G1.3

Global Positioning System (GPS) stations are affected by a plethora of real and system-related signals and errors that occur at various temporal and spatial resolutions. Geophysical changes related to mass redistribution within the Earth system, common mode components, instability of GPS monuments or thermal expansion of ground, all contribute to the GPS-derived displacement time series. Different spatial resolutions that real and system-related errors occur within are covered thanks to the global networks of GPS stations, characterized presently by an unprecedented spatial density. Various temporal resolutions are covered by displacement time series which span even 25 years now, as estimated for the very first stations established. However, since the GPS sensitivity remains unrecognized, retrieving one signal from this wide range of processes may be very uncertain. Up to now, a comparison between GPS-observed displacement time series and displacements predicted by a set of models, as e.g. environmental loading models, was used to demonstrate the accuracy of the model to predict the observed phenomena. Such a comparison is, however, dependent on the accuracy of models and also on the sensitivity of individual GPS stations. We present a new way to identify the GPS sensitivity, which is based on benchmarking of individual GPS stations using statistical clustering approaches. We focus on regional sets of GPS stations located in Europe, where technique-related signals cover real geophysical changes for many GPS permanent stations and those located in South America and Asia, where hydrological and atmospheric loadings dominate other effects. We prove that combining GPS stations into smaller sets improves our understanding of real and system-related signals and errors.

How to cite: Klos, A., Kusche, J., Lenczuk, A., Leszczuk, G., and Bogusz, J.: Benchmarking GPS stations: an improved way to identify the GPS sensitivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-504, https://doi.org/10.5194/egusphere-egu21-504, 2021.

Global navigation satellite system (GNSS) constellations such as GPS, GLONASS, Galileo, and BeiDou and the Japanese regional system QZSS apply various satellite attitude modes during eclipse season, which is the period when the Sun is close to the orbital plane of the satellite. Due to different satellite manufacturers and technological advances over time, these modes can vary between constellations but also between different satellite types within a constellation. For some constellations, namely Galileo and QZSS, the satellite attitude law has been officially published by the satellite operator. For most other GNSS satellite types, researchers have developed attitude models, for example using reverse kinematic precise point positioning, that approximate the actual attitude behaviour.

Outside of eclipse seasons, GNSS satellites generally apply either a nominal yaw-steering or an orbit normal attitude law. While both modes point the antennas towards Earth, the former yaws the satellite around the antenna axis to point the solar panels towards the Sun, while the latter always keeps a fixed yaw angle. When a satellite applying a yaw-steering law is in eclipse season and close to the orbit noon or midnight point, it may have to yaw faster than physically possible to keep the nominal attitude. The various attitude modes used by the satellites aim to prevent this scenario by applying a modified attitude law during this period, for example by yawing at a constant rate around orbit noon/midnight or by switching to orbit normal mode.

Comparisons of attitude files generated by analysis centers of the International GNSS Service (IGS) within the scope of its 3^rd reprocessing campaign show significant differences in some cases. This contribution compares all available attitude models with the aim of finding similarities that allow for generalization, which in turn simplifies the implementation of the various attitude modes into GNSS software packages. The developed functions have been implemented into the open-source software GROOPS (https://github.com/groops-devs/groops), which makes them publicly available and documented.

How to cite: Strasser, S., Banville, S., Kvas, A., Loyer, S., and Mayer-Gürr, T.: Comparison and generalization of GNSS satellite attitude models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7825, https://doi.org/10.5194/egusphere-egu21-7825, 2021.

During the preparations for the International GNSS Service (IGS) contribution to the next reference frame, called repro3, the disclosed pre-launch chamber calibrated Galileo satellite antenna pattern were analyzed. Those tests revealed a discrepancy between the GPS and GLONASS z-component of the phase center offsets (PCO), aligned to the IGS14 scale, and the calibrated Galileo z-PCOs. In order to make the PCOs compatible to the repro3 it was decided to rely on the calibrated Galileo pattern and adjust the GPS and GLONASS PCOs accordingly. Combined with multi-GNSS receiver calibrations for all systems the repro3 might contribute to the scale determination for the next reference frame.

As the repro3 is based on GPS, GLONASS, and Galileo only those three systems have been analyzed leading to the repro3 ANTEX file, containing all used antenna pattern, which is aligned to the Galileo induced scale. In order to extend the repro3 ANTEX file with satellite calibrations for BeiDou and QZSS a dedicated reprocessing based on CODEs MGEX solution is made to assess the available PCOs for those satellites and tests their consistency with the repro3 scale. The results should allow to extend the repro3 ANTEX with the BDS and QZSS pattern for experimental purposes.

How to cite: Villiger, A., Dach, R., Prange, L., and Jäggi, A.: Extension of the repro3 ANTEX file with BeiDou and QZSS satellite antenna pattern, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6287, https://doi.org/10.5194/egusphere-egu21-6287, 2021.

In order to obtain highly precise positions with Global Navigation Satellite Systems (GNSS), it is mandatory to take all error sources adequately into account. This includes phase center corrections (PCC), composed of a phase center offset (PCO) and corresponding azimuthal and elevation-dependent phase center variations (PCV). These corrections have to be applied to the observations since the pattern of the GNSS receiver antennas deviate from an ideal omnidirectional radiation pattern.
The Institut für Erdmessung (IfE) is one of the IGS accepted institutions for absolute antenna calibration. Recently, the operationally calibration procedure has been further developed to a post processing approach. Thus, PCC can also be estimated for all frequencies (including e.g. GPS L2C, L5) and systems like Galileo and Beidou. Additionally, the newly developed approach allows to assess the impact of using different receivers with different settings on an individual calibration. 
Previous studies already have shown, that the geodetic receivers used during the absolute calibration of antennas have an impact on the estimated PCC. However, currently this impact is only analysed at the level of the respective patterns and not in the coordinate domain. Moreover, the results are always only valid for the respective antenna-receiver combination. Therefore, more samples of different combinations are required.
In this contribution, we study calibration results of several antenna-receiver combinations using a zero baseline configuration during the calibration process in order to assess the receiver’s impact due to different signal tracking modes. The resulting PCC are analysed on the pattern level regarding (i) the repeatability of individual calibrations and (ii) differences between different antenna-receiver combinations. Finally, the impact of the different PCC are validated in the coordinate domain by a well controlled short baseline and common clock set-up. Here, again a zero baseline configuration with the identical receivers used during the calibration process is performed. Consequently, the impact of the respective antenna-receiver combination with individually estimated PCC on the positioning is analysed.

How to cite: Kröger, J., Kersten, T., Breva, Y., and Schön, S.: Impact of Multi-GNSS Antenna-Receiver Calibrations in the Coordinate Domain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8507, https://doi.org/10.5194/egusphere-egu21-8507, 2021.

Global navigation satellite systems (GNSS) have been playing an indispensable role in providing positioning, navigation and timing (PNT) services to global users. Over the past few years, GNSS have been rapidly developed with abundant networks, modern constellations, and multi-frequency observations. To take full advantages of multi-constellation and multi-frequency GNSS, several new mathematic models have been developed such as multi-frequency ambiguity resolution (AR) and the uncombined data processing with raw observations. In addition, new GNSS products including the uncalibrated phase delay (UPD), the observable signal bias (OSB), and the integer recovery clock (IRC) have been generated and provided by analysis centers to support advanced GNSS applications.

       However, the increasing number of GNSS observations raises a great challenge to the fast generation of multi-constellation and multi-frequency products. In this study, we proposed an efficient solution to realize the fast updating of multi-GNSS real-time products by making full use of the advanced computing techniques. Firstly, instead of the traditional vector operations, the “level-3 operations” (matrix by matrix) of Basic Liner Algebra Subprograms (BLAS) is used as much as possible in the Least Square (LSQ) processing, which can improve the efficiency due to the central processing unit (CPU) optimization and faster memory data transmission. Furthermore, most steps of multi-GNSS data processing are transformed from serial mode to parallel mode to take advantage of the multi-core CPU architecture and graphics processing unit (GPU) computing resources. Moreover, we choose the OpenBLAS library for matrix computation as it has good performances in parallel environment.

       The proposed method is then validated on a 3.30 GHz AMD CPU with 6 cores. The result demonstrates that the proposed method can substantially improve the processing efficiency for multi-GNSS product generation. For the precise orbit determination (POD) solution with 150 ground stations and 128 satellites (GPS/BDS/Galileo/GLONASS/QZSS) in ionosphere-free (IF) mode, the processing time can be shortened from 50 to 10 minutes, which can guarantee the hourly updating of multi-GNSS ultra-rapid orbit products. The processing time of uncombined POD can also be reduced by about 80%. Meanwhile, the multi-GNSS real-time clock products can be easily generated in 5 seconds or even higher sampling rate. In addition, the processing efficiency of UPD and OSB products can also be increased by 4-6 times.

How to cite: Zheng, H., Chang, H., Yuan, Y., Wang, Q., Li, Y., and Li, X.: An efficient solution for fast generation of multi-GNSS real-time products, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8306, https://doi.org/10.5194/egusphere-egu21-8306, 2021.

Navigation systems have substantially evolved in the last decade. The multi-GNSS constellation including GPS, GLONASS, Galileo, and BeiDou consists of more than a hundred active satellites. To fully exploit their potential, users should be able to take advantage of those systems not only in postprocessing mode employing final solutions but also in real-time. It is also important to make satellite signals highly useful in a real-time regime not only in standard positioning mode but also with the precise positioning technique. That is why real-time products are highly desirable. One of the IGS Analysis Centers that support multi-GNSS real-time solution is CNES which provides not only orbits and clocks but also code and phase biases and VTEC global maps. Over the last few years, real-time products have been changing similarly to navigation systems, which come along with observation availability and calculation strategy changes.

We utilize the signal-in-space ranging error (SISRE) as the main orbit and clock quality indicator. Additionally, SLR observations are used as an independent source of information about orbit quality. Three years of data, between 2017 and 2020, are used to check the progress in the quality of the delivered products to the users through the internet streams provided by CNES.

The progress in the product quality in the test period is obvious and it depends on the satellite system, block or satellite type, time, and the height of the Sun above the orbital plane. The most accurate orbits are available for GPS, however, the very stable atomic clocks of Galileo compensate for systematic errors in Galileo orbits. Consequently, the SISRE for Galileo is lower than that for GPS, equaling 1.6 and 2.3 cm for Galileo and GPS, respectively. The SISRE value for GLONASS, despite the good quality of the orbits, is disturbed by the lower quality of the onboard clocks and is equal to 4-6 cm. The same quality level is for BeiDou-2 MEO and IGSO satellites. Products for BeiDou-2 GEO satellites are less accurate and with poor availability due to a large number of satellite maneuvers, thus they are not very useful for real-time positioning.

For positioning purposes, the presented results may be interesting especially in the context of the proper observation weighting in the multi-GNSS combinations. It is worth mentioning that the quality of the real-time products is not constant and neglecting this fact may bring undesirable positioning errors, especially for long processing campaigns.

How to cite: Kazmierski, K., Zajdel, R., and Sośnica, K.: Multi-GNSS real-time orbit and clock quality changes over time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7160, https://doi.org/10.5194/egusphere-egu21-7160, 2021.

The dawn of Beidou and Galileo as operational Global Navigation Satellite Systems (GNSS) alongside Global Positioning System (GPS) and GLONASS as well as new features that are now present in all GNSS, such as a triple-frequency setup, create new possibilities concerning improved estimation and assessment of various geodetic products. In particular, the multi-GNSS analysis gives an access to a better sky coverage allowing for improved estimation of zenith wet delays (ZWD) and tropospheric gradients (GRD), and can be used to determine integer phase ambiguities. The Multi-GNSS Experiment (MGEX), as realised by the International GNSS Service (IGS), provides orbit, clock and observation data for all operational GNSS. To take advantage of the new capabilities that these constellations bring, space-geodetic software packages have been retrofitted with Multi-GNSS-compliant modules. Based on this, two software packages, namely GipsyX and c5++, are utilised by way of the static Precise Point Positioning (PPP) approach using six months of data, and an assessment of the derived geodetic products is carried out for several GNSS receivers located at the Onsala core site. More specifically, we perform both single-constellation and multi-GNSS data analysis using Kalman filter and least-squares methods and assess the quality of the derived station positions, ZWD and GRD. A combined solution using all GNSS constellations is carried out and the improvement with respect to station position repeatabilities is assessed for each station. Results from the two software packages are compared with respect to each other and the discrepancies are discussed. Inter-system biases, which homogenise the different time scale that each GNSS operates in, and are necessary for the multi-GNSS combination, are estimated and presented. Finally, the applied inter-system weighting and its impact on the derived geodetic products are discussed.

How to cite: Diamantidis, P.-K., Kłopotek, G., Haas, R., and Johansson, J.: Assessment of geodetic products from Multi-GNSS analyses at the Onsala site, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14343, https://doi.org/10.5194/egusphere-egu21-14343, 2021.

The rapid development of multi-GNSS constellations, e.g., Galileo and BeiDou, is catalyzing innovations in high-precision applications. Precise point positioning ambiguity resolution (PPP-AR) has been essential to achieving the highest positioning precision using multi-GNSS data in wide areas. In recent years, several International GNSS Service analysis centers (IGS ACs such as CNES, CODE, WHU) have been providing phase bias products to enable PPP-AR, but whether these AC-specific multi-GNSS (e.g., GPS/Galileo/BeiDou-2/3) products are compatible with each other and whether they can be reconciled for an IGS combination product are pending. In this study, we combined phase bias products from four organizations for GPS/Galileo/BeiDou-2/3 in 2020. All phase bias products are first converted to observable-specific representation and then reconciled with satellite clocks before the combination; their capability of recovering integer undifferenced ambiguities has been always kept after properly addressing inter-system biases and satellite attitude discrepancies. It is found that the RMS of clock alignment residuals are around 6.8, 7.1, 14.9 and 14.6 ps for GPS, Galileo BeiDou-2 and BeiDou-3, respectively. BeiDou products perform worse due largely to sparse tracking networks and deficient orbit models. In a kinematic PPP experiment with 151 global MGEX (Multi-GNSS Experiment) stations, the combined phase bias products provide better or at least equivalent positioning results as opposed to AC specific products. Compared with ambiguity-float solutions, ambiguity-fixed PPP solutions can improve the positioning precision by 29-50% in the east component. With combined phase bias products, the positioning precision of GPS/Galileo/BDS-2/3 PPP-AR solutions can achieve 0.62, 0.64 and 1.90 cm in the east, north and up components, respectively, in contrast to 0.87, 0.88 and 2.60 cm for GPS only PPP-AR solutions.

How to cite: Geng, J., Pan, Y., Yang, S., and Li, P.: Combining multi-GNSS phase bias products for improved undifferenced ambiguity resolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2531, https://doi.org/10.5194/egusphere-egu21-2531, 2021.

PPP-RTK which combines the advantages of real-time kinematic (RTK) and precise point positioning (PPP), is able to provide centimeter-level positioning accuracy with rapid integer ambiguity resolution. In recent years, with the development of BDS and Galileo as well as the modernization of GPS and GLONASS, more than 130 GNSS satellites are available and new-generation GNSS satellites are capable of transmitting signals at three or more frequencies. Multi-GNSS and multi-frequency observations bring more possibilities for enhancing the performance of PPP-RTK. In this contribution, we develop a multi-frequency and multi-GNSS PPP-RTK model aiming to achieve rapid centimeter-level positioning for vehicle navigation in urban environments. The precise undifferenced atmospheric corrections are derived from multi-frequency and multi-GNSS observations of regional networks. Then the corrections are distributed to users to achieve PPP rapid ambiguity resolution. Vehicle experiments in different scenarios such as suburbs, overpasses, tunnels are conducted to validate the proposed method. Our results indicate that the multi-frequency and multi-GNSS PPP-RTK can achieve 2~3 cm positioning accuracy in the horizontal direction, 5~6 cm positioning accuracy in the vertical direction with the time to first fix of 5~7 s. In the urban environments where signals are interrupted frequently, a fast ambiguity recovery can be achieved within 5 s. Moreover, the PPP-RTK performance is significantly improved with multi-GNSS and multi-frequency observations. Compared to GPS-only solution, the positioning accuracy can be improved by 75%, and the fixing percentage can be up to 90% with this new method.

How to cite: Wang, B., Li, X., Huang, J., Feng, G., Lv, H., Han, X., Zhong, Y., and Li, X.: Multi-frequency and multi-GNSS PPP-RTK for vehicle navigation in urban environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9039, https://doi.org/10.5194/egusphere-egu21-9039, 2021.

The U.S. National Geodetic Survey (NGS) has historically processed dual-frequency GPS observations in a double-differenced mode using the legacy software called the Program for the Adjustment of GPS Ephemerides (PAGES). As part of NGS’ modernization efforts, a new software suite named M-PAGES (i.e., Multi-GNSS PAGES) is being developed to replace PAGES. M-PAGES consists of a suite of C++ and Python libraries, programs, and scripts built to process observations from all GNSS constellations. The M-PAGES team has developed a single-difference baseline processing strategy that is suitable for multi-GNSS. This approach avoids the difficulty of forming double-differences across systems or frequencies, which may inhibit integer ambiguity resolution. The M-PAGES suite is expected to deploy to NGS’ Online Positioning User Service (OPUS) later this year. Here, we present the processing strategy being implemented along with a performance evaluation from sample baseline solutions obtained from data collected within the NOAA CORS Network.

How to cite: Stressler, B., Bilich, A., Ogaja, C., and Heck, J.: Multi-GNSS Single-Difference Baseline Processing at NGS with newly developed M-PAGES software, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5556, https://doi.org/10.5194/egusphere-egu21-5556, 2021.

Global Navigation Satellite Systems (GNSS) are effectively used for different applications of Geomatic Engineering. There are lots of model error sources that affect the performance of the point positioning. Especially for the Precise Point Positioning (PPP) technique, which depends on the absolute point positioning, these errors should be modelled since PPP technique utilizes un-differenced and ionosphere-free combinations. Studies about PPP technique show that the effect of tropospheric delay caused by water vapor and dry air in the troposphere, which affects GNSS signals, is an important parameter should be modelled. Total zenith delay consists of both hydrostatic and wet delay. Hydrostatic delay can be accurately estimated by using atmospheric surface pressure and height with empirical models. Although there are many empirical models currently used for the determination of the zenith wet delay, the accuracies of these models are inadequate due to the temporal and spatial variation of atmospheric water vapor. Moreover, the tropospheric delay occurs along the path of GNSS signals and the Mapping Functions (MFs) are used to convert the tropospheric signal delay along the zenith direction to the slant direction. In this study, it is aimed to measure the effect of the globally produced MFs as Niell Mapping Function (NMF), Vienna Mapping Function 1 (VMF1), Global Mapping Function (GMF) and Global Pressure Temperature model 2 (GPT2) for GNSS positioning accuracy. Only GPS satellite system has been taken into account. For the analysis it has planned to process approximately 294 permanent stations from Crustal Dynamics Data Information System (CDDIS) archive with Jet Propulsion Laboratory’s GipsyX v1.2 software. In order to reveal the effect of different season the GPS observations in January, April, July and October, 2018 have been obtained. The solutions were derived for different session durations as 2, 4, 6, 8, 12 and 24 hours for each global MFs and root mean square values have been estimated for each session durations.

Keywords: State-of-the-Art Mapping Function, Troposphere, Precise Point Positioning, Accuracy, GipsyX

How to cite: Durmus, F. C. and Erdogan, B.: The Effect of the State-of-the-Art Mapping Functions on Precise Point Positioning, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9976, https://doi.org/10.5194/egusphere-egu21-9976, 2021.

Because of the inclined-orbit of GNSS constellations that are not cover the Polar Regions, the polar gaps occur between certain latitudes and therefore in these regions the satellite observations are limited around the zenith direction. In addition, from summer to winter season, the daylight and weather conditions vary tremendously in the Polar Regions. In the context of this study, the PPP accuracy performance was tested as a function of winter and summer seasons, GPS-only and GPS&GLONASS constellations, PPP-AR and PPP-Float solution strategies, static and kinematic processing modes, varying occupation times (1h, 2h, 4h, 8h, 12h and daily), and increasing latitudes towards the South Pole at the OHI3, ROTH, MCM4, and AMU2 GNSS stations in the Antarctica continent. Besides, the effect of the ambiguity solution strategies and the used constellations in the process on PPP convergence time was also examined. In the assessment results of the study, it was revealed that the PPP-AR strategy, additional GLONASS system to GPS constellation, and increased occupation times improved the static and kinematic positioning accuracy. Besides, although similar accuracies were obtained in both seasons, the position accuracy was slightly better in winter. Regarding the investigation on convergence time, the PPP-AR solution using the GPS&GLONASS constellations improved the convergence time by 66% comparing to the GPS-only PPP-Float solution. Finally, according to the assessment of the PPP-AR accuracy performance depending on the increasing latitude towards the South Pole, it has been observed that the 2D position accuracy remained stable for three stations except for AMU2. Besides, the vertical position accuracy decreased as it approaches the South Pole and the GLONASS system contributed to the improvement of the accuracy.

How to cite: Erol, S., Mutlu, B., Erol, B., and Çevikalp, M. R.: Static and Pseudo-Kinematic PPP-AR Performance in Antarctic Region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14144, https://doi.org/10.5194/egusphere-egu21-14144, 2021.

Recently researchers revealed that meteorological seasons have an effect on the accuracy of GPS. This modified the conventional prediction formulation in which the accuracy was dependent on observing session duration. However, the available accuracy model is from a major climate zone classification. In this study, we evaluate climatic effects on PPP accuracy from a different climate classification: the widely used Köppen Geiger climate zones. GPS data are obtained from SOPAC (Scripps Orbit and Permanent Array Centre) archives. Synthetic GPS campaigns are generated from the permanent stations of the IGS (International GNSS Service). The data are processed using the PPP module of the NASA/JPL's GipsyX software. The RMS values obtained from the processing solutions are used to determine the effect of climate on PPP accuracy. Eventually, we compare the two climate classifications and present our initial impressions from a core network across the new climate zones.

Keywords: GPS, GNSS, accuracy, PPP, climatic effects

How to cite: Saraçoğlu, A. and Şanlı, D. U.: Climatic Effects on GPS PPP Accuracy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8172, https://doi.org/10.5194/egusphere-egu21-8172, 2021.

This study assesses the quality of multi-constellation GNSS observations of selected Android smartphones namely Huawei P30, Huawei P20 and Huawei P Smart as well as Xiaomi Mi 8 and Xiaomi Mi 9. We investigate the properties of phase ambiguities to anticipate the feasibility of precise positioning with integer ambiguity fixing. The results reveal a significant drop of smartphone carrier-to-noise density ratio (C/N0) with respect to geodetic receivers and discernible differences among constellations and frequency bands. We show that the higher the elevation of the satellite, the larger discrepancy in C/N0 between the geodetic receivers and smartphones. We depict that an elevation dependence of the signal strength is not always the case for the smartphones. We discover that smartphone code pseudoranges are noisier by about one order of magnitude as compared to the geodetic receivers, and that the code signals on L5 and E5a outperform these on L1 and E1, respectively. It was shown that smartphone phase observations are contaminated by the effects that can destroy the integer property and time-constancy of the ambiguities. The long term drifts were detected for GPS L5, Galileo E1, E5a and BDS B1 phase observations of Huawei P30. To isolate the observational noise from low frequency effects we take advantage of time differencing using the variometric approach. These investigations highlight competitive phase noise characteristics for the Xiaomi Mi 8 when compared to the geodetic receivers. We also reveal poor phase signal quality for the Huawei P30 smartphones related to the unexpected long-term drifts of the phase signals. The observation quality assessment is supported with the evaluation of a positioning performance. We proved that it is feasible to obtain a precise solution in a smartphone to smartphone relative positioning mode with fixed ambiguities. Such results move us towards a collaborative precise positioning with smartphones.

How to cite: Paziewski, J., Fortunato, M., Mazzoni, A., Odolinski, R., Li, G., Debelle, M., Warnant, R., and Gong, X.: The quality analysis of GNSS observations tracked by Android smart devices and positioning performance assessment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-334, https://doi.org/10.5194/egusphere-egu21-334, 2021.

Since the release of Android 7.0 in 2016, raw GNSS measurements tracked by smartphones operating with Android can be accessed. Before this date, solely the position solution of the smartphone's internal "black box" algorithm could be further processed in various applications. Now the smartphone's GNSS observations can be used directly to estimate the user position with specialized self-developed algorithms and correction data. Since smartphones are equipped with simple, cost-effective GNSS chips and antennas, they provide challenging, low-quality GNSS measurements. Furthermore, most smartphones on the market offer GNSS measurements on just one frequency. 

Precise Point Positioning (PPP) is one of the most promising processing techniques for Global Navigation Satellite System (GNSS) data. PPP is characterized by the use of precise satellite products (orbits, clocks, and biases) and the application of sophisticated algorithms to estimate the user's position. In contrast to relative positioning methods, PPP does not rely on nearby reference stations or a regional reference network. Furthermore, the concept of PPP is very flexible, which is another advantage considering the challenging nature of (single frequency) GNSS measurements from smartphones.

In this contribution, we present PPP results applying the uncombined model on raw GNSS observations from various smartphone devices. In contrast to the typical use of the ionosphere-free linear combination for PPP, this flexible PPP model applies the raw GNSS observation equations, is suitable for any number of frequencies, and allows the utilization of ionosphere models as an ionospheric constraint. We explore the potential and limitations of using raw GNSS observations from smartphones for PPP to reach a position accuracy at the decimeter level. Therefore, we test different correction data types and algorithms and examine diverse ways to handle the tropospheric and ionospheric delay. The PPP calculations are performed with our self-developed in-house software raPPPid.

How to cite: Glaner, M. F., Gutlederer, K., and Weber, R.: Potential and limitations of processing smartphone GNSS raw observation data in PPP, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3345, https://doi.org/10.5194/egusphere-egu21-3345, 2021.

GNSS antennas are a key factor in precise GNSS positioning. With the increasing availability of low-cost dual-frequency GNSS receivers also the demands on low-cost GNSS antennas increases. Unfortunately, the electronic center of most GNSS antennas is not located in the mechanical Antenna Reference Point (ARP). As a consequence, Phase Center Corrections (PCC) have to be introduced to correct for frequency-dependent signal delays within the antenna system. The PCCs are typically in the range of several millimeters to centimeters. Thus, uncorrected phase center variations can be a significant error source in precise positioning.

For the purpose of antenna calibration, the Institute of Geodesy and Photogrammetry at ETH Zürich acquired a six-axis industrial robot of type KUKA AGILUS KR 6 R900 sixx. In an initial study, the absolute accuracy of the robot has been determined to be better than 1.5 mm (standard deviation). By introducing a set of extended Denavit-Hartenberg parameters, the absolute position accuracy of the robot is further increased to 0.3 mm over the entire workspace and 0.1 mm for a predefined sequence of robot poses, respectively. Therefore, the robot operates well below the phase noise of the GNSS measurements (typically around 1 mm) and is therefore seen as suitable for the calibration of GNSS antennas with sub-millimeter accuracy.

Besides the numerous benefits of absolute field calibration with an industrial robot, several challenges remain if it comes to low-cost GNSS antennas. The main challenges are that for each antenna a specific mounting system has to be built and that low-cost antennas are in general less shielded against multipath (compared to geodetic antennas). Besides, only little information exists about the stability of the electronic reference point and how much the electronic properties change when the antenna is mounted on different platforms (cars, drones, cubesats, etc).

To address the critical issues in low-cost GNSS antenna calibration and study the impact of the PCCs on the positioning solution, a calibration campaign has been initiated at ETH Zürich in autumn 2020. In this campaign, a set of low-cost multi-GNSS dual-frequency patch and loop antennas - suited for centimeter-positioning - has been calibrated and tested. Therefore, in the vicinity of the GNSS reference station (ETH2) the robot has been installed and a sequence of randomized robot poses has been executed in which the ARP of each antenna was defined as rotation point. The GNSS signals recorded during this sequence were processed together with the robot attitude information using the time-differencing approach defined by D. Willi (2019) using a spherical harmonics parameterization.

The PCCs obtained from the calibration campaign were stored in ANTEX files for a subsequent validation. In this presentation, we will highlight the developed calibration procedures for low-cost GNSS antennas, summarize the main results of the calibration and validation campaign, and will give the framework in which a calibration of low-cost GNSS antennas is considered beneficial.

Willi D., GNSS receiver synchronization and antenna calibration, PhD Thesis, ETH Zürich, 2019, https://www.research-collection.ethz.ch/handle/20.500.11850/308750

How to cite: Moeller, G., Piringer, F., Pérez Ortega, M., Presl, R., and Rothacher, M.: Absolute phase center calibration of low-cost GNSS patch antennas – Is it worth the effort?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5700, https://doi.org/10.5194/egusphere-egu21-5700, 2021.

The GNSS Reflectometry is an innovative technique, largely developed in the last years, to monitor local sea level heights. The agreement between sea level measurements derived from SNR-based GNSS-R and tide gauges observations have demonstrated the performance of this approach. In the presented study, we are interested in a subtidal scale phenomenon, the Local Inverse Barometer effect (LIB) which consists in the response of the sea surface to atmospheric pressure changes. The LIB is, in fact, not well modelled in coastal regions where GNSS-R provide continuous observations. The sea level anomaly obtained as the difference between GNSS-R sea level measurements and a tide model, T_TIDE developed by Rich Pawlowicz, is analyzed in order to detect the local inverse barometer effect. For this purpose, we have used 1-year of GNSS data of two antennas of the existing national network, Port-Tudy (Groix island, France) and Lyttelton (eastern coast of the South Island, New-Zealand), where the LIB effect is expected to be significant due to their location outside the equatorial band.

On the whole time series, a trend between the sea level anomaly and the LIB effect can be observed at mid to low frequencies (lower than 0.5 cycle per day). Moreover, high barometric variations caused by the passage of strong depressions lead to good correlations (> 0.7) between these two parameters.

Our results suggest that the GNSS reflectometry allows the observation of subtidal scale phenomena such as the impact of atmospherical variations in complex coastal environments.

How to cite: Gravalon, T., Seoane, L., Darrozes, J., and Ramillien, G.: Can GNSS-R help us to monitor the effects of inverse barometer in coastal areas ?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12499, https://doi.org/10.5194/egusphere-egu21-12499, 2021.

We developed a rapid and precise algorithm to compute Higher-Order Ionospheric Corrections (HOIC) utilizing realistic electron density fields. The electron density field is derived from the International Reference Ionosphere (IRI) and the required magnetic field is the International Geomagnetic Reference Field (IGRF). Direct application of such HOICs is regarded impractical due to the large data volume to be handled. Therefore, we developed a parameterized version; for any location near the Earth's surface (grid with a resolution of 2.5° times 5°) a set of HOICs are computed (various elevation and azimuth angles) and the coefficients of a polynomial expansion (Zernike polynomials) are stored in a look-up-table. These look-up-tables cover the time period 1990-2019 and are available via FTP (ftp://ftp.gfz-potsdam.de/pub/home/GNSS/products/gfz-hoic/). We call this parameterized version GFZ-HOIC. A scalable version utilizing GNSS Total Electron Content (TEC) maps is under construction. A version available for real time applications is foreseen. With such accurate and easy-to-use HOICs available we performed extensive impact studies. For example, we examine how HOIC leak into estimated station coordinates, clocks, zenith delays and tropospheric gradients in Precise Point Positioning (PPP). The study includes a few hundred globally distributed stations and covers the time period 1990-2019. The PPP simulation shows the known significant systematic impact of HOICs on the estimated station y-coordinates and the estimated north-gradient components. In addition, the PPP simulation reveals the significant systematic impact of HOICs on the estimated zenith delays. This impact is not caused by higher-order terms in the formula for the refractive index of the ionosphere. This impact is caused by the ray-path bending effects. These ray-path bending effects are automatically taken into account thanks to the ray-tracing algorithm that is used in the derivation of the HOICs. In conclusion, GFZ-HOICs are both highly accurate and easy-to-use so that we can recommend them for practical applications.

How to cite: Zus, F. and Wickert, J.: Higher-Order Ionospheric Corrections derived from realistic electron density fields , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-824, https://doi.org/10.5194/egusphere-egu21-824, 2021.

The circumpolar ionosphere is recognised as one of the most disturbed region of the ionized part of the atmosphere. The reasons for that are mainly dynamic conditions in the coupled system of the magnetosphere and the ionosphere as well as feeding of the polar plasma from the mid-latitude reservoir. One of the consequences of these phenomenon is the occurrence of large-scale ionospheric structures called polar patches. These are commonly defined as the enhancement of the F-region plasma characterized with a foreground-to-background density ratio larger than 2 and a size up to several hundred kilometres.

In this work we present GNSS-based characteristics of a patch occurrence in the northern hemisphere. The study covers a period of January–May 2014 corresponding to the maximum of the solar activity. The detection of structures was performed with a relative STEC value that is defined as a difference between epoch-wise L4 data and 4^th order polynomial corresponding to background variations of the ionosphere. In order to ensure a continuous monitoring of the ionosphere over the north pole, we used data from ~45 permanent stations. The results prove that ground-based GNSS data can be successfully used in the climatological investigations of polar patches. We found a strong seasonal effect in the occurrence of these structures with the maximum at the turn of February and March and the minimum in May. Such outcomes correspond to variations of a TEC gradient between subauroral and polar regions. This parameter seems to be also responsible for a subdaily pattern of patches observed for particular months. The comparison of GNSS-based results with in-situ SWARM data revealed some differences, which are probably related to different characteristics of the ionosphere provided by both techniques. Furthermore, the study confirms that most of the patches are observed for the negative values of IMF Bz,  whereas IMF By component has no significant impact on the number of analysed structures. 

How to cite: Sieradzki, R. and Paziewski, J.: Statistical investigations of polar patch occurrence during high solar activity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7201, https://doi.org/10.5194/egusphere-egu21-7201, 2021.

In the analysis of GNSS time series, when the sampling frequency and time-series lengths are almost identical, it is possible to highlight a linear relationship between the series repeatabilities (i.e. WRMS) and noise magnitudes. In the literature, linear equations as a function of WRMSs allowed many researchers to estimate the noise magnitudes. However, this was built upon homoskedasticity. We experienced the higher WRMSs, the more erroneous analysis results using the noise magnitudes from the linear equations stated. We hence studied whether or not homoscedasticity clearly describes the modeling errors. To test that, we used the published results of GPS baseline components from the previous work in the literature and realized here that each component forms part of the totality. We introduced all baseline component results as a whole into statistical analysis to check heteroskedasticity. We established null and alternative hypotheses on the residuals which are homoscedastic (H0) or heteroskedastic (HA). We adopted both the Breusch-Pagan test and the Goldfeld-Quandt test to prove heteroskedasticity and obtained p-values for both methods. The p-value, which is the probability measure, equals to almost zero for both test methods, that is, we fail to accept the null hypothesis. Consequently, we can confidently state that the relationship between the WRMSs and the noise magnitudes is heteroskedastic.

Keywords: Noise magnitudes, repeatabilities, heteroskedasticity, time-series analysis

How to cite: Duman, H. and Şanlı, D. U.: Heteroskedasticity between GNSS time-series repeatabilities and noise magnitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4702, https://doi.org/10.5194/egusphere-egu21-4702, 2021.

In 2005, a governmental program for the creation of a geodetic network (SGN) based on GNSS measurements started in the Republic of Uzbekistan. Its main goal is to provide a modern, reliable and accurate geocentric coordinate system for land management, construction, environmental protection and the creation of a spatial database for various sectors of the economy. The SGN established in the country based on the availability of infrastructure and geographical needs and therefore, it does not cover the entire country. SGN consists three levels: reference geodetic points (RGP), high precision satellite geodetic network (SGN-0) points and first class satellite geodetic network (SGN-1) points. Since 2018, a network of 50 Continuously Operating Reference Stations (CORS) has also been developing. The installation of more than 200 GNSS stations in the period from 2005 to 2020 allows the country's scientific community to solve a number of practical geodetic problems. Among them implementation global ITRS system into local area for transition to new national geocentric coordinate system, quasi-geoid determination based on high degree Global Geopotential Models (such as EGM2008, EIGEN-6C4, GECO) and local geodynamic research for stress field modeling.

How to cite: Fazilova, D., Magdiev, H., and Sichugova, L.: GNSS network of Uzbekistan: achievements, prospects and challenges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-842, https://doi.org/10.5194/egusphere-egu21-842, 2021.

The Western Anatolian extensional tectonic regime results in developing a set of approximately E-W trending Horst-Graben morphology. The Gediz graben accommodating many fertile lands is one of the significant tectonic structures associated with that regime. Intensive grape cultivation requiring irrigation has been conducted in these lands for many years, which causes a permanent decrease in the water budget as a consequence of increasing farming activities. Hence, we have aimed to clarify better spatial subsidence of the eastern part of the Gediz graben and performed at first InSAR data to obtain land-surface deformations. Towards the middle of graben, the line-of-sight deformation rates of InSAR from LiCSAR products reach gradually up to nearly 10 cm/yr. To confirm these rates, we monumented four continuous GNSS stations.  One of which was located out of the graben while the rest were at the graben in June 2020. Analysis of such a short time-series does not make sense; however, the vertical displacements for the closest stations to the center of the graben reach up to about 8 cm. while out of the graben station seems to be stable visually. It is worth stating that the givens are biased due most likely to the periodic signals. Consequently, the gradually increasing subsidence rates towards the graben center showed that have not been driven only by tectonic settlements but could also be driven by other phenomena. These results are the first results of the ongoing project no 119Y180 supported by TUBITAK.

Keywords: Land subsidence, GPS, InSAR, Gediz Graben

How to cite: Gül, Y., Duman, H., Hastaoğlu, K. Ö., Poyraz, F., Tiryakioğlu, İ., Erdoğan, H., Doğan, A., and Güler, S.: The first result of geodetic implications on intra-graben subsidence along the eastern part of the Gediz graben, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12489, https://doi.org/10.5194/egusphere-egu21-12489, 2021.