Continuously Operating Reference Station Network Research Articles

Evaluating the Positional Stability Status of the Druk Corsnet: A Preliminary Study

Abstract— The continuously operating reference station (CORS) that provides a Global Navigation Satellite System (GNSS) enables precise location and height data for various applications such as site selection of important engineering projects and constructions and hazard analysis. Similarly, the most common applications of the CORS network, in Bhutan, are for engineering, cadastral, geodetic, and topographical surveys. In this regard, the consistency and dependability of the data from the CORS station are crucial. Moreover, the ability of such applications heavily depends on precise and reliable data from the CORS network. However, Bhutan has not conducted any studies on the stability, in terms of its position, and height fluctuation of the CORS station. Therefore, the goal of this study is to determine the positional and height variation of the 9 CORS stations that are operational in Bhutan between the years 2016 to 2022. Assessing the stability and dynamic behavior of reference stations in a CORS network is part of position variation analysis. It is feasible to identify and quantify a variety of geodetic occurrences by examining the time series of positional analysis. Analysis of height variation focuses on changes in orthometric heights throughout the CORS network. In our study, all the available GNSS CORS data are processed and the displacement in height and position is computed and compared. The analysis consists of a simple method (mean value subtraction) of time series data, which is computed and compared. The major findings show that the absolute mean standard deviation in the position of the stations was 5.49 cm, 6.36 cm, and 4.24 cm vertical (height), ease, and north horizontal component respectively. The findings highlight that the proposed method of determining the variation in height and location of the CORS stations is fairly practical and will help the user community to guide them in using the CORS network across Bhutan wisely. Similarly, the result of this study will be a relevant source of information to organizations such as the National Land Commission Secretariat (NLCS) as a means to augment the system in the future.

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.

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.

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.

Нові виклики для функціонування активних перманентних ГНСС-станцій під час бойових дій на прикладі України

The study presents the current state of GNSS Continuously Operating Reference Stations (CORS) networks and their operational characteristics during the ongoing hostilities in Ukraine. Stable GNSS CORS network operation is crucial not only for agricultural, geodetic, and land management tasks but also for military navigation and topography. The aim of this work is to analyze the impact of hostilities in Ukraine's GNSS network, considering factors like temporary occupation of certain territories, power outages due to missile strikes on energy infrastructure, and GNSS signal jamming using radio-electronic methods in front-line regions. Another objective of this study is to highlight examples of incorrect RTK or VRS operation due to potential errors from radio-electronic jamming or GPS spoofing as well as to provide practical recommendations for surveyors. As a result, the research has analyzed changes in the number of properly functioning GNSS stations from 2021 to 2023 using the GeoTerrace and System.NET networks. These networks cover all regions of Ukraine except the temporarily occupied territories by russia. Daily processing of RINEX files with a sampling interval of 30 seconds from CORS GNSS stations was conducted using the Bernese GNSS v.5.2 software package over three years. It was noted that following the large-scale invasion in February 2022 and through the spring of that year, there was a sharp reduction of about 10% in the number of properly functioning active GNSS stations. Scientific novelty and practical importance. The article presents practical recommendations for users, such as surveyors and land managers, performing GNSS measurements in RTK or VRS modes using permanent stations, to assess the influence of radio-electronic jamming or GPS spoofing on observations. CORS network assessment and daily calculated coordinates of GNSS stations from 2021 to 2023 can be used for future geodynamic research 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.

Real-time high-precision landslide displacement monitoring based on a GNSS CORS network

GNSS technology is an effective all-weather, continuous and real-time method of detecting three-dimensional landslide displacement. However, current positioning methods such as the network real-time kinematic (RTK) technique based on virtual reference station (VRS) technology have some limitations, particularly a sensitivity to atmospheric modeling error. In this study, taking advantage of existing GNSS continuously operating reference stations (CORS), we propose a network RTK algorithm (virtual atmosphere constraint (VAC)) for high-precision landslide monitoring. Network modeling atmosphere delays were used as virtual observations for the terminal position solution, by which their accuracy information can be considered. Additionally, a noise reduction method that combines complementary ensemble empirical mode decomposition and median filtering was proposed to reduce GNSS signal residual errors in the original RTK coordinate time series. To validate the effectiveness of the algorithm, we processed one week of data from two monitoring receivers on the Heifangtai landslide and four surrounding CORS using the VAC and VRS methods. Results show that the original displacement time series processed with VRS was prone to jitter, whereas the VAC method exhibited better accuracy, stability, and ambiguity fixing rates. Horizontal and vertical RMS errors at a stable monitoring point were 1.7 cm and 3.9 cm for VRS-RTK and 0.9 cm and 2.3 cm for VAC-RTK; thus, VAC reduced the error by 40.1% and 20.5%, respectively. After noise reduction, the RMS values of the VAC method at the stable and the other collapsed monitoring point were both reduced to an mm-level accuracy.

A Combination of Low-cost, Dual-frequency, Multi-GNSS Receiver and CORS Network for Precise Positioning Applications in Vietnam

Although Real-time kinematic (RTK) is a positioning technique with high accuracy at the centimeter level, the investment cost for this positioning technique is relatively high. To reduce the investment cost, low-cost, dual-frequency, multi-Global Navigation Satellite System (GNSS) receivers can be considered. In Vietnam, the Continuously Operating Reference Stations (CORS) network has operated and covered the whole country. Therefore, the combination of low-cost, dual-frequency, multi-GNSS receivers and the CORS network can make RTK positioning easier and less expensive. The aim of this study is to evaluate the potential of applying the RTK positioning technique using a low-cost, dual-frequency, multi-GNSS receiver and the CORS network in Vietnam. In this study, four positioning experiments were performed in static and kinematic modes, and at near and far locations in which the rover was about 3 km and 30 km from the CORS station, respectively. Experiments conducted near the CORS station gave good results with the average standard deviation of 0.2 cm for static positioning and average deviation of 2.2 cm for kinematic positioning, and the FIX rate of greater than 95%. Therefore, the experimental results show the excellent potential of RTK positioning using low-cost, dual-frequency, multi-constellation receivers with the CORS network in Vietnam.

High Latitude Ionospheric Gradient Observation Results from a Multi-Scale Network.

In this article, a cluster comprised of eight Continuously Operating Reference Station (CORS) receivers surrounding five supplemental test stations located on much shorter baselines is used to form a composite multi-scale network for the purpose of isolating, extracting, and analyzing ionospheric spatial gradient phenomena. The purpose of this investigation is to characterize the levels of spatial decorrelation between the stations in the cluster during the periods with increased ionospheric activity. The location of the selected receiver cluster is at the auroral zone at night-time (cluster centered at about 69.5° N, 19° E) known to frequently have increased ionospheric activity and observe smaller size of high-density irregularities. As typical CORS networks are relatively sparse, there is a possibility that spatially small-scale ionospheric delay gradients might not be observed by the network/closest receiver cluster but might affect the user, resulting in residual errors affecting system accuracy and integrity. The article presents high level statistical observations based on several hundred manually validated ionospheric spatial gradient events along with low level analysis of specific events with notable temporal/spatial characteristics.

Effects of the Continuously Operating Reference Station (CORS) network on the accuracy, precision, and Time to Fix Ambiguity (TTFA) performance

The geometric design of the Continuously Operating Reference Station (CORS) network is one of the most critical factors that impact accuracy, precision, and Time to Fix Ambiguity (TTFA) performance. In this study, the authors investigate the subject of geometric design by using both local ISKI CORS and national CORS-Turkey (CORS-TR) networks, and they redesign the ISKI CORS network by increasing interstation distances. For three systems, real-time Virtual Reference Station (VRS) solutions have been obtained and tested with Analysis of Variance (ANOVA). As a result, the 8-station ISKI CORS provides the most accurate results in the vertical component. In contrast, for TTFA performance, the CORS-TR network shows worse outcomes than the others. Due to the increase in the interstation distances, the increase in the base length caused worse results in FIX solution ratios for redesigned ISKI CORS. In summary, the authors have stated that the 8-station ISKI CORS network performs better, especially regarding vertical accuracy. Furthermore, the authors state that the TTFA performance and FIX solution ratios are high, and the ISKI CORS geometric design is optimal.

Applying Malawi Continuously Operating Reference Stations (CORS) in GNSS Meteorology

Global Navigation Satellite System (GNSS) signals in the L-band are affected by the non-dispersive neutral atmosphere. Regardless of their center frequency, the L-band code and phase observations are affected by the same measure of delay. GNSS receivers play a significant role in quantifying the zenith tropospheric delay (ZTD) from satellite signals. Malawi has a Continuously Operating Reference Stations (CORS) network which was established to support research in geophysical geodesy and geodynamics. However, the quality of the observations tracked by the CORS has never been tested in terms of its meteorological application. In this paper, the ZTD estimation approach and the evaluation of results from the Global Positioning System (GPS) measurements are presented. The optimal approach of precise point positioning (PPP) was used to estimate ZTD from one-week datasets which were collected from six CORS monuments distributed in the northern and southern regions of Malawi. In addition, the zenith wet delay (ZWD) and zenith hydrostatic delay (ZHD) were also estimated to determine their respective contributions to the total delay in all the stations. Alongside the meteorological parameters, the positioning repeatabilities were also established for all stations. Results indicate that the averaged ZTD, ZWD and ZHD can reach as high as 247mm, 47 mm, and 199 mm, respectively. The minimum ZTD, ZWD, and ZHD for the stations can drop to as low as 220 mm, 24 mm, and 181 mm, respectively. This indicates that the ZHD contributes to more than 90% of the total delay at the stations. For the positioning performance, there was no obvious disparity in the latitude (less than 0.5 cm), longitude (less than 1 cm), and ellipsoidal height repeatabilities (less than 1.5 cm). Thus, the results clearly demonstrate that the Malawi CORS network may be used for GNSS-based meteorological applications using the available geodetic receivers. However, for high precision meteorological applications, Malawi may consider densifying the available network with geodetic grade receivers.

Recovering Regional Groundwater Storage Anomalies by Combining GNSS and Surface Mass Load Data: A Case Study in Western Yunnan

The redistribution of surface mass (e.g., atmosphere, soil water, oceans, and groundwater) can cause load responses, resulting in vertical deformations of the crust. Indeed, the global navigation satellite system (GNSS)-based continuously operating reference stations (CORS) are able to accurately measure the vertical deformation caused by surface mass loads. In this study, the CORS was used to invert groundwater storage anomalies (GWSA), represented by the equivalent water height (EWH), after removing the effect of the non-groundwater surface mass load (atmospheric, groundwater, and non-tidal oceanic loads) from the vertical deformation monitored by CORS. In addition, the global and regional high-resolution surface mass models were combined to calculate the high-precision load deformation field in in western Yunnan using the remove–restore method, thereby obtaining more accurate surface mass load data and improving the accuracy of the inverted GWSA results. In order to assess the feasibility of the CORS inversion for the GWSA used, 66 CORS stations in western Yunnan Province were considered, presenting weekly GWSA data from 10 January 2018 to 31 December 2020. The results revealed significant seasonal variation in GWSA in the study area, showing an amplitude range of −200–200 mm. This approach is based on the already-established CORS network without requiring additional set-up costs. In addition, the reliability of CORS inverse results was assessed using Gravity Recovery and Climate Experiment (GRACE) inverse results and actual groundwater monitoring data. According to the obtained results, GWSA can be monitored by both CORS and GRACE data; however, CORS provided a more effective spatiotemporal resolution of GWSA. Therefore, the CORS network combined with surface mass load data is able to effectively monitor the spatiotemporal dynamics of GWSA in small-scale areas and provides important references for the study of hydrology.

Single-Epoch Ambiguity Resolution of a Large-Scale CORS Network with Multi-Frequency and Multi-Constellation GNSS

Ambiguity resolution at Continuously Operating Reference Station (CORS) network sites is the key step in the whole processing chain of Network Real Time Kinematic (NRTK). An appropriate ambiguity-resolution speed is important, and single-epoch ambiguity resolution has not been realized yet, especially for large-scale CORS. We attempt to realize single-epoch ambiguity resolution for a large-scale CORS network by neglecting tropospheric delay through forming difference between satellites with close mapping functions whether they belong to the same or different GNSSs. As only two frequency bands are shared among GPS, Galileo and BeiDou, the biggest challenge is how to get this single-epoch ambiguity solution for wide-lane combinations of L1 and L5 when the difference is formed between satellites of different GNSSs. The proposed method includes five steps for ambiguity resolution for different combinations: extra wide-lane, wide-lane, inter-GNSS wide-lane, subset narrow-lane and narrow-lane. The single-epoch ambiguity-resolution performance is assessed based on GNSS observations from two long-distance baselines formed with IGS stations, BRUX-REDU and BAUT-LEIJ, separated by distances of approximately 104 km and 151 km, respectively. The numerical results show that the fixing rate of the single-epoch ambiguity resolution can reach more than 90%, so for a large-scale CORS network, single-epoch ambiguity resolution is feasible and can be realized in the future.

Development of continuously operating reference stations in mountainous areas

The purpose of the study is to provide general information on the design, creation and installation of continuously operating reference station (CORS) in mountainous areas (hardware components and CORS configuration, station specifications, design features and creation of a CORS network). The paper lists the main issues that must be taken into account when creating, installing and developing CORS in the mountains - the closure of the radio horizon, multipath, changes in the troposphere at short distances, local geodynamics, problems of receiving and transmitting differential corrections. It is noted that in mountain gorges, canyons, where it is difficult to receive radio signals from GNSS satellites and exchange information between the CORS GNSS receiver and the rover, it is optimal to develop passive geodetic networks using terrestrial methods (polygonometry, trilateration) based on CORS networks.

Reliability of CSRS-PPP for Validating the Egyptian Geodetic Cors Networks

Abstract The development, utilization, and maintenance of continuously operating reference stations (CORS) network are vital in many areas of surveying and geodesy, such as controlling geodetic networks, developing local ionospheric models, and estimating the tectonic plate movements. Accordingly, the Egyptian Surveying Authority (ESA) established a CORS network consisting of 40 stations covering the Nile valley and its delta in 2011. CORS collect global navigation satellite system (GNSS) data. Recently, Egypt has witnessed rapid growth in many infrastructure projects and the development of new cities on a national scale. Therefore, there is an urgent need to investigate the ESA-CORS accuracy; the quality of data from the ESA-CORS must be considered for monitoring continuous tectonic motion, coordinating changes, and for Egypt’s development plan. Contemporary research worldwide identified considerable benefits of the precise point positioning (PPP) solution of dual- or single-frequency GNSS data. This study investigates the reliability of using the CSRS-PPP service for three consecutive observation days of 32 ESA-CORS networks in Egypt and the surrounding six international GNSS services (IGS)-CORS. For ESA-CORS, the PPP solution showed a root mean square error (RMSE) value of 6 mm (standard deviation [SD] = 3–4 mm) in east and north; for the height direction, the solution indicated an RMSE value of 22 mm (SD was about 14 mm). At a confidence level of 95%, this study revealed that SD95% was 2 mm in east and north directions and 6–7 mm for the height direction. This study shows that the PPP solution shown from the ESA-CORS stations is associated with two times better for horizontal and four times for the height direction than the delivered form ESA-CORS stations.

Manifestation of earthquake preparation zone in the ionosphere before 2021 Sonitpur, Assam earthquake revealed by GPS-TEC data

Global Positioning System (GPS) Continuously Operating Reference Station (CORS) data analysis shows that the ionosphere's electron density variability is linked to the deformation and stress accumulation in the Earth's crust. Anomalies in ionosphere total electron content (TEC) variability before 2021 M6.4 Sonitpur, Assam earthquake were detected using L1 and L2 GPS frequencies that showed three distinct abnormalities on April 3, 9, 10, 2021. Pearson's correlation coefficient (r) of TEC decreases in the CORS that lies away from the earthquake epicenter, indicating the possibilities of a positive relationship between TEC variability and earthquake epicenter. TEC concentration also decreases towards the epicenter within the earthquake preparation zone (EPZ). It is also observed that the Pearson's correlation coefficient (r) of TEC decreases linearly near the EPZ. The study demonstrates the possibilities of determining the TEC anomalous zone in the ionosphere that coincides with the EPZ in the crustal rocks. The research indicated the possibilities of magnitude estimation of an impending earthquake based on the TEC anomalous zone in the ionosphere using closely spaced dense CORS network data.

Single Epoch Ambiguity Resolution of Small-Scale CORS with Multi-Frequency GNSS

The network real-time kinematic (RTK) technique uses continuously operating reference stations (CORS) within a geographic area to model the distance dependent errors, allowing users in the area to solve ambiguities. A key step in network RTK is to fix ambiguities between multiple reference stations. When a new satellite rises or when maintenance happens, many unknown parameters are involved in the mathematical model, and traditional methods take some time to estimate the integer ambiguities reliably. The purpose of this study is the single-epoch ambiguity resolution on small-scale CORS network with inter-station distance of around 50 km. A new differencing scheme is developed to explore the full potential of multi-frequency Global Navigation Satellite System (GNSS). In this scheme, a differencing operation is formed between satellites with the closest mapping functions. With the new differencing scheme, tropospheric error can be mostly neglected after the correction, as well as the double-differencing operation. Numerical tests based on two baselines of 49 km and 35 km show that the success rate of ambiguity resolution can reach more than 90%. The single-epoch ambiguity resolution for reference stations brings many benefits to the network RTK service, for example, the instantaneous recovery after maintenance or when a new satellite rises.

REVIEW OF INTERNATIONAL GNSS DATA SHARING POLICY FRAMEWORKS AND PRACTICES

Global Navigation Satellite Systems, or GNSS, is a space technology that has become an important component of positioning, navigation, and timing (PNT) in a broad variety of military and civilian applications. Accordingly, the GNSS is being supported with Continuously Operating Reference Station (CORS) networks which are a common type of GNSS ground-based augmentation infrastructure that governments and industry use to distribute centimetre accurate PNT information throughout the nation or region. In Malaysia, there are few CORS networks currently in services. It was difficult to integrate CORS networks even within the country since the CORS operating came from several different organizations. Furthermore, the CORS data sharing between the public and private sectors in providing precise positioning applications also challenging issue due to the fact that the data contains sensitive information. The relevant policy document on data sharing in the country is still vague. This paper review on existing GNSS data sharing policy framework and practice at the international and national levels. A number of countries from each continents were chosen to be studied further in order to identify the requirements that could be considered for adoption. The goal of this study is to create a clear conceptual framework for GNSS data sharing in the country, as well as to resolve some grey areas between public and private GNSS users.

Regional Crustal Vertical Deformation Driven by Terrestrial Water Load Depending on CORS Network and Environmental Loading Data: A Case Study of Southeast Zhejiang.

Monitoring regional terrestrial water load deformation is of great significance to the dynamic maintenance and hydrodynamic study of the regional benchmark framework. In view of the lack of a spatial interpolation method based on the GNSS (Global Navigation Satellite System) elevation time series for obtaining terrestrial water load deformation information, this paper proposes to employ a CORS (Continuously Operating Reference Stations) network combined with environmental loading data, such as ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric data, the GLDAS (Global Land Data Assimilation System) hydrological model, and MSLA (Mean Sea Level Anomaly) data. Based on the load deformation theory and spherical harmonic analysis method, we took 38 CORS stations in southeast Zhejiang province as an example and comprehensively determined the vertical deformation of the crust as caused by regional terrestrial water load changes from January 2015 to December 2017, and then compared these data with the GRACE (Gravity Recovery and Climate Experiment) satellite. The results show that the vertical deformation value of the terrestrial water load in southeast Zhejiang, as monitored by the CORS network, can reach a centimeter, and the amplitude changes from −1.8 cm to 2.4 cm. The seasonal change is obvious, and the spatial distribution takes a ladder form from inland to coastal regions. The surface vertical deformation caused by groundwater load changes in the east–west–south–north–central sub-regions show obvious fluctuations from 2015 to 2017, and the trends of the five sub-regions are consistent. The amplitude of surface vertical deformation caused by groundwater load change in the west is higher than that in the east. We tested the use of GRACE for the verification of CORS network monitoring results and found a relatively consistent temporal distribution between both data sets after phase delay correction on GRACE, except for in three months—November in 2015, and January and February in 2016. The results show that the comprehensive solution based on the CORS network can effectively improve the monitoring of crustal vertical deformation during regional terrestrial water load change.

Evaluation of methods for connecting InSAR to a terrestrial reference frame in the Latrobe Valley, Australia

Deformation measurements from satellite-borne synthetic aperture radar interferometry (InSAR) are usually measured relative to an arbitrary reference point (RP) of assumed stability over time. For InSAR rates to be reliably interpreted as uplift or subsidence, they must be connected to a defined Earth-centred terrestrial reference frame (TRF), usually made through GNSS continuously operating reference stations (CORS). We adapt and compare three methods of TRF connection proposed by different studies which we term the single CORS RP (SCRP), plane-fit multiple CORS (PFMC), and the multiple CORS RP (MCRP). We generalise equations for these methods, and importantly, develop equations to propagate InSAR and GNSS uncertainties through the transformation process. This is significant, because it is important to not only estimate the InSAR uncertainties, but also to account for the uncertainties that are introduced when connecting to the CORS so as to better inform our interpretation of the deformation field and the limitation of the measurements. We then test these methods using Sentinel-1 data in the Latrobe Valley, Australia. These results indicate that differences among the three TRF connection methods may be greater than their estimated uncertainties. MCRP appears the most reliable method, although it may be limited in large study areas with sparse CORS due to long wavelength InSAR errors and that gaps and/or steps may appear at the spatial limit from the CORS. SCRP relies on the quality of the single CORS connection, but can be validated by unconnected CORS in the study area. The PFMC method is suited to larger areas undergoing slow, constant deformation covering large spatial extents where there are evenly distributed CORS across the study area. Selecting an optimal method of TRF connection is dependent on local site conditions, CORS network geometry and the characteristics of the deformation field. Hence, the choice of TRF connection method should be carefully considered, because different methods may result in significantly different transformed deformation rates. We confirm slow subsidence across the Latrobe Valley relative to the vertical component of the ITRF2014, with localised high subsidence rates near open cut mining activities. Subsidence of ~ -6 mm/year is observed in the adjacent coastal region which may exacerbate relative sea level rise along the coastline, increasing future risks of coastal inundation.