Image Navigation Validation for FY-4A Geostationary Interferometric Infrared Sounder Using the Ground-based Laser Positioning System

Cao, Yun; Shang, Jian; Yang, Lei; Yao, Hongquan; Liu, Chengbao; Zhu, Xiaopeng; Zhao, Xinghui; Huang, Pan; Liu, Jiqiao; Wang, Jing; Zhang, Zhiqing

doi:10.5194/egusphere-2026-1163

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https://doi.org/10.5194/egusphere-2026-1163

Preprints

03 Jul 2026

| 03 Jul 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Image Navigation Validation for FY-4A Geostationary Interferometric Infrared Sounder Using the Ground-based Laser Positioning System

Yun Cao, Jian Shang, Lei Yang, Hongquan Yao, Chengbao Liu, Xiaopeng Zhu, Xinghui Zhao, Pan Huang, Jiqiao Liu, Jing Wang, and Zhiqing Zhang

Abstract. Accurate image navigation validation (INV) is essential for the longwave (LW) bands of the geostationary interferometric infrared sounder (GIIRS) onboard the Fengyun 4A (FY-4A) satellite. However, few scientific peer-reviewed papers have been reported to date. In this study, we present, for the first time worldwide, a satellite-ground integrated INV technique applied to the FY-4A GIIRS’s LW, leveraging the ground-based laser positioning system (GLPS) to assess its navigation precision. The GLPS is designed to precisely and automatically point at the satellite and transmits laser signals whose center wavelength is predesigned and thus can be detected by GIIRS’s LW detectors. Two observation modes are designed for GIIRS when the laser operations are scheduled, namely, dwell observation mode and step scanning mode. For the data analysis, three methods are proposed for different stages of data processing. A simple but universal and efficient algorithm called laser index is proposed for laser signal detection. Two customized algorithms are used for the designed laser operations to obtain the image navigation deviations in dwell observation and step scanning modes. The laser tests demonstrate that east-to-west navigation deviations are within 0.35 pixels, and north-to-south navigation deviations are within 0.45 pixels, which is consistent with the results of equivalent results from the visible band. The result analysis of the laser data demonstrates that the proposed methods are effective and precise, and it is a reliable tool to verify the image navigation results through GLPS.

Received: 02 Mar 2026 – Discussion started: 03 Jul 2026

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Yun Cao, Jian Shang, Lei Yang, Hongquan Yao, Chengbao Liu, Xiaopeng Zhu, Xinghui Zhao, Pan Huang, Jiqiao Liu, Jing Wang, and Zhiqing Zhang

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RC1:
'Comment on egusphere-2026-1163', Anonymous Referee #1, 26 Jul 2026 reply
General comments
This paper describes the world’s first attempt to evaluate image navigation validation (INV) in the longwave (LW) bands of the FY-4A GIIRS using the ground-based laser positioning system (GLPS). It contains engineering innovations that deserve high recognition in the field of remote sensing. The verification concept utilizing multiple stations over long baselines and the introduction of multiple subpixel evaluation methods are of high practical value. However, the current manuscript reads like an unpolished preprint. Errors in the basic physical understanding of the observation geometry, inconsistencies in coordinate systems and parameter settings between the text and figures/tables, and redundant descriptions are found throughout. There are also many unexplained abbreviations, making some portions difficult to understand. Furthermore, there is a critical self-contradiction within the paper. To ensure the reliability and reproducibility of the paper as an academic publication, it is necessary to fundamentally restructure the entire manuscript in accordance with the following comments. Since this manuscript requires significant revisions and the revisions are expected to take time, I recommend rejection at this stage.

Specific comments
The Introduction contains too many details regarding the methods and results (starting with the sentence “For each detector...” at L76) and repeats the same explanations found in Section 4, such as the “weighted gray center method,” “Kriging method,” and number of images, resulting in redundancy. The text in the Introduction should be kept concise, and the duplication with Section 4 should be eliminated.

Table 1: The definitions of wavenumbers are listed in reverse order, such as LW (1650-2250) and MW (700-1130). Please align them with the correct description in L104.

Fig. 1: The caption states, “The units of the numbers in the figure are μm,” but the numbers inside the boxes (15, 47, 48, etc.) are detector index numbers, so this description is inappropriate. Additionally, clearly indicate in the figure the dimensions mentioned in the text: “The first and second columns have a misalignment of 77 μm.” Also, clearly state in the caption that the figure is an enlarged view of the central portion of the 32×4 array.

L118-121: The paragraphs on calibration tasks using blackbodies and the Moon are unrelated to this study (laser-based position verification) and should therefore be deleted.

Fig. 2: Specify the latitudes and longitudes of the Jiamusi station and the Xinjiang station. The description “similar latitudes” (L134) alone is ambiguous.

L134-135: The text argues that the effect of parallax can be ignored because “the latitudes of the Jiamusi and Xinjiang stations are nearly the same,” but the two stations are significantly distant from each other in the longitude direction, and considering the curvature of the Earth, their line-of-sight angles are entirely different. In other words, this statement does not constitute a geometrically valid justification.

L224: Provide a clear definition of the thresholds for the “weak/strong laser” intensity classifications.

Fig. 7: The legend includes “band 389” and “band 392,” which are not mentioned in the text. Please provide a quantitative explanation in the text regarding their relationship to the laser center wavelength.

Fig. 8: Although lines 240-241 define the “x-axis as the EW direction and the y-axis as the NS direction,” the scatter plot in Figure 8 has the x-axis set to the NS direction and the y-axis to the EW direction, making it difficult to interpret. It is recommended that the axes be corrected to match the definition. Additionally, adjusting the aspect ratio of the figure to 1:1 would likely make it easier to interpret.

Fig. 9: Although the differences in interpolation conditions, such as “grid is 4” and “grid is 100”, are shown, the main text lacks an explanation of the intent behind these settings or a discussion of the comparison results. Please state in the main text the reasons and intent for changing the interpolation conditions. Also, clearly specify in the captions which figure represents “Powerful intensity” and which represents “Medium intensity.”

L292: How do “powerful/medium” correspond to “weak/strong” in L224?

L297-302: The comparison with the visible band in Section 5.5 fundamentally contradicts the core motivation for this study as stated by the authors in the Introduction. In L56-61, the authors explicitly state, “It is not appropriate to use the visible band to directly verify the INR of infrared bands.” The very fact that the optical paths are independent should have been the legitimate justification for developing the new GLPS technology in this paper. Nevertheless, at L298, the authors fall into a self-contradiction by using landmark verification results from the visible band, which the authors themselves declared inappropriate, to prove the reliability of GLPS. The authors’ attempt to prove the validity of the proposed method using a method they themselves described as inappropriate as a verification tool undermines the scientific credibility of the paper, and verification using an alternative method is necessary.

L305-313: In the “Conclusion”, a summary and comparison regarding “Dwell observations and Step scanning observations (L90-91)”—which were cited as a contribution of this study—are completely missing. The numerical values presented in the conclusion (EW 0.35 pixels, NS 0.45 pixels) are based solely on the Step scanning results, while the results of the Dwell observations are ignored. The main text must be rewritten to properly conclude the paper’s narrative by including a scientific discussion of the accuracy comparison between the two modes (though there is some mention of this in L287-290) and an explanation of why it was necessary to use both modes in conjunction (even if this was due to observational operational constraints).

L305-313: In the “Conclusion,” there is no discussion of the three methods (laser index, gray centroid method, and Kriging method) presented as the contributions of this study, making the narrative of the paper incomplete.

Technical corrections
It is recommended that the full names of the abbreviations used in the abstract be provided the first time they appear in the main text following the introduction. These include INV, LW, GIIRS, FY-4A, and GLPS.

L35-36 and L98-99: The phrase “the world’s first hyperspectral infrared sounder on a geostationary platform” is repeated, which is redundant. Please rephrase or delete it.

L38: What is the NWP?

L38: image navigation and registration performance (INR)?

L42: What is the GCP？

L56 and L104: “beamsare”→”beams are”?

L73: “um”→”μm”?

L104: “spilt”→”split”?

L172-173 and L175-L176: The phrase “the relative position ...” appears twice; please rephrase or delete it.

L285: In the title of Section 5.4, the text reads “Dwell Observation Data,” but since the content refers to step scanning, please correct it to “Step Scanning Data.”

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Citation: https://doi.org/10.5194/egusphere-2026-1163-RC1

Yun Cao, Jian Shang, Lei Yang, Hongquan Yao, Chengbao Liu, Xiaopeng Zhu, Xinghui Zhao, Pan Huang, Jiqiao Liu, Jing Wang, and Zhiqing Zhang

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Short summary

We proposed the novel validation technique of the image navigation accuracy for FY-4A GIIRS using GLPS. The GLPS points at FY-4A satellite and continuously transmits laser signals. The laser signal is capable of propagating through the atmosphere and being captured by GIIRS. The laser tests demonstrate that EW deviations are within 0.35 pixel, and NS deviations are within 0.45 pixel. This validates that the navigation accuracy of GIIRS is adequate to support the needs of backend applications.


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