Assessing the Performance of Precise Point Positioning (PPP) with the Fully Serviceable Multi-GNSS Constellations: GPS, BDS-3, and Galileo
Round 1
Reviewer 1 Report
In this manuscript, the authors have carried out a comprehensive analysis and validation of positioning performance in terms of positioning accuracy (RMS) and convergence time, which is derived from BDS-3 11 and Galileo precise point positioning (PPP) solutions at a global scale. In my opinion, what has been done is valuable, but the manuscript needs revision.
1） Line 136, the figure 1 should be replaced with figure 2
2） All figures and tables should be checked. Such as, the first letter of every figure title should be capitalized, Units should be added to the table, amount label should be given in the figure 4, figure 1 is only distribution map of selected station. the statement of ‘median RMS of static PPP’ is obviously redundant.
3） How to get the results of figures 2 and 3? By interpolation?
4） In subsection 3.1, That two stations were selected to analyze the Geographical distribution of different systems performance is inappropriate and it is recommended to delete them, Such as the line 180 ‘GPS performance is essentially the same in both the northern and southern hemispheres, whereas Galileo and BDS-3 performance vary significantly between the two hemispheres.’.
5） In subsection 3.3, the experimental design should be simplified, the combinations of GC、GE and CE are enough to analyze their performances.
6） The whole paper needs to be carefully checked. Some descriptions seem a little cumbersome need to be polished.
Author Response
Reply to comments on manuscript entitled “Assessing the performance of precise point positioning (PPP) with the fully serviceable multi-GNSS constellations: GPS, BDS-3, and Galileo”
Thank you very much for your comments. We have revised the manuscript carefully and responded point by point to the comments as below. (C and R indicate comment and response, respectively). Our revisions are highlighted in the revised manuscript with the “Track Changes” function.
C: Line 136, the figure 1 should be replaced with figure 2
R: We are sorry for the mistake and have modified it in the revision.
C: All figures and tables should be checked. Such as, the first letter of every figure title should be capitalized, Units should be added to the table, amount label should be given in the figure 4, figure 1 is only distribution map of selected station. the statement of ‘median RMS of static PPP’ is obviously redundant.
R: Thank you for your comments. We have revised them.
C: How to get the results of figures 2 and 3? By interpolation?
R: According to the satellite position provided by the precise ephemeris, we divide the global longitudes and latitudes into 5x2 grids to calculate the number of visible satellites and PDOP value of each grid.
C: In subsection 3.1, That two stations were selected to analyze the Geographical distribution of different systems performance is inappropriate and it is recommended to delete them, Such as the line 180 ‘GPS performance is essentially the same in both the northern and southern hemispheres, whereas Galileo and BDS-3 performance vary significantly between the two hemispheres.
R: Thanks for your comment. We have delete the corresponding figures and description for the selected two stations.
C: In subsection 3.3, the experimental design should be simplified, the combinations of GC、GE and CE are enough to analyze their performances.
R: Accepted and modified.
C: The whole paper needs to be carefully checked. Some descriptions seem a little cumbersome need to be polished.
R: Thank you for your suggestions. We have already found a native English speaker to help us polish the manuscript.
Reviewer 2 Report
It is of great value to study the performance of PPP with multi-GNSS. The theory of PPP has been properly introduced, and plenty of experiments and a series of analysis have been conducted. The manuscript is supposed to provide valuable reference in both static and kinematic PPP with multi-GNSS. I hope the following comments can help improve the manuscript.
1. I am wondering whether there are some constraints between epochs in kinematic PPP.
2. It seems that the performance of dual-system PPP with EC is worse than that of GPS-only PPP. Could the authors provide some reasonable explanations on this? According to Figures 2 and 3, the number of visible satellites and PDOP with EC should be no worse than those with GPS-only? Does it imply that the EC measurements are significantly outperformed by GPS measurements? By the way, in dual-system experiments, what about the weighting strategies for different systems?
3. It seems that the two stations PTGG and ALIC are frequently used and discussed in the experiments. Are they typical enough to represent the northern and southern hemispheres? I recommend the authors to provide more information about the two stations, such as their latitudes, surroundings, continental or oceanic, which may help make the results more convincing. At least, their name can be displayed in Figure 1. Especially in section 3.1, it seems hasty to conclude that GPS performs better or worse in southern hemisphere than in northern hemisphere (Line 184-185, 225-227), only based on the results of the two single stations in one day. What about the other stations?
4. Could the authors please explain the reason for the worst performance in latitude areas? Although the number of visible satellites are mentioned in Line 342-343, the fewest visible satellites appear in mid- rather than low-latitude areas? More rational explanation is recommended.
5. In Section 3.3, the contents in the figures are repeated too much. The most extreme example is that all the contents in Figures 18 and 19 can be found in the previous figures, and there is nothing new in these two figures. By the way, an obvious mistake is that Figures 18 and 19 are exactly the same. Therefore, I recommend the authors can parallelly put all the results together and make the comparison more direct. It will not only convenient for the readers, but also save paragraphs and pages for some repeating description or discussion, as well as avoid the mistakes as occurred in Figures 18 and 19.
6. The authors please briefly introduce the calculation of RMS and convergence time in the assessment. What is the true value of the station coordinates? Usually, the IGS solutions are used as the reference, and then can the authors know which product do the IGS solutions employ? Does the product used for the reference coordinates make differences in the comparison results? For example, if the reference coordinates are computed with GFZ products, the performance of GFZ might seem better. Is it true or not? And please clarify the standards of convergence.
6. Some minors
(1) Line 173: single-systems -> single-system, and it is not an isolated case. Please check all through the manuscript.
(2) Line 269: what dose “bar” mean?
(3) The units used in the tables are not specified, although it is self-evident.
(4) Line 452: Galileo-based -> BDS-3-based
(5) Some of the figure captions are not explicit, such as “same as …”, which are not very readable. Please follow the editors’ advice, and just neglect my comments if the editor think it’s OK.
Author Response
Reply to comments on manuscript entitled “Assessing the performance of precise point positioning (PPP) with the fully serviceable multi-GNSS constellations: GPS, BDS-3, and Galileo”
Thank you very much for your comments. We have revised the manuscript carefully and responded point by point to the comments as below. (C and R indicate comment and response, respectively). Our revisions are highlighted in the revised manuscript with the “Track Changes” function.
C: I am wondering whether there are some constraints between epochs in kinematic PPP.
R: For kinematic PPP, the station coordinates and receiver clock offsets are usually modeled as white noise process, hence, there are no constraints between epochs for these parameters. The zenith wet tropospheric delays and carrier phase ambiguities are estimated as random walk and constant process, respectively. Some constraints between epochs will be added on these parameters.
C: It seems that the performance of dual-system PPP with EC is worse than that of GPS-only PPP. Could the authors provide some reasonable explanations on this? According to Figures 2 and 3, the number of visible satellites and PDOP with EC should be no worse than those with GPS-only? Does it imply that the EC measurements are significantly outperformed by GPS measurements? By the way, in dual-system experiments, what about the weighting strategies for different systems?
R: Since the modeling precision of solar radiation pressure, satellite and receiver antenna phase corrections for the new satellite systems (i.e., Galileo and BDS-3) is still limited, the positioning performance of Galileo-only or BDS3-only is more or less worse than that of GPS-only. In the case that both are worse than GPS, the combination of the two systems is not necessarily better than GPS, which is determined by the characteristics of filtering or least square estimation.
For the dual systems, the weighting strategies for different systems is 1:1.
C: It seems that the two stations PTGG and ALIC are frequently used and discussed in the experiments. Are they typical enough to represent the northern and southern hemispheres? I recommend the authors to provide more information about the two stations, such as their latitudes, surroundings, continental or oceanic, which may help make the results more convincing. At least, their name can be displayed in Figure 1. Especially in section 3.1, it seems hasty to conclude that GPS performs better or worse in southern hemisphere than in northern hemisphere (Line 184-185, 225-227), only based on the results of the two single stations in one day. What about the other stations?
R: Thanks for your comments. The selected two stations really can't fully explain the problem. The first reviewer recommended us to delete them. Hence, we have delete the two stations and the corresponding descriptions in the revision.
C: Could the authors please explain the reason for the worst performance in latitude areas? Although the number of visible satellites are mentioned in Line 342-343, the fewest visible satellites appear in mid- rather than low-latitude areas? More rational explanation is recommended.
R: PPP especially for the kinematic PPP is highly dependent on the precision and quality of the precise satellite orbit and clock products, which are estimated from the GNSS observations distributed on the surface of the Earth. The differences in the positioning performance of PPP at different latitudes is likely to be related to the quality of the observed satellite orbits and clock products.
C: In Section 3.3, the contents in the figures are repeated too much. The most extreme example is that all the contents in Figures 18 and 19 can be found in the previous figures, and there is nothing new in these two figures. By the way, an obvious mistake is that Figures 18 and 19 are exactly the same. Therefore, I recommend the authors can parallelly put all the results together and make the comparison more direct. It will not only convenient for the readers, but also save paragraphs and pages for some repeating description or discussion, as well as avoid the mistakes as occurred in Figures 18 and 19.
R: Thank you for your comments. We have modified them in the revision.
C: The authors please briefly introduce the calculation of RMS and convergence time in the assessment. What is the true value of the station coordinates? Usually, the IGS solutions are used as the reference, and then can the authors know which product do the IGS solutions employ? Does the product used for the reference coordinates make differences in the comparison results? For example, if the reference coordinates are computed with GFZ products, the performance of GFZ might seem better. Is it true or not? And please clarify the standards of convergence.
R: Here, we define “convergence” as obtaining a 3D positioning error less than the predefined threshold at the current epoch and the following twenty epochs, which has been adopted from Li and Zhang (2014). Only when the positioning errors of all twenty epochs are within the threshold, we consider that the position has converged at the current epoch. Here, the threshold is 1 dm, which has been suggested by Lou et al. (2016).
For positioning accuracy (RMS), it is calculated through the same convergence period; here, we have chosen 2 h after the start of PPP filtering for each test.
We use IGS weekly SINEX solutions as reference, and they are relatively strong and authoritative reference results, which are often used in PPP analysis.
C: Line 173: single-systems -> single-system, and it is not an isolated case. Please check all through the manuscript.
R: Accepted and modified.
C: Line 269: what dose “bar” mean?
R: We are sorry for the wrong expression. We have used ‘use’ instead of ‘bar’ in the revision.
C: The units used in the tables are not specified, although it is self-evident.
R: Sorry that we overlooked it. We have added them in the revision.
C: Line 452: Galileo-based -> BDS-3-based
R: Accepted and modified.
C: Some of the figure captions are not explicit, such as “same as …”, which are not very readable. Please follow the editors’ advice, and just neglect my comments if the editor think it’s OK.
R: Thanks for your comments. We have revised the figure captions.
Reviewer 3 Report
Manuscript Number: remotesensing-2136692
Full Title: Assessing the performance of precise point positioning (PPP) with the fully serviceable multi-GNSS constellations: GPS, BDS-3, and Galileo
General comment
The article submitted to Remote Sensing MDPI by Zunyao Hou and Feng Zhou studied multi GNSS constellation GPS, BDS-3, and Galileo, in order to provide a comprehensive assessment for the different system PPP solutions on a global scale.
The paper, analysed in original submission, has been edited according to the RS MDPI instructions for authors and is set up correctly in all its paragraphs. In my opinion, the study is adequately original and the methodological approach is rigorous. The methods and tools are described with details to allow another researcher to reproduce the results.
For this reviewer is unambiguous, however, that the study shows a considerable effort on the part of authors to implement the project and its potential applications. The content of the paper is potentially interesting, and with improvement, it might be published.
However, in its current form it is not acceptable, in my opinion (especially for a high quality journal like RS MDPI) for several reasons, that follow in the specific comments. On the basis of what has been mentioned, I permit to suggest (line by line), in agreement with the notes for MDPI RS reviewers:
Specific comment
With regret, the paper shows some limitations that need to be resolved by the authors with a new and completely revised version of the manuscript.
In particular the authors must follow these recommendations:
1. Line 28, the introduction partially summarises the scientific background and needs to be expanded with other works in the literature, I suggest (but is not mandatory): DOI: 10.3390/rs14163930, DOI: https://doi.org/10.1186/s43020-020-00014-y, DOI:10.1088/1361-6501/ab69d5;
2. Line 70, please chech “a” after “.”;
3. Line 122, please check “(https://github.com/zhouforme0318/GAMPII-GOOD)”; in instruction for authors it be reported in a different manner in the references;
4. Line 124, please check (https://www.ngs.noaa.gov/gps-toolbox/GAMP.htm); in instruction for authors it be reported in a different manner in the references;
5. Line 125. I suggest the authors to include other works with different open source software for PPP processing, such as: DOI: 10.3390/app10165420, DOI: 10.33012/2019.17128;
6. Line 136, the authors correctly show the number of satellites and the PDOP for the day 1 September 2022. Why only this day since the dataset refers to a month? Shouldn't other days be shown, e.g. after one week, two, three? The authors must justify this choice to the reader in the text;
7. Line 168, figure 4, the authors should also indicate in the caption G (I suppose GPS) and E (I suppose Galileo), for the reader;
8. Line 173-175, the authors give a summary of only two CORS out of 143. Why not also include others? I suggest indicating at least five more CORS with similar properties. It is also appropriate for the text to be more readable that the CORS PTGG and ALIC is showed with the characteristics (name, location, type of instrumentation used, etc.);
9. Line 220, please see previous comment 7;
10. Line 267, please see previous comment 7;
11. Line 313, please check the size of the captions;
12. Line 368, please see previous comment 7;
13. Line 392, please see previous comment 7;
14. Line 411, please see previous comment 7;
15. Line 429, please see previous comment 7;
16. Line 446, please see previous comment 7;
17. Line 464, please see previous comment 7.
In conclusion the work deserves publication after major revisions, in my opinion. I will be available to authors for the assessment of the manuscript at the subsequent submission.
Best regards
Author Response
Reply to comments on manuscript entitled “Assessing the performance of precise point positioning (PPP) with the fully serviceable multi-GNSS constellations: GPS, BDS-3, and Galileo”
Thank you very much for your comments. We have revised the manuscript carefully and responded point by point to the comments as below. (C and R indicate comment and response, respectively). Our revisions are highlighted in the revised manuscript with the “Track Changes” function.
C: Line 28, the introduction partially summarises the scientific background and needs to be expanded with other works in the literature, I suggest (but is not mandatory): DOI: 10.3390/rs14163930, DOI: https://doi.org/10.1186/s43020-020-00014-y, DOI:10.1088/1361-6501/ab69d5.
R: We have added them as reference, and the corresponding descriptions are also added in the revision.
C: Line 70, please chech “a” after “.”.
R: Accepted and modified.
C: Line 122, please check “(https://github.com/zhouforme0318/GAMPII-GOOD)”; in instruction for authors it be reported in a different manner in the references.
R: Thanks for your comment. We have modified it in the revision.
C: Line 124, please check (https://www.ngs.noaa.gov/gps-toolbox/GAMP.htm); in instruction for authors it be reported in a different manner in the references.
R: Accepted and modified.
C: Line 125. I suggest the authors to include other works with different open source software for PPP processing, such as: DOI: 10.3390/app10165420, DOI: 10.33012/2019.17128.
R: Thank you for your suggestions. The main purpose of this study is to comprehensively evaluate the global positioning performance of the new emerging BDS-3 and Galileo systems and the overall comparative analysis of PPP with GPS. Different software for PPP processing may have some differences, mainly caused by the data processing and quality control strategies. In future work we will involve more in-depth research on this aspect.
C: Line 136, the authors correctly show the number of satellites and the PDOP for the day 1 September 2022. Why only this day since the dataset refers to a month? Shouldn't other days be shown, e.g. after one week, two, three? The authors must justify this choice to the reader in the text
R: We used the GNSS data of a month in September for PPP processing, and September 1st is the beginning of this month. For the number of satellites and PDOP values, the daily variation is negligible, which is related to the orbital period of the satellite system.
C: Line 168, figure 4, the authors should also indicate in the caption G (I suppose GPS) and E (I suppose Galileo), for the reader.
R: Accepted and modified.
C: Line 173-175, the authors give a summary of only two CORS out of 143. Why not also include others? I suggest indicating at least five more CORS with similar properties. It is also appropriate for the text to be more readable that the CORS PTGG and ALIC is showed with the characteristics (name, location, type of instrumentation used, etc.).
R: Thanks for your comments. The selected two stations really can't fully explain the problem. The first reviewer recommended us to delete them. Hence, we have delete the two stations and the corresponding descriptions in the revision.
C: Line 220, please see previous comment 7; Line 267, please see previous comment 7; Line 368, please see previous comment 7; Line 392, please see previous comment 7; Line 411, please see previous comment 7; Line 429, please see previous comment 7; Line 446, please see previous comment 7; Line 464, please see previous comment 7.
R: Accepted and modified.
C: Line 313, please check the size of the captions.
R: Accepted and modified.
Round 2
Reviewer 3 Report
Dear Aauthors,
in the new version of the paper all requests for explanation have been made.
I propose to accept in this form.
Best regards
