Drag and Attitude Control for the Next Generation Gravity Mission
, Luca Massotti 2, João Teixeira da Encarnação 3,4 and Bruno Christophe 5
Round 1
Reviewer 1 Report
The paper presents an analysis about the importance of accelerometer observations for the planned NGGM mission in particular in context of the necessity of drag control capabilities and inter-satellite pointing accuracy. It is concluded that as a minimum a drag compensation in flight direction and a stable inter-satellite pointing is required in order to achieve the NGGM mission goals. The paper is very well structured and written and references are provided wherever necessary. It fits very well to the topic of the special issue and provides important information for the development of the NGGM mission. Therefore, I highly recommend publication of the paper after a minor revision addressing my comments below.
Chapter 2.1: Line 127 to 139: For the requirements reference is made to a Mission Requirements Document, which is not publicly available. In other words, the reader has no access to this information. There is provided table 1 with some selected requirements, which is o.k., but may be not sufficient. In this paragraph it should be mentioned from where these requirements have been derived. I guess they are taken from ref. [1], [16] and [18]. If this is correct reference to these publications shall be made here instead to the ESA document. This is very important as all citations need to be accessible by the reader. Please find a good solution for this.
Chapter 2.1: Line 151 to 154: This sentence is hardly understandable. May be rephrase it with two clear sentences.
Chapter 2.1: Line 159 to 163: I think the general message is a bit misleading. In principle all scenarios measure the full signal disregarding the orbit configuration. When separating the signals, the spatial and temporal sampling (i.e. the orbit scenario) is important. I propose to reformulate this sentence.
Chapter 2.2: Line 195 to 198: The message is wrong. With as shorter repeat period the long wavelengths can be better resolved, but not the details in the gravity field as the spatial sampling is not good enough. I know what the authors mean, but the sentence says something different. Please reformulate.
Chapter 2.2: Line 223 to 225: I think the statement that the distance is chosen as a function of the target resolution is wrong. If the authors think this is correct they should provide a proof or a reference. So far I know the range has to be within specific limits in order to achieve good results, but there is no real functional relation. If I am wrong, then the function shall be provided in the paper.
Chapter 2.3: Line 239 to 242: I think the requirements are somehow driven by the expected technical performances of the 2 main instruments. Otherwise I would prefer a flat error spectrum over all frequencies. I know that this is unrealistic, but as it is written right now the reader could think that these are the ultimate requirements. Please reformulate this slightly in order to clarify the issue.
Chapter 3.2: Figure 11: I guess the red curve represents the science readout noise for the z and y axis. This is not mentioned in the figure legend. In any case I think it should be mentioned that the accelerometer frame has a different orientation as the satellite frame. The reader might get confused as the x-axis is slightly less sensitive and I hardly can imagine that this axis should point towards the flight direction with the largest signal. Please clarify this somewhere.
Chapter 3.4: I have a major comment to this chapter. This chapter is very technical and partially too detailed. But at the end a better description or interpretation of the resulting Figure 16 is required. This is the main input to the follow-up simulations and therefore it should be explained in detail. In Fig. 16 also the control requirement is shown. Where does this come from? How is this determined? Please consider to revise the current content slightly and focus on the most important elements. Then add a clear description of Figure 16 answering also my questions above.
Chapter 4: Before starting with the detailed descriptions of the simulation results I think a short summary of the simulation approaches is required. For the reader it is difficult to follow the results, which have been obtained from the results of chapter 3 without having some basic information about the approach, which was used. So I ask the authors to include a paragraph describing the simulation strategies used in the following sub-chapters.
Chapter 4.1: Lines 492 to 498: As we know the gravity potential decreases with the square of the distance from the Earth surface. Where does this relation shown in line 496 comes from? Either provide a reference or explain it in a better way. What is r in your definition? Orbit radius is from the center of the Earth, but I have some doubts if this is meant. Please modify this accordingly.
Chapter 4.2: This is just a remark about the LPT: This is a very simplified way to propagate orbit perturbations to gravity potential variations and might not provide the complete insight. I know that the authors already added a remark about this, but may be I would propose to state in the conclusions that more sophisticated simulation techniques need to be applied in order to confirm these conclusions.
Chapter 4.2: Lines 552 to 556: I do not understand this message as I couldn´t identify the laser interferometer errors in Figure 21. May be I misunderstand this message, but this is an indicator that it needs to be improved.
Chapter 4.2: Line 564: Lower degrees are not equivalent with higher spatial resolution, but with lower spatial resolution. I think this needs to be corrected.
Chapter 4.2: This table is not really required as Fig. 21 provides this information as well.
Chapter 4.2: Lines 576 to 578: It is correct that C20 in GRACE/GRACE-FO are not well estimated. But, I don´t think that this is related to no drag control or bad pointing. The instrument performances of GRACE and GRACE-FO is significantly worse than planned for NGGM, and the altitude is relatively high, therefore I think this problem is less relevant for these missions. I think the main reason for being not able to observe C20 is related to the fact that usually range-rate (1st derivatives) are used as observations. From derivatives it is usually hardly possible to estimate the full signal. May be avoid to speculate about C20 as I think also with a drag control and fine pointing C20 will not be observable very good.
Section 5: Line 586: I think there is a typo: “fulling”.
References:
Please check references number 8, 9, 11, 21, 22, 32. These references are abstracts or some kind of internal reports. For most cases there are publications available, which can be cited. Please replace them with real publications. Abstracts should not be used as references. Related to reference 19 please refer to my initial comment above.
Author Response
The answers to the specific comments are provided in the attached file "Author's Reply to Reviewer 1 Report.pdf"
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Authors provide a very comprehensive and detailed analysis of drag and altitude control for NGGM. The simulation analysis provide very clear results by means of achievable spatial resolution. I believe that article can be accepted in the current form.
Author Response
The authors thank the reviewer for its report.
Reviewer 3 Report
This paper deals with critical design for the Next Generation Gravity Mission. In this paper a constellation of 2 pairs of satellites is proposed, and the performance of accelerometer is deeply explored in gravity field solution.
This paper is well structured, with enough computation to support the conclusion, and will play an important role in NGGM mission design.
Two minor points:
1. Better to repot Figure 12, there are not clear like other figures;
2. Line 496, Spherical Harmonic -> spherical harmonic.
Author Response
The answers to the specific comments are provided in the attachment "Author's Reply to Reviewer 3 Report.pdf"
Author Response File: Author Response.pdf
Reviewer 4 Report
General comments
The subject of the article fits to the scope of the Remote Sensing journal. The authors discuss the issue of drag and attitude control, specifically in the contexts of the Next Generation Gravity Mission (NGGM) – the result of their research within the Phase A System Study of the NGGM, under ESA contract, which certainly is worthy of investigation. In particular, based on advanced simulations and extensive analysis they show the necessity of a suitable drag-free environment and a precise and stable pointing that would be a complementary “observable” to the one of a high-performance accelerometer.
The paper is very well written, its length is appropriate and the use of the English is concise and clear. The results and discussions are clearly presented.
The manuscript needs, however some improvements.
1. The abstract should provide more information on what is presented in the paper. It consists of too general statements and does not sufficiently describe the contents of the paper.
2. The introduction sets the scope of the paper. It gives a review of the current status of the subject area but it should also refer to the earlier original publications pioneering the issue of satellite-to satellite mission for geodesy, published e.g. in the Bulletin of Geodesique.
3. More explicit advantage of NGGM potential as compared with GRACE-FO one would be beneficial, e.g. in terms of what could be gained more from NGGM data than from GRACE-FO data.
4, Quality of Figures 12 (433) and 16 (456) needs improvement.
Detail comments:
154-155 – the sentence “A future gravity mission dedicated to mass change in the Earth system, as studied in the context of a Next Generation” should be reformulated to “A future gravity mission dedicated to monitor mass change in the Earth system, as studied in the context of a Next Generation” The word “monitor” or “detect” was missing.
222 “A satellite-to-satellite reference distance d = 220 km has been selected for NGGM”. How the reference distance d = 220 km has been selected? Why just 220 km? It should be explained and referred to the literature, probably as well as the principles of ll SST.
574-575 - in the sentence “This may not seem relevant to the observability of the HIS signal, but such large different may play an important role in the estimation of the low degrees, ..” – “different” should be replaced with “difference”.
The manuscript could be accepted after the revisions specified above.
Author Response
The answers to the specific comments are provided in the attachment "Author's Reply to Reviewer 4 Report.pdf"
Author Response File: Author Response.pdf
