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Öğe Analysis of Displacement Response of the Ermenek Dam Monitored by an Integrated Geodetic and Pendulum System(Springer International Publishing Ag, 2018) Alcay, Salih; Yigit, Cemal Ozer; Inal, Cevat; Ceylan, AyhanThis paper studies the structural behavior of the Ermenek arch dam, the second highest dam in Turkey. The investigated period covers the stage of the last-quarter part of first filling (1 and 1/2 year) and the begging of its operational lifetime (1year). Displacement responses of the dam to temperature load and water load are assessed in detail based on both geodetic and pendulum monitoring records. In addition, the present study explores the possibility and advantage of integrating pendulum-measured signals with geodetic-measured signals for the dam deformation monitoring. The analysis results reveal that pendulum-derived displacements at four gallery levels, along the vertical cross section of the dam, which exhibit periodicity and linear trend with different slope due to seasonal temperature oscillation and linearly increased reservoir level, respectively. Comparative evaluation of the results shows that there is very good agreement between geodetic-derived displacement in radial direction at the middle of the crest and corresponding pendulum-derived displacement.Öğe Analysis of global TEC prediction performance of IRI-PLAS model(Elsevier Sci Ltd, 2019) Alcay, Salih; Oztan, GurkanThe International Reference Ionosphere (IRI) empirical model provides valuable data for many fields including space and navigation applications. Since the IRI model gives the ionospheric parameters in the altitude range from 50 km to 2000 km, researchers focused on the IRI-PLAS model which is the plasmasphere extension of the IRI model. In this study, Total Electron Content (TEC) prediction performance of the IRI-PLAS model was examined at a global scale using the location of globally distributed 9 IGS stations. Besides the long term (01.01.2015-31.12.2015) behavior of the model, TEC predictions during the equinox and solstice days of 2014-2017 were also tested. IRI-PLAS-TEC values were examined in comparison with GPS-TEC data. Hourly interval of yearly profile exhibits that when the geomagnetic and solar active days are ignored, differences between IRI-PLAS-TEC and GPS-TEC are rather small (similar to 2-3 TECU) at stations in the northern hemisphere, generally similar to 4-5 TECU level at the southern hemisphere stations and reaching above 10 TECU for few hours. While the IRI-PLAS-TEC generally overestimates the GPS-TEC at southern hemisphere stations during quiet days, the model-derived TEC underestimates GPS-TEC during solar active days. IRI-PLAS-TEC and GPS-TEC values exhibit similar trend for the equinoxes 21 March and 23 September which refer equivalent conditions. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.Öğe ANALYSIS OF IONOSPHERIC TEC VARIATIONS RESPONSE TO THE Mw 7.2 VAN EARTHQUAKE(Acad Sci Czech Republic Inst Rock Structure & Mechanics, 2016) Alcay, SalihThis paper investigates the pre-earthquake ionospheric variations using Total Electron Content (TEC) of Precise Point Positioning (PPP) based regional ionospheric maps and Global Ionospheric Maps (GIMs) during the 7.2 Mw Van earthquake which occurred on October 23, 2011. TEC Maps around the epicenter from 10 days before to 4 days after the earthquake were examined. In addition, time series of the TEC values were evaluated and results showed that the ionospheric TEC was increased obviously 3 days before and 2 days after the earthquake. These variations of the TEC were possibly associated with the seismic activity and geomagnetic storm respectively.Öğe ANALYSIS OF IONOSPHERIC TEC VARIATIONS RESPONSE TO THE Mw 7.2 VAN EARTHQUAKE(Acad Sci Czech Republic Inst Rock Structure & Mechanics, 2016) Alcay, SalihThis paper investigates the pre-earthquake ionospheric variations using Total Electron Content (TEC) of Precise Point Positioning (PPP) based regional ionospheric maps and Global Ionospheric Maps (GIMs) during the 7.2 Mw Van earthquake which occurred on October 23, 2011. TEC Maps around the epicenter from 10 days before to 4 days after the earthquake were examined. In addition, time series of the TEC values were evaluated and results showed that the ionospheric TEC was increased obviously 3 days before and 2 days after the earthquake. These variations of the TEC were possibly associated with the seismic activity and geomagnetic storm respectively.Öğe Analysis of the TEC Prediction Performance of IRI-2016 Model in the Mid-Latitude Region(Maik Nauka/Interperiodica/Springer, 2021) Alcay, SalihThis paper investigates the performance of International Reference Ionosphere (IRI-2016) in predicting the variation of the Total Electron Content (TEC) over six IGS stations in the mid-latitude region. Three of the stations are in the northern hemisphere and three in the southern hemisphere. IRI-2016 TEC values are estimated using various Ne Topside, Ne F-peak and F-peak height options during different geomagnetic and solar activity conditions. Three different ionospheric conditions are considered; geomagnetic active-solar quiet days (June 23, 2015; October 7, 2015), geomagnetic-solar quiet days (May 23, 2015; July 19, 2015) and geomagnetic quiet-solar active days (July 8, 2014; December 18, 2014). Model derived TEC values are compared with the GPS-TEC data, which is used as a reference value. The overall results show that TEC predictions using IRI-2001 (Ne-Topside) and URSI (Ne F-peak) options provide better agreement with GPS-TEC values for the summer days. Whereas IRI01-corr-NeQuick (Ne-Topside) with CCIR (Ne F-peak) options predict well for the winter days. Besides, evaluation of the results reveals insignificant differences (<1 TECU) among three F-peak height option (AMTB-2013, SHU-2015 and BSE-1979) solutions. Furthermore, regardless of which IRI parameter is used during active days, the differences between IRI-TEC and GPS-TEC reach high values at some stations at some time intervals.Öğe Anamolous Ionospheric TEC Variations Prior to the Indonesian Earthquake (M 7.1) of November 15, 2014(Maik Nauka/Interperiodica/Springer, 2017) Alcay, SalihThis paper investigates preearthquake ionospheric variations with the use of TEC of Global Ionospheric Maps (GIMs) and regional maps based on Precise Point Positioning (PPP) during the 7.1-M Indonesian earthquake that occurred on November 15, 2014. TEC maps corresponding to 10 days before to 4 days after the event were examined. In addition, a time series of TEC values according to the PPP maps were also evaluated. In addition to GIMs, it was possible to detect TEC variations with PPP maps. The results showed that ionospheric TEC decreased strikingly 4 days prior to the earthquake. This TEC variation was highly likely related to seismic activity.Öğe Comparative analysis of real-time precise point positioning method in terms of positioning and zenith tropospheric delay estimation(Taylor & Francis Ltd, 2023) Atiz, Omer Faruk; Alcay, Salih; Ogutcu, Sermet; Bugdayci, IlkayThe positioning performance of widely used real-time precise point positioning (RT-PPP) software packages BNC, RTKLIB, and PPP-WIZARD were tested in terms of convergence time and accuracy. The convergence time of PPP-WIZARD solutions is reduced by ambiguity resolution (AR). The GPS + GLONASS + GALILEO (GRE) mode improved the convergence time of GPS + GALILEO (GE) mode by 22.0%, 15.5%, 17.1%, and 11.4% for the BNC, RTKLIB, PPP-WIZARD (AR) and PPP-WIZARD, respectively. For the GRE mode, RMSEs of the BNC, RTKLIB, PPP-WIZARD (AR), and PPP-WIZARD software packages in the horizontal/vertical component are 3.8/5.6, 2.6/6.2, 3.3/6.5, 4.3/7.0 cm, respectively. In comparison with the IGS-ZTD (International GNSS Service ZTD), BNC, RTKLIB, PPP-WIZARD (AR), and PPP-WIZARD solutions show a mean bias of 0.28, -0.72, 2.80, and 2.83 cm, respectively in GE mode. The GRE mode reduced the RMSEs of the ZTD estimations of BNC, RTKLIB, PPP-WIZARD (AR) and PPP-WIZARD by 2.9%, 5.1%, 0.6%, and 0.4% respectively.Öğe Comparative analysis of the performance of Multi-GNSS RTK: A case study in Turkey(Selcuk Univ Press, 2022) Atiz, Omer Faruk; Konukseven, Ceren; Ogutcu, Sermet; Alcay, SalihThe Real Time Kinematic (RTK) method is widely used in the land surveying. Whereas RTK method has the advantage of practical use, positioning accuracy depends mostly on the baseline length due to the atmospheric errors. In general, RTK measurements are made by using GPS and GLONASS satellite systems. For this reason, the positioning performance of the technique is adversely affected under restricted satellite geometry conditions such as urban canyons. At present, most receivers on the market have the ability to track signals of Galileo and BeiDou satellites. Therefore, in this study, the positioning performance of RTK with different satellite combinations (GPS-only, GPS+GLONASS, GPS+GLONASS+GALILEO+BeiDou) was examined with a comparative approach. A field test was carried out considering approximately 20, 40, 60, and 80 km length of baselines. Three different cut off elevation angles - namely, 10 degrees, 20 degrees, and 30 degrees - were chosen for the field test. The results were investigated in terms of accuracy and precision. Also, the ground truth coordinates of the rovers were obtained by post-processing relative method using GAMIT/GLOBK software. The results showed that multi-GNSS combinations provided better repeatability at the 10 degrees cut off angle option. The accuracy of GPS-only solutions varied between 0.63/2.17 cm and 2.40/4.94 cm for horizontal and vertical components, respectively. However, the multi-GNSS combinations did not have a remarkable superiority in terms of position accuracy even at high satellite cut off angle (30 degrees) compared to the GPS-only RTK.Öğe Comparison of IRI_PLAS and IRI_2012 model predictions with GPS_TEC measurements in different latitude regions(Ist Nazionale Di Geofisica E Vulcanologia, 2017) Alcay, Salih; Oztan, Gurkan; Selvi, Huseyin ZahitThe International Reference Ionosphere (IRI) is an empirical model for providing ionospheric parameters, including Total Electron Content (TEC), electron density, electron and ion temperature etc., in the altitude range from 50 km to 2000 km. Since the IRI model is limited up to 2000 km, IRI_PLAS model, plasmasphere extension of the IRI model, was proposed by the researchers. This paper investigates the TEC prediction performance of IRI_PLAS and IRI_2012 models by comparing GPS TEC data, in different latitude regions for magnetically active and quiet days. TEC data over 9 International GNSS Service ( IGS) stations, located in different latitude regions, are used for the comparison. Evaluation of the diurnal results reveals good agreement with correlation coefficient >0.9 between GPS_TEC and empirical models for the quiet day irrespectively of the latitudinal data used. However, while the differences are not relatively large in most part of the active days, they reach high level, above 30 TECu, in some parts of the days.Öğe Displacement monitoring performance of relative positioning and Precise Point Positioning (PPP) methods using simulation apparatus(Elsevier Sci Ltd, 2019) Alcay, Salih; Ogutcu, Sermet; Kalayci, Ibrahim; Yigit, Cemal OzerBesides the classical geodetic methods, GPS (Global Positioning System) based positioning methods are widely used for monitoring crustal, structural, ground etc., deformations in recent years. Currently, two main GPS positioning methods are used: Relative and Precise Point Positioning (PPP) methods. It is crucial to know which amount of displacement can be detected with these two methods in order to inform their usability according to the types of deformation. Therefore, this study conducted to investigate horizontal and vertical displacement monitoring performance and capability of determining the direction of displacements of both methods using a developed displacement simulator apparatus. For this purpose, 20 simulated displacement tests were handled. Besides the 24 h data sets, 12 h, 8 h, 4 h and 2 h subsets were considered to examine the influence of short time spans. Each data sets were processed using GAMIT/GLOBK and GIPSY/OASIS scientific software for relative and PPP applications respectively and derived displacements were compared to the simulated (true) displacements. Then statistical significance test was applied. Results of the experiment show that using 24 h data sets, relative method can determine up to 6.0 mm horizontal displacement and 12.3 mm vertical displacement, while PPP method can detect 8.1 mm and 19.2 mm displacements in horizontal and vertical directions respectively. Minimum detected displacements are found to grow larger as time spans are shortened. (C) 2018 COSPAR. Published by Elsevier Ltd. All rights reserved.Öğe GPS plus Galileo + BDS-3 medium to long-range single-baseline RTK: an alternative for network-based RTK?(Cambridge Univ Press, 2023) Ogutcu, Sermet; Alcay, Salih; Ozdemir, Behlul Numan; Duman, Huseyin; Koray, Ulkunur; Konukseven, Ceren; Bilal, Nesibe GulThanks to the development of the real-time kinematic (RTK) algorithm and the emerging Global Navigation Satellite System (GNSS), especially for Galileo and BeiDou-3, reliable positioning accuracy for medium and long-baseline RTK became possible globally. Moreover, with the development of the GNSS receiver hardware, baseline length limitations due to radio-based communications are removed thanks to internet-based communication. In this work, single-baseline RTK, incorporated partial ambiguity resolution with troposphere and ionosphere weighting, using GPS (G), Galileo (E), BeiDou-3 (C3) and multi-GNSS (GE and GEC3), is conducted with real GNSS data of EUREF Permanent GNSS network under three different cutoff angles (10 degrees, 20 degrees, and 30 degrees) for six different lengths of baselines (similar to 50, similar to 150, similar to 250, similar to 350, similar to 450, and similar to 550 km). The results show that the multi-GNSS RTK solution significantly contributed to the positioning accuracy and convergence time of the single-system RTK solutions. Based on the results, non-available epoch-wise solutions for the high-degree cutoff angles are more obvious for the single-system RTK, whereas multi-GNSS solutions provide 100% solutions for each cutoff angle and baseline. The results indicate that instantaneous and a few epochs single-epoch ambiguity resolution is feasible for 50, 150, 250 and 350 km baseline lengths for multi-GNSS RTK. Based on the positioning results, horizontal-vertical positioning improvements of multi-GNSS RTK (GEC3) compared with the single-system GPS RTK are found as 50%-37%, 40%-35%, 55%-47%, 53%-54%, 57%-49% and 57%-49% for 50, 150, 250, 350, 450 and 550 km, respectively, under a 10 degrees cutoff angle. For 20 degrees and 30 degrees cutoff angles, the accuracy improvements are much higher. The convergence time improvements (n/e/u) of multi-GNSS RTK (GEC3) compared with the single-system GPS RTK are found as 86/92/75%, 77/67/72%, 75/77/83%, 53/56/52%, 69/49/62%, and 52/45/39% for 50, 150, 250, 350, 450 and 550 km, respectively, under a 10 degrees cutoff angleÖğe Investigation of ionospheric TEC anomalies caused by space weather conditions(Springer, 2020) Alcay, Salih; Gungor, MerveThis study examines the influence of space weather conditions on ionospheric TEC variations at different latitudes around the world. According to the space weather condition indices (geomagnetic storm-activity indices, solar activity indices, magnetic field change indices, plasma density and proton flux indices) different time periods were chosen. GPS observations were used to obtain TEC data derived from International GNSS Service (IGS) stations located at northern and southern hemisphere latitude regions (equatorial, mid-latitude and high latitude). GPS TEC data were evaluated associated with the space weather conditions using 15-day running median statistical analysis method. The results showed that besides the commonly used geomagnetic storm-activity and solar activity indices, plasma density, magnetic field change and particle flux indices are also important in determining whether there is any TEC anomaly in the ionosphere. This study illustrated that all indices should be examined in the applications associated with the ionosphere and particularly in the research of pre-earthquake ionospheric anomalies.Öğe Investigation of the performance of Real-Time Precise Point Positioning (RT-PPP) method using different software packages(Geomatik Journal, 2021) Alcay, Salih; Atiz, Omer FarukThe Precise Point Positioning technique (PPP), which emerged as an alternative to the conventional relative positioning method, has been widely used in many applications in recent years. The PPP method is a special case of absolute positioning method, in this method the static and kinematic position can be determined with high accuracy by using single GNSS receiver. On the other hand, thanks to the real-time precise orbit and clock products made available by different establishments and organizations (International GNSS Service-IGS, Federal Agency for Cartography and Geodesy-BKG, European Space Agency-ESA, European Reference Frame-EUREF, German Research Centre for Geosciences-GFZ etc.), PPP method has started to be used in applications that require real-time position information. The high interest in this method, which is called real-time-PPP (RT-PPP), has accelerated the development of the method. Many software have been developed in this context. In this study, the positioning performance of RT-PPP method was investigated using BKG/BNC (BKG Ntrip Client), RTKLIB/RTKNAVI (Real Time Kinematic Library) and PPP-WIZARD (PPP With Integer and Zero-difference Ambiguity Resolution Demonstrator) software. For this purpose, real time coordinate values of ISTA IGS station, located in Istanbul, belongs to the observation period of about 25 hours were obtained in 1 second epoch interval. The obtained results were compared with the reference coordinates of the station, and examined in terms of accuracy and precision. According to the results, it has been observed that the convergence time sometimes exceeds 1 hour, and after this time, an accuracy of approximately +/- 10 cm in horizontal component and +/- 20 cm in vertical component could be obtained with BKG/BNC and RTKLIB/RTKNAVI. However the accuracy of the coordinates obtained using the PPP-WIZARD software is over +/- 30 cm due to the large number of disconnections and subsequent convergence.Öğe Network based performance of GPS-only and combined GPS/GLONASS positioning under different sky view conditions(Springer, 2017) Alcay, Salih; Yigit, Cemal OzerThis paper offers a comprehensive assessment of network-based positioning results using GPS-only and combined GPS/GLONASS data under open to limited sky view conditions. Data analysis is performed in a short (4 h) and a long (24 h) term spanning observations from five GNSS stations using 10 degrees, 20 degrees, 30 degrees and 40 degrees elevation cut-off angles, which simulate the availability of satellites in different sky view conditions. Evaluation of the results reveals insignificant differences between the accuracy of the GPS-only and combined GPS/GLONASS solutions for daily observation in all sky view conditions. However the clear advantage of integrating GPS and GLONASS observation in a GNSS differential solution is apparent for short (4 h) observation periods, particularly at the highest elevation cut-off angle (40 degrees).Öğe Performance Evaluation of Real-Time Precise Point Positioning Method(Iop Publishing Ltd, 2017) Alcay, Salih; Turgut, MuzeyyenPost-Processed Precise Point Positioning (PPP) is a well-known zero-difference positioning method which provides accurate and precise results. After the experimental tests, IGS Real Time Service (RTS) officially provided real time orbit and clock products for the GNSS community that allows real-time (RT) PPP applications. Different software packages can be used for RT-PPP. In this study, in order to evaluate the performance of RT-PPP, 3 IGS stations are used. Results, obtained by using BKG Ntrip Client (BNC) Software v2.12, are examined in terms of both accuracy and precision.Öğe Performance investigation of LAMBDA and bootstrapping methods for PPP narrow-lane ambiguity resolution(Taylor & Francis Ltd, 2021) Atiz, Omer Faruk; Ogutcu, Sermet; Alcay, Salih; Li, Pan; Bugdayci, IlkayPrecise point positioning with ambiguity resolution (PPP-AR) is a powerful tool for geodetic and time-constrained applications that require high precision. The performance of PPP-AR highly depends on the reliability of the correct integer carrier-phase ambiguity estimation. In this study, the performance of narrow-lane ambiguity resolution of PPP using the Least-squares AMBiguity Decorrelation (LAMBDA) and bootstrapping methods is extensively investigated using real data from 55 IGS stations over one-month in 2020. Static PPP with 24-, 12-, 8-, 4-, 2-, 1- and 1/2-h sessions using two different cutoff angles (7 degrees and 30 degrees) was conducted with three PPP modes: i.e. ambiguity-float and two kinds of ambiguity-fixed PPP using the LAMBDA and bootstrapping methods for narrow-lane AR, respectively. The results show that the LAMBDA method can produce more reliable results for 2 hour and shorter observation sessions compared with the bootstrapping method using a 7 degrees cutoff angle. For a 30 degrees cutoff angle, the LAMBDA method outperforms the bootstrapping method for observation sessions of 4 h and less. For long observation times, the bootstrapping method produced much more accurate coordinates compared with the LAMBDA method without considering the wrong fixes cases. The results also show that occurrences of fixing the wrong integer ambiguities using the bootstrapping method are higher than that of the LAMBDA method.Öğe Performance investigation of Trimble RTX correction service with multi-GNSS constellation(Taylor & Francis Ltd, 2023) Atiz, Omer Faruk; Shakor, Abbas Qader; Ogutcu, Sermet; Alcay, SalihIn the middle of 2011, Trimble introduced the RTX correction service to support RT-PPP applications. In this study, the performance of Trimble RTX correction service is investigated over a one-month period using five different GNSS constellations - namely, GPS-only, GPS + GLONASS, GPS + Galileo, GPS + GLONASS + Galileo, and GPS + GLONASS + Galileo + BeiDou. The results show that positioning accuracy and convergence time are significantly improved with the use of the multi-GNSS constellation compared with the GPS-only solution. The result of the converged positioning accuracy indicates that the GPS + GLONASS + Galileo + BeiDou combination improves the accuracy by 63%, 54%, and 60% for north, east, and up components, respectively, compared with the GPS-only solution. The mean convergence time is reduced by the GPS + GLONASS+ Galileo + BeiDou combination by 70%, 71%, and 38.6% compared with the GPS-only solution in the north, east, and up components, respectively. 1.2 (north), 1.5 (east), and 2.3 cm (up) root mean square errors (RMSEs) of converged positioning from the GPS + GLONASS + Galileo + BeiDou combination are computed.Öğe Static and kinematic PPP-AR performance of low-cost GNSS receiver in monitoring displacements(Elsevier Sci Ltd, 2023) Ogutcu, Sermet; Alcay, Salih; Duman, Huseyin; Ozdemir, Behlul Numan; Konukseven, CerenRecently, low-cost GNSS receivers have played a significant role in displacement monitoring studies due to the increasing availability of mass-market applications. In this work, the performance of static and kinematic PPP-AR using a low-cost u-blox ZED-F9P GNSS receiver in monitoring displacements is investigated by comparing it with a geodetic-grade GNSS receiver using a displacement simula-tion apparatus. The capability to determine the horizontal displacement direction is also investigated for both receivers. The results showed that one-sigma horizontal / vertical RMSEs computed from the true displacements and computed displacements are 1.4 / 5.6 mm and 2.6 / 8.4 mm for the geodetic and u-blox receivers, respectively, using 24-h data with static GPS + GLONASS PPP-AR. It is found that GLONASS contribution to GPS-only static PPP is more evident for the u-blox receiver compared with the geodetic one. RMSEs are found to be higher as observation times are decreased. The post-processed kinematic test results showed that minimum 10 / 20 and 20 / 30 mm horizontal / vertical dynamic displacements can be visually detected as an offset from the daily time series for the geodetic and u-blox receivers, respectively.(c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.
