Theoretical Concept for a Mobile Underwater Radio-Navigation System Using Pseudolite Buoys

Round 1

Reviewer 1 Report

New Mobile Underwater Positioning System –Concept, Signal Design, Propagation, and
Performance Evaluation
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Abstract needs to be rewritten

Concepts of localisation and methods adopted needs to be explained in more detailed way

How is latency measured and affects the performance of the system

Overall a good manuscript and can be published with minor revisions.

 

Author Response

Thank you very much for all your time and effort that you have dedicated to providing your valuable feedback on our manuscript. We are grateful for your insightful comments on our paper. Please find attached the detail responses to your comments. 

Sincerely,

Anja Grosch

Author Response File: Author Response.docx

Reviewer 2 Report

I was very excited to read the paper, and I like the idea. However, the following concerns should be addressed before publication.

General issues:

• The concept and findings in the paper are of very theoretic nature only and lack any practical proof of concept. The main issue the reviewer has with this is that it is absolutely unclear, how well the theoretic concept will work in reality. Likewise, the figures in the evaluation give little access to real-world performance and validity. There should be at least some proof-of-concept study in the paper.
• At the end of the last paragraph in the Introduction, the authors claim that acoustic devices are too expensive and cannot be exported. Firstly, there are inexpensive devices, and secondly, exporting them is possible under certain considerations (e.g., staying within a certain frequency band). So I think this is a somewhat too broad and unfair statement, because the authors do not consider export regulations/restrictions for radio devices. A discussion on this should be added to the paper (e.g., in conjunction with the final paragraph of Sect. 2.2).
• The errors reported (Fig. 11) appear rather large given the number of buoys and low depth (>7m). Can this be improved anyhow? Unless the reviewer is overlooking something, an error (std. dev.) of roughly 7m at 0-20m depth for 5 buoys and 10m radius is enormous.
• While the authors claim small system size at several points in the paper, the reviewer could not find any comment on expected/demanded system size. What dimensions do the authors envision for their system? The authors should quantify the requirements (e.g., at the beginning of Sect. 2, lines 99pp).
• The power consumption of 100W is high compared to acoustic communication (there are low-power devices with <5W consumption achieving similar range and ranging accuracy). Even though these large numbers only apply to the senders (buoys), the reviewer wonders how this can be justified.
• The authors claim that acoustic devices lead to acoustic noise and may harm mammals. This may surely be an issue, but the reviewer requests more justification in the paper (output levels, hearing thresholds, etc.). Moreover, the reviewer wonders about any potential risks of a 100W radio transmitter (or even multiple of them).
• In line 310, the authors assume that GNSS errors are zero-mean. Can they please comment on why this is the case? What if reflections/obstacles lead to different errors at the GNSS receivers?
• The estimation of the phase (e.g., Eq. 3) requires a high time resolution and, thus, fast sampling of the received signal. However, this issue is not addressed in the paper and should be added (80kHz => 12.5us period, what sampling rate is required to estimate the phase with sufficient precision?).
• The choice of the relative buoy coordinates (page 7, lines 218pp) is unclear. Firstly, why do the authors need a z-coordinate (planar surface, low vertical variation anyway)? Secondly, the given range (ca. 100m) and an 8-bit data type yield very poor resolution (ca. 40cm). Can the authors please comment on this? What is the effect on localization accuracy?
• It is unclear, why a 90 degree shifted sine-wave (data) would be orthogonal to a cosine (ranging) in Eq. 9. Please comment.
• Sect. 2 evolves a bit too quickly with little explanation. Equations pop up a bit surprisingly and the reader is referred to third-party sources or the appendix. It would be preferrable to make the section more focused and self-explanatory.

Minor / Paper organization issues:

• The term "pseudolite" is explained too late (in Section 2, but is used several times before).
• The phrase "very important" in the Introduction is too generic and void. Please me more concise. Why is positioning "very important"?
• Please use equal axes scales (Fig. 12) very better comparability of results.
• The caption of Fig. 11 is on the following page.
• Fig. 2 is in the middle of the page (it should be at either top or bottom to not interfere with the reading/text flow).

 

Author Response

Thank you very much for all your time and effort that you have dedicated to providing your valuable feedback on our manuscript. We are grateful for your insightful comments on our paper. Please find attached the detail responses to your comments. 

Sincerely,

Anja Grosch

Author Response File: Author Response.docx

Reviewer 3 Report

The authors present an interesting concept that is worthy of study and eventual publication; however, some significant practical issues need to be addressed.  Alternately, or ideally in addition, an existence proof in the form of a fielded system or single-beacon trials would assuage this reviewer's concerns about the system's practically achievable performance.

I am particularly concerned about the effect of beam pattern on both the transmitter and receiver side.  The attitude of each platform will affect the amplitude of the transmitted and received signal, and hence affect the range estimate.  The paper does not discuss this effect.  If it is not important, then a quantitative argument should be added to support this.

A second concern is the effect of water column variability, particularly temperature variability in the vertical.  Multi-degree variability is present on the scale of meters in fresh water environments and some salt water environments as well, though 10 m is probably a more relevant scale in the open ocean, for which this system is less suited.  If the associated change in conductivity is irrelevant, a quantitative argument to that effect should be presented.

Finally, the claim of low cost is not substantiated.  I do not have expertise in the RF domain, so it is not that I particularly doubt the claim, but the authors provide no evidence to back it up, e.g., a bill of materials.  A related claim, of practicality, would seem to conflict with the need for a 1 m aperture antenna?  Perhaps I misunderstand how aperture scales into physical size for the antenna envisioned.

Ultimately this is a paper study, and as such must be more thorough with respect to the sources of error analyzed.  I my view, experimental results would not only dramatically strengthen the paper, but would also reduce the need for a comprehensive study of error sources.  A full buoy system does not need to be built to satisfy this requirement.  A single fixed transmitter and mobile receiver, the latter with precise ground truth positioning, would be sufficient to evaluate the predictive skill of the analysis presented here. 

A number of additional comments appear in the attached annotated manuscript.

Comments for author File: Comments.pdf

Author Response

Thank you very much for all your time and effort that you have dedicated to providing your valuable feedback on our manuscript. We are grateful for your insightful comments on our paper. Please find attached the detail responses to your comments. 

Sincerely,

Anja Grosch

Author Response File: Author Response.docx

Reviewer 4 Report

This is an exciting and very important line of research that appears to have immediate utility for relatively accurate positioning underwater. Based on the innovative approach, I suspect these researchers will determine methods to provide even higher positional accuracy in the longer term. 

It would be nice to have some metrics for an ocean-based saltwater scenario, assuming at least limited testing was done in that setting.

Strong Work!

Author Response

Thank you very much for all your time and effort that you have dedicated to providing your valuable feedback on our manuscript. We are grateful for your insightful comments on our paper. Please find attached the detail responses to your comments. 

Sincerely,

Anja Grosch

Author Response File: Author Response.docx