| 06 Oct 2025
Quality aspects of Fengyun3 D/E radio occultation bending angle products
Abstract. This study systematically evaluated the quality of ionosphere-corrected bending angles from Fengyun3 (FY3) D/E satellites (equipped with GPS and BDS receivers) using ERA5 data as references and MetOp products as comparisons. The quality of subsequent retrieved optimized bending angles, refractivity, and temperature were also analysed. Ionosphere-corrected bending angle were assessed via two approaches: outlier detection across 10–80 km and bias/noise quantification. Overall quality evaluation showed that FY3 ionosphere-corrected bending angles were consistent with MetOp below 40 km. Above 40 km, FY3 bending angles exhibited larger errors than MetOp. In outlier detection, MetOp had nearly 0 % bad profiles, followed by FY3D (<10 %), while FY3E (GPS/BDS) had about 20 % bad profiles. FY3E-GPS bending angles are prone to have large outliers in the height range of 35–50 km. For bias and noise quantification, the daily mean biases and noise levels of FY3 satellites were higher than those of MetOp. Specifically, FY3E-GPS showed notable large daily mean biases of about −0.4 μrad and most of these biases are in rising RO events. FY3D and FY3E-BDS ranked second, with biases of approximately −0.1 μrad. MetOp had the smallest biases, at around –0.05 μrad. Regarding noises, FY3D, FY3E-GPS exhibited comparable noise levels, at roughly 2.5 μrad; FY3E-BDS had lower noises of 1.5 μrad. MetOp noises are smallest which are about 1.0 μrad. Due to the larger biases and noises at high altitudes, FY3’s optimized bending angles were strongly corrected by background bending angles. Refractivity and temperature were also influenced by the strong correction of optimized bending angle. In summary, FY3 ionosphere-bending angles show high quality below 40 km. However, at high altitudes, further efforts are required to improve the quality issue for improving FY3 data’s utility in numerical weather prediction and climate studies, especially for stratospheric applications.
The authors present a thorough investigation of the RO data quality of the FY3 satellites, using as reference ERA5 data and comparing it to a high-quality RO receiver (Metop-C). The quality is assessed using both ionospheric-corrected and statistically optimized bending angles. Comparisons are also performed for derived refractivity and temperature profiles.
The results are useful and they could benefit from some changes and more information on a number of topics, listed below under “Main comments”. Included are also minor comments and a list of language suggestions.
Main comments:
Consider adding a description of the FY3 mission (e.g., orbital configurations, main features of the different satellite buses, description of the RO antennas, etc.). Having this information would make it easier to assess statements like the one on Ln 180-181 (“This suggests that the GNOS BDS RO receiver produces smaller noises than the GNOS GPS RO receiver”) Based on this sentence, it looks like the BDS receiver is different from the GPS receiver. Is it so, or is it a GPS-only VS a GPS-and-BDS receiver?
Ln 186: It is mentioned that the spikes have been reduced in the latest version of the Fengyun data. This is very good, and some description of how this result has been achieved and possibly a figure illustrating it would be very nice.
Figure 1, panel (c): It is rather unusual to have more rising than setting occultations. Can you double-check that the data is correct? If this is the case, please comment on this unexpected asymmetry.
Ln 303: Please provide a description of how you obtain the statistically optimized results. Are you using the ROPP for this?
Ln 381/382: You mention that the orbit determination (OD) and clock estimation of the FY3 is of very high quality. This is an important piece of information and it requires at least the references that support it.
To further support the statement that the OD is not the cause of the residual biases and noises, it would be very useful to see figures similar to, e.g., Fig. 2, but where the datasets are split by rising VS setting products. OD errors often show up as rising VS setting biases above ~ 50 km.
Figures are not of pdf quality, so when zooming they become blurred.
The y-labels of Figure 8 (panels (a) and (b)) are wrong.
Minor comments:
Ln 71: Missing reference “Anthes, 2024”. The correct one is this: https://journals.ametsoc.org/view/journals/bams/105/8/BAMS-D-23-0326.1.xml
In several places the citation style is incorrect. E.g., in ln 79, “Liao et al., 2016b” should be “Liao et al. (2016b)”
Paragraph starting on line 76: This paragraph presents a comprehensive list of literature. However, only for the paper by Liao et al. (2016b) a short description of the results is provided. Please report concise descriptions of the results of the other papers as well.
Ln 94: What is the NOMEX experiment? Do you mean the ROMEX? If so, please note that the data-collection part has already been completed (see reference in the comment to Ln 71 above).
Paragraph ln 135-144: Please provide a brief description of the schemes mentioned here. This would help the reader understand how your approach is more valid and/or relevant for FY3. If it makes no sense to do so because, e.g., they are all empirically based, provide a qualitative description of them.
Ln 153: You are referring to a non-peer-reviewed publication. It’s ok but at least do include the relevant equations.
Ln 157: I wouldn’t use “It is believed” but something like “We assume that if one of the bending anglesx …”.
Data availability: you indicate that FY3 data comes from the CMA reprocessing campaign and the Metop-C from the ICDRs. Is there an official way to access the CMA-reprocessed data? Could you indicate where the ICDRs are available?
Edits/Language
Ln 21: Substitute “which are about” with “at about”
Ln 24: Consider shortening “required to improve the quality issue for improving” to just “for improving”
Ln 30: Substitute “As propagating” with “While propagating”
Ln 31: Not sure what “formulated” here means
Ln 33: By “of can” do you mean “of water wapour”?
Ln 35: “2022” should be “2002”
Ln 36: Remove “to be used”
Ln 39: “all weather” should be “all-weather”
Ln 42: “system” should be plural
Ln 44: Substitute “In the meanwhile” with “Meanwhile”
Ln 45: Substitute "detecting” with “detection”
Ln 47: “and also tropical cyclones” is missing a verb or noun (e.g., “detection”, “identification”)
Ln 70: Remove “few” from “In recent few years”
Ln 70: Remove comma after “(ROMEX)”
Ln 71: Substitute “of three months’ period” with “over a three-month period”
Ln 74: Remove “improving”
Ln 76: Either “in stratospheric regions” or “in the stratospheric region”
Ln 77: “bending angles” should be “bending angles”
Ln 78: Substitute “There are current a lot of work” with “Several works”
Ln 87: “a couple of reports” is informal. Use instead “two”
Ln 87: Remove “in” from “in analysing”
Ln 89: “i.e.,” should be “e.g.,”
Ln 90/91: Consider rewriting the sentence as: “Unlike the case of statistical optimization and 1dVar (Li et al., 2013, 2019), the retrieval of bending angles is not influenced by background models”
Ln 92: “are useful” should be “is useful”
Ln 92: Repetition: substitute the second instance of “understanding” with “assessing”
Ln 92: “subsequent retrieved” should be “subsequently retrieved”
Ln 96: “examination” should be “examining”
Ln 97: “subsequent retrieved” should be “subsequently retrieved”
Ln 98: Please list both parties of this joint work, not only CMA.
Ln 148: Remove “: QF1, QF2, and QF3 are used to detect outliers”, since it’s a repetition of the first part of the sentence.
Ln 161: “exceed threshold” should be “exceeds the threshold”
Ln 161-163: Please consider rephrasing as: “QF0 indicates a profile of good quality, i.e., where no quality issues have been detected. QF8 indicates that at least one of the seven quality flags has a non-zero value.”
Ln 176: “observations” should probably be “latitudinal bands” or something similar.
Ln 182: “largest errors” should be “the largest errors”
Ln 213: “also include quality” should be “also includes the quality”
Ln 214: “set a strict criteria” should be “set as a strict criteria”
Ln 214: “solely use” should be “solely using”
Ln 255: “altitude” should be “latitude”
Ln 286: “are mainly comes” should be "mainly come” (or “are mainly coming”)
Ln 287: “suggest a positive” should be “suggests positive”
Ln 288: “data are used” should be “data used”
Ln 313: “sources” should be “source”
Ln 362: “to 15%” should be “15%” (two instances on this line)
Ln 364/365: “biases are around” should be “biases around” (two instances)
Ln 376: “were given” should be “given”