Diffraction Tomography of the Disturbed Ionosphere Based on GPS Scintillation Data

A new inverse diffraction algorithm has been developed to estimate the fine-scale ionospheric electron density variations that cause equatorial scintillation in GPS signals. This work is part of an effort to image the disturbed ionosphere in order to better understand scintillation from a scientific standpoint. The new algorithm relies on a physical model of how electron density irregularities affect received GPS signals in ground-based receivers. The model is based on a thin phase-screen approximation of the ionosphere that accounts for non-normal incidence of the GPS signal. The heart of the model is a modified Huygens-Fresnel integral that is evaluated using FFT-based techniques. The forward model computes the received in-phase and quadrature accumulations as functions of a "frozen", drifting 1-dimensional vertical electron density profile. Inversion of this model is performed using nonlinear least-squares techniques. The final algorithm estimates electron density profiles based on measured accumulations from a dual-frequency GPS receiver. The algorithm has been applied to weak scintillation data. It has demonstrated an ability to fit dual-frequency amplitude and phase data based on a single electron density profile.