Improving the Abel inversion by adding ground GPS data to LEO radio occultations in ionospheric sounding

GPS radio occultations allow the sounding of the Earth's atmosphere (i.e. t roposphere and ionosphere). The basic observable of this technique is the a dditional delay, due to the refractivity index, of a radio signal when pass ing through the atmosphere. This additional delay is proportional to the in tegrated refractivity, in such a way that we can obtain an estimation of th e vertical refractivity profiles using observations at different elevation angles by solving an inverse problem. Traditionally, the solution of this i nverse problem is obtained by using the Abel inversion algorithm assuming a refractivity index that only depends on the altitude. In this paper we pre sent a modified Abel inversion algorithm for ionospheric sounding that over comes the spherical symmetry assumption of the traditional Abel inversion a lgorithm. Processing a set of simulated data and 1 day of real data with th is algorithm, a clear improvement over the traditional one can be obtained when comparing the derived critical frequencies with the ionosonde measurem ents. It is also shown that this improvement is sufficient to measure criti cal frequencies associated with the ionospheric E layer.