  In this report we analyze the feasibility of ionospheric monitoring using
GNSS technology. The focus will be on the use of LEO GNSS data, exploiting GNSS
Reflections, Navigation and Occultation TEC measurements. In order to attack
this question, we have simulated GNSS ionospheric TEC data as it would be
measured from a polar LEO (exploiting Navigation, Occultation and Reflection
TEC data) and IGS ground stations, through the use of a climatic ionospheric
model (we have explored both NeQuick and PIM). We have then developed a new
tomographic approach inspired on the physics of the hydrogen atom, which has
been compared to previous successful but somewhat awkward methods (using a
voxel representation) and employed to retrieve the Electronic Density field
from the simulated TEC data. These tomographic inversion results using
simulated data demonstrate the significant impact of GNSS-R and GNSS-NO data:
3D ionospheric Electron Density fields are retrieved over the oceans quite
accurately, even as, in the spirit of this initial study, the simulation and
inversion approaches avoided intensive computation and sophisticated
algorithmic elements (spatio-temporal smoothing). We conclude that GNSS-R data
can contribute significantly to the GIOS (Global/GNSS Ionospheric Observation
System).
