BDS and Galileo: Global Ionosphere Modeling and the Comparison to GPS and GLONASS

Round 1

Reviewer 1 Report (Previous Reviewer 3)

The authors did a considerable work and the revised manuscript is significantly improved compared to the previous version. Their response is also quite comprehensive and covers in detail most of the raised issues. Especially, the apparent "better performance of Galileo" has been thoroughly investigated and light was shed into this topic.

The newly added text contains some minor syntax error which should be corrected, e.g.:
- Line 508: The colon after DCB should be deleted and the two sentences should be merged.
- Line 513: "month" instead of "mouth"
- line 539: Consider giving the number of MEO satellites as you are doing for GEO and IGSO.
- Line 540: "After comparing with C2I-C6I, C2I-C7I DCB is less stable": Please check the syntax.

To conclude, after making the above minor corrections the article can be considered for publication.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report (New Reviewer)

This manuscript explains the global ionospheric modeling of GPS, GLONASS, Galileo, and BDS systems under ionospheric quiet and active conditions in the year 2021 using a 15-order spherical harmonic function algorithm. The results show that the ionospheric modeling performance of GPS, GLONASS, and Galileo is comparable, while the BDS still needs to be improved. It also analyzes the characteristics of the ionospheric outliers and attributes them to the distribution of the stations. The work done in the manuscript is relatively significant and would be of interest to the ionospheric community. However, some revisions are suggested to improve the quality of the manuscript. Hence, I suggest a minor revision to implement the comments before acceptance of the manuscript.

Specific Comments:

·       Line 82 Please fix the citation error related to reference database [Error! Reference 82 source not found.,21]

·       Line 87 Around this line, I expect the authors should explain the ionospheric model parameters embedded in the GNSS systems such as Klobuchar in GPS and NeQuick-G in Galileo. I suggest referring and citing https://doi.org/10.1016/j.asr.2021.11.025 as it discusses about Klobuchar modeled ionospheric TEC from five GNSS constellations and highlights the ionosphere models for single frequency solutions.

·       Line 93 and discussion elsewhere It would be more interesting if the authors emphasized the plasmaspheric contributions in GPS-based ionospheric observations. The contemporary global and regional ionospheric model shortcomings must be highlighted. The authors may cite and refer https://doi.org/10.1016/j.actaastro.2020.12.005 by debating the above.

·       Line 200 The reason for selecting the particular period in 2021 in the study is not clarified. Moreover, it would have been better if the authors selected two sets of data (Equinox and Solstice)

·       Line 387 If TEC values are negative in some grid points due to the absence of observations and a few numbers of IPPs, what about the immediate positive values at adjacent grids of such cases where the magnitude is actually less due to fewer observations/IPPs ? Clarify the errors introduced due to scaling up all negative values irrespective of their magnitudes to zero levels.

·       Some figures are not clearly visible; the authors may need to change the color combination and resolution.

 

·       The discussion section could have been more intense by referring to the past works with respect to the outputs from the present study to underline the improvements. 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report (New Reviewer)

Dear Authors,

The comments for the article have been uploaded to the system as a PDF file.

 

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

It is of great value to study global ionosphere modeling. This paper analyzes the global ionospheric modeling accuracy under different geomagnetic conditions based on the 15th order spherical harmonic function. It especially takes into account the BDS-based ionospheric modeling with and without GEO satellites, which is of great significance in distinct the performance of BDS in global ionospheric modeling. The authors have made some contributions to this topic, however, there are still many inadequacies in this manuscript. I hope the following comments can help to improve the manuscript.

1) Line 105: "IPP" should be given its full name when it first appears.

 

2) Line 163: The number of stations mentioned here does not match what is described elsewhere in the paper, please check.

 

3) Figure 2 is not referenced in the text. Please add.

 

4) Line 267: Separate the sentence here and start a new sentence, “same as for CODE. The BIAS of”.

 

5) Line 391: “The biases of the BDS ionospheric model biases” should be “The biases of the BDS ionospheric model”.

 

6) Line 412: “and the insufficient number an of IPPs” is confusing, please check.

 

7) Line 503: “the satellite DCB value” should be “the satellite DCB values”.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Reviewer comments on manuscript # remotesensing-1919689

“BDS and Galileo: Global Ionosphere Modeling and the Comparison to GPS and GLONASS”

General Comments

The manuscript compares the performance of Global Ionospheric Models made on basis of BDS and Galileo GNSS systems with respect to similar ones based on GPS and GLONASS data.

At a glance, the manuscript needs a huge improvement regarding its editing content and organization. There are several grammatical issues which makes its reading and understanding very challenging even for expert readers. Additionally, from the methodological point of view, I would expect to get more insight from a twenty-three pages article.

Unfortunately, there are many drawbacks in its organization that decrease the manuscript's worth. The conclusions section needs to be improved in order to stress the author's key findings. In its present form, the conclusions seems to be pretty obvious and contains term definitions that will be moved to the main body of the section 3.

I need to stress that the conclusions section is intended to summarize the key findings of the paper, not to add new knowledge.

In my modest opinion, the grammar needs to be strongly improved in order to avoid mechanical repetition of some terms and enrich the use of the language. This will contribute to ease the reading and shorten the whole manuscript length. In this way, I recommend submitting the next version to a native English style reviewer.

Particular Comments

Minor Comments are marked in the annotated PDF file enclosed Referencing seems adequate, but its management through the main text need to be improved.  

General Assessment
In virtue of the aforementioned comments, I will recommend rejecting and encourage resubmitting a thoroughly revised version.

 

 

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper deals with an active research topic, namely the computation of Global Ionospheric Models from multi-GNSS observations. The paper is generally well structured and prepared. The amount of work behind the article is satisfactory. The usage of English is good, the references are appropriate and the title describes well the study. However, there are some weaknesses that should be addressed, as described in the following:

Experiment design
The authors use the average BIAS (Fig 6.) to assess the performance of each system. This is not a fair comparison due to the smaller number of BDS stations. BDS appears to have a systematic difference. In order to ensure that no unmodelled errors are hidden behind these increased BIASES for BDS, the comparison should be made for a small area where the BDS IPP coverage is comparable to that of the other systems. For example, looking at Fig.3 the coverage in Europe is comparable for all four GNSS. However, looking at Fig 5. we see that the BIAS for BDS is considerably higher compared to GPS, GLONASS and Galileo also in Europe. This fact is questioning the results for BDS.
The authors should address this in detail and explain how can this happen. Otherwise, doubts may arise about unmodelled errors, errors in the calculations or in the methodology for the BDS satellites. I would suggest: a) checking the calculations (DCBs of BDS ect.) and b) computing a more detailed model using higher order (>15^th) spherical harmonic functions. This could be done in an area (for example in Europe) where the stations network is very dense. If the high BIAS values remain, it seems rather unlike that the large BIAS is due to sparse network conditions.

Data analysis - Conclusions
In lines 347-348 it is stated: “The biases of the BDS ionospheric model are 0~4 TECU larger than those of the other systems, and the biases in the southern hemisphere are smaller than those in the Northern hemisphere.”
In lines 385-391 it is stated: “The biases of the BDS ionospheric model are significantly smaller in the mid-latitude and high-latitude regions of the northern hemisphere than those of in the southern hemisphere. The biases in the northern hemisphere are basically stable within 4 TECU,  while the biases in the southern hemisphere are in the range of 4-8 TECU. The biases in the southern hemisphere are the largest and the main reason is that there are fewer stations in the high and mid-latitude regions of the southern hemisphere, resulting in less IPPs than the northern hemisphere.”
It comes out that in the case of the quite period the biases are smaller in the southern hemisphere, whereas in the active period the opposite is true. The second outcome is attributed by the authors to the fewer stations of the southern hemisphere. However, the station distribution is common for both time periods (quiet and active). If the reason is indeed the poor station distribution, one would expect that the biggest biases would appear for both periods in the southern hemisphere. This should be addressed carefully by the authors.

The discussion of the appearance of 0-values seems to consist another weakness of the article. In line 410-412 the 0-values of BDS are attributed to the few stations tracking BDS. However, if we look at Figure 1, we see that the Galileo stations are very sparse in some areas (e.g. south America and Antarctica). However, comparing figures 9a and 9 c, we see that Galileo performs considerably better than GPS (which has the densest stations network). Thus, the sparse stations seem not to be the cause for the 0-values, as the authors claim. Thus, the authors conclusions may be questioned. A way to investigate the origin of both high biases and 0-values would be the comparison of the VTEC obtained by different GNSSs (e.g GPS vs BDS, GPS vs GAL etc.) at IPPs that are very close to each other. For neighbor IPPs the VTEC values from all GNSSs should be very close to each other. If not, an error in the calculations or in the modelling (especially for BDS) is quite possible. Is believe such an investigation should be made and the results should be added in the article.

Related to the above issue is maybe also the following: Comparing Fig. 1 with Fig. 3 some conflicts seem to appear. For example, in Fig. 1 only five stations in Antarctica seem to track Galileo (blue color) whereas in Fig. 3c eight Galileo stations appear stations in Antarctica. Similar conflicts appear also for other GNSS. Please check thoroughly.

Looking at Figure 18 we clearly see that the differences of BDS DCBs are much higher for BDS-2 satellites. Are the authors using also the BDS-2 satellites for the computation of VTEC? If yes, this could partly explain the high number of 0-values and (to some extend) the increased biases w.r.t. CODE. In this case I suggest to recompute the BDS GIM using only the BDS-3 satellites.

Typos and other corrections
There are some minor corrections to be done, as described in detail in the separate attached file.

Final recommendation
The paper can be considered for publication, after major revision.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Manuscript Number: remotesensing-1919689

Full Title: BDS and Galileo: Global Ionosphere Modeling and the Comparison to GPS and GLONASS

 The Article submitted to Remote Sensing MDPI by Hu Wang et al. studies the ionosphere modelling based on the 15th order spherical harmonic function, of Galileo, BDS, GPS and Glonass systems. The paper is analyzed in original submission. The work has been prepared in accordance with the authors' instructions, with the only exception of the Discussion section, which is missing but mandatory in RS.

The work is an enjoyable read, also because it covers very interesting topics from a scientific point of view and I believe that with the necessary amendments it can be published on RS.

Here are my suggestions for improving the work:

1)      Line 20-48, please check the instructions for authors to RS and verify that the abstract should be written as one paragraph (of about 300 words). In my opinion should be reduced and summarised;

2)      Line 51-78, I permit to suggest to insert here the Reference Frames of each individual constellation (GPS, Glonass, Galileo, Beidou) as well as the Frequency/MHz and Observation Codes, in order to give more thoroughness to the topic. See (but is not mandatory): DOI 10.1007/s00190-008-0300-3, DOI 10.1007/PL00012776, DOI 10.1016/j.asr.2010.02.001, DOI 10.1007/978-981-13-7759-4_7, https://doi.org/10.3390/rs14163930; 

3)      Line 82, please define PPP;

4)      Line 105, please define IPP;

5)      Line 108, please define DCB;

6)      Line 120, please define STD;

7)      Line 163, please chech “340”, earlier you reported 364 stations;

8)      Line 169, please modify “In addition”, was just written up;

9)      Line 197, please increases the size of the figure, it is not readable for the reader;

10)  Line 214, please modify the figure, is not readable the labels for the reader;

11)  Line 218, please insert a reference when you reported an equation;

12)  Line 233, please see previous comment 11;

13)  Line 239, please see previous comment 11;

14)  Line 241, please see previous comment 11;

15)  Line 255, please see previous comment 11;

16)  Line 271, please divide Result from Analysis and insert after Result the Discussion section, which is missing but mandatory in RS;

17)  Line 341-344, please removes the spaces (e.g. 30°N - 60°N and so on);

18)  Line 354, please increases the size of the figure, it is not readable for the reader;

19)  Line 398, please increases the size of the figure, it is not readable for the reader;

20)  Line 444, please define “Area” in the table. Is NH Northern hemisphere?

21)  Line 523, please insert here the Discussion Section.

I will be available to authors for the assessment of the manuscript at the subsequent submission.

Best regards

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The revised document consists an improvement of the initially submitted version. The authors responded successfully to the detailed comments (with a single exception. See in the attached file the comment for line 50). They also responded adequately to some of the comments w.r.t. methodological questions, experiment control and validity of the drawn conclusions. However, some of these comments are not satisfactory addressed. I mainly refer to point 3 of my first review.
For point 3 the authors did not provide any evidence that the VTEC values they estimate from the different GNNSs are compatible to each other. The authors did not make the suggested check, i.e. to compare the VTEC values obtained by different GNSS (e.g GPS vs BDS, GPS vs GAL etc.) at nearby IPPs to see if they are close to each other, as they should be. This is an important (and not time consuming) validation test, which I was expect to see. Furthermore, I can hardly accept the authors’ answer regarding the better performance of Galileo in certain areas of the southern hemisphere. The authors claim in their response that “Galileo performs considerably better than GPS due to the fact that more Galileo satellites than GPS were observed by stations in the high latitudes of the Southern Hemisphere at that time.” I strongly doubt about this “fact” for three reasons:
1) In 2021 the total number of visible GPS satellites was 31, whereas only 24 Galileo satellites were available. Based on the orbital characteristics of GPS & Galileo, it seems rather unlikely that more Galileo satellites were visible. This is confirmed also by the following two facts.
2) As we can see comparing Figures 3 (a) and (c), the IPPs of GPS are clearly more than the IPPs of Galileo, which means that more GPS satellites were visible.
3) Computing the number of visible satellites for August 2021 at latitude S 49 deg and S 68 deg, we clearly see that the visible Galileo satellites are less than the visible GPS satellites (the plots are given in the pdf file with the detailed comments).

General recommendation
In the absence of the afore mentioned questionable points, I would recommend the acceptance of the paper with minor corrections (actually only the correction for line 50). However, I’m not convinced about some explanations given by the authors and the interpretation of their result, particularly regarding the better performance of Galileo. Thus, I am not fully convinced that the article satisfies the high standards of the journal. So, it remains to the editor to judge if the article is suitable for “Remote Sensing”.  

Comments for author File: Comments.pdf

Reviewer 4 Report

Second revision of the work: the authors have thoroughly revised their work, which is now near publication in RS.

Only point 2 of the previous revision has been partially addressed, and in particular I request that it be included in the text Reference Frames of each individual constellation (GPS, Glonass, Galileo, Beidou), for the readers.

Best regards