Pseudorange Bias Analysis and Preliminary Service Performance Evaluation of BDSBAS

Round 1

Reviewer 1 Report

Manuscript Number: remotesensing-1386342

Full Title: Pseudorange bias analysis and preliminary service performance evaluation of BDSBAS

General comment

The work submitted to RS concerns pseudorange bias errors of BDSBAS and their effect on the performance of the service. Although the topics is of considerable interest to the scientific community, this reviewer does not recognise any originality in the work, since numerous studies have been carried out in this field and are not even taken as references. Unfortunately, I cannot recommend publication in this version for the reasons that follow in the specific comments.

Specific comment

With regret, the paper sounds a rather disordered for a scientific application on RS MDPI, since first of all, it does not follow the instructions for authors, which include Introduction, Materials and Methods, Results, Discussion, Conclusions, and so on.

Secondly, I cannot understand why the text first mentions a study limited to the range August 25 to August 31, 2020 and then mentions a period between September 1, 2020 to September 30, 2020.

In addition, the results of some tables (3, 4, 5) only refer to the mean value and no statistical analysis is carried out on the data. The figures are also presented in low resolution and appear to be taken from screenshots of data processing software. The equations are not easily readable for the reader, not least because the materials and methods section is not present. Discussion section is also missing and is mandatory in RS.

Last but not least, there are only 15 articles in the bibliography and I think that this is not possible for submitting a paper in an journal like Remote Sensing.

In my opinion, the authors have not provided enough novelty, either in terms of the methodology or the case study, to consider this manuscript as useful for the scientific community.

From all the above it is my belief that the present manuscript is not up to standard to be published in Remote Sensing, and my recommendation is to Reject the manuscript.

 

Technical comment

In relation to the previous points, I would like to advise the authors to deeply modify the structure of the work as follows, for future submission:

• read the authors' instructions and set up the paper according to the following scheme: Introduction, Materials and Methods, Results, Discussion, Conclusions;
• the section “Introduction” must be literature review revised to be more in-depth and informative, with particular attention to other articles concerning similar approaches on the topic; I attach the list of the most cited paper on the topic;
• lines 45, 54, 59, 68 and so on, the reference must be included in the [ ], see instructions for authors;
• line 84, please move table 1 in the another section of the paper, is not appropriate to include it in the introduction;
• line 101, please insert here the section “Materials and Methods”;
• line 102-104, please reformulate, I do not understand the purpose of the sentence;
• line 136, eq. 1 and so on, is not readable, perhaps because it is not written in a word editor;
• line 168, why “G29” was selected as the reference satellite? Authors have to persuade the reviewer and the reader of their selection, so it should be specified in the text;
• line 170, please define the range of the time series for the readers;
• line 175, figure 1, please modify is of low quality and too small;
• line 177, figure 2, please modify is of low quality and too small;
• line 183, Table 2, why do you only report the mean value? To make the results consistent you have to do a thorough statistical analysis, so change the table;
• line 231, why did you select only one week of data? You have to justify this option to the reader;
• line 253, figure 5, please insert the words above the figures in the caption;
• line 285, please define the five station. Are they part of the worldwide Crustal Movement Observation Network of China (CMONC)? Please modify in the text for the readers;
• line 287, now the range is between September 1, 2020 to September 30, 2020. Why do you use a different range from the first one? Please modify in the text for the readers;
• line 290, table 3, please refer to previous comment 12;
• line 292, table 4, please refer to previous comment 12;
• line 372, figure 7, please modify is of low quality and too small; also insert in the caption the datum (WGS 1984, UTM, ESPG) for the reader;
• line 374, figure 8, please refer to previous comment 19;
• line 375-377, please reformulate, I do not understand the purpose of the sentence; also consider that the reader can be from anywhere in the world and not know where the borders between Mongolia and China are;
• line 385, table 5, please refer to previous comment 12;
• line 394-395, please shows the results of all five stations you have analysed, for the reader;
• line 416, figure 11, please modify and shows the results of all five stations you have analysed, for the reader;
• line 418, figure 12, please insert the five station that you used; also insert in the caption the datum (WGS 1984, UTM, ESPG) for the reader;
• line 436, please insert Discussion section, is mandatory in RS.

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

Best regards

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Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript discusses two aspects of the BDSSBAS system: the handling of the GPS code biases and the positioning accuracy, availability and integrity that can be achieved by the system.
My main concern is a lack of rigor and clarity.   In my opinion, the following points need to be improved before warranting publication.

Table 1: explain the source.  Where do these numbers come from and what do they represent?

Line 109: “Since the receiver clock error cannot absorb the errors between the satellite segment and the propagation segment”.  What is the “propagation segment”?

  
Line 129: “biases of different sizes and symbols”.  What is the meaning of “symbols” here?

Equation (1): what is the difference between DCB/IFB and \alpha_r^i.   Would the pseudorange bias (\alpha_r^i) not absorb DCB/IFB?  What is the reasoning behind separating these terms.  To be explained.

Line 139: IFB/DCB refer to biases between two frequencies/signals.  Equation (1) is about frequency Lj, so what is the IFB/DCB to be considered here? Is it with respect to L1?  To be clarified.

Line 157: “track that spans at least 24hours”.  A satellite track is a few hours max.  Explain in a few words how this is extended to 24hours.

Equation (3).  DCB should be IFB.

Figure 1&2: it is explained earlier that tracks of 24 hours are used.  Here only a few hours are shown.  Isn’t there any plot illustrating the 24-hour tracks?
Figure 1&2: the “biases” are difficult to see because of long transients.  Is there any measure taken to remove the effect of those transients? This deserves a note in the paper, as it does not seem possible to extract meaningful biases from these plots.

Table 2: do these values correspond to the data shown in Figure 1&2, or to a different dataset?

Equation (5).  In this section, there is a confusion between L1CA and L1P.  For example, P_L1CA in equation (5) is described in the text as the pseudorange of L1P.  This needs to be clarified.  It seems from the text that the L1CA/L2P combination is made.  However, the clock model transmitted by the GPS satellites refer to the L1P/L2P combination, and there are known biases between GPS L1CA and L1P.  This aspect must be discussed and clarified as it is an essential contribution to the biases.

Section 2: It would be useful to have a note indicating why only the GPS biases are considered.  Is this because the BDS biases are smaller, or just because only GPS has been studied?

Equation (7): how can the pseudorange depend on d_corr?  d_corr is a correction to be applied to a pseudorange, it is not a term contributing to the pseudorange.  This equation seems incorrect.

Line 285: “five monitoring station”.  The paper should show the position of these 5 stations so that users can interpret the results.  Only the position of station 4 is shown in Figure 12

Line 302: “The integrity parameters are the detection of difference corrections”.   This is not understood.  To be rewritten or clarified.

Line 395: “Station 4 (right)”.  Figure 9 does not show data from Station 4, to be corrected.

Figure 10: show the AL level on these figures.

 

Some typos:
Line 199, title of Fig 5: typo in pseudorange
Line 220: “is the IFB and alpha between is the pseudorange”

Line 249: “one station is compared shown in Fig. 5”
Line 278: typo in coordinates
Line 287: “solving corrections are selected”

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper shows the pseudorange bias errors estimated in the BDSBAS monitoring receivers and their effect on BDSBAS differential corrections. The service accuracy, integrity and availability of BDSBAS single-frequency service and DFMC service were also analyzed with real data and the results are very interesting. However, the paper needs to be revised and improved in the following aspects before it is published: There are many grammatical errors in English. The expression of parameter terms in the paper should be standardized and unified, like α_(p1-pc) in Eq 6 should be α_(p1-pl1l2), consistent with the above text. Why do some lines have obvious convergence process at the beginning of the arc in Fig. 2? Please change “IF combination” to “L1P/L2P combination” in Fig. 3. The pseudorange biases shown in Fig. 4 are the results from L1C/A signal, L1/L2 combination or the difference between pseudorange biases in single signal and pseudorange biases in dual-frequency combination? I would like to modify the 2.2 section title from “Calculation method ” to “Correction methods for BDSBAS differential corrections” I would like to change the first sentence above Fig. 5 from “With the different satellite clock corrections with and without pseudorange bias correction, the positioning accuracies of one station is compared shown in Fig. 5.” to “With two kinds of satellite clock corrections, one kind is modified with pseudorange biases and the other is not, the differential positioning accuracies for a monitoring receiver are compared shown in Fig. 5”. Please explain which correction is the long-term correction in Eq (8) and Eq(9). Please explain the difference between the parameters dcor in Eq(8) and Eq(9). Please change the “dual-frequency service” to “DFMC service”.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear authors,
thank you for correctly editing the paper in accordance with my review.
I propose to accept in present form

Best regards

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have made significant changes in their methodology, introducing concepts which were not addressed in their initial submission (e.g. the difference between L1CA and L1P).   The problem is that the introduction of these new concepts is not consistent throughout the paper, e.g. the code bias is defined in the introduction as dependent on receiver parameters (filtering, etc), and suddenly the major source of bias appears to be the L1P-L1CA mismatch .  It is also not properly traced with the "track changes".  In addition, the text still needs English proofreading to correct the many grammatical errors.

In my opinion, this manuscript is not a second revision of the previous manuscript, but a new paper.   I would recommend the authors to try to improve the consistency of their methodology (getting the focus on L1CA-L1P differences from the start), and to start a new submission.

Author Response

Please see the attachment.

Author Response File: Author Response.docx