometry Synthetic Aperture Radar (InSAR) provides unique capabilities to map regional/global topography and deformation of the Earth’s surface and has led to a broad spectrum of deformation monitoring applications. In order to adapt to various challenging monitoring environments, researchers have made tremendous innovations to deal with issues such as atmospheric and ionospheric effects, loss of coherence due to large displacements, geometric distortions and unwrapping errors. Owing to recent technical and methodological advances, the Earth’s surface deformation, ranging from earthquake ruptures, volcanic eruptions, landslides, glaciers, to groundwater storage variations, mining subsidence and infrastructure instability can now be mapped anywhere in the world at high spatial and temporal resolutions. This special issue received a set of contributions highlighting recent advances in methodologies and applications of InSAR to ground deformation monitoring. We aim to present overviews of both the state of the art of SAR/InSAR techniques and the next generation of applications across the broad range of deformation monitoring applications.

To aid single-frequency GNSS users, the Neustrelitz Total Electron Content Model (NTCM) has been proposed as an adequate solution to mitigate propagation errors besides the GPS Klobuchar and Galileo NeQuick models. Using the three effective ionization coefficients broadcast in the Galileo navigation message as driver parameters, the version NTCM-GlAzpar, in general, performs equal to or better than NeQuickG when compared to the total electron content domain. In this work, we performed a global statistical validation of the NTCM-GlAzpar model in the position domain by comparing its results with the results of the Klobuchar and NeQuickG models for the first time. For this purpose, we used the GNSS analysis tool gLAB and its customization capabilities in the Standard Point Positioning mode. The data used for model validation correspond to a one-month period of perturbed solar and geomagnetic activity (December 2014) and another one-month period of quiet conditions (December 2019). The data have a worldwide coverage with up to 73 IGS stations. The statistical analysis of the hourly average 3D position error shows that the root mean squared (RMS) values of the Klobuchar model are 6.71 and 2.75 m for the perturbed and quiet conditions, respectively, whereas the NeQuickG model has RMS values of 4.61 and 2.35 m. In comparison, the corresponding RMS values of 4.36 and 2.32 m of the NTCM-GlAzpar model confirm its better positioning performance for both periods. However, we identify also that the performance of NTCM-GlAzpar slightly worsens toward higher latitudes and at night local time. Simple software adaptations and a low computational cost make NTCM-GlAzpar an alternative practicable algorithm to improve the accuracy of GNSS single-frequency applications. [ABSTRACT FROM AUTHOR]

Interferometry Synthetic Aperture Radar ( InSAR) provides unique capabilities to map regional/ global topography and deformation of the Earth's surface and has led to a broad spectrum of deformation monitoring applications. In order to adapt to various challenging monitoring environments, researchers have made tremendous innovations to deal with issues such as atmospheric and ionospheric effects, loss of coherence due to large displacements, geometric distortions and unwrapping errors. Owing to recent technical and methodological advances, the Earth ' s surface deformation, ranging from earthquake ruptures, volcanic eruptions, landslides, glaciers, to groundwater storage variations, mining subsidence and infrastructure instability can now be mapped anywhere in the world at high spatial and temporal resolutions. This special issue received a set of contributions highlighting recent advances in methodologies and applications of InSAR to ground deformation monitoring. We aim to present overviews of both the state of the art of SAR/InSAR techniques and the next generation of applications across the broad range of deformation monitoring applications. [ABSTRACT FROM AUTHOR]

Abstract I have been conducting a study of postseismic deformation following the 2016 Kumamoto earthquake using ALOS-2/PALSAR-2 acquired till 2018. I apply ionospheric correction to interferograms of ALOS-2/PALSAR-2. L-band SAR gives us high coherence enough to reveal surface deformation even in vegetated or mountainous area for pairs of images acquired more than 2 years. Postseismic deformation following the Kumamoto earthquake exceeds 10 cm during 2 years at some spots in and around Kumamoto city and Aso caldera. Westward motion of ~ 6 cm/year was dominant on the southeast side of the Hinagu fault, while westward shift was detected on both sides of the Futagawa fault. The area of latter deformation seems to have correlation with distribution of pyroclastic flow deposits. Significant uplift was found around the eastern Futagawa fault and on the southwestern frank of Aso caldera, whose rate reaches 4 cm/year. There are sharp changes across several coseismic surface ruptures such as Futagawa, Hinagu, and Idenokuchi faults. Rapid subsidence between Futagawa and Idenokuchi faults also found. It is confirmed that local subsidence continued along the Suizenji fault, which newly appeared during the mainshock in Kumamoto City. Subsidence with westward shift of up to 4 cm/year was also found in Aso caldera. Time constant of postseismic decay ranges from 1 month to 600 days at selected points, but that postseismic deformation during the first epochs or two is dominant at point in the Kumamoto Plain. This result suggests multiple source of deformation. Westward motion around the Hinagu fault may be explained with right lateral afterslip on the shallow part of this fault. Subsidence along the Suizenji fault can be attributed to normal faulting on dipping westward. Deformation around the Hinagu and Idenokuchi faults cannot be explained with right lateral afterslip of Futagawa fault, which requires other sources. Deformation in northern part of Aso caldera might be the result of right lateral afterslip on a possible buried fault.

Glacial isostatic adjustment (GIA) is the solid Earth's viscoelastic response to ice sheet deglaciations. Studies of GIA provide insights into crustal and mantle rheology, mass redistribution, relative paleo sea-level variations, and polar wander. In North America, GIA causes large-scale and low-gradient vertical displacement, but limited ground-based measurements in some places hinder accurate detection of such signals. This study proposes a workflow to extract GIA-induced land deformation using interferometric synthetic aperture radar (InSAR) time-series over the southern Hudson Bay land region. The workflow incorporates several model-based corrections to account for multiple elastic lithospheric responses and atmospheric phase disturbances, significantly reducing variances in the estimated time-series. It further separates the GIA signals from residual short-period noise through a spatiotemporal filter, as demonstrated in variogram analyses. With the workflow we produced the first high-resolution InSAR-based GIA vertical velocity map in the southern Hudson Bay land area at an estimated uncertainty of 2 mm/yr. Our result exhibits first-order agreement with contemporary GIA model predictions, and suggests that higher-level complexities such as 3D rheology structures or spatially uneven loading dynamics may exist in this region. • First InSAR-derived high-res GIA-induced land deformation in Hudson Bay. • A comprehensive workflow with model-based corrections and spatiotemporal filtering. • Noise contribution from different confounding factors is analyzed. • Variograms show significant reduction of noise at different processing stages. • InSAR-derived velocity shows a different gradient pattern from ICE-6G_D predictions. [ABSTRACT FROM AUTHOR]

Ice motion is an essential element for accurately evaluating glacier mass balance. Interferometric synthetic aperture radar (InSAR) has been widely applied for monitoring ice motion with high precision and wide coverage in the Antarctic. However, the ionospheric effects can significantly impact InSAR-based ice-motion measurements. At low radar frequencies in particular, the ionospheric effects have been regarded as a serious source of noise in L-band SAR data. The split-spectrum method (SSM) is commonly used for correcting the ionospheric effects of the InSAR technique. However, it requires spatial filtering with the relatively large factors used to scale the sub-bands’ interferograms, which often results in an unwrapped phase error. In this paper, a reformulation of the split-spectrum method (RSSM) is introduced to correct the ionospheric effects in the Grove Mountains of East Antarctica, which have slow ice flow and frequent ionosphere changes. The results show that RSSM can effectively correct the ionospheric effects of InSAR-based ice-motion measurements. To evaluate the ability of ionospheric correction using RSSM, the result of ionospheric correction derived from SSM is compared with the results of RSSM. In addition, ionosphere-corrected ice motion is also compared with GPS and MEaSUREs. The results show that the ionosphere-corrected ice velocities are in good agreement with GPS observations and MEaSUREs. The average ice velocity from the InSAR time series is compared to that from MEaSUREs, and the average ionosphere-corrected ice velocity error reduces 43.9% in SSM and 51.1% in RSSM, respectively. The ionosphere-corrected ice velocity error is the most significant, reducing 86.9% in SSM and 90.4% in RSSM from 1 November 2007 to 19 December 2007. The results show that the ability of RSSM to correct ionospheric effects is slightly better than that of SSM. Therefore, we deduce that the RSSM offers a feasible way to correct ionospheric effects in InSAR-based ice-motion measurements in Antarctica.

A multi-frequency Global Navigation Satellite System (GNSS) provides greater opportunities for positioning and navigation applications, particularly the BeiDou Global Navigation Satellite System (BDS-3) satellites. However, multi-frequency signals import more pseudorange channels, which introduce more multi-channel Differential Code Biases (DCBs). The satellite and receiver DCBs from the new BDS-3 signals are not clear. In this study, 9 DCB types of the new BDS-3 signals from 30-days Multi-GNSS Experiment (MGEX) observations are estimated and investigated. Compared with the DCB values provided by the Chinese Academy of Science (CAS) products, the mean bias and root mean squares (RMS) error of new BDS-3 satellite DCBs are within ±0.20 and 0.30 ns, respectively. The satellite DCBs are mostly within ±0.40 ns with respect to the product of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The four sets of constructed closure errors and their mean values are within ±0.30 ns and ±0.15 ns, respectively. The mean standard deviation (STD) of the estimated satellite DCBs is less than 0.10 ns. In particular, our estimated satellite DCBs are more stable than DCB products provided by CAS and DLR. Unlike satellite DCBs, the receiver DCBs have poor compliance and show an obvious relationship with the geographic latitude when compared to the CAS products. The STDs of our estimated receiver DCBs are less than 1.00 ns. According to different types of receiver DCBs, the distribution of STDs indicates that the coefficient of the ionospheric correction has an influence on the stability of the receiver DCBs under the ionosphere with the same accuracy level. In addition, the type of receiver shows no regular effects on the stability of receiver DCBs.

I have been conducting a study of postseismic deformation following the 2016 Kumamoto earthquake using ALOS-2/PALSAR-2 acquired till 2018. I apply ionospheric correction to interferograms of ALOS-2/PALSAR-2. L-band SAR gives us high coherence enough to reveal surface deformation even in vegetated or mountainous area for pairs of images acquired more than 2 years. Postseismic deformation following the Kumamoto earthquake exceeds 10 cm during 2 years at some spots in and around Kumamoto city and Aso caldera. Westward motion of ~ 6 cm/year was dominant on the southeast side of the Hinagu fault, while westward shift was detected on both sides of the Futagawa fault. The area of latter deformation seems to have correlation with distribution of pyroclastic flow deposits. Significant uplift was found around the eastern Futagawa fault and on the southwestern frank of Aso caldera, whose rate reaches 4 cm/year. There are sharp changes across several coseismic surface ruptures such as Futagawa, Hinagu, and Idenokuchi faults. Rapid subsidence between Futagawa and Idenokuchi faults also found. It is confirmed that local subsidence continued along the Suizenji fault, which newly appeared during the mainshock in Kumamoto City. Subsidence with westward shift of up to 4 cm/year was also found in Aso caldera. Time constant of postseismic decay ranges from 1 month to 600 days at selected points, but that postseismic deformation during the first epochs or two is dominant at point in the Kumamoto Plain. This result suggests multiple source of deformation. Westward motion around the Hinagu fault may be explained with right lateral afterslip on the shallow part of this fault. Subsidence along the Suizenji fault can be attributed to normal faulting on dipping westward. Deformation around the Hinagu and Idenokuchi faults cannot be explained with right lateral afterslip of Futagawa fault, which requires other sources. Deformation in northern part of Aso caldera might be the result of right lateral afterslip on a possible buried fault. [ABSTRACT FROM AUTHOR]

The main objective of the European Geostationary Navigation Overlay System (EGNOS) is to improve the positioning accuracy by correcting several error sources affecting the Global Positioning System (GPS) and to provide integrity information to GPS signals for users in real time. This research presents analysis used to investigate improvement in the performance of single-frequency GPS positioning using EGNOS corrections in Algeria. In this study, we performed position measurements with two calculation approaches, the first based on GPS single-point positioning and the second using EGNOS differential corrections. Positioning accuracy was determined by comparison with the known precise coordinates of the sites; and then the improved ionospheric correction using EGNOS was investigated. The results revealed that GPS + EGNOS performance was significantly improved compared with GPS alone, when measurements of horizontal and vertical accuracy were taken into account, and that the EGNOS corrections improved east and north components slightly, and the up component significantly. [ABSTRACT FROM AUTHOR]

The Quasi-Zenith Satellite System (QZSS) established by the Japan Aerospace Exploration Agency mainly serves the Asia-Pacific region and its surrounding areas. Currently, four in-orbit satellites provide services. Most users of GNSS in the mass market use single-frequency (SF) receivers owing to the low cost. Therefore, it is meaningful to analyze and evaluate the contribution of the QZSS to SF precise point positioning (PPP) of GPS/BDS/GLONASS/Galileo systems with the emergence of GNSS and QZSS. This study compares the performances of three SF PPP models, namely the GRoup and PHase Ionospheric Correction (GRAPHIC) model, GRAPHIC with code observation model, and an ionosphere-constrained model, and evaluated the contribution of the QZSS to the SF PPP of GPS/BDS/GLONASS/Galileo systems. Moreover, the influence of code bias on the SF PPP of the BDS system is also analyzed. A two-week dataset (DOY 013–026, 2019) from 10 stations of the MGEX network is selected for validation, and the results show that: (1) For cut-off elevation angles of 15, 20, and 25°, the convergence times for the static SF PPP of GLONASS + QZSS are reduced by 4.3, 30.8, and 12.7%, respectively, and the positioning accuracy is similar compared with that of the GLONASS system. Compared with the BDS single system, the convergence times for the static SF PPP of BDS + QZSS under 15 and 25° are reduced by 37.6 and 39.2%, the horizontal positioning accuracies are improved by 18.6 and 14.1%, and the vertical components are improved by 13.9 and 21.4%, respectively. At cut-off elevation angles of 15, 20, and 25°, the positioning accuracy and precision of GPS/BDS/GLONASS/Galileo + QZSS is similar to that of GPS/BDS/GLONASS/Galileo. And the convergence times are reduced by 7.4 and 4.3% at cut-off elevation angles of 20 and 25°, respectively. In imitating dynamic PPP, the QZSS significantly improves the positioning accuracy of BDS and GLONASS. However, QZSS has little effect on the GPS-only, Galileo-only and GPS/BDS/GLONASS/Galileo. (2) The code bias of BDS IGSO and MEO cannot be ignored in SF PPP. In static SF PPP, taking the frequency band of B1I whose multipath combination is the largest among the frequency bands as an example, the vertical component has a systematic bias of approximately 0.4–1.0 m. After correcting the code bias, the positioning error in the vertical component is lower than 0.2 m, and the positioning accuracy in the horizontal component are improved accordingly. (3) The SF PPP model with ionosphere constraints has a better convergence speed, while the positioning accuracy of the three models is nearly equal. Therefore the GRAPHIC model can be used to get good positioning accuracy in the absence of external ionosphere products, but its convergence speed is slower. [ABSTRACT FROM AUTHOR]

Ionospheric disturbance is one of the most serious factors that limit the accuracy of deformation measurement of interferometric synthetic aperture radar (InSAR) especially for spaceborne SAR systems in low frequency such as L-band. Especially in imaging modes with a large coverage, ScanSAR mode for example, ionospheric disturbance phase screen almost submerges other useful geographical information. In this study, two ALOS-2 (L-band) full-aperture ScanSAR images (October 2015 and October 2016) are used to study ground deformation in Beijing from 2015 to 2016. The split range-spectrum technique is adopted to extract ionospheric distortion. The authors splice the differential interferograms of adjacent subswaths by resampling them to perform the overall ionospheric extraction. Also non-local filtering is used to improve the coherence of interferograms. Spatial deformation distribution that was obtained by differential interferometric synthetic aperture radar (D-InSAR) technique using the two ALOS-2 ScanSAR images is highly consistent with the result obtained by persistent scatterer interferometric synthetic aperture radar (PS-InSAR) technique using Sentinel Tops mode images (October 2015–February 2017). The comparison results show the effectiveness of the ionospheric distortion extraction method used in this study. Moreover, the spatial distribution is also highly consistent with the results before 2014, which means that the deformation trend in Beijing continues.

Ionospheric disturbance is one of the most serious factors that limit the accuracy of deformation measurement of interferometric synthetic aperture radar (InSAR) especially for spaceborne SAR systems in low frequency such as L-band. Especially in imaging modes with a large coverage, ScanSAR mode for example, ionospheric disturbance phase screen almost submerges other useful geographical information. In this study, two ALOS-2 (L-band) full-aperture ScanSAR images (October 2015 and October 2016) are used to study ground deformation in Beijing from 2015 to 2016. The split range-spectrum technique is adopted to extract ionospheric distortion. The authors splice the differential interferograms of adjacent subswaths by resampling them to perform the overall ionospheric extraction. Also non-local filtering is used to improve the coherence of interferograms. Spatial deformation distribution that was obtained by differential interferometric synthetic aperture radar (D-InSAR) technique using the two ALOS-2 ScanSAR images is highly consistent with the result obtained by persistent scatterer interferometric synthetic aperture radar (PS-InSAR) technique using Sentinel Tops mode images (October 2015–February 2017). The comparison results show the effectiveness of the ionospheric distortion extraction method used in this study. Moreover, the spatial distribution is also highly consistent with the results before 2014, which means that the deformation trend in Beijing continues. [ABSTRACT FROM AUTHOR]

The L-band synthetic aperture radar (SAR) interferometry (InSAR) technique has a lower accuracy due to ionospheric phase distortions. Recently, a multiple-aperture interferometry (MAI)-based ionospheric correction method has been proposed. Using four types of ionosphere-distorted interferograms, the performance of the correction method was evaluated and then analyzed the feasibility of the correction method. The test interferograms contained severe azimuth streaking, low-frequency ionospheric phase distortion and drastic phase change due to the ionosphere. The results showed that (i) the existence and magnitude of ionospheric phase distortions can be recognized from MAI interferograms and (ii) the MAI-based ionospheric correction method efficiently reduced severe azimuth streaking and low-frequency distortion but did not mitigate the drastic phase change perfectly. It is allowed (i) to determine whether a given SAR interferogram has an ionospheric distortion and (ii) to predict whether the ionospheric distortion can be corrected by using the MAI-based ionospheric correction method. [ABSTRACT FROM AUTHOR]

Abstract In the last 20 years, and in particular in the last decade, the availability of propagation data for GNSS has increased substantially. In this sense, the ionosphere has been sounded with a large number of receivers that provide an enormous amount of ionospheric data. Moreover, the maturity of the models has also been increased in the same period of time. As an example, IGS has ionospheric maps from GNSS data back to 1998, which would allow for the correlation of these data with other quantities relevant for the user and space weather (such as Solar Flux and Kp). These large datasets would account for almost half a billion points to be analyzed. With the advent and explosion of Big Data algorithms to analyze large databases and find correlations with different kinds of data, and the availability of open source code libraries (for example, the TensorFlow libraries from Google that are used in this paper), the possibility of merging these two worlds has been widely opened. In this paper, a proof of concept for a single frequency correction algorithm based in GNSS GIM vTEC and Fully Connected Neural Networks is provided. Different Neural Network architectures have been tested, including shallow (one hidden layer) and deep (up to five hidden layers) Neural Network models. The error in training data of such models ranges from 50% to 1% depending on the architecture used. Moreover, it is shown that by adjusting a Neural Network with data from 2005 to 2009 but tested with data from 2016 to 2017, Neural Network models could be suitable for the forecast of vTEC for single frequency users. The results indicate that this kind of model can be used in combination with the Galileo Signal-in-Space (SiS) NeQuick G parameters. This combination provides a broadcast model with equivalent performances to NeQuick G and better than GPS ICA for the years 2016 and 2017, showing a 3D position Root Mean Squared (RMS) error of approximately 2 m. [ABSTRACT FROM AUTHOR]

Abstract We developed the methodology for the optimal estimation of global ionospheric coefficients of the current Global Navigation Satellite Systems (GNSSs), including the eight- and ten-parameter Klobuchar-like as well as NeQuick models. The ionospheric coefficients of those correction models are calculated from two sets of globally distributed tracking stations of the International GNSS Services (IGS). Performance of the re-estimated Klobuchar-like and NeQuick coefficients are validated during 2002–2014 over the continental and oceanic areas, respectively. Over the continental areas, GPS TECs derived from 40 ground GPS receivers are selected as reference. The eight-, ten-parameter Klobuchar-like and NeQuick models can mitigate the ionospheric delay by 65.8, 67.3 and 75.0%, respectively. Over the global oceans, the independent TECs derived from Jason-1&2 altimeters are used as reference. The re-estimated ionospheric correction models can mitigate 56.1–66.7% of the delay errors. Compared to the original GPS Ionospheric Correction Algorithm (ICA), performance of those eight-, ten-parameter Klobuchar-like and NeQuick models has improved 3.4, 5.9 and 13.4% during the whole test period, respectively. The methodology developed here takes the advantage of high-quality ionospheric TECs derived from the global network of GNSS receivers. The re-estimated ionospheric coefficients can be used as precise ionospheric products to monitor and assess GNSS broadcast ionospheric parameters and to improve the performance of various single-frequency GNSS applications. [ABSTRACT FROM AUTHOR]

Interferometric synthetic aperture radar (InSAR) products may be significantly distorted by microwave signals traveling through the ionosphere, especially with long wavelengths. The split-spectrum method (SSM) is used to separate the ionospheric and the nondispersive phase terms with lower and higher spectral sub-band interferogram images. However, the ionospheric path delay phase is very delicate to the synthetic aperture radar (SAR) parameters including orbit vectors, slant range, and target height. In this paper, we get the impact of SAR parameter errors on the ionospheric phase by two steps. The first step is getting the derivates of geolocation with reference to SAR parameters based on the range-Doppler (RD) imaging model and the second step is calculating the derivates of the ionospheric phase delay with respect to geometric positioning. Through the numerical simulation, we demonstrate that the deviation of ionospheric phase has a linear relationship with SAR parameter errors. The experimental results show that the estimation of SAR parameters should be accurate enough since the parameter errors significantly affect the performance of ionospheric correction. The root mean square error (RMSE) between the corrected differential interferometric SAR (DInSAR) phase with SAR parameter errors and the corrected DInSAR phase without parameter errors varies from centimeter to decimeter level with the L-band data acquired by the Advanced Land Observing Satellite (ALOS) Phased Array type L-band SAR (PALSAR) over Antofagasta, Chile. Furthermore, the effectiveness of SSM can be improved when SAR parameters are accurately estimated.

Users of the Global Positioning System (GPS) utilize the Ionospheric Correction Algorithm (ICA) also known as Klobuchar model for correcting ionospheric signal delay or range error. Recently, we developed an ionosphere correction algorithm called NTCM-Klobpar model for single frequency GNSS applications. The model is driven by a parameter computed from GPS Klobuchar model and consecutively can be used instead of the GPS Klobuchar model for ionospheric corrections. In the presented work we compare the positioning solutions obtained using NTCM-Klobpar with those using the Klobuchar model. Our investigation using worldwide ground GPS data from a quiet and a perturbed ionospheric and geomagnetic activity period of 17 days each shows that the 24-hour prediction performance of the NTCM-Klobpar is better than the GPS Klobuchar model in global average. The root mean squared deviation of the 3D position errors are found to be about 0.24 and 0.45 m less for the NTCM-Klobpar compared to the GPS Klobuchar model during quiet and perturbed condition, respectively. The presented algorithm has the potential to continuously improve the accuracy of GPS single frequency mass market devices with only little software modification.

This study shows the way of ionospheric data usage obtained from 2 Tables using D2 NAV message Chinese BeiDou system. It was identified particular algorithms for Table's calculation. Authors also suggested data Table 2 modification and special algorithms useful for that Table. It was given calculated examples for BeiDou system Tables as well as for new suggested system. [ABSTRACT FROM AUTHOR]

Originally designed for applications over the ocean, satellite altimetry has been proven to be a useful tool for hydrologic studies. Altimeter products, mainly conceived for oceanographic studies, often fail to provide atmospheric corrections suitable for inland water studies. The focus of this paper is the analysis of the main issues related with the atmospheric corrections that need to be applied to the altimeter range to get precise water level heights. Using the corrections provided on the Radar Altimeter Database System, the main errors present in the dry and wet tropospheric corrections and in the ionospheric correction of the various satellites are reported. It has been shown that the model-based tropospheric corrections are not modeled properly and in a consistent way in the various altimetric products. While over the ocean, the dry tropospheric correction (DTC) is one of the most precise range corrections, in some of the present altimeter products, it is the correction with the largest errors over continental water regions, causing large biases of several decimeters, and along-track interpolation errors up to several centimeters, both with small temporal variations. The wet tropospheric correction (WTC) from the on-board microwave radiometers is hampered by the contamination on the radiometer measurements of the surrounding lands, making it usable only in the central parts of large lakes. In addition, the WTC from atmospheric models may also have large errors when it is provided at sea level instead of surface height. These errors cannot be corrected by the user, since no accurate expression exists for the height variation of the WTC. Alternative and accurate corrections can be computed from in situ data, e.g., DTC from surface pressure at barometric stations and WTC from Global Navigation Satellite System permanent stations. The latter approach is particularly favorable for small lakes and reservoirs, where GNSS-derived WTC at a single location can be representative of the whole lake. For non-timely critical studies, for consistency and stability, model-derived tropospheric corrections from European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis ERA Interim, properly computed at surface height, are recommended. The instrument-based dual-frequency ionospheric correction may have errors related with the land contamination in the Ku and C/S bands, making it more suitable to use a model-based correction. The most suitable model-based ionospheric correction is the Jet Propulsion Laboratory (JPL) global ionosphere map (GIM) model, available after 1998, properly scaled to the altimeter height. Most altimeter products provide the GIM correction unreduced for the total electron content extending above the altitude of these satellites, thus overestimating the ionospheric correction by about 8%. Prior to 1998, the NIC09 (NOAA Ionosphere Climatology 2009) climatology provides the best accuracy.

The article presents a proposal to make simultaneous allowance for both ionospheric and tropospheric corrections in differential synthetic aperture radar interferometry (DInSAR) measurements. Atmospheric delay in the interferometric phase may cause the detection of terrain-surface changes to be impossible or significantly distorted. This fact remains of special importance in the case of surface changes that show limited amplitude and spatial range. Two areas were chosen to verify the validity of the proposed solution. The first area includes terrains affected by underground copper-ore mining activity (Poland), which shows high induced seismic activity. Mining tremors recorded in this area cause the terrain surface to locally subside. The authors analyzed three tremors that were recorded in 2016, 2017, and 2019. Each of the tremors exceeded a magnitude of Mw 4.0. The second area is located in the coastal region of Chile, in the Cardenal Caro province. In this case, the authors focused on a series of three earthquakes recorded on 11 March 2010. The strongest of the earthquakes was of Mw 7.0 magnitude. In the first case, calculations were based on obtained data from the Sentinel 1 satellites, and in the second case from the ALOS-1 satellite. It is demonstrated that simultaneous allowance for both the tropospheric and ionospheric corrections significantly improves the final results. The authors were also able to use the analyzed cases to demonstrate that implementation of the corrections does not have negative influence on the range and magnitude of local ground-surface deformations. At the same time, such implementation minimizes local displacement fluctuations and reduces displacement values in areas affected by deformations. The examples used in the article served to show that tropospheric correction is mainly responsible for global corrections (i.e., within the whole analyzed spatial range), while ionospheric correction reduces local fluctuations.

To meet the demands of civil aviation and other precise navigation applications, several satellite-based augmentation systems (SBASs) have been developed around the world, such as the Wide Area Augmentation System (WAAS) for North America, the European Geostationary Navigation Overlay Service (EGNOS) for Europe, the Multi-functional Satellite Augmentation System (MSAS) for Japan, the GPS (Global Positioning System) Aided GEO Augmented Navigation (GAGAN) for India, and the System for Differential Corrections and Monitoring (SDCM) for Russia. The SBASs broadcast messages to correct satellite orbit, clock, and ionosphere errors to augment the GPS positioning performance. In this paper, SBAS orbit, clock and ionospheric corrections are evaluated. Specifically, the orbit, clock and ionospheric corrections derived from SBAS messages are comprehensively evaluated using data collected from the above mentioned systems over 181 consective days. The evaluation indicates that the EGNOS outperforms other systems with signal-in-space range error (SISRE) at 0.645 m and ionospheric correction accuracy at 0.491 m, respectively. Meanwhile, the accuracy of SDCM is comparable to EGNOS with SISRE of 0.650 m and ionospheric correction accuracy of 0.523 m. For WAAS, the SISRE is 0.954 m and the accuracy of ionospheric correction is 0.505 m. The accuracies of the SBAS corrections from the MSAS and GAGAN systems, however, are significantly worse than those of others. The SISREs are 1.931 and 1.325 m and the accuracies of ionospheric corrections are 0.795 and 0.858 m, for MSAS and GAGAN, respectively. At the same time, GPS broadcast orbit, clock and ionospheric corrections are also evaluated. The results show that there are no significant improvements in the SISRE of the broadcast navigation data by applying SBAS corrections. On the other hand, the accuracy of SBAS ionospheric corrections is still much better than GPS broadcast ionospheric corrections, which could still be beneficial for single-frequency users.

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Ionospheric total electron content (TEC) disturbances can seriously influence the signal of low-frequency spaceborne synthetic aperture radar (SAR) systems, e.g., Advanced Land Observation Satellite (ALOS)-phased array-type L-band synthetic aperture radar (PALSAR). With regard to coseismic studies using interferometric synthetic aperture radar (InSAR), it is vital to mitigate the ionospheric artifacts in the contaminated coseismic interferogram. In this paper, we propose a new method for the integral constant calculation, and we then aim to improve the estimation of the ionospheric phase screen (IPS). The proposed method is based on both azimuth and range displacement field maps. At present, the azimuth displacement field can be generated by an offset-tracking procedure or multiple-aperture InSAR (MAI), but the range displacement field can only be estimated by an offset-tracking procedure. We applied ALOS-PALSAR data that were acquired before and after the 2008 Wenchuan earthquake and the 2010 Darfield earthquake to test the proposed method. This case study further showed that ionospheric azimuth streaks were clearly visible in the azimuth displacement field maps of these two cases, one of which was generated using the MAI algorithm and the other using an offset-tracking algorithm. The results confirmed that the long-wavelength ionospheric artifacts in the non-coseismic regions could be corrected by the use of the proposed method. The line-of-sight (LOS) displacement corrections of these two cases, Wenchuan and Darfield, were in the range of $-35.9$ to 21.0 cm and $-6.6$ to 10.0 cm along the LOS direction, respectively. [ABSTRACT FROM PUBLISHER]

Broadcast ionospheric model is currently an effective approach to mitigate the ionospheric time delay for real-time Global Navigation Satellite System (GNSS) single-frequency users. Klobuchar coefficients transmitted in Global Positioning System (GPS) navigation message have been widely used in various GNSS positioning and navigation applications; however, this model can only reduce the ionospheric error by approximately 50% in mid-latitudes. With the emerging BeiDou and Galileo, as well as the modernization of GPS and GLONASS, more precise ionospheric correction models or algorithms are required by GNSS single-frequency users. Numerical analysis of the initial phase and nighttime term in Klobuchar algorithm demonstrates that more parameters should be introduced to better describe the variation of nighttime ionospheric total electron content (TEC). In view of this, several schemes are proposed for the improvement of Klobuchar algorithm. Performance of these improved Klobuchar-like models are validated over the continental and oceanic regions during high (2002) and low (2006) levels of solar activities, respectively. Over the continental region, GPS TEC generated from 35 International GNSS Service (IGS) and the Crust Movement Observation Network of China (CMONOC) stations are used as references. Over the oceanic region, TEC data from TOPEX/Poseidon and JASON-1 altimeters are used for comparison. A ten-parameter Klobuchar-like model, which describes the nighttime term as a linear function of geomagnetic latitude, is finally proposed for GNSS single-frequency ionospheric corrections. Compared to GPS TEC, while GPS broadcast model can correct for 55.0% and 49.5% of the ionospheric delay for the year 2002 and 2006, respectively, the proposed ten-parameter Klobuchar-like model can reduce the ionospheric error by 68.4% and 64.7% for the same period. Compared to TOPEX/Poseidon and JASON-1 TEC, the improved ten-parameter Klobuchar-like model can mitigate the ionospheric delay by 61.1% and 64.3% in 2002 and 2006, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the Global Ionospheric Maps (GIMs) during 1998–2012 have been adopted as the primary source of data to generate the Galileo-like ionospheric correction parameters. Taking the previous day GIMs as ingested dataset and the current day GIMs as reference, we have computed the cumulative distribution function (CDF) of TEC (Total Electron Content) error less than 5 TECU (TEC Unit, 1TECU = 10 16 electron/m 2 ) or 20% (denoted as zi 20 ) to study the performance of NeQuick 2 in providing global ionospheric correction. The statistical analysis is carried out to study how the performance changes with seasons, solar activities and geographic latitudes. Our studies showed that: (1) in general, the NeQuick 2 model driven by the previous day GIMs agrees well with the current day GIMs, with monthly averaged zi 20 exceeds 60%. (2) The performance of GIMs-driven-NeQuick model varies with geographic latitudes. The model behaves better in mid-latitudes than in low-latitudes, which is more prominent in northern hemisphere. (3) There is a nonlinear relationship between zi 20 and F10.7. When the solar activity level is low or medium, zi 20 tends to decrease with increasing solar activities. However, when F10.7 increases further to exceed a certain threshold, zi 20 generally shows either saturation or an increase for high solar epoch. The zi 20 decreases with F10.7 more sharply in mid-latitudes than in low-latitudes, in the southern hemisphere than in the northern hemisphere. (4) The seasonal behavior of zi 20 in different latitudinal belts are quite different. In the southern hemisphere, zi 20 mainly presents annual variation, and it is largest in winter and smallest in summer. In the northern hemisphere, the semi-annual variation is dominant, and zi 20 is larger in solstice than in equinox. During the High Solar Activity years, the zi 20 is much larger in summer than in winter in the low-latitude regions. [ABSTRACT FROM AUTHOR]

Because of the characteristics of its topographic distribution, it is hard for China to set up a Grid-based ionospheric correction model. In order to obtain accurate regional ionospheric correction, the wide area ionospheric correction triangular partition method is proposed. A triangular partition of the ionospheric delay model for mid-latitude in China was established using five ionosphere reference stations of the China seismo-ionospheric groundbase monitoring network. The precision of the method was examined by eight reference stations, the results show that, in the case of the existing network layout for the triangular partition of the internal area, ionospheric delay error can be improved to 90%; For the external triangular partition, hundreds of kilometers from the area, this method can also achieve correction accuracy of more than 80%. At the same time, GNSS data from four IGS tracking stations in United States and Canada were used to test this method. The results also verify the feasibility of this method, with important reference value to the wide area single-frequency ionospheric delay error correction. [ABSTRACT FROM AUTHOR]

Originally designed for applications over the ocean, satellite altimetry has been proven to be a useful tool for hydrologic studies. Altimeter products, mainly conceived for oceanographic studies, often fail to provide atmospheric corrections suitable for inland water studies. The focus of this paper is the analysis of the main issues related with the atmospheric corrections that need to be applied to the altimeter range to get precise water level heights. Using the corrections provided on the Radar Altimeter Database System, the main errors present in the dry and wet tropospheric corrections and in the ionospheric correction of the various satellites are reported. It has been shown that the model-based tropospheric corrections are not modeled properly and in a consistent way in the various altimetric products. While over the ocean, the dry tropospheric correction (DTC) is one of the most precise range corrections, in some of the present altimeter products, it is the correction with the largest errors over continental water regions, causing large biases of several decimeters, and along-track interpolation errors up to several centimeters, both with small temporal variations. The wet tropospheric correction (WTC) from the on-board microwave radiometers is hampered by the contamination on the radiometer measurements of the surrounding lands, making it usable only in the central parts of large lakes. In addition, the WTC from atmospheric models may also have large errors when it is provided at sea level instead of surface height. These errors cannot be corrected by the user, since no accurate expression exists for the height variation of the WTC. Alternative and accurate corrections can be computed from in situ data, e.g., DTC from surface pressure at barometric stations and WTC from Global Navigation Satellite System permanent stations. The latter approach is particularly favorable for small lakes and reservoirs, where GNSS-derived WTC at a single location can be representative of the whole lake. For non-timely critical studies, for consistency and stability, model-derived tropospheric corrections from European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis ERA Interim, properly computed at surface height, are recommended. The instrument-based dual-frequency ionospheric correction may have errors related with the land contamination in the Ku and C/S bands, making it more suitable to use a model-based correction. The most suitable model-based ionospheric correction is the Jet Propulsion Laboratory (JPL) global ionosphere map (GIM) model, available after 1998, properly scaled to the altimeter height. Most altimeter products provide the GIM correction unreduced for the total electron content extending above the altitude of these satellites, thus overestimating the ionospheric correction by about 8%. Prior to 1998, the NIC09 (NOAA Ionosphere Climatology 2009) climatology provides the best accuracy. [ABSTRACT FROM AUTHOR]

The accuracy of L-band synthetic aperture radar (SAR) differential interferometry (InSAR) on crustal deformation studies is largely compromised by ionosphere path delays on the radar signals. The ionosphere effects cause severe ionospheric distortion such as azimuth streaking and long wavelength phase distortion similar to orbital ramp error. Effective detection and correction of ionospheric phase distortion from L-band InSAR images are necessary to measure and accurately interpret surface displacement. In this paper, we investigate the performance improvement of L-band InSAR interseismic deformation measurements in southern California through the joint correction of both ionosphere noise and orbital error. Our results show that this method can effectively remove orbit and ionosphere phase distortions. In comparison with in situ GPS measurements, the achieved InSAR measurement accuracy is improved from ~ 30 mm to ~ 10 mm by the proposed joint correction method. We show that, after the joint correction, the remaining atmosphere noise can be further mitigated through stacking, leading to an RMS error of ~ 4.7 mm/year in resultant line-of-sight velocity, as compared with ~ 11.3 mm/year before the correction. Our results demonstrate that the proposed joint correction technique provides a promising way to jointly correct orbital and ionospheric artifacts in L-band InSAR studies of crustal deformation. [ABSTRACT FROM PUBLISHER]

A modified ionospheric correction method and the corresponding approximate algorithm for spaceborne single-frequency Global Positioning System (GPS) users are proposed in this study. Single Layer Model (SLM) mapping function for spaceborne GPS was analyzed. SLM mapping functions at different altitudes were calculated. Ionospheric Pierce Point (IPP) trajectories of the dlft station (An IGS station located at the longitude of 4°23′15.22′′E and the latitude of 51°59′9.63′′N, in the TU Delft University, The Netherlands.) and the GRACE satellite were computed with the corresponding single layer height of 350 and 500 km, respectively. The Klobuchar model was used to compute ionospheric delays for the dlft station, and modified Klobuchar model, together with scale factors, was used to compute the fractional ionospheric corrections above the GRACE altitudes. Calculation results were validated using dual-frequency observations. The study shows that the single layer height needs to be changed from 350 to 500 km according to the altitude of GRACE. Approximate forms of Earth angle and slant factor developed for modified Klobuchar model are applicable to GRACE, with accuracy adequate to preserve the essential elements required to compute ionospheric delays. Results show that the Klobuchar model is effective for ground GPS, and the modified Klobuchar model corrects more than 80% on average of the ionospheric delays for spaceborne single-frequency GPS. [ABSTRACT FROM AUTHOR]