Design of Mega-Constellations for Global Uniform Coverage with Inter-Satellite Links

Round 1

Reviewer 1 Report

The paper presents a mega-constellation design process to attain global coverage while controlling collision avoidance approaches (analytically assessed, which is a very interesting contribution), and inter-satellite link considerations. In general, the paper is very well written and easy to follow. (Note this reviewer was not able to fully follow the math, so I cannot judge equation’s correctness.) The background overview on existing constellation types and design approaches is interesting and valuable. In the opination of this reviewer, the publication of this paper should be pursued.

 

A suggestion to improve the impact of the paper Is to at least discuss about the stability of the designed orbits, in terms of station-keeping requirements. Besides the global coverage, ISL conditions, and collision avoidance, fuel consumption should be an important figure of merit. Note I am not suggesting a modification to the proposed scheme, but a general discussion on the role of station-keeping either on a qualitative level, or as a possible consideration for future extensions.

 

I also leave some minor cosmetic details that should be addressed to ensure a high-quality and conceptually clear final version of the manuscript.

 

1) (Abstract) “Earth” should be capitalized.

2) (Abstract) A configuration design scheme is said to be proposed, which is then reffered to as a method to obtain said scheme. This could be clearer, especially in the abstract.

3) (Abstract) Same orbital "layer" is used, when the abstract seems to reference orbital planes: it is stated that all the constellation shares the same altitude/heigth. Clarify what layers are.

4) (Abstract) "Relatively uniform coverage" can be replaced with actual results in terms of percentages achieved or a more precise statement depicting what was achieved.

5) (Abstract) The use of Hybrid Constellation as a term conveys the idea of two or more constellation types combined, when only Walker type constellations are said to be used. This can be though as a mission using several constellations of the same type, rather than one big "Hybrid" constellation.

6) (Abstract) It is not clear what [...] Greater number of coverage repetitions [...] means. Is this revisit percentage or actual orbit repeat period?

7) (Lines 28-31) define what is large scale or what realistic possibility means in this context, since many constellations comprised of a considerable number of satellites have already been deployed, as it is stated later in the paragraph.

8) (Lines 31-32) phrasing is not clear. Are you reffering to Constellation/Mission names or Companies?

9) (Lines 42-45) needs citation.

10) (Lines 46-47) Walker constellations are a design pattern choice, not necessarily the most representative or significantly superior in every aspect (line 90).

11) (Introduction) Review in terms of phrasing and grammar. Mind the use of singular and plurals.

12) (Line 153) What is rapid coverage?

13) (Lines 151-156) may need citation.

14) (Section 2.1, (2)) As indicated in my comment above, it should be emphasized that orbital maintainance using Orbital Control Systems are assumed in your model, as you propose a constellation in the order of thousands of satellites at the same height

while stating that mitigating collisions is an important part of your research.

15) (Section 2.3) Mind the position and size of the figure. Depict S in the figure. Clarify what Earth model and/or approximations are you using.

16) (Section 2.4) Clarify the acronym RAAN.

17) (Section 3.1) The title should be Visibility Conditions Between Satellites, also in line 420 the term visibility should be used.

18) (Lines 491-495) mind the phrasing. Element w (omega) has no impact in the orbit's shape.

19) (Section 4.1) the title is not accurate in reflecting the subsection's content, which are simulation considerations and results.

20) (Table 1) Use the N,F,P acronyms in column's names.

Author Response

Dear reviewer,

We are really grateful to you for the critical comments and constructive recommendations. We feel lucky that our manuscript went to you as the valuable comments not only helped us with the improvement of our manuscript, but also raised some thoughtful suggestions. We will revise our paper according to your comments one by one. If we have not explained them clearly to you, please allow us to continue improving them. Thanks again.

Author Response File: Author Response.docx

Reviewer 2 Report

The constellation design method is not disruptive, but the inclusion of intersatellite link design in the study and the design scalability
for large number of satellites deserves publication.
The article should be considered for publication at least as short note.

General comments:
The introduction is globally satisfactory but the English shall be improved.
The algorithms for the design procedure shall be presented more clearly.
Furthermore, the work does not consider orbital perturbations while considering the collision avoidance constraint.
In view of a very large number of satellites and the different inclinations of the sub-constellations, the collision avoidance constraint may be endangered.
For instance due to J2, the argument of ascending node and mean anomaly drift non homogeneously.
It is suggested to either include in this work orbital perturbations or at least mention explicitly the problem.

Introduction:
Line 52 (and others): The expression "The literature" to refer to someone's work is awkward.
Line 131, 142 and 165: These paragraphs appear to be a kind of repetition of what has been said before. They shall be better integrated in the text.

Sec 2:
Line 207: M0 and Omega0 appearing here shall be referred to as "datum values" rather than "initial time values". 
Line 209: Rewarding is needed for the sentence, it's not clear.
Line 228: The rotation matrix in Eq 6 seems to be incorrectly transposed
Line 236: Eq 9 is not correct, X_ECEF contribution is missing.
Line 263: Last part of the sentence shall be rephrased, it's not clear.
Line 277: I suggest the use of "prograde" instead of "anterograde", it's more common.
Line 295: Eq 14 and 15, it is not clear the start value of the j-th average density (D) and number of satellites (n) for the complete mixed constellation C.
    Do you start from a default 0 value? you consider the complete as the one made of all sub-constellations up to the j-th iteration?
Line 301: Eq 16, why does it appear a fixed subscript 2 for S at each iteration?
Line 306: Eq 17, shall the right hand side be a lower-case n?
Line 336: check sentence meaning
Line 345 (and others): Use "argument of latitude" instead of "latitude argument", the latter may generate confusion with the simple latitude (delta) used in the paper.
Line 351: Eq 19, typo, there should be a sin of i_2 instead of u_2
Line 354: Eq 20, typo, there should be a ratio of cosines instead of sines, i.e. cos(i)/cos(delta)
Line 358: check alpha subscripts, shall be Omega_1 + alpha_1
Line 365: Eqs 24 and 25, doublecheck the signs, some of those within the parenthesis seems not correct.
Line 401: Eq 33, subscript for 2 shall be "max".

Sec 3:
Line 462: At step 4 the rows of D'_max shall be sorted. It is unclear with respect to what they are sorted (as it is, the matrix is made of zeros and ones).

Sec 4:
Line 504: Check sentence meaning, it is unclear.
Line 629: Paragraph starting from this line appears to be a repetition and shall be better harmonized with the rest of the section.

Conclusions:
Line 661: It cannot be said that collision avoidance is "effectively" addressed as the effect of orbital perturbations are not taken into account (although significant).

Author Response

Dear reviewer,

We are really grateful to you for the critical comments and constructive recommendations. We feel lucky that our manuscript went to you as the valuable comments not only helped us with the improvement of our manuscript, but also raised some thoughtful suggestions. We will revise our paper according to your comments one by one. If we have not explained them clearly to you, please allow us to continue improving them. Thanks again.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The article has sufficiently improved its quality.

One last minor comment about a notation crash at line 303 of the revised version. The average density D_i_c1 use i as a subscript to indicate (I supposed) a different running index than j, but i is already reserved for "inclination".
This shall be corrected or clarified.

Author Response

Dear reviewer,

We are really grateful to you for the constructive recommendations. Your comments are very helpful to improve the quality of our paper. We have made necessary modifications according to your suggestions, and we hope to meet your approval. Thanks again.

Author Response File: Author Response.docx