A three-dimensional time-dependent algorithm for ionospheric imaging using GPS

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