On-orbit calibration and performance validation of the Yunyao polarimetric radio occultation system

<p>Polarimetric radio occultation (PRO) extends the capability of standard radio occultation (RO) by providing not only the conventional thermodynamic profiles but also information on clouds and precipitation. The differential phase (<span class="inline-formula">ΔΦ</span>) is the cumulative phase shift between horizontal and vertical polarizations observed from PRO caused by aspherical hydrometeors along the propagation path, typically measured in millimeters. In early 2025, Yunyao Aerospace Technology Co., Ltd. successfully launched the first Chinese low Earth orbit satellite equipped with a PRO payload, generating over 500 measurements per day. Based on this mission, we established an end-to-end PRO data processing chain incorporating on-orbit calibration and tailored for operational applications. We analysed approximately 53 000 events collected between March and June 2025, in conjunction with the Integrated Multi-satellite Retrievals for Global Precipitation Measurement (GPM) precipitation product (IMERG). The results show that <span class="inline-formula">ΔΦ</span> remains close to zero under non-precipitating conditions but exhibits distinct peaks at 3–5 km altitude when traversing precipitation layers, with amplitudes strongly correlated with path-averaged rainfall rates. Thresholds of 1, 2, and 5 mm h<span class="inline-formula">⁻¹</span> are proposed as indicators of precipitation sensitivity, detection confidence, and heavy-rain events, respectively, and a <span class="inline-formula">ΔΦ</span>-to-rainfall intensity mapping table is derived to quantify this relationship. Yunyao PRO data preserve the thermodynamic retrieval quality of conventional RO while enabling effective precipitation detection, thereby providing important data support for the theoretical, technical and data research on the transition of meteorological observations from “temperature, humidity and pressure” observations to new types of observations such as precipitation.</p>