When the low-Earth orbit (LEO) satellite is low, the contribution of the Total Electron Contents (TEC) above the satellite is not neglectable for inversion of GPS-LEO ionospheric occultation.

By defining a total reflection coefficient of natual EM field over multi layered medium, the forward and inversion equations for nature soutce EM sounding are established.

The total variation constraint item is added to the wave equation inversion objective functional, to improve the stability and the resolution of inversion.