Ionospheric Impact on GNSS Reflectometry: A correction approach for the PRETTY satellite data

Abstract

The ionosphere, spanning 60 to 2000 km above the Earth’s surface, plays a crucial role in Global Navigation Satellite System (GNSS) signal propagation, as signals traverse this layer on their path from the GNSS satellite to the receiver. In GNSS Reflectometry (GNSS-R), coherent observations are prominent in regions with smooth reflecting surfaces and grazing elevation angles (5° - 30°). However, within this elevation range, higher ionospheric effects (e.g., delay biases) are expected due to the longer path signals travel through the atmosphere. Dual-frequency receivers can mitigate first-order ionospheric effects by using an ionosphere-free linear combination of code or carrier measurements. Single-frequency receivers, on the other hand, rely on a model to compensate for ionospheric refraction. In this study, the Neustrelitz Electron Density Model (NEDM2020) has been employed to estimate the slant total electron content (slant TEC) along the direct, incident, and reflected ray paths. The reflection events have been simulated using the orbit data from the Spire Global CubeSat constellation. In preparation for the single-frequency GNSS-R ESA “PRETTY” mission data, this study conducts a comprehensive characterization of relative ionospheric delay, Doppler shift, and variations in the heights at which the maximum electron density is found along the ray paths. The investigation spans different elevation angle ranges, latitude-dependent regions, diurnal changes, and solar activity conditions. The results span a wide range of slant TEC values from 10 TECU between the reflection point and receiving satellite at moderate elevations (15°) to 300 TECU between transmitter and receiver (direct path) at very low elevations (5°). These results correspond to periods of low solar activity (March 2021). The ongoing study focuses on identifying and correcting the ionosphere impact in satellite data of the CyGNSS and PRETTY missions based on the developed simulation scheme.