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This paper reports on the development of a real-time three-dimensional ionospheric tomography technique using GNSS measurements. GPS measurement data from 200 GPS receivers distributed throughout Japan are analyzed in real time to calculate ionospheric Total Electron Contents (TECs). The three-dimensional ionospheric electron density distribution was reconstructed using the tomography technique from the slant TEC data. The tomography results were found to be in good agreement with both the electron density profiles obtained by radio occultation measurements by the COSMIC satellite and the ionospheric peak densities obtained by vertical radio sounding measurements (ionosondes). The real-time ionospheric tomography system commenced operation in March 2016 and it continuously produces three-dimensional electron density distributions over Japan every 15 min with latency of about 6 min. This system is a very powerful tool for monitoring the ionosphere in real time, which could both improve the performance of GNSS-based systems and further the understanding of ionospheric variations.

3-D ionospheric tomography by using measurements of ground GNSS networks is a powerful tool to investigate ionospheric behavior. Recently, a real-time 3-D ionosphere tomography system has been developed [1] to reconstruct 3-D ionospheric density profiles over Japan. 3-D ionospheric density profiles over Japan estimated by a real-time 3-D tomography system, which is available from March 2016 were used to generate ionospheric correction for single-frequency stand-alone positioning. On both magnetic quiet and disturbed days (14 November 2017 and 25 October 2017, respectively), stand-alone positioning corrected by real-time tomography resulted in mean vertical errors equivalent to those of dual-frequency positioning. The RMS vertical errors of stand-alone positioning corrected by real-time tomography were better than those of dual-frequency positioning and equivalent to those of uncorrected single-frequency positioning. More analysis on data sets at different location, in different seasons, and in different magnetic conditions are planned. Although there is no specific plan, it is also interesting to apply the same method to other locations where GNSS receiver networks with the similar receiver density as used in this study.