Model-Based Analysis of Ionospheric Effects in Grazing Angle Reflectometry from Space

Abstract

The ionosphere constitutes a layer within the Earth's upper atmosphere that becomes ionized due to solar radiation. It plays a pivotal role in the propagation of signals from the Global Navigation Satellite System (GNSS), as these signals traverse the ionosphere while traveling from GNSS satellites to receivers. The irregularities in ionospheric electron density can significantly impact GNSS signals, leading to signal delays and scintillations. Ground-based atmospheric sounding techniques, involving continuously operating reference station (CORS) networks, combined with GNSS receivers positioned on low Earth orbit (LEO) satellites to measure refracted radio signals through GNSS Radio Occultation (GNSS-RO), constitute the foundational framework of GNSS meteorology. GNSS Reflectometry (GNSS-R) presents a promising technique for atmospheric and ionospheric sounding, particularly in locations lacking GNSS ground stations or GNSS-RO observations. In anticipation of the ESA CubeSat Reflectometry mission "PRETTY," this study aims to characterize ionospheric effects by analyzing varying grazing elevation angles, distinct latitude-based regions, and diurnal temporal variations. The investigation employs simulations using authentic metadata from Spire Global Inc.'s Lemur-2 CubeSat constellation for the orbits on March 1, 2021. The first-order ionospheric delays are estimated along each ray path (incident, reflected, and direct) by deriving the slant total electron content (sTEC) from the Neustrelitz Electron Density Model (NEDM2020) and the NeQuick Model. The study findings reveal significant fluctuations in crucial ionospheric parameters. Specifically, the slant Total Electron Content (sTEC) displays variations of up to approximately 300 TECUs, underscoring the dynamic nature of electron density in the ionosphere. Moreover, the relative ionospheric delay exhibits variations of 19 meters, providing insight into the influence of ionospheric effects on signal propagation paths. Additionally, Doppler shifts demonstrate deviations of 2 Hz, accentuating the frequency changes resulting from ionospheric interactions. The investigation delves into the spatial dimension by exploring the height of the electron density peak. The results depict variations spanning from 215 to 330 kilometers, signifying the diverse altitudinal effects of the ionosphere on GNSS signals. These variations are intricately influenced by various parameters, including the grazing elevation angle of the signal, the geographical location of the event, and the time of day during which the observations occur.