Global Positioning System (GPS) satellite receivers provide a world-wide network of phase and group delaymeasurements. The combination of two-frequency measurements can be used to derive the integral of the electronconcentration along each satellite-to-receiver path, a parameter known as the Total Electron Content (TEC). At thisstage these slant TEC data are diffi cult to interpret as they originate from a combination of a temporally changingionosphere and spatially changing observation geometry. In this paper TEC data are inverted to evaluate the underlyingdistribution and time evolution of electron concentration. Accordingly, a new three-dimensional, time-dependentalgorithm is presented here for imaging ionospheric electron concentration using GPS signals. The inversion resultsin a three-dimensional movie rather than a static image of the electron-concentration distribution. The technique isdemonstrated using simulated ground-based GPS data from actual measurement geometry over Europe.
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