Yer Tabanlı Destek Sistemleri İçin Uyarlamalı Bölgesel İyonküre İzleme Yöntemi Geliştirilmesi

Abstract

Ground-Based Augmentation Systems (GBAS) improve positioning precision of GPS receivers on-air platform landing systems. GBAS consists of multiple GPS receivers placed on known locations within a region. A central processing station process other GPS receivers’ data and generate a correction signal. Ionosphere Radio navigation signals are affected from the ionosphere due to its complex structure, such as different delays. These ionospheric delays cause errors up to 40 m in position. Reginal Ionosphere Monitoring Algorithms are used to detect positioning errors and correct them in GBAS. Slant Total Electron Content (STEC) delays are critical parameters for the observation of ionospheric variations in regional ionospheric monitoring algorithms. STEC is estimated using dual-frequency GPS receiver recordings and satellite biases which is crucial observable for electron density in the ionosphere. An ionospheric disturbance is observed as sinusoidal wave-like disturbances or sudden increase in STEC values. Ionospheric gradient is the ratio of difference of slant delays and distance of two close GPS receivers. Safe threshold levels are calculated using ionospheric gradients from GPS receiver pairs which look at exact satellite durations from diurnal to several years. Changes that exceed the threshold level are categorized as ionospheric. However, existing systems cannot estimate and track parameters of the detected disturbances. Therefore, gradient threshold values are fixed according to direction and time. Safe threshold levels dramatically increase the false detection possibilities of ionospheric monitoring algorithms. In addition, monitoring algorithms estimates are valid within 150 km diameter bounded local regions. This study proposes a systematic and stochastic method that can work by covering larger regions in near real-time, which can be input to ionosphere monitoring algorithms to detect disturbances and perform parameter estimations. Firstly, the Differential Rate of TEC (DROT) is computed for GPS receivers in TNPGN-Active Network to analyze temporal variations in regional threats. DROTS are categorized according to their amplitudes, classified as threat categories. Temporal occurrences are investigated. It is observed that there are periodic trends that temporarily extend from 8-9 hours to 15-16 days, whose frequency of occurrence is consistent with the literature. Besides, ionospheric gradients are calculated for each satellite and receiver pair using existing gradient calculation methods in ionospheric monitoring algorithms. Stochastic trends are analyzed using Parametric Probability Density Functions PDFs, estimated for calculated ionospheric gradients for satellite track directions, time intervals, and different receiver distance groups. The maximum value of ionospheric gradients reaches up to 25 mm/km, and gradients depend on space, time, and directions. A very detailed database of disturbances includes spatio-temporal disturbance variations of regional ionosphere of Turkey using ionospheric gradients and DROTS. The first step of ionospheric thread monitoring algorithm development high resolution near real-time STEC maps are obtained by using IONOLAB-SMAP based on Kriging interpolation. Using IONOLAB-SMAP ionospheric STEC maps are estimated within 15-minute intervals in which ionosphere is stationary in a wide sense. The second step is an estimation of disturbance parameters using variational mode decomposition for direction, amplitude, and wavelengths of disturbances. The performance of the developed algorithm is investigated by using synthetic surfaces, which are generated based on the behavior of ionosphere using information estimated from the analyses from ionospheric gradients and DROTS. Furthermore, the developed algorithm is validated using St.Patrick’s Day Storm (17 March 2015), which is reported intense ionospheric storm day in which multiple large-scale disturbances have occurred. Estimates are consistent with the observations reported in the literature using STEC data estimated from the European and Turkish ionosphere. Multiple wave-like disturbances are traveling from polar to equatorial regions are detected and estimated. Besides apparent wavelengths of estimates, disturbances are 32°, 16° and 5-8° in latitude, with different energy levels.