Umut Sezen

Hacettepe University, Engüri, Ankara, Turkey

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Publications (28)12.84 Total impact

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    ABSTRACT: Global and regional behavior of the ionosphere is an important component of space weather. The peak height and critical frequency of ionospheric layer for the maximum ionization, namely, hmF2 and foF2, and the total number of electrons on a ray path, Total Electron Content (TEC), are the most investigated and monitored values of ionosphere in capturing and observing ionospheric variability. Typically ionospheric models such as International Reference Ionosphere (IRI) can provide electron density profile, critical parameters of ionospheric layers and Ionospheric electron content for a given location, date and time. Yet, IRI model is limited by only foF2 STORM option in reflecting the dynamics of ionospheric/plasmaspheric/geomagnetic storms. Global Ionospheric Maps (GIM) are provided by IGS analysis centers for global TEC distribution estimated from ground-based GPS stations that can capture the actual dynamics of ionosphere and plasmasphere, but this service is not available for other ionospheric observables. In this study, a unique and original space weather service is introduced as IRI-PLAS-MAP from www.ionolab.org. IRI-PLAS-MAP assimilates the GIM-TEC values into IRI Extended to Plasmasphere (IRI-Plas) model for a user defined region, date and time period, thus integrating the ionospheric dynamics into the model. The user can choose to compute IRI-PLAS-TEC, -foF2, -hmF2 or the W-index (an indicator of ionospheric disturbance) by assimilating either one-hour time resolution UHRG GIM or two-hour time resolution JPL GIM. IRI-PLAS-MAP can provide hmF2, foF2, TEC and W-index maps in both IGS standard IONEX text file format and graphical format. The desired IRI-PLAS-MAPs can be downloaded through the link that is sent to the user e-mail when the files are ready. This unique service allows the users from all disciplines to observe, model and monitor ionospheric parameters of space weather.
    Advances in Space Research 04/2015; 103(8). DOI:10.1016/j.asr.2014.10.016 · 1.35 Impact Factor
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    ABSTRACT: In this study, research activities of IONOLAB group on remote sensing and 2-D imaging of ionosphere in the last 10 years will be summarized. In TUBITAK EEEAG 105E171 and 109E055 projects, a novel Total Electron Content (TEC) estimation method, IONOLAB-TEC, is developed using the dual frequency Global Positioning System (GPS) pseudo range and phase delay recordings. The IONOLAB-TEC computation is provided from www.ionolab.org as an important and unique Space Weather service that can estimate robust, reliable and accurate single station TEC values. The sparse estimates of GPS-TEC in space and time are interpolated regionally and globally and TNPGN-Active IONOLAB-TEC values are mapped using novel space-time interpolation methods. Ionospheric climatic model IRI-Plas is used as a background for fast and robust electron density distributions first time in literature. Critical Ionosperic parameters are interpolated to obtain global and regional distributions. Critical frequency and height maps are provided in www.ionolab.org as another Space Weather Service. The important contributions of IONOLAB group will continue in TUBITAK 112E568 project.
    2014 22nd Signal Processing and Communications Applications Conference (SIU); 04/2014
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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; 12(1). DOI:10.1002/2013SW000998 · 2.22 Impact Factor
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    ABSTRACT: F2-layer is the most important and characteristic layer of the ionosphere in the propagation of high frequency (HF) waves due to the highest level of conductivity in the propagation path. In this study, the relation of Total Electron Content (TEC) with the maximum ionization height (hmF2) and the critical frequency (foF2) of F2-layer are investigated within their defined parametric range using the IRI model extended towards the plasmasphere (IRI-Plas). These two parameters are optimized using daily observed GPS-TEC (IONOLAB-TEC) in an iterational loop through Non-Linear Least Squares (NLSQ) optimization while keeping the physical correlation between hmF2 and foF2 parameters. Optimization performance is examined for daily (24-hour) and hourly TEC optimizations separately. It is observed that hourly TEC optimization produces results with much smaller estimation errors. As a result of the hourly optimization, we obtain the hourly hmF2 and foF2 estimates as they are the optimization parameters. Obtained hmF2 and foF2 estimates are compared with the ionosonde estimates for various low, middle and high latitude locations for both quite and disturbed days of ionosphere. The results show that hmF2 and foF2 estimates obtained from IRI-Plas optimization (IRI-Plas-Opt) and ionosonde are very much in agreement with each other. These results also signify that IRI-Plas provides a reliable background model for ionosphere. With the proposed method, it is possible to build a virtual ionosonde via optimization of IRI-Plas model using the observed TEC values.
    IEEE Antennas and Propagation Magazine 10/2013; 61(10):5264-5273. DOI:10.1109/TAP.2013.2275153 · 1.15 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). DOI:10.1002/swe.20054 · 2.22 Impact Factor
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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
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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: Publication details, including instructions for authors and subscription information:
    Geomatics, Natural Hazards and Risk 11/2012; 3(1):1-8. · 0.62 Impact Factor
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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.
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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.
  • 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.
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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 02/2012; 3(1):1-8. DOI:10.1080/19475705.2011.638027 · 0.62 Impact Factor
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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
  • Gokhan Taskin, Umut Sezen
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    ABSTRACT: ulere uygulanarak g¨ olgesi kaldõrõlmõsg ¨ or¨ unt¨ uler elde edilmisv e sunulmustur. ABSTRACT Shadow can cause problems for image processing algorithms like object tracking and image segmentation. It is observed that, whenever a target enters a shadowed area, object tracking algo- rithms fail. To overcome that kind of problems, various methods are produced to find and remove shadows on images. In this paper a different model, which compensate both contrast and illumination for shadows, is developed. By applying this model on images containing shadows, corresponding shadow-free im- ages are obtained and presented.
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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.
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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
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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.
  • 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; DOI:10.1109/TSP.2009.2016228 · 3.20 Impact Factor

Publication Stats

55 Citations
12.84 Total Impact Points

Institutions

  • 2008–2015
    • Hacettepe University
      • Department of Electrical and Electronics Engineering
      Engüri, Ankara, Turkey
  • 2007
    • Ankara University
      • Department of Electronic Engineering
      Engüri, Ankara, Turkey