Challenges for the PRETTY Satellite Mission: Characterizing Grazing Angle Ionospheric Effects in Space-borne GNSS Reflectometry.

Abstract

The altimetric application of Earth-reflected GNSS signals is one of the initial ideas brought up in the field of space-based GNSS reflectometry. There are two challenging points for this application: (1) GNSS signals do not have the large bandwidth of code modulation that other radar altimeters have to reach sub-decimeter precision, (2) coherent carrier phase observations may provide better precision, however, their application is limited by the signal propagation conditions (power loss due to surface roughness or delay biases due to atmospheric variability). In this study challenges of propagation conditions are analyzed to prepare altimetric retrievals of the ESA "PRETTY" mission that will be launched in October 2023. The PRETTY mission concentrates on grazing elevation angles and smooth reflecting surfaces to facilitate the use of coherent carrier phase observations. In a first step the climatology of sea surface roughness conditions is analyzed on a global scale to select regions with highest probability of coherent observation for ocean altimetry. The results show high probability over the calm waters of the Indonesian sea. In a second step the variability of ionospheric conditions for PRETTY like mission is studied. The aim is to comprehensively characterize ionospheric parameters (delay, Doppler shift, and peak height of electron density) under relevant conditions: elevation angles within the grazing range (5° to 30°), different zones of latitude (North, Tropic, South), diurnal variations (daytime and nighttime), and low and high solar activity, represented by the solar flux index F10.7 at 75 and 180, respectively. The ionospheric parameters are simulated assuming reflection events of the Spire Global Inc.'s Lemur-2 CubeSat constellation with observation geometries similar to the one of PRETTY. The electron density along the ray path of each reflection event is determined using empirical models (Neustrelitz Electron Density Model - NEDM2020 and NeQuick 2 model). For the low solar activity, the simulations reveal up to about 300 TECU for rays propagating on the longest ionospheric path, biases (for L5 signals) of 35 meters in relative ionospheric delay, and 2 Hz in Doppler shifts. The peak height of electron density profiles ranges from 215 to 330 km. Considering model uncertainties we have to expect that significant residuals (delays in the meter range) will remain after model-based ionospheric correction. This challenge for altimetry could be a chance, as well, to investigate ionosphere monitoring opportunities.