Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking

Round 1

Reviewer 1 Report

This is clearly a valuable review paper and is a very comprehensive study into published material on the worldwide RADAR capability for tracking near-Earth orbiting space debris. The study of space debris is an expanding research topic, and it is useful to have in one paper a catalogue of leading facilities and their capabilities as itemised in many data plots.

The information for each facility is taken from the very comprehensive list of references.

I have marked on a copy of your manuscript some suggestions for small changes to help with the overall understanding of your work.

Comments for author File: Comments.pdf

Author Response

COMMENT

This is clearly a valuable review paper and is a very comprehensive study into published material on the worldwide RADAR capability for tracking near-Earth orbiting space debris. The study of space debris is an expanding research topic, and it is useful to have in one paper a catalogue of leading facilities and their capabilities as itemised in many data plots.

The information for each facility is taken from the very comprehensive list of references.

I have marked on a copy of your manuscript some suggestions for small changes to help with the overall understanding of your work.

ANSWER:

We would like to thank you for your precious suggestions. The paper has been revised addressing all the English grammar corrections from the reviewer.

Salient comments:

COMMENT 1

This sentence needs clarification. I cannot guess what is meant here:

“It can be noticed an expectable drop in small objects sensitivity by all radars”

ANSWER 1

We apologize for being unclear. The sentence has been rephrased as follows: “A lower sensitivity in detecting small objects can be noticed for all radars”.

COMMENT 2

Row 281: Maybe include here that the Arecibo dish has been closed down with effect from late 2020?

ANSWER 2

Thank you for the clarification. We have added the following sentence in the revised paper: “Sadly, in late 2020, the NSF has declared the Arecibo Radiotelescope closed, due to some structural flaws. As if that were not enough, the antenna structure dramatically collapsed on itself on December.”.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present a comprehensive review article on the use of radar for tracking and monitoring space debris. The article includes a comprehensive set of references to relevant papers in the open literature. The article is well written, although some minor grammatical editing is needed. The following are my recommendations on how the paper can be further improved:

1. I think that it would be helpful to include an overview of the principles of using radar for space debris detection and tracking, along with some of the key radar system design trade-offs. This would be appropriate for a review article, and would probably fit nicely right after the introduction section. A bit of this is mentioned in the conclusion section, but I think a deeper discussion would be useful.
2. I suggest that the authors should consider creating an overview table that summarizes all of the radar systems that they mention, along with key characteristics, such as radar wavelength, operational mode, minimum detectable target size, etc. This might create the opportunity for the authors to point out new insights into how radar performance has improved over the years, which could be very interesting to the audience of this paper. This type of overview analysis would help to generate additional insights from the information collected in this article.
3. I suggest that the authors consider re-organizing the body of the paper to categorize the various systems by radar design instead of by country of origin. This type of organization would be more appropriate for a scientific journal, and also would help readers with comparing/contrasting how radar design affects debris monitoring results. If such a re-organization is not desired, I would suggest that the authors should at least explain in the introduction section why they have chosen to organize their discussion by country.
4. The authors can consider including some discussion of recent developments in radar technology that are relevant to debris monitoring. How is the technology expected to evolve and improve over the next decade? This type of discussion would be interesting to readers of this type of review paper.

Author Response

COMMENT

The authors present a comprehensive review article on the use of radar for tracking and monitoring space debris. The article includes a comprehensive set of references to relevant papers in the open literature. The article is well written, although some minor grammatical editing is needed.

ANSWER

Thanks for pointing out this deficiency. English grammar has been revised throughout the paper.

The following are my recommendations on how the paper can be further improved:

COMMENT 1:

I think that it would be helpful to include an overview of the principles of using radar for space debris detection and tracking, along with some of the key radar system design trade-offs. This would be appropriate for a review article, and would probably fit nicely right after the introduction section. A bit of this is mentioned in the conclusion section, but I think a deeper discussion would be useful.

ANSWER 1:

Thank you for this comment. Your suggestion helped us to improve the overall quality of the paper. As requested, a new section of the paper has been added after the introduction, with a general overview of radar principles and configurations used in space debris monitoring and tracking. In order to highlight the new addition, a brief sentence has been also inserted in the introduction:

“An overview of radar principles and configurations typically used in space debris monitoring and tracking is reported in section 2.”

COMMENT 2:

I suggest that the authors should consider creating an overview table that summarizes all of the radar systems that they mention, along with key characteristics, such as radar wavelength, operational mode, minimum detectable target size, etc. This might create the opportunity for the authors to point out new insights into how radar performance has improved over the years, which could be very interesting to the audience of this paper. This type of overview analysis would help to generate additional insights from the information collected in this article.

ANSWER 2:

We would like to thank the reviewer for this valuable comment. The summary table has been added (table 3 of the revised paper) and briefly discussed in the conclusion section (now “Conclusion and Future Perspectives” in the revised paper).

COMMENT 3:

I suggest that the authors consider re-organizing the body of the paper to categorize the various systems by radar design instead of by country of origin. This type of organization would be more appropriate for a scientific journal, and also would help readers with comparing/contrasting how radar design affects debris monitoring results. If such a re-organization is not desired, I would suggest that the authors should at least explain in the introduction section why they have chosen to organize their discussion by country.

ANSWER 3:

Thank you for the comment. Actually, the classification based on the geographic origin was a thoughtful choice. Nonetheless, we perfectly understand the concern of the reviewer, as the solution suggested is also absolutely viable. We would like to better clarify the reasons behind this choice. As reported in other papers ([R1], [R2], [R3]), the country-based discrimination, is very often used to remark the fact that a specific sensor is part of a more extensive network and to highlight its belonging to the local space agency. Especially in space debris monitoring, the concept of Space Surveillance Network is pivotal in the comprehension of the strategies adopted to face the planetary threat. As a further evidence of this assumption, we would like to point out that over the years the most important measurement campaigns on space debris have not been performed by single sensors but by coordinated networks that shared the acquired data. This solution allows for a comparison of the flux of debris registered in different locations, improving the knowledge of orbital information about catalogued and uncatalogued objects. The concept has been stressed in the revised paper by adding the following sentence at the end of the introduction section:

“This classification choice is aimed to identify a specific sensor within a wider network, to highlight its belonging to the local space agency and/or government organization, thus helping the reader to understand the operating context of these sensors.”.

[R1] Console, A. Command and Control of a Multinational Space Surveillance and Tracking Network. The Joint Air Power Competence Center 2019, pp. 31-39.

[R2] Vallado, D. A.; Griesbach, J. D. Simulating Space Surveillance Networks. Paper AAS 11-580 presented at the AAS/AIAA Astrodynamics Specialist Conference, Girdwood, AK, USA, 31 July-4 August 2011.

[R3] Weeden, B.; Cefola, P.; Sankaran, J. Global Space Situational Awareness. Advanced Maui Optical and Space Surveillance (AMOS) Conference, Maui, HA, USA, 14-17 September 2010.

COMMENT 4:

The authors can consider including some discussion of recent developments in radar technology that are relevant to debris monitoring. How is the technology expected to evolve and improve over the next decade? This type of discussion would be interesting to readers of this type of review paper.

ANSWER 4:

We appreciate the relevant comment. In order to address it we have modified the conclusion section (now “Conclusion and Future Perspectives” in the revised paper), including at the end of this section a discussion about recent developments, future evolutions and improvements over the technology behind space debris measurements.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have thoroughly addressed my comments, and I think that the revised manuscript is definitely better than the original draft.

I have a few minor comments:

Line 78: "recognized as mono-static" should be changed to "defined as mono-static", to be consistent with the wording in the next sentence.

Line 93: "mechanically point the target" should be clarified to say "mechanically point the beam at the target".

Line 778: For grammatical correctness, "will become more performing" should be changed to "will have improved performance".

Author Response

Dear Reviewer,

thank you again for your time and comments,

we have made the suggested correction in the revised paper (see lines 78, 93, and 765),

Best regards,

Giorgio Montisci