Continuously Operating Reference Stations Research Articles

FEASIBILITY STUDY OF GNSS-ENABLED GCP MARKER FOR CONTROL ESTABLISHMENT FOR DRONE SURVEYING

Abstract. This study explores the potential of GNSS-enabled Ground Control Point (GCP) markers as an alternative to traditional survey-grade receivers in observing GCPs for drone surveys. This is because an average drone flight and the observation period for rapid static are both 15 minutes. A GNSS-enabled GCP is already in the market through Propeller Aeropoints but is not available in the Philippines. Previous independent studies on the accuracy of such markers are limited as it is only focused on the accuracy. There are other considerations in using this technique such as the practicality of the marker, its usability with public Continuously Operating Reference Station (CORS) networks such as PAGeNet, and whether using it in conjunction with passive monuments will significantly affect the accuracy. The researchers designed and assembled a low-cost alternative using a u-blox module to determine the markers’ applicability and effectiveness. This study was limited to using three (3) GCPs to simulate the minimum number required to georeference in 2D. Accuracy remains an important criterion so observations will be compared with known positions. Three drone surveys were conducted to further evaluate its performance. The marker achieved 3rd Order geodetic control and demonstrated up to one standard deviation of error using statistical analysis. However, the lack of control points affected the accuracy observed in the checkpoints. This study focused on low-cost GNSS-enabled GCP marker since the commercial Aeropoints markers are not vailable in the country. Implementing this methodology in drone surveying could potentially increase survey efficiency in the Philippines and aid in the country's cadastral mapping efforts.

A Two-Step Regional Ionospheric Modeling Approach for PPP-RTK.

In the precise point positioning/real-time kinematic (PPP-RTK) technique, high-precision ionospheric delay correction information is an important prerequisite for rapid PPP convergence. The commonly used ionospheric modeling approaches in the PPP-RTKs only take the trend term of the ionospheric total electron content (TEC) variations into account. As a result, the residual ionospheric delay still affects the positioning solutions. In this study, we propose a two-step regional ionospheric modeling approach that involves combining a polynomial fitting model (PFM) and a Kriging interpolation (KI) model. In the first step, a polynomial fitting method is used to model the trend term of the ionospheric TEC variations. In the second step, a KI method is used to compensate for the residual term of the ionospheric TEC variations. Datasets collected from continuously operating reference stations (CORSs) in Hunan Province, China, are used to validate the PFM/KI method by comparing with a single PFM method and a combined PFM and inverse distance weighting interpolation (IDWI) method. The experimental results show that the two-step PFM/KI modeled ionospheric delay achieves an average root mean square (RMS) error of 1.8 cm, which is improved by about 48% and 23% when compared with the PFM and PFM/IDWI methods, respectively. Regarding the positioning performance, the PPP-RTK with the PFM/KI method takes an average of 1.8 min or 4.0 min to converge to a positioning accuracy of 1.3 cm or 2.5 cm in the horizontal and vertical directions, respectively. The convergence times are decreased by about 18% and 14% in the horizontal direction and 9% and 5% in the vertical direction over the PFM and the PFM/IDWI methods, respectively.

Analysis of 3D positions processed from CORS in southern Ghana

Differential mode of processing GNSS in the preferred choice for high precision assignments in areas where there are active reference stations. Studies into the performance of these continuously operating reference stations (CORS) have mainly been centered on either session times or baseline lengths. Active CORS have emerged replacing the old passive stations. However, these innovations in the science of spatial data acquisition needs to be monitored for networks strength and sensitivity. In this study, GPS data from five CORS stations for 12 days were used and processed with RTKLib. The research was restricted to baseline lengths of 2.5 to 13.5 km with varying session lengths starting from 1 hour to 24 hours. 3D error for each baseline and 3D RMS for each group of baselines were subsequently computed. A plot of the results was analyzed and compared. This comparison reveals that Geodetic accuracy (below 1cm) is achieved after 4 hours of observation session showing that the precision of Rtklib baseline solution can reach sub-centimeter level and therefore can yield reliable results in almost any professional project.

Inside into GNSS CORS infrastructure setup in Malaysia

Global Navigation Satellite Systems is a space-based technology that are used in positioning, navigation, and timing (PNT) applications. Accordingly, the Continuously Operating Reference Station (CORS) networks, which are a common type of GNSS ground-based augmentation infrastructure are used to distribute PNT information with centimetre accuracy. In Malaysia, multiple CORS networks are operated independently by various government agencies, universities, and industries. Numerous users have licenced CORS data and services for their positioning and navigation applications. There may be issues with the existing CORS operational model among public and industrial positioning and navigation applications that need to be considered. This paper reviewed the country's CORS management and technical issues and discussed the Higgins model and quadruple helix concept to local CORS operations. A conceptual framework for sustainable development of CORS infrastructure and data is proposed to support the social economy context of the nation.

Mitigation of tropospheric delay induced errors in TS-InSAR ground deformation monitoring

ABSTRACT Interferometric Synthetic Aperture Radar (InSAR) is capable of detecting crust deformation. However, the accuracy is limited by spatiotemporal changes in the lower troposphere. In this paper, we constructed a periodic zenith total delay negative exponential function (PZTD-NEF) model of atmospheric spatiotemporal variation characteristics based on ERA-5 data to alleviate the temporal oscillation bias introduced by tropospheric delay and improve the accuracy of time series InSAR (TS-InSAR) inversion of surface deformation. We evaluated the model’s performance using the phase standard deviation (STD), atmospheric delay correlation coefficient with topography and the spatial structure function. The results were compared with a linear topography-dependent empirical model, generic atmospheric correction online service (GACOS) and ERA-5 methods. Our method reduces the STD of the phase of 83% of the interferograms by 12.8%. For vertical stratification delay correction, the correlation between the proposed method and the Linear, GACOS, and ERA-5 reached 0.734, 0.708, and 0.729, respectively. We found that accounting for spatiotemporal variation characteristics of tropospheric delay can alleviate the seasonal oscillations of vertical stratification delay and improve the accuracy of the deformation time series solution by 40.04%. We also used the Kunming continuous operation reference station system (KMCORS) to verify the displacement results of our method.

Geomagnetic storm effect on equatorial ionosphere over Sri Lanka through total electron content observations from continuously operating reference stations network during Mar–Apr 2022

Abstract The technological advancements in the current era have highlighted the increasing significance of satellite-based positioning, navigation, and timing services in a wide range of dynamic and critical applications. This has led to significant efforts towards enhancing the performance of global navigation satellite systems (GNSS) operating under challenging ionospheric conditions. The Sri Lankan ionosphere region is a focal point of equatorial aeronomy scientists, being situated in the southernmost landmass of the Indian longitude sector within the vicinity of the magnetic equator where a combination of electric, wind, and temperature dynamics exerts a substantial influence on the ionosphere but was relatively unexplored in the past. In the present work, we employed a Kriging interpolation technique on the total electron content (TEC) variables from ten GNSS receivers operating under the Continuously Operating Reference Stations (CORS) network in Sri Lanka first ever of its kind to deliver two-dimensional regional ionospheric TEC maps at hourly intervals, both during quiet and disturbed ionospheric conditions in the equinoctial March and April months of 2022. The latitudinal variation patterns are discernable from the hourly TEC maps. Furthermore, a comparative analysis of the performance of GNSS-derived TEC with that of the routinely published Global Ionospheric Maps (GIMs) confirms overestimation characteristics of the latter irrespective of the local time of observation. The generated regional ionospheric maps are fairly responsive to the onset of the storm and the recovery phase thereafter. The extent of nighttime ionospheric irregularity is also probed through the rate of TEC index (ROTI) variations, demonstrating that the irregularities were insignificant during the selected storm event.

The current situation of Russia’s GNSS continuous operating reference station network and thinking on future development

The GNSS (Global Navigation Satellite Systems) continuous operating reference station network not only provides a reliable technical foundation and data support for achieving high-precision navigation and positioning services, but also can be integrated with other Earth observation systems (such as laser ranging, gravity measurement, etc.) to improve the accuracy and stability of Earth observation data. It is used to establish regional and global coordinate frameworks and provide data to support scientific research such as seismology, geo-tectonics, and oceanography. This article first systematically introduces the construction, development history and current situation of the Russian GNSS continuous operating reference station network, and then carefully analyzes the construction, operation, and application status and existing problems of the Russian reference station network. Finally, after careful consideration, it discusses the problems and challenges faced in the planning of reference station network construction, equipment localization, and national unified reference station network construction, and provides some suggestions and ideas.

Machine learning assisted crustal velocity proxy: A case study over the Tibetan Plateau and its surroundings

So far only a few individuals have attempted to use Machine Learning approaches to anticipate GPS site velocity for crustal deformation research. Generally, a dense network of Continuously Operating Reference Station (CORS) is employed to monitor crustal deformation continuously. Campaign-mode GPS surveys are often used to densify the existing CORS network. Even sometimes, it is very challenging to establish a station in the location of our interest due to logistical problems and regional geographical considerations. However, this process is expensive, and crustal movement studies are eventually hampered by data missing issues owing to logistical restrictions. Thus, to obtain velocity vectors at the desired locations, we implemented machine learning (ML) techniques, such as support vector machines, decision trees and Gaussian processing regression, to accomplish the crustal movement precisely. We inspected data from 1271 permanent continuous and campaign-mode GPS stations, located on the Tibetan plateau and its environs. This study demonstrates the effectiveness of these ML techniques in forecasting velocity vectors (easting velocity (VE) and northing velocity (VN)) and enhancing plate movement characterisation. The correlation coefficient between (CR > 0.98 at training and CR > 0.96 at test phase) predicted and actual velocity vectors are satisfactory, making these ML predictive models considerably reliable for estimating geodetic velocity vectors. This ML algorithm demonstrates a remarkable achievement in the field of geodetic study in a cost-effective manner.

GNSS positioning accuracy performance assessments on 1st and 2nd generation SBAS signals in Thailand

Abstract This research evaluates the performance of the first and second-generation satellite-based augmentation system (SBAS) GAGAN and BDSBAS services in Thailand. The study initially analyses GNSS observations from 40 local continuously operating reference stations (CORS) over the past 12 months in 2022, providing initial horizontal and vertical accuracies at 2.03 and 3.66 m respectively with the single point positioning (SPP) mode. The positioning accuracies are 2.27 m horizontally and 2.54 m vertically as of GAGAN, while 2.94 m horizontally and 3.90 m vertically as of BDSBAS with the first-generation system. Since the 1st generation SBAS performance is affected by the ionosphere, especially in the equatorial and auroral regions, the ionosphere-free combination is applied in the SPP algorithm as well as the 2nd generation SBAS with the Dual-Frequency Multi-Constellation (DFMC) capable receivers for BDSBAS only. The SPP accuracies are 1.51 m horizontally and 3.26 m vertically, where the BDSBAS results are 2.16 m horizontally and 4.28 m vertically. Demonstrated results show that the positioning accuracy cannot be improved significantly when applying the 1st generation GAGAN and BDSBAS systems and the 2nd generation BDSBAS system in Thailand due to the low number of common satellites available, especially when using the SBAS outside their ground tracking network; therefore, it is expected to apply the GNSS observation and computed satellite error correction from the regional ground tracking network to enhance the performance of the 2nd generation SBAS. The positioning accuracy result could be achieved at sub-metre level, which will greatly benefit high-accuracy applications such as air, land, and sea navigation in the region.

Analysis of differences in accuracy of positioning tied to various CORS networks in Poland: Case study

Abstract Network Real Time Kinematic (NRTK) measurements are currently the most popular surveying method in geodesy. In most countries, there are networks of Continuously Operating Reference Stations (CORS), which form the core of the terrestrial infrastructure that allows for NRTK measurements. In many countries, including Poland, several CORS networks operate in parallel and independently. The paper presents the characteristics of the CORS network in Poland. The results of several day NRTK and Real Time Kinematic (RTK) test measurements performed tied to five CORS networks operating in Poland: ASG-EUPOS, NadowskiNET, SmartNet, TPINETpro, VRSNet.pl, were subjected to a comparative analysis. VRS, FKP, MAC and POJ streams were used in the test measurements. The research mainly concerned the possibility of the occurrence of systematic errors when NRTK and RTK measurements were tied to different CORS networks for the survey of the same points. Conclusions from the comparative analysis of the accuracy and precision of the NRTK and RTK measurement results for each coordinate were also included.

Research on Reliable Long-Baseline NRTK Positioning Method Considering Ionospheric Residual Interpolation Uncertainty

In the past few decades, network real-time kinematic (NRTK) positioning technology has developed rapidly. Generally, in the continuously operating reference stations (CORS) network, within a moderate baseline length, e.g., 80–100 km, atmospheric delay can be effectively processed through regional modeling and, thus, can support almost instantaneous centimeter-level NRTK positioning. However, in long-baseline CORS networks, especially during the active period of the ionosphere, ionospheric delays cannot be fully eliminated through modeling, leading to decreased NRTK positioning accuracy. To address this issue, this study proposes a long-baseline NRTK positioning method considering ionospheric residual interpolation uncertainty (IRIU). The method utilizes the ionospheric residual interpolation standard deviation (IRISTD) calculated during atmospheric delay modeling, then fits an IRISTD-related stochastic model through the fitting of the absolute values of the ionospheric delay modeling residuals and IRISTD. Finally, based on the ionosphere-weighted model, the IRISTD processed by the stochastic model is used to constrain the ionospheric pseudo-observations. This method achieves good comprehensive performance in handling ionospheric delay and model strength, and the advantage is validated through experiments using CORS data with baseline lengths ranging from 54 km to 106 km in western China and from 84 km to 180 km in AUSCORS data. Quantitative results demonstrate that, across the three sets of experiments, the proposed ionosphere-weighted model achieves an average increase in the fixed rate of 16.9% compared to the ionosphere-fixed model and 25.6% compared to the ionosphere-float model. In terms of positioning accuracy, the proposed model yields average improvements of 67.4%, 76.4%, and 66.0% in the N/E/U directions, respectively, compared to the ionosphere-fixed model, and average improvements of 21.0%, 32.0%, and 24.4%, respectively, compared to the ionosphere-float model. Overall, the proposed method can achieve better NRTK positioning performance in situations where ionospheric delay modeling is inaccurate, such as long baselines and ionospheric activity.

A Review of Selected Applications of GNSS CORS and Related Experiences at the University of Palermo (Italy)

Services from the Continuously Operating Reference Stations (CORS) of the Global Navigation Satellite System (GNSS) provide data and insights to a range of research areas such as physical sciences, engineering, earth and planetary sciences, computer science, and environmental science. Even though these fields are varied, they are all linked through the GNSS operational application. GNSS CORS have historically been deployed for three-dimensional positioning but also for the establishment of local and global reference systems and the measurement of ionospheric and tropospheric errors. In addition to these studies, CORS is uncovering new, emerging scientific applications. These include real-time monitoring of land subsidence via network real-time kinematics (NRTK) or precise point positioning (PPP), structural health monitoring (SHM), earthquake and volcanology monitoring, GNSS reflectometry (GNSS-R) for mapping soil moisture content, precision farming with affordable receivers, and zenith total delay to aid hydrology and meteorology. The flexibility of CORS infrastructure and services has paved the way for new research areas. The aim of this study is to present a curated selection of scientific papers on prevalent topics such as network monitoring, reference frames, and structure monitoring (like dams), along with an evaluation of CORS performance. Concurrently, it reports on the scientific endeavours undertaken by the Geomatics Research Group at the University of Palermo in the realm of GNSS CORS over the past 15 years.

LOW-COST MULTI-FREQUENCY GNSS RECEIVERS: PERFORMANCE EVALUATION FOR POSITIONING AND NAVIGATION

Abstract. In recent years, the possibility of using interoperable global constellations, the growing number of Continuously Operating Reference Stations (CORS) and the technological progress of instrumentation, computing algorithms and GNSS products are significantly marking the evolution of the various satellite survey techniques and the diffusion of mass-market technologies contributing to innovation transfers in different sectors including smart cities, smart mobility, connected automated driving, precision farming and others (Egea-Roca et al., 2022).Currently, the study of low-cost GNSS systems for navigation and precision positioning especially utilised in monitoring applications is the focus of numerous research activities (Joubert et al., 2020; Raza et al., 2022; Bellone et al., 2016; Hamza et al., 2020).The aim of this work is to test the performance of some of the latest generation multi-constellation and multi-frequency GNSS medium and low-cost sensors, evaluating their possible application in the mentioned fields. Differential and undifferential techniques were compared (Dardanelli et al., 2021; Ocalan et al.,2016); Precise Point Positioning (PPP) has become a valid alternative to differential methods allowing to obtain comparable accuracy offering greater flexibility (Lin, 2021). The multi-constellation permanent stations network GPS-Umbria was utilised for differential mode tests (Radicioni and Stoppini, 2019).The tests were carried out in different modes (static and kinematic) and operating conditions; various intermediate and low-cost sensors were employed, while the data of a high precision geodetic receiver were used as reference for the comparison of the different solutions.

Performance assessment of continuously operating reference stations using the deemed rover analysis approach with NRTK GNSS technique in Thailand

The study initiated an evaluation of the GNSS continuously operating reference stations (CORS) performance for Network-based Real-Time Kinematic (NRTK) positioning in Thailand, using the deemed rover analysis approach. Fifty CORS were selected as rovers with varying network geometries. Results indicated the highest performance, 79.9% of fixed solutions, for an average triangle side length of 54 km, while the lowest performance, 29.9%, was observed for a 93 km length with accuracies at a decimetre level. These initial explorations suggest that reducing side lengths enhances fixed solution efficiency and positioning accuracy at rovers. Occasional metre-level errors occurred, potentially linked to Internet instability.

Tomographic Reconstruction of Atmospheric Water Vapor Profiles Using Multi-GNSS Observations

Continuously operating reference stations (CORS) provide augmentation services for the highly accurate, cm-level GNSS positioning needs of land surveyors, agriculture, and even autonomous vehicles. These stations have accurate coordinates, thus they can be used to estimate the signal delay caused by the neutral atmosphere including the atmospheric water vapor. The estimated zenith wet delay (ZWD) is in a close correlation with the integrated water vapor in the atmospheric column. Since a ground station tracks several satellites at every epoch, one could also estimate the slant tropospheric delays, which can provide information on the spatial distribution of the atmospheric water vapor, too. This paper introduces a near real-time multi-GNSS processing approach to estimate slant wet tropospheric delays and a coupled tomographic reconstruction technique to estimate the 3D wet refractivity model that can be assimilated in numerical weather models. The estimated zenith tropospheric delays (ZTDs) and tropospheric gradients are used to restore the slant wet delays (SWD) affecting the observed satellite-receiver range. The SWDs are used as input for a tomographic reconstruction algorithm providing the wet refractivities in a pre-defined voxel model. The derived refractivity profiles have been validated with radiosonde observations. The results show that our GNSS tomography approach could reconstruct the refractivities with the uncertainty of 10 ppm below 3 km of altitude and of 0.3 ppm at the altitude of 10 km in terms of standard deviation.

Analysis of vertical seismic deformation of the 2018 Palu earthquake using Global Navigation Satellite System (GNSS) data

The vertical seismic deformation of the Palu earthquake magnitude of 7.4 on the Richter Scale, which occurred on September 28, 2018, has been analyzed using InaCORS (Indonesian Continuously Operating Reference Stations) stations. The stations used are CBAL, PALP, CTOL, CMLI, CPAL, and CBIT. The analysis was conducted using GAMIT, GLBOK, and GMT software. The analysis focused on the period of 100 days, starting from August 6, 2018 (DoY 218) to November 28, 2018 (DoY 318), encompassing the preseismic, coseismic, and post-seismic phases. During the preseismic phase (DoY 218-270), the InaCORS stations experienced a slight deformation ranging from -10.9353 mm to 6.5525 mm. Tectonic manifestations influenced the direction of movement in Sulawesi. The deformation significantly increased in the coseismic phase (DoY 271-272). The InaCORS stations experienced deformations ranging from -169.8 mm to 8.755 mm, which differed in the direction from the preseismic phase. The stations in the northern region predominantly moved towards the north, while those in the southern region moved towards the south. During the post-seismic phase (DoY 273-318), the movement of the InaCORS stations followed the direction of deformation observed in the coseismic phase. However, the displacement was relatively smaller, ranging from 11.5909 mm to 17.23733 mm.

An Efficient BDS-3 Long-Range Undifferenced Network RTK Positioning Algorithm

In 2020, the BeiDou-3 global navigation satellite system (BDS-3) was officially completed and put into service. Currently, network real-time kinematic (RTK) technology is considered the main means through which to improve the positioning accuracy of the BeiDou navigation satellite system (BDS). This paper proposes a long-range undifferenced network RTK (URTK) algorithm, based on multi-frequency observation data of the BDS. First, the multi-frequency phase integer ambiguity resolution (AR) model considering atmospheric error parameters is designed, and the multi-frequency phase integer ambiguity of the long-range BDS reference station is determined. Then, the undifferenced integer ambiguity of each reference station is obtained, using linear variation based on the accurately determined phase integer ambiguity between reference stations, and the undifferenced observation error of each reference station is calculated. Considering the weakening spatial correlation of the observation errors between long-range stations, undifferenced classification error corrections of a reference station network are separated, according to different error characteristics. Finally, the inverse distance weighting method is employed to calculate the classification undifferenced error correction of the rover station. The rover station corrects the observation error through applying the undifferenced error correction to achieve high-precision positioning. The measured data of a long-range continuous operation reference station (CORS) network are selected for an experiment. The results show that the proposed algorithm can quickly and accurately realize the resolution of the BDS integer ambiguity of a reference station network and establish an undifferenced area error correction model in order to achieve accurate classification of undifferenced error correction values for a rover station. In China, the BDS-3 is superior to the global positioning system (GPS) in terms of the satellite number, position dilution of precision (PDOP) value, AR success rate, stability, and convergence time. The results show that the AR success rate, stability, and convergence time increase with the operational frequency, and the BDS-3 can achieve centimeter-level positioning of single-system rover stations without relying on the GPS.

Determination and assessment of GNSS-derived precipitable water vapor in Indonesia using Ina-CORS

This work is the first attempt to investigate the accuracy and reliability of the Ina-CORS (Indonesian Continuously Operating Reference Stations) for meteorological applications. We used the standard procedures to determine and assess the precipitable water vapor (PWV) obtained from the Global Navigation Satellite System (GNSS) measurements. The GNSS-derived zenith tropospheric delay (ZTD) was estimated using the GAMIT software, and it was then converted to PWV using surface pressure measurements and the water vapor weighted mean temperature. The results were compared against those derived from the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF ERA5) data.Comparisons against the ECMWF ERA5 data reveal the following findings. Values for the mean bias and root mean square deviation (RMSD) of the GNSS-derived ZTD over the period 2011–2020 are −6.6 mm and 14.1 mm, respectively, representing the total percentage error of about 0.4%. The total errors from the surface pressure measurements and the calculations of water vapor weighted mean temperature introduce errors to the PWV values with the magnitude less than 1 mm. The GNSS-derived PWV values over Indonesia are comparable with those derived from the ECMWF ERA5 data. The mean bias and RMSD of PWV values are −0.60 mm and 2.59 mm, respectively. In addition, spatio-temporal variations of the GNSS-derived PWV can capture the main atmospheric features that are consistent with the other results deduced from long-term rainfall data. One of the features is the annual and semiannual cycles of PWV that largely correspond to the annual migration of the Asian-Australian monsoon system. Based on these results, we can conclude that the Ina-CORS is capable enough for accurate monitoring of the Indonesian weather and climate variations.

Estimation of tropospheric parameters with GNSS smartphones in a differential approach

With the introduction of the operating system Android 7 Nougat in the year 2016, it became possible to access Global Navigation Satellite System (GNSS) code and carrier phase observations. These observations can be processed with the state-of-the-art GNSS processing software packages, which allows an in-depth evaluation of the smartphone’s GNSS performance. The availability of carrier phase observations enables sub-decimeter-level positioning. A few years ago, smartphones wearing dual-frequency GNSS chipsets hit the mass market. In this study, we investigate the capability of such a device for the estimation of tropospheric delays. Static measurements carried out over the period of two weeks are performed using a Google Pixel 4 XL smartphone. The measurements are processed using relative positioning methods with a baseline length of about 33 kilometers, where a continuously operating reference station (CORS) acts as a base. The estimated differential zenith tropospheric wet delay (dZWD), obtained for the smartphone are then combined with absolute values computed at the reference station, in order to obtain time series of Zenith Total Delay (ZTD). Using this method, we demonstrate that high-precision ZTDs can be successfully determined from smartphone GNSS observations. When comparing the estimated tropospheric delays with those determined at a nearby geodetic receiver to assess the accuracy of the acquired time series of ZTD, differences in the range of few millimeters to a centimeter are visible. We examine the impact of various error sources, such as antenna phase center variations and residual effects of the ionosphere. Given that the obtained accuracies are at the level of a centimeter and below, the suggested method shows the potential to resolve small-scale tropospheric structures in near real-time, and thus, could be an interesting data source for numerical weather prediction models or related GNSS crowdsourcing projects.

An ERA5 based local modelling of weighted mean temperature over hilly region in India for improved spatiotemporal analysis of extreme weather event using GNSS PWV

Precipitable water vapor (PWV) from Global Navigation Satellite System (GNSS) offers the benefits of high spatiotemporal resolution logging-consistency and all-weather operability. Estimation of water vapor weighted mean temperature (Tm) is a crucial part and a major error source in extracting GNSS PWV from atmospheric delay. Empirical modelling of Tm is a convenient approach, especially in cases where, the meteorology community suffers from inefficient and inconsistent logging of temperature and humidity profiles etc., in the existing meteorological observing systems, particularly for harsh weather conditions. In a first, a novel site specific model (Tm-SSM) is developed for the atmospheric weighted mean temperature (Tm) for 13 CORS (continuously operating reference stations) in the Survey of India (SOI) network, recently established in the hilly state of Uttarakhand (UK) using the ERA5 reanalysis data from 2016 to 2018. GNSS observations at 10 CORS are then used to observe the spatiotemporal characteristics of PWV from Tm-SSM during the heavy rainfall event in October 2021. The newly developed Tm-SSM performs better with the mean bias error (MBE) and root mean square error (RMSE) of 0.498 and 2.9 K, respectively, when compared with other globally accepted models. The Tm-SSM PWV estimates are well correlated with the precipitation data from AWS stations. The newly developed Tm-SSM is well conditioned and highly stable, with the mean stability indicator value as low as 1.043%. Further, a close relationship can be observed from the spatiotemporal characteristics of GNSS PWV during heavy rainfall event, with the in-situ precipitation measurements. It is apparent from the results that PWV steadily builds up before the arrival of rain, with sharp peaks in PWV observed just before rainfall event. It then, follows a downward slope through which it decreases to a stable value of around 30 mm when the rainfall ceases. The fluctuations in the PWV from Tm-SSM and the passage of the severe mesoscale convective system in a complex terrain such as UK were shown to have high spatial and temporal correlation, which is significant for the monitoring and forecasting such extreme weather. The findings imply that GNSS PWV represents the compendious signature of the water vapor dynamics during severe weather, demonstrating the potential to improve the early detection and sensing of severe weather.