Ionospheric delay in GNSS reflectometry: studying satellite and sounding rocket scenarios

Abstract

The ionosphere induced delay in GNSS reflectometry (GNSS-R) observations is a promising research topic. Early concepts for space-borne application of GNSS-R in the 1990's pointed out already that precise altimetric monitoring over the global oceans and sea-ice areas can profit from additional GNSS-R data to mitigate ionospheric bias. Studying the ionospheric impact in GNSS-R can be challenging due to the different locations where the signal path crosses the ionospheric layers. The study has to consider the incident, reflected and the direct ray that connect transmitter, specular point and receiver. The height of the receiver platform plays an important role as the relevant refractive delays originate often from the atmosphere below. Satellite-based observations, with receiver heights usually above 350 km, experience a significantly higher ionospheric delay compared to airborne observations that at most can reach receiver heights of 20 km. In this study we investigate ionospheric delays in GNSS-R for receiver heights in the range between 10 km and 600 km, it means from common aircraft flight levels to typical low Earth orbits. This range is most important for ionospheric research as it starts below the relevant ionospheric layers (E, F) and ends considerably above typical F-layer peak heights (250-350 km). The major objective of this study is to clarify how changes in the height of ionospheric layers will affect delay and Doppler observations in GNSS-R for different receiver heights. Simulations are conducted for receivers in two GNSS-R scenarios: (1) on the PRETTY satellite (orbit height: ~550 km) and (2) on a MAPHEUS sounding rocket (with heights: 10 to 240 km). The simulation results show that for PRETTY orbit heights the budget of ionospheric delays is especially sensitive to the F-layer location (more precisely to its penetration by the direct ray). This F-layer sensitivity occurs in grazing angle geometry. It will be further quantified in the on-going study. These simulation results will help to evaluate whether GNSS-R data from a satellite like PRETTY (acquiring data in orbit since Feb. 2024) or from a sounding rocket like MAPHEUS (launch planned for Nov. 2024) can be used to monitor the variability of ionospheric layers.