Recently, a national CORS network, called CORS-TR network has been established by Istanbul Kultur University in Turkey and Northern Cyprus. The project was sponsored by the Scientific and Technological Research Council of Turkey (TUBITAK). The network was designed to fulfill several important tasks ranging from geodetic measurements, crustal deformation studies to modeling the atmosphere (troposphere and ionosphere) over Turkey. The network consists of 147 reference stations and three control centers collecting the GNSS data and broadcasting Real Time Kinematic (RTK) corrections.
Overview of CORS-TR
The establishment of conventional and RTK CORS networks has opened a new era worldwide (Rizos 2007; Grejner-Brzezinska et al., 2007). The network based CORS network project, known as the CORS-TR project, was initiated by Istanbul Kultur University (IKU) in association with the General Directorate of Land Registration and Cadastre (GDLRC) and the General Command of Mapping (GCM).
The CORS-TR project officially started in May 2006 and completed in May 2009. The main goals of this undertaking were a) establishing network-based CORS-TR stations functioning 24/7 with RTK capabilities, b) modeling the atmosphere (troposphere and ionosphere) over Turkey contributing to atmospheric studies and weather predictions, with extension to signal and communication studies, c) providing mm-level accuracy for monitoring plate tectonics, measuring deformations and contributing to earthquake prediction and early warning systems (Brownjohn et al. 2004), d) determining datum transformation parameters between the old European Datum 50 (ED50) and International Terrestrial Reference Frame / ITRFxx (Kempe et al. 2006).
Prior to the establishment of CORS-TR, it was decided to perform some benchmark tests. Actually, there are numerous RTK test networks established worldwide at different locations and of various sizes mainly affected by its geographic locations, infrastructure and type of receivers used. Some good examples of such networks are: St. Lawrence River and Southeastern Brazil Tests (Lachapelle et al. 2001) and Sydnet Project Test (Kinlyside 2006).
A comprehensive prototype test was carried out in Turkey in the Marmara region (roughly 300 x 150 km, as shown in Figure 1) in order to i) optimize the CORS–TR network design, ii) test different RTK techniques with modern GNSS (Global Navigation Satellite System) receivers and study the effectiveness of the control center software packages.
Three network configurations were selected, each consisting of six CORS points: NET-120, NET-90 and NET-60 with interstation distances of 120, 90 and 60 km respectively. The three companies participating in the tests were Leica, Topcon, and Trimble. The field work was carried out during August – October 2006. The survey characteristics and its results were published in (Eren et. al., 2009).
Over 20 researchers from several universities in Turkey took part in the CORS-TR project. The first phase involved a literature study and on-site analysis of existing implementations. As a result of this phase it was decided to carry out the benchmark test presented above. This test allowed for the analysis of CORS systems, communication infrastructure and mathematical models for the datum conversion. Using the results of this test and other studies the final CORS network of 147 reference stations (spread at 80-100 km offsets) was designed (Figure 2).
CORS-TR’s reference communication infrastructure for all stations and the control center make use of ADSL with GSM EDGE/3G as the alternative method. This system uses globally accepted VRS, FKP and MAC techniques. The reference stations can use signals from GPS, GLONASS and Galileo. The satellite data recorded by the reference stations are relayed to the CORS control center.
The results of data analyzed with the network principles are sent out to the roving stations (Eren et. al., 2008). Correction values are broadcasted from the control center to the users via RTCM3.x. It is well known that conventional RTK techniques are generally used for rovers up to 5-10 km from base stations (Figure 3). In case of network-based CORS, this range can go up to 40-50 km, even longer (Figure 4), thus CORS-TR covering the entire Turkey with RTK services. The reference stations are installed on the ground and on the roof of large but low-story governmental buildings (cadastre offices, meteorological observation stations, universities etc.) from where the technical support can be supplied. .
The three control centers of the CORS-TR system were placed in GDLRC, GCM and IKU for governmental, military and scientific studies, respectively. The reference stations and the control centers have been in use since September 2008. Currently, there are more than 2000 RTK users of the network. . This marks the start of a new era for mapping, cadastre and all geographic information technologies and research in Turkey. 24 hours a day, all across the country users of this system are able to determine their coordinates to within centimeters in real-time or to within millimeters after post-processing.
CORS-TR and Tectonic Studies
It is a well known fact that the Anatolian plate has been squeezed by the Arabian plate towards the Eurasian plate and earthquakes have been great natural hazard in Turkey that threaten the country socially and economically. Hence, it is crucial to have the knowledge in the characteristic and dynamics of the tectonic fault lines to mitigate the earthquake hazard.
This mission seems to be partly accomplished by the outcomes of CORS-TR network. That could even be demonstrated by looking the results of GNSS data process obtained from a short period measurement data. On the other hand, to improve the monitoring capabilities, it is needed to extend the current studies to additional data acquisition and processing techniques.
Earthquake prone countries like Turkey are likely to take precautionary measures in advance to mitigate the results of natural hazards. There are many different study areas: some focus on analyzing the characteristics of strong ground motions, others on developing structural design guidelines, and still others on understanding the plate mechanisms that cause earthquakes.
During recent decades, the studies aimed learning the plate mechanisms accelerated after the introduction of the space-based application tools such as GNSS and radar image acquisition systems. We exhibit here the versatility of the space based monitoring systems and their immediate use in Turkey where the active fault lines and surface deformations (subsidence) exist.
Turkey is located on Anatolian plate where the Arabian, Eurasian and the African plates conjunct (Figure 5). The recent studies showed that the Anatolian plate has been squeezed by Eurasian and Arabian plates due to the global plate tectonics (McClusky et al, 2000). As a result, all around the Anatolian plate, the high slip rates as much as 24mm/year have been observed (Meade et al, 2002).
At the end of November 2010, All CORS-TR data recorded from the stations during November 2008 and October 2010 were processed using the GAMIT/GLOBK program. The results of the processing displayed that the CORS-TR network has the great potential of displaying the tectonic movement of the Anatolian plate as seen in Figure 6.
The GAMIT/GLOBK process of 147 CORS stations has already represented the motion of the Anatolian plate with respect to the Eurasian plate. The maximum displacement estimated is 19.9 mm/year (southwest direction) and 23.2 mm/year (northwest direction) at FINI (Antalya) and UDER (Erzurum) stations, respectively. Keeping in mind that several researchers have commented that the Anatolian Plate has a slip rate of 24 mm/year after studying many years of data, the good result from processing 24 months of CORS-TR observations encourages the reliability, stability and effectiveness of such a young network.
Istanbul – Supersite
The “Geohazard Supersite” Initiative was launched in order to “stimulate an international effort to study selected sites by establishing open access to relevant datasets according to GEO principles fostering the collaboration between all partners and end-users (Geo Group on Earth Observations, 2010)”. The following cities were selected as Geohazard Supersites: Tokyo (Japan). Los Angeles (USA), Vancouver-Seattle (Canada-USA), and Istanbul (Turkey).
Super sites data set consists of geodetic, geophysical, geological, seismic and chemical data (Figure 7). Data set In Istanbul, the priority shall be given to GNSS surveys and InSAR techniques.
Synthetic Aperture Radar (SAR) images and their interferometric applications have been complementary to very specific scientific studies like tectonic, volcanic, subsidence etc.
SAR interferometric techniques, which uses images acquired with two repeat passes over the same scene, are being used increasingly to monitor landslides and vertical deformation.
Even if the GNSS data at points where the ground deformation exist provide information, it is not possible to estimate the ground deformation surface of the area. That is due to the discrete scattering of the observation points; hence, additional data acquisition techniques are needed. At this point, the use of InSAR would yield the surface deformation results.
The results of the InSAR process provide the relative deformation values those are due to either seismic excitation or subsidence. CORS-TR stations located throughout Turkey and northern Cyprus yield the precise coordinates to be used in remote sensing studies, particularly InSAR studies by providing the offset and trend.
The use of the InSAR results calibrated with the GNSS observations would solve many problematic cases in Turkey (Uzel et al., 2010). Owing to reasons outlined above, Istanbul Kultur University in corporation with Istanbul Technical and Bogazici Universities has taken part in the Supersites initiative that was started by the European Space Agency (ESA), NASA and the National Science Foundation (NSF). The initiative promotes the Earth science data from its participants to be used in InSAR and GNSS crustal deformation measurements studies.
Therefore, besides the processing of the GNSS data from CORS-TR network, in parallel, there is an on-going study to establish an additional monitoring mechanism that uses InSAR technique in Turkey. The new study concentrates not only in the earthquake prone Istanbul area but also in the Konya region where distinctive subsidence occurs.
CORS-TR was established in 2009 in a country where seismic activity is high due to tectonic dynamics. The network consists of 147 reference stations and covers the whole Turkey (including Anatolian Plate) and the northern part of Cyprus.
The GNSS observation campaigns provided valuable information on tectonic plate movement in Turkey.
CORS-TR, together with other techniques such as InSAR, shall contribute to studies regarding tectonic movements, disaster management and early warnings.
CORS-TR may be integrated with the CORS networks of neighboring countries in order to give a better understanding of the characteristics of the Eurasian, African, Arabian plates as well as the Anatolian plate and northern Cyprus.
Participation to supersite study in Istanbul shall extend the monitoring of plate tectonics and subsidence from discrete observation points to wide areas in the region, using InSAR and other techniques.
Turgut Uzel, Ph.D., Professor in Civil Engineering Department
Kamil Eren, Ph.D., Professor, Director of GEOMER in Istanbul Kultur University
Ahmet A. Dindar, Ph.D., Researcher, GEOMER
More information http://cors-tr.iku.edu.tr
Brownjohn, J.M.W., Rizos, C., Tan G.H., Pan T.C. (2004). "Real-time long-term monitoring and static and dynamic displacements of an office tower, combining RTK GPS and accelerometer data." 1st FIG Int. Symp. on Engineering Surveys for Construction Works & Structural Eng., Nottingham, U.K., 28 June - 1 July, paper TS1.4, CD-ROM procs.
Eren K., Uzel T., Gulal E., Yildirim O., Cingoz O (2009), “Results from a Comprehensive Global Navigation Satellite System Test in the CORS-TR Network: Case Study”, JOURNAL OF SURVEYING ENGINEERING © ASCE / FEBRUARY 2009
Eren K., Uzel T. (2009). “CORS-TR Project (in Turkish)”, Final Report, Istanbul Kultur University, Istanbul, Turkey.
Geo Group on Earth Observations, (2010), “The Geohazard Supersites, White Paper and Implementation Plan”, Third Draft, FA, 10 June 2010
Grejner-Brzezinska, D.A., Kashani, I. Wielgosz, P., Smith, D.A., Spencer, P.S.J., Robertson, D.S., and Mader, G.L. (2007). “Efficiency and reliability of ambiguity resolution in network-based real-time Kinematic GPS.”, J. Surv. Eng., 133(2), 56-65.
Kempe, C., Alfredsson A., Engberg, L. E., and Lilje, M. (2006). "Correction Model to Rectify Distorted Co-ordinate Systems", XXIII FIG Congress, Munich, Germany, October 8-13, 10p.
Kinlyside, D. (2006). Sydney Project, Land & Property Information Division Department of Lands, Bathurst NSW 2795, Australia.
Lachapelle, G, Fortes L.P., Cannon, M.E., Alves, P., Townsend, B. (2001). "RTK Accuracy Enhancements with a Reference Network−Based Approach." The Third International Symposium on Mobile Mapping Technology, Cairo, Egypt, January 3-5, CD-ROM.
McClusky, S., S. Bassalanian, A. Barka, C. Demir, S. Ergintav, I. Georgiev, O. Gurkan, M. Hamburger, K. Hurst, H.-G. Hans-Gert, K. Karstens, G. Kekelidze, R. King, V. Kotzev, O. Lenk, S. Mahmoud, A. Mishin, M. Nadariya, A. Ouzounis, D. Paradissis, Y. Peter, M. Prilepin, R. Relinger, I. Sanli, H. Seeger, A. Tealeb, M.N. Toksoz, G. Veis, (2002), “Global Positioning system constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus”, Journal of Geophysical Research, 105 (B3), pp. 5695-5719.
Meade, B.J., Hager B.H., McClusky, S., Reilinger, R., Ergintav, S., Lenk, O., Barka, A., Ozener, H., (2002), “Estimates of Seismic Potential in the Marmara Sea Region from Block Models of Secular Deformation Constrained by GPS Measurements”, Bulletin of the Seismological Society of America, Volume 92, Number 1, pp. 208-215. Rizos, C. (2007). "Alternative to current GPS-RTK services and some implications for CORS infrastructure and operations," GPS Solut., 11(3):151-158. Uzel T., Eren K., Dindar A. (2010), “Monitoring Plate Tectonics and Subsidence in Turkey by CORS-TR and InSAR”, FIG Congress 2010, Facing the Challenges – Building the Capacity, Sydney, Australia, 11-16 April 2010