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Near real-time one-dimensional vertical electron density profiles are determined from GPS-derived total electron content (TEC) data by means of the iterative conjugate gradient projection method (CGP). Electron density profiles are determined in near realtime (within minutes of the time of measurement) from short time series of slant TEC (STEC) approximately 5 minutes. Measured STEC values are obtained from dual frequency data from a single GPS satellite at a single dual frequency receiver station. Both code-based TEC derived from the P-observable (Ptec) and phase-based TEC derived from the carrier phase observable (Ltec) are used in the solution. The CGP method addresses the ill-posed inverse problem of determining the electron density profiles from TEC measurements through the application of a side constraint to the acceptable solution. This is an iterative method which approximates the solution of a least squares problem through a converging sequence of solutions. The accuracy of the results is verified by comparison to electron density determined from the ionograms measured with Digisondes (Pushkov Institute of Terrestrial Magnetizm, Ionosphere and Radio Wave Propagation, Russian Academy of Science) located at Troizk, Moscow region (55.5N, 37.3E). The results of a hardware-software complex intended for monitoring the Earth's ionosphere according to navigation satellite systems are presented. The anomalous behavior of the critical frequency of the F2-layer ionosphere at latitudes 57-59 degrees observed in December 2014 is detected.
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