Browsing by Author "Sezen, U."
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Item Open Access Determining F2 layer parameters via optimization using IRI model and IONOLAB TEC estimations(IEEE, 2011) Şahin, O.; Sezen, U.; Arıkan, F.; Arıkan, OrhanWe 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.Item Open Access Dynamo equation solution using Finite Volume Method for midlatitude ionosphere(KeAi Communications Co., 2018) Arikan, F.; Sezen, U.; Arıkan, OrhanIonosphere is the layer of atmosphere which plays an important role both in space based navigation, positioning and communication systems and HF signals. The structure of the electron density is a function of spatio-temporal variables. The electrodynamic medium is also influenced with earth's magnetic field, atmospheric chemistry and plasma flow and diffusion under earth's gravitation. Thus, the unified dynamo equation for the ionosphere is a second order partial differential equation for quasi-static electric potential with variable spatial coefficients. In this study, the inhomogeneous and anisotropic nature of ionosphere that can be formulated as a divergence equation is solved numerically using Finite Volume Method for the first time. The ionosphere and the operators are discretized for the midlatitude region and the solution domain is investigated for Dirichlet type boundary conditions that are built in into the diffusion equation. The analysis indicates that FVM can be a powerful tool in obtaining parametric electrostatic potential distribution in ionosphere.Item Open Access Estimation of hmF2 and foF2 communication parameters of ionosphere F2-layer üsing GPS data and IRI-plas model(IEEE, 2013-10) Sezen, U.; Sahin, O.; Arikan, F.; Arıkan, OrhanF2-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.Item Open Access Estimation of single station interfrequency receiver bias using GPS-TEC(Wiley-Blackwell Publishing, Inc., 2008) Arikan, F.; Nayir, H.; Sezen, U.; Arıkan, OrhanDual-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. Copyright 2008 by the American Geophysical Union.Item Open Access Investigation of ionospheric precursors of 23 October 2011 Mw=7.2 earthquake in Van, Turkey(EGU, 2012-04) Deviren, M. N.; Arıkan, F.; Sezen, U.; Arıkan, OrhanIn 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.Item Open Access Ionolab grubunun iyonküre uzaktan algılama ve 2-b görüntüleme çalışmaları(IEEE, 2014-04) Arıkan, F.; Toker, C.; Sezen, U.; Deviren, M. N.; Çilibaş, O.; Arıkan, OrhanBu çalışmada, IONOLAB grubunun son 10 yıldır iyonküre uzaktan algılaması ve 2-B görüntüleme çalışmaları özetlenecektir. TÜBİTAK EEEAG 105E171 ve 109E055 projelerinde, çift frekanslı Yerküresel Konumlama Sistemi (YKS) alıcılarının sözde menzil ve faz gecikmesi kayıtlarından özgün Toplam Elektron İçeriği (TEİ) kestirim yöntemi IONOLAB-TEC geliştirilmiştir. Önemli bir Uzay Havası hizmeti olarak www.ionolab.org sitesinden tüm araştırmacılara açılan IONOLAB-TEC, dünyada ilk ve tek gürbüz, güvenilir ve hassas tek istasyon için TEİ kestirimleri yapabilmektedir. Uzayda ve zamanda seyrek YKS-TEİ kestirimlerinin bölgesel ve yerküresel aradeğerlemesi için çalışmalar yapılmış ve Türkiye üzerindeki TUSAGA-Aktif istasyon ağından IONOLAB-TEC yöntemi ile elde edilen Toplam Elektron İçeriği (TEİ) kestirimleri kullanılarak otomatik yüksek çözünürlüklü 2-B TEİ görüntüleri elde edilmiştir. IRI-Plas iyonküre iklimsel modeli altyapısıyla literatürde ilk kez hızlı ve gürbüz elektron yoğunluğu dağılımları elde edilmiş ve iyonküre model parametreleri özgün aradeğerleme ile birleştirilmiştir. www.ionolab.org sitesinde iyonküre kritik frekans ve yükseklik haritaları sunulmaktadır. IONOLAB grubunun bu önemli katkıları TÜBİTAK EEEAG 112E568 projesi kapsamında devam etmektedir.Item Open Access NVIS HF signal propagation in ionosphere using calculus of variations(KeAi Communications Co., 2018) Sezen, U.; Arikan, F.; Arıkan, OrhanModeling Near Vertical Incidence Sounding (NVIS) High Frequency (HF) signal propagation in the ionosphere is important. Because, ionosondes which are special types of radars probing the ionosphere with certain HF frequencies (between 2 and 30 MHz), work mostly in NVIS mode (where elevation angle is between 89 and 90°). In this work, we are going to propose a new method for NVIS wave propagation in the ionosphere by discretizing the NVIS wave propagation path into mediums in which the refractive index changes linearly, where we solve the ray propagation in each medium analytically using calculus of variations and use Snell's Law at medium changes. The main advantage of the proposed solution is the reduced computational complexity and time. This algorithm can be used to simulate and compare the behavior of vertical ionosondes together with other ray tracing algorithms.Item Open Access Observed Ionospheric Effects of 23 October 2011 Van, Turkey Earthquake(Taylor and Francis, 2012-01-17) Arikan, F.; Deviren, M.N.; Lenk, O.; Sezen, U.; Arıkan, OrhanOn 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. © 2012 Taylor and Francis Group, LLC.Item Open Access Online user‐friendly slant total electron content computation from IRI‐Plas: IRI‐Plas‐STEC(Wiley-Blackwell Publishing, Inc., 2014-01) Tuna, H.; Arıkan, Orhan; Arikan, F.; Gulyaeva, T. L.; Sezen, U.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 International GNSS Service (IGS) or EUREF Permanent Network (EPN). 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 e-mail as both text and plots. Key Points Online, automatic, Slant Total Electron Content is computed as IRI-Plas-TEC IRI-Plas-STEC is available from www.ionolab.org for desired date, hour, and location IRI-Plas-STEC is provided via e-mail in both graphical and text format ©2014. American Geophysical Union. All Rights Reserved.Item Open Access Online, automatic, near-real time estimation of GPS-TEC: IONOLAB-TEC(IEEE, 2013) Sezen, U.; Arikan, F.; Arıkan, Orhan; Ugurlu, O.; Sadeghimorad, A.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. Key PointsOnline, automatic, near real-time GPS-TEC is estimated as IONOLAB-TECIONOLAB-TEC is available from www.ionolab.org for single/multiple stations/daysIONOLAB-TEC is available in graphics and as excel file ©2013. American Geophysical Union. All Rights Reserved.Item Open Access Optimization of F2 layer parameters using IRI-Plas and IONOLAB-TEC(IEEE, 2011) Şahin O.; Sezen, U.; Arikan F.; Arıkan, OrhanIn 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. © 2011 IEEE.Item Open Access Optimization of F2 layer parameters using IRI-Plas model and IONOLAB Total Electron Content(IEEE, 2011) Sahin O.; Sezen, U.; Arikan F.; Arıkan, Orhan; Aktug, B.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. © 2011 IEEE.Item Open Access Space weather activities of IONOLAB group using TNPGN GPS Network(IEEE, 2011) Aktug, B.; Lenk O.; Kurt, M.; Parmaksiz, E.; Ozdemir, S.; Arikan F.; Sezen, U.; Toker, C.; Arıkan, OrhanCharacterization 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. © 2011 IEEE.Item Open Access Space weather studies of IONOLAB group(IEEE, 2016) Arıkan, F.; Sezen, U.; Toker, C.; Artuner, H.; Bulu, G.; Demir, U.; Erdem, E.; Arıkan, Orhan; Tuna, Hakan; Gulyaeva, T. L.; Karatay, S.; Mosna, Z.IONOLAB is an interdisciplinary research group dedicated for handling the challenges of near earth environment on communication, positioning and remote sensing systems. IONOLAB group contributes to the space weather studies by developing state-of-the-art analysis and imaging techniques. On the website of IONOLAB group, www.ionolab.org, four unique space weather services, namely, IONOLAB-TEC, IRI-PLAS-2015, IRI-PLAS-MAP and IRI-PLAS-STEC, are provided in a user friendly graphical interface unit. Newly developed algorithm for ionospheric tomography, IONOLAB-CIT, provides not only 3-D electron density but also tracking of ionospheric state with high reliability and fidelity. The algorithm for ray tracing through ionosphere, IONOLAB-RAY, provides a simulation environment in all communication bands. The background ionosphere is generated in voxels where IRI-Plas electron density is used to obtain refractive index. One unique feature is the possible update of ionospheric state by insertion of Total Electron Content (TEC) values into IRI-Plas. Both ordinary and extraordinary paths can be traced with high ray and low ray scenarios for any desired date, time and transmitter location. 2-D regional interpolation and mapping algorithm, IONOLAB-MAP, is another tool of IONOLAB group where automatic TEC maps with Kriging algorithm are generated from GPS network with high spatio-temporal resolution. IONOLAB group continues its studies in all aspects of ionospheric and plasmaspheric signal propagation, imaging and mapping.Item Open Access Space-time interpolation and automatic mapping of TEC using TNPGN-active(2011-08) Arıkan, F.; Arıkan, Orhan; Sezen, U.; Toker, C.; Aktug, B.; Lenk, O.; Kurt ,M.; Parmaksız, E.Turkish National Permanent GPS Network (TNPGN) is the Reference Station Network of 146 continuously-operating 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. © 2011 IEEE.