Grazing angle GNSS reflectometry with the PRETTY satellite: an opportunity to resolve the structure of ionospheric layers

The Passive REflecTometry and dosimeTrY (PRETTY) satellite has been launched into a low Earth orbit, of about 570 km altitude, on 9th October 2023. Its main payload is dedicated to reflectometry using signals of Global Navigation Satellite Systems (GNSS). The on-board reflectometry receiver provides delay maps of the Earth-reflected signal. The main objective of the mission is to exploit the delay maps for ocean and sea ice altimetry.

Observations are recorded in grazing angle geometry. It means that incident and reflected ray reach a maximum elevation angle at the reflection point of 15°, higher elevation observations are out of scope. These grazing angle observations have advantages compared to higher ones: reduced roughness effect on the reflected signal, higher coherent reflection power and wider coverage of reflection points over the ocean. However, grazing geometry brings also challenges: a reduced altimetric sensitivity (to surface height changes), as well as, higher magnitude and complexity of atmospheric delays.

The presented study modifies the initial altimetric idea and uses reflectometry data with known surface height to investigate the structure of ionospheric layers. The study concentrates on four PRETTY delay maps recorded over Arctic sea ice. Correcting the delay for other effects, especially surface height and troposphere delay, reveals characteristic ionospheric delay profiles. These profiles, of the reflected signal delay relative to the direct signal, reach a minimum in the grazing angle range before they vanish in the limit of tangent Earth reflection. Both, retrieved profiles, from PRETTY observations, and simulated profiles, assuming ionospheric electron density of the Neustrelitz Electron Density Model (NEDM), confirm the characteristic minimum of the profiles.

For a deeper analysis of the profiles we run simulations with configurable electron density distribution according to Chapman layers and the PRETTY satellite geometry. Dominating F-layers are assumed with peak heights from 250 to 350 km. The corresponding profiles show minima from about 2° to about 4° of elevation. The inversion of peak height and other parameters of ionospheric layers from delay profiles will be developed further in the on-going study. An extended validation of results, with different ionosphere models, shows an overall agreement in the existence of local minima. However, uncertainties among the models persist with minima positions deviating by 1 or 2 degrees. It indicates space for improvement of existing models.