General Comments:

In my opinion this is a valuable and well written contribution and the athors are to be congratulated: I very much enjoyed reading this manuscript.

The authors describe an experiment that aims to test combined GNSS reflectometry and refractometry to derive snow water equivalent (SWE), snow density and surface mass balance (SMB) at an Antarctic ice shelf site. The method described is largely automatic, high resolution in space and time, and relatively low cost. The research topic is very important as the authors note. The method offers an alternative to labour intensive and destructive manual measurements or (incomplete) automatic weather station approaches. It has great potential and the article is both highly relevant to the journal and of high quality. 

Specific Comments:

The main challenge the authors faced in comparing different measurement series was the difference in the footprint of the individual measurements and, therefore, the impact of surface roughness on the results. As a reader I would like to see this issue discussed further, ideally supported by some data. The authors could add a more rigorous analysis of uncertainties. At present it is more or less covered by two sentences (section 5. line 252-257). I would also like to know if outliers were filtered from the laser data? Lasers are subject to errors caused by blowing snow at the snow surface.

Why do the high-end and low cost receiver time series diverge after August 2022? I think a hypothesis is needed (section 4.2, line 194). I note that according to figure 6 the low-cost data better tracks the manual density measurements after Aug. 2022 (section 4.3, line 228). This is not mentioned in the text and this omission should, in my option, be addressed. I would also expect the high-end receiver to better correlated with the laser data as the former seems directly under the latter while the low-cost antenna is further from the laser footprint.

Section 4.1: I would like to know if there are relationships between GNSS satellite zenith and aspect, and the errors. Is there are directional component, related to, for example, the effect of the mast and prevailing wind on surface roughness? Should tiltmeters and power monitoring be added to the setup?  

Would a comparison of your results and accuracies from different measurements (manual density, SWE; surface height/accumulation from lasers and sonic rangers) be appropriate using the literature? Perhaps as a short paragraph at the end of the discussion to provide additional context.

Technical Corrections:

In several places, especially in sections 2 and 3 you use the term "ground" to describe the ice shelf surface. Please use the term "surface" as it is not strictly speaking ground.  

In section 2.1 it would be useful to know which reference ellipsoid was used: was it the same for all instruments? If not could there be an effect on data quality?

p4. l80: could the arm for the buried GNSS antennas bend? Was it rigid?

 

p5. l10; eq. 1: should h not be δh if it is a difference or change?

p.6 fig. 3. For the sake of completeness please explain the difference between cyan and mauve ellipses or what they represent (which antennas they represent). How were they calculated?

p7. eq. 3. Please remind the reader that m is SWE and b is accumulation. I have a short memory.

p9. fig. 4. (also figures 5 and 6): would it not be better to have sub-figures a) and b) across the top and c) and d) along the bottom of these figures? This might be pedantry, I know.

The manuscript shows an interesting and well done study in the growing field of using GNSS for all kinds of environmental research and monitoring. The manuscript is well written and describes the set-up, methods and results mostly adequately. In general, I don’t see any major issues with the manuscript, but some details should be clarified.

What is the reference antenna? It might be the same Leica AS10 as the high-end rover, but it should be mentioned. Same antennas usually give results matching better, thus some of the difference between high-end and low-cost rovers might stem from this. Also the antenna calibration might be a thing to check, if you want to improve the results, at least no calibration tables were mentioned in the text.

How is the mast attached to the moving ice, i.e. how is the stability of the monument ensured? I was also wondering, along with a colleague earlier, how you can ensure that the mast or the lever is not deforming inside the ice?

One thing that could be added would be the number of satellites observed by the GNSS antennas. How many satellites are left when the elevation angle is limited for the reflectometry? And how many satellites there are generally available? This is one factor affecting the quality of the results as well and could maybe explain if there were problems with the data.

Some terminology is used abundantly, for example, line 129 “vertical Up (height) component U”. For a geodesist, one would be enough, up component, height, or vertical component. Also the symbol m seems to mean both mass (in equations) and the slope (in tables).

I agree with colleague on the points regarding 1) the order of figures 4 and 5, a and b on top row and c and d below is much easier to read, and 2) some explanation why the results seem to lie nicely on top of each other before August 2022 and diverge after that.