General Comments:

This is an interesting manuscript that aims to illustrate how inexpensive lidar units developed for the automotive industry can be used for geoscience applications. The authors do a nice job at outlining the technical aspects of the lidar units they compare.

My general suggestions for improvements to the paper are to do a better job of setting up the problem in the introduction. In particular, can you better describe why users would want to use these systems versus techniques like photogrammetry.  Your underground examples make it obvious as to why you need lidar, but I think you need to clearly explain that to readers.  In addition, you should expand the intro to show the breadth of how people have been using lidar in the geosciences ranging from controlled outdoor studies (e.g., Rapstine et al. 2020; Rengers et al., 2021) to natural observations (e.g., Rosser et al. 2005) to damage assessments (Olsen and Kayen, 2013).

Another rather large suggestion is to try to gear the paper to seem relevant far into the future. Right now, there are references in euros and references to years when technology is expected to be developed, but those references will seem irrelevant 10 years from now. If you could re-frame the tone to have a long-view (at least a decade) I think it will seem more relevant.

In addition to these general suggestions, I have provided several line comments below.

Olsen, M. J., & Kayen, R. (2013). Post-earthquake and tsunami 3D laser scanning forensic investigations. In Forensic Engineering 2012: Gateway to a Safer Tomorrow (pp. 477-486).

Rapstine, T. D., Rengers, F. K., Allstadt, K. E., Iverson, R. M., Smith, J. B., Obryk, M. K., ... & Olsen, M. J. (2020). Reconstructing the velocity and deformation of a rapid landslide using multiview video. Journal of Geophysical Research: Earth Surface, 125(8), e2019JF005348.

Rengers, F. K., Rapstine, T. D., Olsen, M., Allstadt, K. E., Iverson, R. M., Leshchinsky, B., ... & Smith, J. B. (2021). Using High Sample Rate Lidar to Measure Debris-Flow Velocity and Surface Geometry. Environmental & Engineering Geoscience, 27(1), 113-126.

Rosser, N. J., Petley, D. N., Lim, M., Dunning, S. A., & Allison, R. J. (2005). Terrestrial laser scanning for monitoring the process of hard rock coastal cliff erosion. Quarterly Journal of Engineering Geology and Hydrogeology, 38(4), 363-375.

Specific Comments:

Line 19: Explain why you used such a long-range lidar for such confined areas?

24: This is a minor comment, but I don’t see how you get the “LI” in MOLISENS. Don’t you need to put “Lidar” in there, for example, Mobile Lidar Sensor system.

28: Here and elsewhere, when you say automotive lidar, it is a little unclear. What you mean is that you are using small lidar sensors that were originally designed for the automotive industry. But it is a little confusing because people might think you mean lidar mounted on an automobile. So try using a more descriptive term like lidar developed for the automotive industry.

43-44: I think you should avoid things like prices and saying when an instrument is expected to be out. Here you write “late 2022”, but what if your paper isn’t out in 2022? What if the instrument isn’t ever released.

145: Can you explain how the GPS works in a cave?

206: The sentences here: “We found that…” should be in the results.

218: Where you have written ( etc) what are the other things that you are measuring? I’m more interested in that, than the information in Table 1. Consider putting those measurements in a table.

226: where you say “How many clusters of points in a 0.5m radius exist between 5 and 10 in THE x direction …” What are the units you are refering to when you say between 5 and 10?

345-361 Add this information to the intro.

Figure 1. Add labels (a, b, and c) for the sub-figures here. Also in (b) consider adding something for scale, such as a pen.

Figure 5. Add a colorbar in 5d.

Figure 6. Use a more complete sentence and be more descriptive in the caption for (a). (b) Add a colorbar, label the hand-rail, and try using something like the EDL filter to make the point cloud more visible in the figure.

Figure 7: Show location map of where the glacier is located (similar to inset in 5a). Add a colorbar to all figures.

Table 1: Is this table necessary? The specs feel somewhat out-of-step with a journal article.

This is an interesting paper presenting a methodology for developing an automotive lidar unit for use as a mobile lidar unit with geoscience applications.  The manuscript does a nice job of explaining the components of the system and the testing that was done to find the applicable range and resolution. Perhaps the one component that I feel the paper is missing is a comparison of a point cloud results from the two test areas with a point cloud generated from the Riegl scanner used for the initial testing. The authors do highlight why this would be impractical for the entire area at the Lurgrotte Semriach, but even a small section would provide useful evidence for how comparable the mobile unit is to a more well-known tripod-based unit in a complex natural setting.  

Line 24: remove “allows to build” and replace with “builds”

Line 33: Three systems are mentioned (Lidar, Radar, and Camera) so remove the word “both”

Line 87:  I may be misunderstanding something, but here you note that the system can be powered by AC/DC adapter, but in line 91 you note that it requires external batteries to be mobile.  If the appeal of this system is that it is mobile doesn’t that exclude the AC/DC power option.

Table 1:  I know the previous reviewer mentioned removing this table, and while I agree it may not be necessary, I do appreciate knowing the size and weight of the unit, it is much lighter than many others available, and this certainly increases the number of potential users.

Line 94:  please consider writing out your acronyms, in particular IMU.  It was easy enough to find what this was, but for those of us not familiar with these acronyms, writing them out can be helpful for clarifying their purpose.

Line 100:  It appears that HAT is typically a capitalized acronym.  Also, this could again be written out once to help clarify what it is.

Line 217: here it is noted that the sensor records point cloud information, yet earlier in line 173 you noted that the lidar doesn’t produce the point cloud, but rather the raw data are timestamp, measurement ID, and range. I suspect these are discussing two separate steps, but there is some additional information may help clarify, as both lines appear to be discussing the Lidar sensor.

Line 289:  I am curious about the errors in ice.  Perhaps these errors are well covered in the literature and there can be citations, but if not, maybe there can be a few more details.  I would imagine that because light can move through ice it might create some errors in the data. Perhaps this is accounted for in the lower return intensity described later, but it might be helpful to address outright how the MOLISENS system compares to others when surveying ice.  

Line 294:  There appears to be a few words missing after “These are the price, size, weight, and robustness…”, or perhaps there should be a semicolon connecting this with the previous sentence.

Line 324:  “A useful” not “an useful”

Line 329:  These sentences seem to imply that the lower intensity returns for ice surfaces may be better for change detection.  If this is true, could you provide more information as to why.  If this isn’t meant to be implied, consider removing the word “hence”, and possibly moving this line.

Line 335:  Does this line suggest that Structure from Motion (SfM) would be a preferred method of monitoring coastal bluffs?  This is the first mention of SfM, it might be helpful to have a line or two about the advantages of this system in comparison that one, particularly where you might have good GPS control.

Line 336-339:  This sounds amazing, I look forward to this technology becoming more widely accessible.

As a general comment about the discussion and conclusions, river systems are mentioned a few times as an application for this technology, but I suspect the laser isn’t powerful enough to penetrate through water.  Similarly, there is no mention of multiple returns, so I suspect this isn’t penetrating through vegetation.  It may be worth noting these caveats specifically depending on the anticipated audience.