Browsing by Author "Toker, C."
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Item Open Access Application of modified particle swarm optimization method for parameter extraction of 2-D TEC mapping(EGU, 2012-04) Toker, C.; Gökdağ, Y. E.; Arıkan, F.; Arıkan, OrhanIonosphere is a very important part of Space Weather. Modeling and monitoring of ionospheric variability is a major part of satellite communication, navigation and positioning systems. Total Electron Content (TEC), which is defined as the line integral of the electron density along a ray path, is one of the parameters to investigate the ionospheric variability. Dual-frequency GPS receivers, with their world wide availability and efficiency in TEC estimation, have become a major source of global and regional TEC modeling. When Global Ionospheric Maps (GIM) of International GPS Service (IGS) centers (http://iono.jpl.nasa.gov/gim.html) are investigated, it can be observed that regional ionosphere along the midlatitude regions can be modeled as a constant, linear or a quadratic surface. Globally, especially around the magnetic equator, the TEC surfaces resemble twisted and dispersed single centered or double centered Gaussian functions. Particle Swarm Optimization (PSO) proved itself as a fast converging and an effective optimization tool in various diverse fields. Yet, in order to apply this optimization technique into TEC modeling, the method has to be modified for higher efficiency and accuracy in extraction of geophysical parameters such as model parameters of TEC surfaces. In this study, a modified PSO (mPSO) method is applied to regional and global synthetic TEC surfaces. The synthetic surfaces that represent the trend and small scale variability of various ionospheric states are necessary to compare the performance of mPSO over number of iterations, accuracy in parameter estimation and overall surface reconstruction. The Cramer-Rao bounds for each surface type and model are also investigated and performance of mPSO are tested with respect to these bounds. For global models, the sample points that are used in optimization are obtained using IGS receiver network. For regional TEC models, regional networks such as Turkish National Permanent GPS Network (TNPGN-Active) receiver sites are used. The regional TEC models are grouped into constant (one parameter), linear (two parameters), and quadratic (six parameters) surfaces which are functions of latitude and longitude. Global models require seven parameters for single centered Gaussian and 13 parameters for double centered Gaussian function. The error criterion is the normalized percentage error for both the surface and the parameters. It is observed that mPSO is very successful in parameter extraction of various regional and global models. The normalized reconstruction error varies from 10-4 for constant surfaces to 10-3 for quadratic surfaces in regional models, sampled with regional networks. Even for the cases of a severe geomagnetic storm that affects measurements globally, with IGS network, the reconstruction error is on the order of 10-1 even though individual parameters have higher normalized errors. The modified PSO technique proved itself to be a useful tool for parameter extraction of more complicated TEC models.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 KD haberleşme için iyonkürenin plazma frekansı -yükseklik profilinin matematiksel modellenmesi(IEEE, 2014-04) Toker, C.; Arıkan F.; Arıkan, Orhanİyonkürenin plazma frekansı - yükseklik profili özellikle gökdalgası ile yapılan KD haberleşmesini önemli derecede etkilemektedir. Bu çalışmada, belirli bir coğrafik bölge üzerinden bulunan bu profilin, en küçük kareler yöntemi ile doğrusal ve küresel düzlem modellerine oturtulmakta incelenmiştir. IRI-PlasG ile gerçeğe yakın profiller elde edilerek, bahsi geçen modellerin Ankara çevresindeki bir bölge için bu profillere uygunluğu gösterilmektedirItem Open Access Parameter estimation for synthetic TEC surfaces by using Particle Swarm Optimization(IEEE, 2012) Gökdaǧ, Y.E.; Arikan F.; Toker, C.; Arıkan, OrhanIn this study, parameter estimation is made for global ionospheric Total Electron Content (TEC) on both noiseless and noisy synthetic surfaces by using modified Particle Swarm Optimization (PSO). In addition, the improvements made in the PSO algorithm to obtain better results are presented. Trend functions that best regionally and globally represent the quiet and distorted ionosphere are given. For noisy trend surfaces, additive white Gaussian noise is added on trend surfaces according to latitude. International GPS System stations (IGS) are used for regional sampling whereas TNPGN-Active stations are used for both regional and global sampling. A brief discussion of PSO and its improvements for modified PSO is provided. Performance and error criterias are determined for the results of noisy and noiseless dual-core Gaussian trend surfaces. © 2012 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.