Analysis of Precise Orbit Determination of BDS-3 MEO and IGSO Satellites Based on Several Dual-Frequency Measurement Combinations

Round 1

Reviewer 1 Report

1. The idea of BDS-3 satellite precise orbit determination using different dual-frequency measurement combinations, especially B2b signal, has unique originality. New signal pairs have shown superiority than old B1IB3I combination not only in data analysis, but also in orbit validation results. 

2. Author should introduce BDS-3/BDS-3e satellite POD research work and BDS-3 orbit products from MGEX ACs in greater detail to enrich the background research investigation. 

3. Recommend to specifically explain the BDS-3 POD process in the manuscript with extra diagram or flow-chart.

4．As the typical representative result of C/N0 in figure (2), and MPs code noise in figure (4), how about the MP information of C22, C23, C38, C39 for the consistence displayed? Meanwhile, the statistical results of MP and C/N0 in Table 2 may lost the unit information?

5. What platform or software is used to conduct the SLR validation?

6. The title of all figures should be centered and aligned. They were in line 70-72, line 118, line 128, line 142, line 161-162 and line 186-187.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Paper presents interesting research and very well prepared. Test procedure is clear and justified. Text is written in a logical and thoughtful way, creating a coherent whole. Below are some remarks, after taking into account which text will be suitable for a publication:

（1）“Satellite laser ranging (SLR) residuals indicate that the mean RMS is 3-4 cm for the four BDS-3 MEOs for most of the solutions, while the mean RMS value for B1IB3I combination is a few millimeters worse”. In the abstract, it needs to be clear what the “most of the solutions” refer to?

(2) In Table 1, why use GPS L1/L2 values adopted for all dual-frequency measurement combinations of BDS-3?

(3) In Section 3.2.1, the authors need to add the values of w1 and w2 used for IGSO and MEO satellites, respectively.

(4) In Section 3.2.1, the authors need to add a detailed description of CODE and GFZ solutions. Moreover, please explain what B1CB2a/CODE, B1CB2b/CODE, B1CB2a/GFZ and B1CB2b/GFZ refer to?

(5) “Meanwhile, B1IB3I/CODE and B1IB3I/GFZ are about one centimeter worse for both MEOs and IGSOs” . Please add an explanation.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

It is of great value to study the POD of BDS-3 satellites with multi-frequency measurements. The authors have made some novel contributions in experiments. I hope the following comments can help improve the manuscript.

1. Methodology and equations:

(1) In Eq(2), the carrier phase measurements also contain ambiguities. So do the CC combinations. Could the authors please provide more information on how to deal with the ambiguities to obtain the results in Figure 4, especially when there are cycle slips?

(2) The meanings of the letters R, A and C in Eq(3) are not mentioned . I think the sentence below Eq(3) should be “R, A and C represent the satellite orbit differences in radial, along-track and cross-track directions, respectively”.

2. Experiment and results

(1) The units in Figure 4 and Table 2 are all missing. In addition, the meanings of the MP columns in Table 2 are not explicit. Does the MP represent the average, maximum, standard deviation or something else of the multipath effects?

(2) Since it takes up several pages to analyze the data quality, i.e. C/N0, MP and pseudorange noise, is it possible to show this important part of work also in the title of the paper? For the authors’ reference only, and it is up to the authors. I also recommend that the correlation between the data analysis results and the orbit quality results can be strengthened in the analysis and/or discussion parts, which will highlight the significance of the data analysis in this work and make the whole manuscript more logically rigorous.

3. Language and format

There are some typos or grammatical errors, please check all through the manuscript and polish the language. For example,

(1) Both “altitude angle”(Line 115) and “elevation” are used. It would be better to uniformly adopt “elevation (angle)”.

(2) Line 121: “to be” should be removed.

(3) Line 126: “are” should be removed.

(4) Line 130: “series” should be removed.

(5) Line 154-155: denote(s), difference

(6) I think “B1I/B3I” might be better than “B1IB3I”. For the authors’ reference only.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

The manuscript presents interesting study outcomes on precise orbit determination dual-frequency measurement combinations. The research can be successfully applied in practice and strengthen the current state of the art. However, this issue is insufficiently exposed and needs to be improved. 

The primary impression after reading the text is that the authors present the result of an experiment, which is pure modeling. Each discovered method - predominantly theoretical- in the applied or natural sciences- needs validation. Regarding that, a reference to practical applications is mandatory. At least, it should be clarified what influence the proposed method has on the navigation systems. 

 Lines 84-95 contain formulas. It should be clarified whether it is the authors' finding or did they just cite them. I presume it's a literature overview partially confirmed by the position [33]. Nevertheless, it needs clarification.

The text demonstrates some technical errors, e.g., missing spaces (see lines 79, 89, etc.). It needs correction as well as providing language proofreading.

My most significant concern regarding the references is that the authors base their studies mostly on domestic (Chinese) authors. It's understandable; hence the cited authors demonstrate significant achievements in the study field, but except for them, there are numerous studies worldwide focusing in a similar subject. Please consider that fact before publishing.

The authors use the word 'to adopt meaning probably 'to adapt'. Hence, 'adopted' means something else than 'adapted'. Please check it thoroughly and correct it.

The conclusions end, leaving readers with a bit of a shortage. Pure results are discussed with no conclusions or recommendations. I would encourage the authors to extend the section by providing the readers with some more conclusions.

Should the text be corrected respectively, it should be submitted for a final checking cycle.

Author Response

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Author Response File: Author Response.docx

Reviewer 5 Report

The paper is well written, the layout is clear and the results highlight clearly the novelty of the work, which propose a new methodology for a Precise Orbit Determination (POD), based on the use of additional signals with respect to the existing techniques.

From a scientific point of view, the paper shows in an exhaustive way the goodness of the outcomes and of the presented methodology. I found it interesting.

However, some parameters and technical concepts are presented in a very concise way. Thus, to improve the readability of the paper, I suggest, where it is possible, to explain the meaning of the variables involved in the work and extend their definitions. Many achronims are introduced and they need to be described a little.

Similar, the orbital part, which also may be taken into account for introducing some useful considerations, is also presented in a very concise manner. Thus, to improve the quality of the paper and make it more complete, I suggest to stress a little this part, presenting the orbit elements of the used orbits and their relationship (advantages and drawbacks) with the indexes utilized in the work.

Also, given that some recent papers are exploiting the geometrical mutual inter-satellite visibility for improving the orbit determination, I am wondering if the POD proposed in this work may take advantage from the Inter-Satellite Link (ISL). Concerning this, the authors could spend some sentences in the Introduction to adress this point. Useful references can be found in the following papers:

Geometrical approach for an optimal inter-satellite visibility, M. Cinelli, E. Ortore, G. Laneve, C. Circi, Astrodynamics, volume 5, pp. 237–248 (2021).  

Xiong, K., Wei, C. L., Liu, L. D. Autonomous navigation for a group of satellites with star sensors and intersatellite links. Acta Astronautica, 2013, 86: 10–23.

Yu, F., He, Z., Xu, N. Autonomous navigation for GPS using inter-satellite ranging and relative direction measurements. Acta Astronautica, 2019, 160: 646–655.  

Finally, in my understanding, there is a minutia at pag. 3 line 83: probably in the sentence “which called MP probal…” it is better to write “which is called MP”.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

Dear Authors,

Thank you very much for your answers to my comments. I appreciate them and think your manuscript demonstrates a much better outcome now. However, I still have one small concern about Figure 1. I suggest you enlarge the symbols because, in the present form, they are much too small and hence - hard to distinguish. 

Except for that, I do not have any other objections, so - when you correct the figure, your text can proceed with publishing. Good luck!