the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Sep 2025
On-Orbit Calibration and Performance Validation of the Yunyao Polarimetric Radio Occultation System
Abstract. 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. 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 tailored for operational applications and 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 the differential phase (ΔΦ) 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⁻¹ are proposed as indicators of precipitation sensitivity, detection confidence, and heavy-rain events, respectively, and a ΔΦ-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.
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RC1: 'Comment on egusphere-2025-4362', Anonymous Referee #1, 09 Oct 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4362/egusphere-2025-4362-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2025-4362-RC1 -
AC1: 'Reply on RC1', Sai Xia, 24 Oct 2025
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We sincerely thank the reviewer for their valuable comments and constructive suggestions. We have carefully addressed each of the points raised. Below, we provide a point-by-point response to the specific comments:
(1) Regarding the suggestion to enhance the contextual motivation:
We have incorporated additional content to better situate our work within the current industrial and scientific context. The revised text now reads:“At present, Chinese commercial aerospace enterprises are actively laying out in this field. Yunyao Aerospace Technology Co., Ltd. is committed to integrating polarized occultation technology into its high-timeliness meteorological constellation. The point is to make up for the deficiency of traditional occultation observations in detecting the microphysical processes of water vapor and precipitation. By capturing ∆Φ caused by aspherical hydrometeors within the cloud and rain area, PRO can significantly enhance the constraints on water vapor condensation and phase transition paths during severe convective weather processes, thereby improving the accuracy of numerical models in short-term and imminent precipitation prediction.”
(2) Regarding the definition of ∆Φ in the abstract:
We have clarified the definition of the differential phase at the beginning of the abstract as follows:“The differential phase (∆Φ) 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.”
(3) Regarding the expansion of atmospheric science applications in the conclusion:
We have expanded the conclusion to better reflect the broader potential of PRO in atmospheric science:“Beyond precipitation detection, PRO observations have broader potential in atmospheric science. The sensitivity of ΔΦ to aspherical hydrometeors enables its use in discriminating precipitation types and in identifying mixed-phase and ice-dominated cloud regions. Combined with conventional RO profiles, PRO can constrain not only thermodynamic structures but also microphysical processes aloft, providing a pathway to improve cloud parameterizations in weather and climate models. Furthermore, the high spatiotemporal sampling of PRO constellations supports the analysis of moist processes in data assimilation systems, potentially enhancing the accuracy of short-term precipitation forecasts and the representation of latent heating in tropical cyclones and mesoscale convective systems. As a cost-effective extension of existing RO infrastructure, PRO is poised to bridge gaps between thermodynamic sounding and precipitation observation, advancing the integrated profiling of the moist atmosphere.”
Responses to Specific Comments:
- (L40) – Definition and explanation of differential phase in the abstract
We have redefined the differential phase in the abstract, specifying that it is derived from the difference between the H- and V-polarized signals obtained from PRO observations. The differential phase directly reflects the scattering characteristics of hydrometeors along the propagation path and, through statistical analysis, can indirectly indicate rainfall rate.
- (L138) – Definition of differential phase in the introduction
Similarly, the introduction now clearly defines the differential phase, emphasizing that both H- and V-polarized observations are acquired from the PRO technique.
- (L264) – Attribution of phase difference to hydrometeors
The relevant sentence has been revised to explicitly state that the differential phase is induced by hydrometeors.
- (Fig. 10) – Addition of relevant content
We have added appropriate content and labels related to Figure 10 as suggested.
- (L355) – Attenuation of differential phase near the surface
The attenuation of the differential phase near the surface occurs when the radio ray traverses an insufficient thickness of the precipitation layer, leading to a reduced integrated phase shift. Although radio occultation signals near the surface are often affected by multipath interference and low signal-to-noise ratio—which can degrade data quality—the segment in question has undergone quality control and truncation. This behavior is not typical in a general sense; rather, it depends on the actual penetration depth of the ray through the precipitating region.
- (Fig. 12) – Ray profiles corresponding to the highest and second-highest differential phase values
The ray profile with the highest differential phase and the one with the second-highest correspond to the ray paths whose tangent points are located at approximately 3 km and 2 km, respectively. This is influenced by the relative motion and geometry between the GNSS satellite and the low-Earth orbit receiver. Although these two rays are close in their tangent altitudes, they are distinct and not superimposed.
Technical Revisions
All technical issues and methodological points raised have been addressed and corresponding modifications have been made throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-4362-AC1
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AC1: 'Reply on RC1', Sai Xia, 24 Oct 2025
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RC2: 'Comment on egusphere-2025-4362', Anonymous Referee #2, 28 Oct 2025
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This paper describes the new polarimetric RO data obtained from the Yunyao satellites, the processing/retrieval of these data, and the validation of these data. Overall, I find this paper to be well written and contains new results that are of interests to the community. I recommend its publication with minor revision addressing the followings:
- Please comment on whether the Yunyao PRO data are available or planned to be made available to the public.
- Did you try to look for collocations of Yunyao and PAZ occultations? It would greatly enhance the validation study to include such examples.
- Fig 5a: there appears to be two “holes” in the local time distribution at lower altitudes. Why?
- There are rather detailed descriptions of the processing not relating to the polarimetric processing (e.g, Sections 2.2.1 and 2.2.2). Are these described elsewhere (e.g. Yue et al. 2025)? If so, I suggest shortening them here.
- Some basic receiver characteristics would be useful, such as SNR (H, V, Combined), highest vs lowest tracking altitudes, and closed-loop to open-loop transition point. Are Yunyao tracking setting occultations only?
- (7): Why is delta alpha corrected set to be constant below 20 km? Please explain.
- L235: Could you explain how H and V are combined in the processing?
- L280: What’s the “fixed circular regions” used in Katona et al.? How’s that different from the approach used here?
- 10e: from the plot, it looks like BDS occultations do not penetrate as deep below 5 km. Is that right? Any reason if that’s the case?
- Figures 11 and 12. Please revise the captions to provide better descriptions.
- L360: it would be useful to show temperature along with Figures 11 and 12 which would provide some info about the ice vs liquid water. Also I assume the raypaths shown are straight line without bending included. Is the bending enough to change the interpretations?
- Lines 401-403: “when the occultation tangent point is near 5-6 km, the ray path achieves a longer effective propagation distance within a relatively homogenous clouds and precipitation region, thereby maximizing the integrated contribution to Delta Phi.” I don’t follow this. Please explain support for this statement.
- L470: “stable temperature and pressure retrievals” What do you mean by “stable” here?
Citation: https://doi.org/10.5194/egusphere-2025-4362-RC2
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