Comparison of spherical harmonic and B spline models for the vertical total electron content

Most of the geodetic ionosphere models describe the electron density of the Earth's atmosphere by global maps of the vertical total electron content (VTEC) for fixed time intervals (e.g., 2 h) assuming a single‐layer model and using a spherical harmonic expansion up to a specific degree n . However, it is well known that spherical harmonic models are not suited for representing data of heterogeneous density and quality. As a consequence, data gaps cannot be handled appropriately. In this paper we present a different approach for modeling VTEC generally depending on space and time defining the 3‐D system of base functions as tensor products of trigonometric B spline functions for the longitude and two sets of endpoint‐interpolating B spline functions for latitude and time, respectively. We compare this approach to a spherical harmonic expansion with similar resolution and show how data gaps influence the accuracy of VTEC maps even in areas with good data coverage.