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    A comparative study for accuracy assessment of PPP technique using GPS and GLONASS in urban areas
    (Elsevier B.V., 2015) Alkan, Reha Metin; İlçi, Veli; Ozulu, İbrahim Murat; Saka, Mehmet Halis
    In recent years, the usage of the Precise Point Positioning (PPP) has increased due to its ease of use, providing high accuracy as well as reducing the field operational costs by using only a single GNSS receiver. Moreover, it does not require any additional reference station data or CORS-like network data. It is a positioning technique that has been extensively investigated and used in a variety of applications, mainly for conventional surveying applications and time-transfer, ionospheric and tropospheric characterization. In GNSS positioning, limited satellite visibility in obstructed areas and poor satellite geometry adversely affect the accuracy of results. With the revitalization of the GLONASS system reaching to full global coverage, PPP studies have focused on combined GPS and GLONASS solutions. The goal of this study is to investigate the usability of PPP technique in urban areas with GPS-only and GPS + GLONASS data by the use of online-PPP services for sub-decimeter surveying applications. For this purpose, test measurements were conducted in Çorum province of Turkey. Collected data were evaluated with two commonly used PPP services, namely CSRS-PPP operated by the Geodetic Survey Division of Natural Resources Canada (NRCan) and magicGNSS/PPP developed by Spanish GMV Aerospace and Defense Co. Results obtained from this study revealed that for some cases, the use of GLONASS with GPS raises the number of satellites used in the solution twice as much as compared to GPS-only. However, unless satellites geometry gets better, the use of GLONASS has no significant role in strengthening the accuracy of results. The results indicate that an increase in the number of satellites is vital especially in urban areas where minimum number of satellites for a solution may not be obtained. Also, it was observed that when the sufficient numbers of GPS satellites with good geometry are available combining GPS together with GLONASS may not bring any significant contribution to the solution. © 2015 Elsevier Ltd. All rights reserved.
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    Decimeter-level positioning in dynamic applications with a single GPS receiver
    (Kluwer Academic Publishers, 2014) Saka, Mehmet Halis; Alkan, Reha Metin
    Although the ongoing GPS modernization studies provide better autonomous positioning accuracy (i.e. a few meters), this accuracy level is not enough for many surveying applications. Depending on the developments in satellite-based positioning technology and GNSS data availability and analysis techniques, some new approaches and algorithms have been proposed in the world of global positioning to enhance the autonomous positioning accuracy. In this study, a precise positioning technique, named “self-differential method”, is introduced using the dual frequency GPS data collected by a single geodetic-grade GPS receiver. The technique uses iono-free carrier phase observations, precise satellite ephemerides and satellite clock data. In order to validate the performance of the introduced method in a dynamic environment two kinematic tests were conducted at Haliç Bay in Istanbul, Turkey. The results show that the method is capable of achieving decimeter level of positioning accuracy without the need of an additional reference station or any other data while reducing the field operational costs. © 2014, Akadémiai Kiadó.
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    Kinematic precise point positioning using GPS and GLONASS measurements in marine environments
    (Elsevier B.V., 2017) Alkan, Reha Metin; Saka, Mehmet Halis; Ozulu, İbrahim Murat; İlçi, Veli
    The Precise Point Positioning (PPP) technique has received increased attention in recent years. When employing PPP, users need only a single GNSS receiver data to obtain a cm to dm accuracy level on a global datum, both in static and kinematic modes. This technique has become more popular due to its easy use, simple field operations, no base station(s) requirement, and provides cost effective high accuracy positioning. In satellite-based positioning, the accuracy and reliability of the positioning results are strongly dependent on the number and geometry of visible satellites, and quality of the observations. This issue is more important when the measurements are conducted in environments where satellite signals are blocked or degraded in some places like narrow channels, ravines, congested harbors, coastal areas with intense urbanization, inland waterways, in waters having severe terrain obstructions or surrounded by high mountains. To overcome this limitation, a combination of GPS and GLONASS measurements are proposed, especially to achieve more reliable and accurate solutions by increasing the number of visible satellites. The additional satellite system's observations like GLONASS, are expected to enhance the positioning accuracy and solution availability where especially not enough numbers of GPS satellites are visible. As of today, precise orbits and clock data are available not only for GPS but also for GLONASS. This provides an opportunity to apply the PPP technique to GPS and GLONASS observations. In this study, performance of the PPP method is assessed in a dynamic environment using GPS-only and GPS + GLONASS observations. Accordingly, a kinematic test is carried out at the Obruk Lake Dam in the Çorum province, of Turkey. The collected data is processed using a globally popular on-line processing service: the Canadian Spatial Reference System-Precise Point Positioning Service (CSRS-PPP) operated by the Geodetic Survey Division of Natural Resources Canada (NRCan). The results show that the combined GPS + GLONASS data produced almost the same level of accuracy with the GPS-only data if there is a sufficient number of GPS satellites with good geometry. In the extreme cases, such as the elevation of satellites above 40 degrees, the GPS cannot provide a solution. However, when data and both satellites systems are combined, it is then possible to provide the solution. The results also reveal that a dm-level of accuracy can be achieved with the PPP technique in a dynamic environment. This accuracy level largely meets the requirements of many marine applications, including precise hydrographic surveying, marine geodesy, navigation and oceanography. © 2017 Elsevier Ltd
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    The determination of interseismic, coseismic and postseismic deformations caused by the Gökçeada-Samothraki earthquake (2014, Mw: 6.9) based on GNSS data
    (Elsevier Ltd, 2017) Tiryakioğlu, İbrahim; Yiğit, Cemal Özer; Yavaşoğlu, Hasan Hakan; Saka, Mehmet Halis; Alkan, Reha Metin
    Since the 1990s, seismic deformations have been commonly determined using the Global Navigation Satellite System (GNSS). Recently, the GNSS systems have become even more powerful with the use of new technologies in innovative studies. In this study, the GNSS data was used to investigate interseismic, coseismic and postseismic deformation and velocity of the Gökçeada-Samothraki earthquake (Mw = 6.9) that occurred on May 24, 2014. The data was obtained at 30 s (0.033 Hz) and 1 s (1 Hz) intervals from the GNSS receivers in the network of Continuously Operating Reference Stations, Turkey (CORS-TR). For the interseismic period, the daily coordinate time series of 12 stations located within 90–250 km of the earthquake epicenter was evaluated for the displacement of stations over a period of approximately 2000 days prior to the day of the earthquakes, from October 1, 2008 to May 23, 2014. In order to analyze the ground motion displacement during the Gökçeada-Samothraki earthquake, 1 Hz data from 8 continuous GNSS stations was processed using precise point positioning (PPP) and relative positioning methods to estimate the epoch-by-epoch positions of the stations. During the earthquake, coseismic displacements of approximately 7 and 30 mm were detected in the NW direction at the YENC and CANA stations, respectively. However, at the IPSA station, a coseismic deformation of 20 mm was observed in the NE direction. There were no significant changes at the other stations during the earthquake. For the postseismic period, the daily coordinate time series of the 12 stations were evaluated for station displacements for 570 days after the day of the earthquakes, from May 24, 2014 to January 1, 2016. The results demonstrated that no significant postseismic deformation with the exception of the EDIR station. An abnormal deformation caused by local factors was determined at the EDIR station. In this study, the PPP and the relative solution were also compared in terms of capturing the earthquake wave motion. The results demonstrated that the PPP-based solutions showed good agreement with those of relative positioning in terms of the ability to capture coseismic displacement. © 2017 Elsevier Ltd