Peer review of: Descent trajectory reconstruction and landing site positioning of Chang’E-4 on the lunar farside (2019)


RR2
Reviewers' comments:
Reviewer #1 (Remarks to the Author): I had no issues with the updated paper.
Reviewer #2 (Remarks to the Author): Review of Paper
     „Descent Trajectory reconstruction and landing site positioning of Chang’E-4 on Moon’s farside“ by
 Jianjun Liu and Co-authors
 Again, this is a most interesting report on the first historic landing on the lunar far-side, quite
 appropriate for publication in Nature.
 The authors have responded very effectively on the comments by the referees. Also, wording and
 phrasing has very much improved.
 I only have minor comments and remaining requests for clarifications. I recommend publication of
 the exciting material, pending on these clarifications.
 Specifically:
It is almost the only effective way to reconstruct ... -- > We effectively reconstruct ...
Fig. 4b: numbers on the color-bar: Shouldn't the elevation be km?
Please check numbers on scale bars. Shouldn’t the scale bar in 4c be in km?
Fig. 5 caption
 Abstract
 series of image data -- > series of images
 radio measurements -- > radio tracking
 using photogrammetric processed images -- > using photogrammetrically processed images -- >
 (or) using photogrammetric image processing techniques
  Fig. 1 d and e: please label the axis (m?). Please mark the North direction (or say in the caption).
 Note: There is no need to give the scale bar (0, 100, 200 m) when the relevant data are obvious
 from the axes.
  Considering the elevation profile given in Fig 6: wouldn’t it be useful to show the approach
 trajectory on Fig. 4b?
 Fig. 4c and d: Judging from the size of the rectangles, shouldn't the resolution of 4c be better than
 the resolution of 4d? The caption says it the other way.
 Besides, craters in Fig 4c are difficult to relate to Figures 4d,e, or f.
  Fig. 5 b. It is not clear, where the North arrow is pointed in this perspective viewing. Looking at
 Fig. 5a: From where was the image 5b taken? Which one is the crater in the background seen in
 5b?
  
There are 5 craters around the landing site -- > Five prominent craters around the landing site are
 marked
 Perhaps, one should add before the last sentence: Prominent craters are marked, as in (a). (Note that Fig 5a is almost identical to Fig. 3d)
  The landing position was determined according to (a), and the position of the lander’s leg was
 judged by (b).
 -- >
 While landing position was determined from orbital data (a), the orientation of the lander was
 determined using images taken from the ground (b).
 Text above Fig. 6:
 the lander attitude was adjusted to a vertical attitude (?)
 -- > the lander attitude was adjusted to enable thrusting for near-vertical descent
 Please add: Elevation is given with respect to ... (?)

Figure 7 caption
 The designed trajectories -- > “designed” ?
 The dotted border -- > The marked rectangle
 The reconstruction trajectories .... -- > Magnified portion of the reconstructed trajectory
Please add: Altitude is given with respect to final landing site level.
  Page 12 and Fig. 9:
 Perhaps, it is better to say: stones – > rocks
Fig. 9: what sizes are the rocks?

Reviewer #3 (Remarks to the Author):
The authors have not adequately responded to the biggest issue: Their implication that the 433 meter discrepancy between the manuscript coordinates and the LROC team coordinates for the Chang’e 4 lander can be explained by a more accurate Chang’e 4 derivation (and other associated mission data).
Text from the rebuttal.
“Therefore, the position of the CE-4 landing site determined in this study is
authentic, which can reveal the actual position on the lunar farside. At the
same time, in order to easily find the position of the CE-4 landing site, the
pixel coordinates of the landing site were given in multiple LCAM images,
which will be convenient for subsequent applications.
In lines 195-196 of the manuscript, we change old description (“obviously,
the positioning data of this study is more complete and detailed, the results
are more accurate.”) to “The position of the CE-4 landing site determined in
this study is authentic, which can reveal the actual location on the lunar
farside. In this study, the CE-4 descent trajectory was recovered, and the
landing site position was accurately determined. The pixel coordinates of the
landing site were given in multiple LCAM images.”
From the revised manuscript lone 203: “...and the landing site position was accurately determined.”
[Note – I do not understand the use of the word authentic in the rebuttal and the revised manuscript, I think this might be a translation issue?]
- The authors state that the absolute uncertainty of their reference frame is "decameter level" (line 312). (Reference 21 says "21-97 m" as does the methods section.)
- The reviewer originally pointed out that the LROC frame of reference uncertainty is about 20 meters (though the manuscript under review does not mention that fact). See the papers mentioned in the original review that document the coordinate framework for LRO (Lemoine et al. 2014; Mazarico et al. 2012; Mazarico et al. 2013) and LROC (Speyerer et al., 2016, Wagner et al 2017, Henriksen et al 2017).
- The authors acknowledge that the LROC team coordinates disagree by 433 m from their estimate (line 200), and state that this is "within the positional deviation range between the CE2TMap2015 and LRO terrain data [20,21] (see Methods section for details)." (Note: The Methods section says nothing about the LRO coordinate reference frame.) http://ch.whu.edu.cn/EN/volumn/volumn_1422.shtml
- Reference 21 (http://ch.whu.edu.cn/EN/volumn/volumn_1422.shtml) does seem to be the source of their statement that the LRO and Chang’2 laser coordinates agree within
uncertainty. Reference 21 also has a figure (pasted below) that shows that either the CE2TMap2015 or the GLD100/LOLA DEMs are grossly in error on the farside, disagreeing by ~500 m horizontally over the CE-4 landing site (and at least that much over most of the much of
  
the farside). Neither reference 21 nor the paper under review give any rationale for why they assume that CE2TMap2015 is more accurate. Note that Reference 20 is in press and not available to this reviewer.
Figure 3 and 4 from Li, C. L., Liu, J. J., Ren, X., et al. Lunar Global High precision Terrain Reconstruction Based on Chang’e-2 Stereo Images. Geomatics and information Science of Wuhan University . 43 (4): 486-495 (2018). Note that Google Translate was used to help this non-Chinese reading reviewer read the publication.
Bottom line: The authors have not adequately responded to the biggest issue: Their implication that the 433 meter discrepancy between the manuscript coordinates and the LROC team coordinates for the Chang’e 4 lander can be explained by a more accurate Chang’e 4 derivation (and other associated mission data).