Aim. The widespread use of global navigation satellite systems (GNSS) has led to the development of new methods designed to determine and accumulate the index of ionosphere ionization (VTEC). Using these data it is possible to significantly improve the accuracy and reliability of determining the coordinates of the observation point. Therefore, the task of constructing a model of ionization index is relevant. Method. To construct a spatial model, we used the spherical Legendre functions of the first kind of real order with integer degree as a basic system of functions. We found the magnitude of order using the Sturm-Liouville theory since it depended on the size of the investigated region. Such system of functions form two orthogonal systems of functions in the region under study (the sphere segment), but does not have recurrence relations between functions, therefore, it is necessary to use function expansion in a hypergeometric series to find them. Also in order to find unknown coefficients of the model it is necessary to use the Tikhonov regularization parameter, since the matrix of normal equations will not be stable. For calculating the time model of the ionosphere the coefficients of different spatial models were expanded in series of power polynomials. Results. Based on the data of the ionization parameter values obtained of 19 permanent stations of the ZAKPOS network using the Trimble Pivot Platform software, the spatial-temporal model of this parameter was constructed using the Legendre spherical functions up to the 3rd order as well as with power polynomials up to 3rd order. The standard deviation between the measured and model values of the VTEC parameter does not exceed 1TECU. The scientific novelty and practical significance. We developed algorithm for construction of the space-time model of the ionosphere parameter. A ionosphere model of high resolution is obtained, which can be used to solve geodetic tasks in order to provide the necessary accuracy in determining the coordinates of the point, as well as to study and forecast the space weather.
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