Ionospheric Delay

The ionosphere is the zone of the terrestrial atmosphere that extends itself from about 60 km until more than 2 000 km in high. As it name says, it contains a partially ionised media, as result of the X and UV rays of Solar Radiation and the incidence of charged particles. The propagation speed of the GNSS electromagnetic signals in the ionosphere depends on its electron density (see below), which is typically driven by two main processes: during the day, the sun radiation causes ionisation of neutral atoms producing free electrons and ions. On the night a recombination process happens, where free electrons are recombined with ions producing neutral particles, which leads to a reduction of the electron density. A media where the angular frequency ω and the wave number k are not proportional, is a dispersive media (i.e., the waves propagation speed and thence, the refractive index depends on the frequency). This is the case with the ionosphere where ω and k are related, in a first approximation, by [Crawford, 1968]: