Atmospheric Remote Sensing with Airborne and Spaceborne GNSS Reflectometry: from Tropospheric to Ionospheric Applications

Kurzfassung

Since its introduction in the 1990s, GNSS Reflectometry (GNSS-R) has emerged as a revolutionary remote sensing technique, demonstrating vast potential for characterizing surface properties across various applications and environments. Today, GNSS-R continues to evolve, supported by increasing operational spaceborne missions such as PRETTY, CYGNSS, SMAP GNSS-R, and TRITON, alongside upcoming missions like HydroGNSS. These missions aim to advance scientific knowledge and generate commercial products, such as those from the LEMUR constellation operated by Spire Global Inc. In addition to its surface-related applications, GNSS-R offers significant potential for atmospheric monitoring by exploiting the differences between the direct signals, which provide information above the receiver, and reflected signals, which capture information below the receiver. However, compared to its extensive application in surface studies, the use of GNSS-R for atmospheric research remains relatively underdeveloped, presenting an opportunity for further exploration to enhance its capabilities in ionospheric and tropospheric studies. This dissertation addresses this opportunity initially through airborne experiments, which serve as an effective tool for testing concepts and refining methodologies. Airborne GNSS-R data are utilized to demonstrate the feasibility of tropospheric parameter retrieval, specifically Zenith Total Delay (ZTD), over coastal waters. The proposed method yielded promising results, with relative deviations between 5% and 24% compared to the typical ZTD value of 2.3 m at sea level, highlighting the potential for tropospheric monitoring using coherent phase observations. For ionospheric studies, this thesis begins by conducting simulations to characterize ionospheric effects on GNSS-R signals at grazing angles, leveraging orbital data from low Earth orbit CubeSats and climatological three-dimensional electron density models. These efforts provided significant insights into model-based ionospheric delays, Doppler shifts, and variations in electron density peak height across diverse scenarios, including elevation ranges from 5° to 30°, established geographic regions, local times, and solar activity levels for spaceborne applications. The analysis further compares the findings from these simulations with GNSS-R code delay observations from the PRETTY mission. Results demonstrated deviations between 1.28 m and 4.96 m when compared with state-of-the-art climatological ionospheric models. Additionally, by applying a Chapman F-layer fitting, the observations provide valuable insights into the vertical structure of the ionosphere, showing differences in peak electron density height of ±15 km compared to values obtained from ionosondes and EISCAT ground stations. This thesis contributes to advancing GNSS-R as a potential tool for atmospheric and ionospheric monitoring by demonstrating its capability to retrieve tropospheric parameters and characterize ionospheric effects, while also highlighting opportunities and challenges for future research.