Tianmu-1 Constellation GNSS-R In-Orbit Performance: Spatiotemporal Characteristics, Product Applications, and Polarimetric Features

This article introduces the Chinese Tianmu-1 Global Navigation Satellite System Reflectometry (GNSS-R) constellation of 22 small satellites launched in 2023–2024 and comprehensively evaluates the latest version of the in-orbit data. First, the mission design and instrument technology are described, which largely builds on the FengYun-3/GNOS-II missions. Notable innovations include full GNSS compatibility and dual-polarization antenna. Then, the spatiotemporal characteristics of the constellation are analyzed—specifically, coverage percentage (CP) and mean revisit time at different latitudes. Next, the accuracy of its science products including ocean surface winds and land soil moisture has been assessed, with two application cases demonstrating the mission’s utility for monitoring tropical cyclones (TCs) and flooding. Finally, this article, for the first time, evaluates Tianmu’s polarized observations including horizontal (H), vertical (V), left-hand circularly polarized (LHCP), and right-hand circularly polarized (RHCP). Analysis of the signal-to-noise ratio (SNR) and reflectivity shows that the dual-polarimetric observations follow the trend of theoretical models and hold promise for advancing land remote sensing.