Ionospheric Impact on GNSS Reflectometry in the Tropical Region: A Simulation Study with NEDM model

The ionosphere is a layer of Earth's upper atmosphere that is ionized by solar radiation. It plays a crucial role in the propagation of Global Navigation Satellite System (GNSS) signals, as these signals pass through the ionosphere on their way from the GNSS satellite to the receiver. The irregularities in the ionospheric electron density may have a significant impact on the GNSS signals, causing delays and phase and amplitude scintillations.

GNSS reflectometry (GNSS-R) is a promising technique for atmospheric sounding. Multiple studies have been successfully conducted in the recent decade by using GNSS-R ground-based, airborne and spaceborne data e.g., to estimate ionospheric disturbances from the reflected signals. However, further investigations are needed to precisely characterize ionospheric effects for GNSS-R altimetric applications.

This study presents simulation results of ionospheric delay for reflection events in tropical regions. The first-order ionospheric effects are estimated along the ray paths by deriving the slant total electron content from the Neustrelitz Electron Density Model (NEDM). The geometry of the simulated events refers to reflectometry records of the SPIRE satellite constellation and the satellite navigation system GPS on 2021/03/01.

Initial analysis has shown promising results. As solar activity increases (indicated by solar radio flux F10.7 index), an increase in the total ionospheric phase delay is evident.  Between 0h and 8h local time, there is a delay of 2 to 10 meters. For the time interval from 8h to 16h, the delay is from 14 up to 22 meters, with the maximum at noon. In the sunset period from 16h to 24h, the ionospheric delay reduces from 9 to 3 meters, respectively. The height above the Earth’s surface at which the highest amount of electron content is found along the ray path is ~290 km. This altitude corresponds to the F-region which has the highest concentration of free electrons. The analyzed events correspond to elevation angles from 5 to 30 degrees. The highest ionospheric delay is found at elevation angles between 10 and 20 degrees also depending on the local time.