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Session A4: Atmospheric Effects, GNSS Remote Sensing, and Scientific Applications

Assessing Grazing GNSS-R Ionospheric Measurements Over Equatorial Oceans Using IRI and JASON TEC
Brian Breitsch, Madeline Evans, Jade Morton, University of Colorado, Boulder
Location: Royal Ballroom DF
Date/Time: Thursday, Jan. 29, 11:26 a.m.

Global Navigation Satellite System (GNSS) receivers on low Earth-orbiting (LEO) satellites are crucial for remote sensing of the ionosphere’s total electron content (TEC). While established overhead and occultation geometries, like those used by the COSMIC-2 constellation, are standard, grazing GNSS reflectometry (GNSS-R) has emerged as a compelling new technique. By receiving signals reflected at low grazing angles from calm ocean or smooth ice surfaces, GNSS-R provides valuable carrier phase measurements that can be used to estimate relative TEC when dual-frequency measurements or precise models of signal delay parameters are available [1]. This method has recently been used to observe polar TEC enhancements and traveling ionospheric disturbances (TIDs) from the 2022 Tonga volcano eruption [2], offering unprecedented coverage over ocean and polar regions where conventional observations are sparse.
The most abundant source of these grazing GNSS-R measurements is Spire Global’s LEO cubesat fleet. However, data collected over low-latitude oceans, such as the Indonesian seas, present unique challenges. Signal reception is often intermittent due to ocean surface roughness and interruptions from islands, complicating the reconstruction of the signal carrier phase compared to more continuous tracks over polar ice. Our recent work focuses on improving estimation for this type of data [3], and although results thus far show general agreement with coarse Global Ionosphere Maps (GIMs), the validity of medium-to-small-scale ionospheric structures observed in the GNSS-R data remains uncertain. GIMs are primarily derived from ground-based data and are often unreliable or interpolated over vast ocean expanses, and recent work by [4] identifies significant differences between TEC modeled by global GIMs versus TEC estimated using global measurements from COSMIC radio occultation or or altimeter measurements. It is therefore necessary to find valid measurements over the ocean that can be used for validation of grazing GNSS-R TEC.
To address this validation challenge, this work proposes a direct comparison between Spire’s grazing GNSS-R measurements and co-located TEC data from the JASON-3 altimeter. Altimeters provide reliable vertical TEC (VTEC) estimates directly over the ocean, making them an ideal data source for validating the grazing angle slant TEC (STEC) retrievals. For this work, we will survey data from 2023 and 2024 to identify co-located measurement opportunities, focusing on the low-latitude region over the Indonesian Seas. By mapping the JASON-3 VTEC onto the grazing GNSS-R signal path geometry, we will perform a rigorous validation of the GNSS-R ionospheric measurements over the ocean. We also address estimation of the topside ionosphere TEC, both using an auxiliary GNSS satellite and through use of a topside VTEC mapping function. The overall slopes and any particular structures of the grazing reflection tracks will be compared with those obtained through the mapping procedure. We expect this work to provide context and evidence for how reliable low-latitude grazing GNSS-R measurements can be when used for TEC estimation.

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