U. Sezen

Hacettepe University, Engüri, Ankara, Turkey

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Publications (23)8.68 Total impact

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    ABSTRACT: [1] Slant Total Electron Content (STEC), the total number of free electrons on a ray path, is an important space weather observable. STEC is the main input for Computerized Ionospheric Tomography (CIT). STEC can be estimated using the dual-frequency GPS receivers. GPS-STEC contains the space weather variability, yet the estimates are prone to measurement and instrument errors that are not related to the physical structure of the ionosphere. International Reference Ionosphere Extended to Plasmasphere (IRI-Plas) is the international standard climatic model of ionosphere and plasmasphere, providing vertical electron density profiles for a desired date, time and location. IRI-Plas is used as the background model in CIT. Computation of STEC from IRI-Plas is a tedious task for researchers due to extensive geodetic calculations and IRI-Plas runs. In this study, IONOLAB group introduces a new space weather service to facilitate the computation of STEC from IRI-Plas (IRI-Plas-STEC) at www.ionolab.org. The IRI-Plas-STEC can be computed online for a desired location, date, hour, elevation and azimuth angle. The user-friendly interface also provides means for computation of IRI-STEC for a desired location and date to indicate the variability in hour of the day, elevation or azimuth angles. The desired location can be chosen as a GPS receiver in IGS or EUREF networks. Also instead of specifying elevation and azimuth angles, the user can directly choose from the GPS satellites and obtain IRI-Plas-STEC for a desired date and/or hour. The computed IRI-Plas-STEC values are presented directly on the screen or via email as both text and plots.
    Space Weather 01/2014; · 1.85 Impact Factor
  • IEEE Antennas and Propagation Magazine 10/2013; 61(10):5264-5273. · 1.18 Impact Factor
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    ABSTRACT: [1] The variability of space weather can best be captured using total electron content (TEC), which corresponds to total number of electrons on a ray path. The dual-frequency ground based GPS receivers provide a cost-effective means for monitoring TEC. Computation of TEC for a single GPS station is a challenge due to various unknowns and ambiguities such as inter-frequency receiver bias and satellite bias, choice of mapping function, and peak height of ionosphere for ionospheric piercing point. In this study, IONOLAB group introduces a robust, automatic, online computation routine near-real time TEC, IONOLAB-TEC, for IGS and/or EUREF stations from www.ionolab.org. The user can choose online one station or multiple stations, date or dates for the computation. The IONOLAB-TEC values can be compared with TEC estimates from IGS analysis centers. The output can be obtained either in graphical form, or IONOLAB-TEC estimates can be provided in an excel file. The service is easy to use with a graphical user interface. This unique and original space weather application is provided online, and IONOLAB-TEC estimates are downloaded automatically to the user defined directories under user defined filenames.
    Space Weather 05/2013; 11(5). · 1.85 Impact Factor
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    ABSTRACT: Ionosphere, which plays an important role in both High Frequency and satellite communication, can be described by Total Electron Content (TEC), and critical layer parameters in frequency (fo) and peak height (hm). International Reference Ionosphere Extended to Plasmasphere (IRI-Plas), is a convenient empirical deterministic model of ionosphere and plasmasphere. In this study, IRI-Plas is modified through an optimization algorithm, where hourly Global Ionospheric Maps (GIM), are used as the control variable. The optimization algorithm, IRI-Plas-Opt, minimizes the difference between the GIM-TEC and the IRI-Plas TEC with Non-linear Least SQuare (NLSQ) and changes F2 layer critical frequency and peak height values that are input to IRI-Plas. The optimization algorithm is performed on all grid points of the hourly GIM and high resolution ionospheric parameters are obtained. The globals maps of IRI-Plas-Opt reflect the current state of ionospheric parameters and therefore enables the investigation of ionosphere and plasmasphere during a geomagnetic storm.
    Recent Advances in Space Technologies (RAST), 2013 6th International Conference on; 01/2013
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    ABSTRACT: Ionosphere plays an important role in High Frequency (HF) communications. In this paper, Global Ionospheric Maps (GIM) of Total Electron Content (TEC), maximum ionization height (hmF2) and critical frequency (foF2) of F2-layer are presented. These maps are obtained from the Nonlinear Least Squares (NLSQ) optimization of the International Reference Ionosphere (IRI) model extended to the Plasmasphere (IRI-Plas) using the one-hour resolution GIM-TEC (UHR) maps provided by the Polytechnical University of Catalonia (UPC) as the reference input.
    Information Fusion (FUSION), 2013 16th International Conference on; 01/2013
  • 07/2012;
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    ABSTRACT: In recent years, a strong coupling between ionospheric disturbances and seismic activity has been observed through the increase of ion temperatures, critical frequencies of ionospheric layers and Total Electron Content (TEC) before high magnitude earthquakes. TEC is defined as total number of electrons over a ray path through the ionosphere. TEC can be estimated in a cost-effective way with dual-frequency Global Positioning Satellite (GPS) System receivers. The unit of TEC is given by TECU where 1 TECU = 1016 el/m2. In this study, the disturbances in daily TEC values before 23 October 2011, Mw=7.2 Earthquake in Van, Turkey are investigated using Turkish National Permanent GPS Network (TNPGN-Active). Earthquake Day Period (EDP) is chosen between October 1 and 31, 2011. Daily TEC values, for each station and each day, are estimated as IONOLAB-TEC (www.ionolab.org ) with 30 s time resolution. EDP-TEC values are compared with an Average Quiet Day TEC (AQDT) which is obtained by averaging the TEC values between 25 and 28 March, 2011. Statistical comparison is accomplished using Symmetric Kullback-Leibler Divergence (SKLD), which is also a method for measuring entropy of a system. It has been previously observed that SKLD is a better method for measuring the amount of disturbances compared to L2 norm and cross-correlation coefficient. AQDT is also compared with magnetically Quiet Day Period (QDP) from 25 to 28 April, 2011, during which Kp and Dst indices indicate a very quiet ionospheric and magnetospheric period. Also, in order to measure the variability between the consecutive days, TEC values for each day during EDP and QDP are compared with the TEC values of the following day. A third measure of W-index is also applied to identify the local disturbances in the ionosphere, where TEC of a given day is compared to the median of seven days prior to the day of investigation logarithmically. Since W-index is obtained for each epoch, the within-the-day variability can also be monitored. It has been observed that peak TEC values for all stations in TNPGN increase 10 to 15 TECU two days prior to the earthquake. The SKLD values for comparison of EDP and AQDT also peak on 21st of October, 2011, two days prior to the earthquake. Since ionospheric disturbance can be observed on all days prior to the earthquake, comparison of TEC for consecutive days for each station using SKLD does not provide extra information. The W-index values indicate that there may be small scale variability for stations closer to the earthquake epicenter. When compared with previous earthquakes that occurred in Turkey with magnitudes 4.5 and 5.2 on Richter scale, this 7.2 magnitude earthquake has been felt as an ionospheric disturbance for stations especially on Northern Anatolian Fault. The results also indicate the need for constant monitoring and statistical decision theory for detection of earthquake precursors. This study is supported by TUBITAK EEEAG Grant 109E055.
    04/2012;
  • U. Sezen, F. Arikan
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    ABSTRACT: Accurate and reliable estimation of ionospheric parameters are very important for correct functioning of communication, navigation and positioning satellite systems. In recent years, dual-frequency GPS receivers are widely used for estimation of Total Electron Content (TEC), which is defined as the line integral of the electron density along a ray path. Since both electron density and TEC are functions of solar, geomagnetic, gravitational and seismic activity, any disturbance along the ray path can be detected using GPS receiver observables. It is observed that, with the development of recent sophisticated receivers, disruptions due to the receiver antenna, hardware or outside obstructions are minimized. Most of the observed sudden disturbances are signal phase lock losses due to ionosphere. These sudden phase shifts are named as cycle slips and if not corrected, they may lead to positioning errors or incorrect TEC estimates. There are many methods in the literature that deal with cycle slips and their repairs, yet these methods are not matured to detect all kinds of cycle slips. Most algorithms require double differencing, and/or complicated Kalman Filters, Wavelet transforms, Neural Network models, and integration of external INS systems. In this study, we propose a fast and efficient algorithm for identifying the cycle slips on individual observables, classifying them for future investigations and finally repairing them for more accurate and reliable TEC estimates. The algorithm traces the pseudorange and phase observables and computes the geometry free combinations of L4 and P4. The sudden disturbances on L1, L2, P1, C1 and P2 are classified and noted for further use. Most of the cases, the disruptions are on phase observables, yet for a few occasions, a sudden disturbance is also observed on pseudorange observables. The algorithm, then, checks the epoch section where P4 exists continually. When a disruption on L1 or L2 occurs, it becomes evident on L4. When P4 and L4 sections are compared with each other, with the use of a common base, the sudden disruptions up to three epochs can be corrected using second order interpolation. For disruptions that continue for more than three epochs are considered to be separate sections and treated within that epoch section. Any cycle slip occuring within an epoch section is corrected efficiently using thresholds based on cumulative mean of the derivatives. With the efficient repair of cycle slips, Slant TEC (STEC) values can be reliably estimated, and by categorizing the cycle slips with respect to the observables, satellite and epoch, the reason of disturbance can be identified. The cycle slip detection and repair algorithm is incorporated into the web version of IONOLAB-TEC and they can be reached presently from the site www.ionolab.org. This study is supported by TUBITAK EEEAG under Grant No: 109E055.
    04/2012;
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    ABSTRACT: Collaboration patterns of scholars have been the subject of many studies. This paper investigates the collaboration patterns of the Turkish scholars’publications within the citation indexes. Turkey’s contribution to the world’s scientifi c literature has increased signifi cantly during the recent years. It is important to understand the collaboration types in scholarly communication in order to derive a legitimate scientifi c publication policy in Turkey. In this context, the following research questions have been addressed: 1. Is the multiple authorship prevalent in the Turkish publications? 2. Does the collaboration rate change by year? 3. What is the distribution of collaboration types (intranational/international) authored by Turkish scholars? 4. Does the rate and type of collaboration diff er across the disciplines? 5. Which countries are the most important collaborative partners of Turkish scholars? Based on the analysis of fi ndings, we found that Turkish scholars generally collaborate intranationally.
    CollNet Journal of Scientometrics and Information Management. 01/2012;
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    ABSTRACT: On 23 October 2011, a very strong earthquake with a magnitude of Mw = 7.2 shook Eastern Anatolia, and tremors were felt up to 500 km from the epicentre. In this study, we present an early analysis of ionospheric disturbance due to this earthquake using Global Positioning Satellite-Total Electron Content (GPS-TEC). The variability with respect to average quiet day TEC (AQDT) and variability between the consecutive days are measured with symmetric Kullback–Leibler divergence (SKLD). A significant variability in total electron content (TEC) is observed from the GPS stations in the 150 km neighbourhood of the epicentre eight and nine days prior to the earthquake. An ionospheric disturbance is observed from GPS stations even more than 1,000 km to the epicentre, especially those on the North Anatolian fault (NAF). The present results support the existence of lithosphere–atmosphere–ionosphere coupling (LAIC) associated with Van, Turkey earthquake.
    Geomatics, Natural Hazards and Risk. 01/2012; 3(1):1-8.
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    ABSTRACT: In this study, the relation of the maximum ionization height (HmF2) and the critical frequency (FoF2) of F2 layer is examined within their parametric range through the International Reference Ionosphere extended towards the plasmasphere (IRI-Plas) model and the IONOLAB-TEC. HmF2 and FoF2 are optimized using an iterational loop through Non-Linear Least Squares method. HmF2 and FoF2 are obtained for various locations including Turkey for the same quiet day. Results are compared with ionosonde data where available. This study enables the modification and update of empirical and deterministic IRI Model to include instantaneous variability of the ionosphere.
    General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
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    ABSTRACT: Characterization and constant monitoring of variability of the ionosphere is of utmost importance for the performance improvement of HF communication, Satellite communication, navigation and guidance systems, Low Earth Orbit (LEO) satellite systems, Space Craft exit and entry into the atmosphere and space weather. Turkish National Permanent GPS Network (TNPGN) is the Reference Station Network of 146 continuously-operating GNSS stations of which are distributed uniformly across Turkey and North Cyprus Turkish Republic since May 2009. IONOLAB group is currently investigating new techniques for space-time interpolation, and automatic mapping of TEC through a TUBITAK research grant. It is utmost importance to develop regional stochastic models for correction of ionospheric delay in geodetic systems and also form a scientific basis for communication link characterization. This study is a brief summary of the efforts of IONOLAB group in monitoring of space weather, and correction of geodetic positioning errors due to ionosphere using TNPGN.
    Recent Advances in Space Technologies (RAST), 2011 5th International Conference on; 07/2011
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    ABSTRACT: Turkish National Permanent GPS Network (TNPGN) is the Reference Station Network of 146 continuouslyoperating GNSS stations o which are distributed uniformly across Turkey and North Cyprus Turkish Republic since May 2009. IONOLAB group, formed by researchers and students in Hacettepe University, Bilkent University and General Command of Mapping is currently investigating new techniques for space-time interpolation, and automatic mapping of TEC through a TUBITAK research grant. This study presents the developments in monitoring of space weather, and correction of geodetic positioning errors due to ionosphere using TNPGN.
    01/2011;
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    ABSTRACT: In this study, the relation of the maximum ionization height (HmF2) and the critical frequency (FoF2) of F2 layer is examined within their parametric range through the International Reference Ionosphere extended towards the plasmasphere (IRI-Plas) model and the IONOLAB-TEC (Total Electron Content) observations. HmF2 and FoF2 are optimized using an iterational loop through Non-Linear Least Squares method by also using a physical relation constraint between these two parameters. Performance evaluation of optimization algorithm is performed separately for the cases running IRI-Plas with optimized parameters and TEC input; only with optimized parameters; only with TEC and finally with no optimized parameter and TEC input. As a conclusion, it is seen that using optimized parameters and TEC together as input produces best IRI-TEC estimates. But also using only optimized parameters (without TEC update) gives estimates with also very low RMS errors and is suitable to use in optimizations. HmF2 and FoF2 estimates are obtained separately for a quiet day, positively corrupted day, negatively corrupted day, a northern latitude and a southern latitude. HmF2 and FoF2 estimation results are compared with ionosonde data where available. This study enables the modification and update of empirical and deterministic IRI Model to include instantaneous variability of the ionosphere.
    01/2011;
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    ABSTRACT: We know that F2 layer of the ionosphere is most important layer in the progaration of high frequency (HF) waves. In this study, The relation of the height (HmF2) and the critical frequency (FoF2) of F2 layer—among the parameters of the Internation Reference Ionesphere (IRI) model—to the Total Electron Content (TEC) structure of ionosphere is investigated within their defined parametric range. These two parameters are then optimized using IONOLAB TEC estimations. Performance of the optimization algorithm is examined seperately for the cases of processing daily (24-hour) and hourly TEC data. It is observed that using hourly data produce results with much smaller errors. By using this optimization method, the height and the critical frequency of F2 layer are obtained for countries located on low and high latitudes including Turkey for the same quite day. Results are compared with ionosonde data and it is observed that error norms were in an acceptable range. By this way it is attained the more realistic electrical structure of ionosphere. 1. GİRİŞ
    01/2011;
  • U. Sezen
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    ABSTRACT: In this paper, perfect reconstruction polyphase infinite impulse response (IIR) filter banks involving causal and anticausal inverses are examined for finite-length signals. First, a novel and efficient nonexpansive perfect reconstruction algorithm based on the state-space implementation is presented. Then the proposed method is extended to support linear signal extensions at the boundaries in a nonexpansive manner. The powerfulness of the proposed algorithm is demonstrated with the image compression results.
    IEEE Transactions on Signal Processing 07/2009; · 2.81 Impact Factor
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    ABSTRACT: Space Weather (SW) is the concept of changing environmental conditions in outer space and affect Earth and its technological systems. SW is a consequence of the solar activities and the coupling of solar energy on Earth's atmosphere due to the Earth's magnetic field. The monitoring and prediction of SW has utmost importance for HF communication, Satellite communication, navigation and guidance systems, Low Earth Orbit (LEO) satellite systems, Space Craft exit and entry into the atmosphere. Ionosphere is the plasma layer of the atmosphere that is ionized by solar radiation and it is a key player of SW. Ionosphere is a temporally and spatially varying, dispersive, anisotropic and inhomogeneous medium that is characterized primarily by its electron density distribution. IONOLAB is a group of researchers of various disciplines, getting together to handle challenges of the Earth's ionosphere. The team has researchers from Hacettepe University and Bilkent University, Department of Electrical and Electronics Engineering and General Command of Mapping of Turkish Army. One of the most important contributions of IONOLAB group is the automated web-based computation service for Total Electron Content (TEC). TEC corresponds to the line integral of electron density distribution on a given path. TEC can also be expressed as the amount of free electrons within 1 m2 cross-sectional area of the cylinder on the ray path. Global Position System (GPS) provides a cost-effective medium for monitoring of ionosphere using the signals recorded by stationary GPS receivers in estimating TEC. IONOLAB group has developed IONOLAB-TEC for reliable and robust estimates for all latitudes and both calm and disturbed days by using RINEX, IONEX and satellite ephemeris data provided from the IGS centers. IONOLAB-TEC consists of a regularized signal estimation algorithm which combines signals from all GPS satellites for a given instant and a given receiver, for a desired time period or for 24 hours, with 30 s time resolution. IONOLAB-TEC values also include the receiver differential code bias (DCB) for each GPS station estimated uniquely by the IONOLAB-BIAS algorithm. The web based computation program is written in JAVA and it is provided both in Turkish and English at www.ionolab.org. The IONOLAB-TEC computation requires no installation or licensing on the client side. The application has a layered design. Developed components are modular that allows possible changes regarding the estimation method can be easily adapted. Same flexibility is also provided for the data access. Also, presentation of estimation data is architected to support different client types. Currently, the user can login to the IONOLAB-TEC web site and choose the desired location and dates on-line for TEC estimation. The carrier phase leveled TEC estimates of IONOLAB-TEC are provided for the chosen station/s and for the chosen day/s along with two-hourly GIM-TEC estimates of IGS centers. The output is provided in the user designated form either in graphs or an excel data sheet. The IONOLAB-TEC provides robust, reliable, and high resolution TEC estimates and provides a medium for comparison of the GIM-TEC values from the IGS centers.
    03/2009; 11:5188.
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    ABSTRACT: Dual-frequency Global Positioning System (GPS) receivers present a plausible and cost-effective way of computing Total Electron Content (TEC). For accurate estimates of TEC, frequency-dependent satellite and receiver instrumental biases should be removed from GPS measurements properly. Although instrumental satellite bias values are widely available through the internet from various International GPS Service (IGS) analysis centers, receiver biases (also known as differential code biases or interfrequency biases) are provided only for a very few GPS stations and a select number of days. This makes it very difficult to compute TEC for a single station. In this study, an online, single station receiver bias estimation algorithm, IONOLAB-BIAS, is developed and implemented to obtain daily and monthly averages of receiver bias. The algorithm is successfully applied to both quiet and disturbed days of the ionosphere for stations positioned in high-latitude, midlatitude, and equatorial regions. The receiver bias estimates are compared with two of the basic methods in the literature that can be applied off-line, and also with the receiver bias values provided from the IGS centers for a select number of stations. It is observed that IONOLAB-BIAS is in excellent accordance with the sparse estimates from the IGS centers for all ionospheric states and regions. IONOLAB-BIAS has a high potential to be an alternative receiver bias computation algorithm with its ease of implementation and accurate estimates for any single station GPS-TEC.
    Radio Science 01/2008; 43(4). · 1.00 Impact Factor
  • O. Ugurlu, U. Sezen, A.Z. Alkar
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    ABSTRACT: Total Electron Content (TEC), is widely used in monitoring ionospheric effects. TEC is expressed as the amount of free electrons within 1 m<sup>2</sup> cross-sectional area of the region between ground and ionosphere. Although IRI and IGS analysis centers can generate and present TEC estimations, either the temporal resolution is low or estimations are based on empirical data. IONOLAB (www.ionolab.org) method provides robust estimations with high temporal resolution. In this study, TEC estimations by IONOLAB are presented by an easily accessible, user friendly interface. Developed application makes TEC estimation for given GPS station and time period. User input required is minimal. Observation data needed for estimation is retrieved from IGS data centers in RENEX format. Hardware biases are automatically downloaded from CODE. Developed application is the first one that provides TEC estimations with a temporal resolution of 30 s. Web based IONOLAB is an easily accessible system that requires no installation on the client side. The application has a layered design. By means of this modular design, possible changes regarding the estimation method can be easily adapted. Same flexibility is also provided for the data access. The presentation of estimation data is architected to support different client types. Estimation data can be presented in different output media.
    Recent Advances in Space Technologies, 2007. RAST '07. 3rd International Conference on; 07/2007
  • O. Ugurlu, Umut Sezen, I. Sayin
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    ABSTRACT: Because of the effects on radio waves, monitoring the ionosphere gains significant importance. Monitoring of the ionosphere becomes possible by estimating Total Electron Content (TEC) which is the major variable expressing ionospheric effects. Although International Reference Ionosphere (IRI) and International GNSS Service (IGS) analysis centers can generate TEC estimates, these estimates either have low temporal resolution or are based on emprical data. IONOLAB (www.ionolab.org) TEC estimation method provides robust estimations with high temporal resolution compared to IGS analysis centers. In this work, robust TEC estimations with high temporal resolution generated by IONOLAB method are presented by an easily accessible, user-friendly application. The application has a layered modular design. By means of this modular design, possible changes regarding the estimation method can be easily adapted.
    01/2007;