Real time reactualizing of the ionospheric electron density profile

The ionospheric medium characterized by its density profile presents an important spatial and temporal variation. At present the access to the profile is obtained either by a modeling of a great number of previously acquired values or by measurements of propagation delays obtained with vertical or oblique soundings working at 2 to 25 MHz. The operation consisting in changing measurements to the density profile is called inversion. For most applications the temporal variation of the ionosphere necessitates obtaining the density profile at least every 15 min. The passage from one to another profile corresponds to what we call reactualizing. The aim of this paper is to propose a method for reactualizing the density profile which rests on both estimated models of propagation (and hence on profile) which initialize the inversion (and which could be assimilated to a previous measurement) and on the measurements of group delay times and/or goniometry for known connections with a limited number of frequencies. The first inversion trials show the originality of the method and also display the following difficulties: important number of parameters, instability, sensitivity to initial conditions (which requires reliable ionospheric forecasts). Numerous HF applications (radiolocation, HF radar, oblique sounding) justify however the interest of this study. The complexity of the problem necessitates a learning phase described briefly in this paper