Reflection point calculations and atmospheric delay corrections for PRETTY observations

Abstract

This thesis explores the PRETTY satellite mission, a CubeSat supported by the European Space Agency (ESA) designed to measure sea and ice levels using altimetry and electromagnetic signals from GNSS satellites. It was the first reflectometry mission approved by ESA. The PRETTY satellite employs GNSS-R techniques to capture both direct and reflected GNSS signals to ascertain sea surface altitude. Unlike traditional radar systems that require both transmitting and receiving antennas, GNSS-R uses already existing GNSS signals, reducing energy and space requirements. Despite initial delays due to software and radio interference, PRETTY successfully operates in orbit at 560 km altitude, utilizing a 'slant geometry' approach for signal reflection. In this thesis, the directly received signal and the one reflected off the Earth's surface were both utilized to compute the reflection point position and the tropospheric delay in the reflected signal. To calculate the specular points of PRETTY, three different methods were used: Gleason, Wagner and Klokocnik, and the Binary method. Additionally, two different versions of the Vienna Mapping Function, VMF1 and VMF3, as well as the Global Mapping Function and the ATom software package developed at TU Wien, were employed to compute tropospheric corrections. The results and performances of different methods and algorithms were extensively compared by applying various statistical techniques. In summary, the spheroidal binary approach is determined to be the most effective and efficient technique for calculating specular points. When it comes to computing tropospheric corrections, the VMF performs slightly better (especially in the extreme southern region over Antarctica and the extreme northern region over Greenland) in comparison to the ATom software package. Accumulating more data will enable cross-validation with other reflectometry and conventional altimetry missions, thus assessing the limits of PRETTY's innovative Earth observation method. Consequently, it will be interesting to observe PRETTY's performance relative to these other missions. To enhance PRETTY's accuracy, the next step in determining the reflection point might involve investigating the delay caused by the ionosphere in an electromagnetic signal. Since PRETTY is located within the ionosphere, with parts above and below it, basic single-layer models would be inadequate for calculating corrections. Models currently available, such as the NeQuick ionosphere electron density model, take into account the entire ionosphere (TEC). Hence, alternative methods would need to be explored.