Lake Altimetry Using Long Coherent Integration Carrier Phase Measurements from Airborne GNSS Reflectometry
Round 1
Reviewer 1 Report
The manuscript realized long coherent integration carrier phase measurements processing. And, combined field data of Carcans-Hourtin lake in France, the experiments showed good results on the reflection surface height retrieving. This manuscript is also well written overall. Thus, I think this manuscript could be accept for publication after the following issues are addressed.
2.1 describes “Flights at 300, 500, 1000, 2000 and 5000 ft were performed”, But there are only 300, 500, 1000, 2000 results. Where is the result at 5000 feet. Perhaps more data will also show interesting results.
Line 20: In this article, the term "feet" is utilized to describe height; I personally advise to use unit meter instead.
Line 93: What’s the meaning of the RF? Abbreviations, upon their first occurrence in the text, should be accompanied by their corresponding meanings.
The font size in Figure 8 is too small.
Line 348 and Line 356: where is the figure 15?
Fig 14 lacks a textual description of e, please check it.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper focuses on enhancing the possibilities and applications of GNSS reflectometry, specifically using carrier phase measurements for water altimetry. It describes an innovative signal processing approach involving signal correlation and extended integration time, applied to data collected over a lake to determine reflection surface height and compare it with in-situ GNSS buoy measurements, showing a difference of less than 1 cm for certain GNSS signals and consistent geoid slope estimation.
The authors are recommended to expand their method of ambiguty solving in terms of uncertainty assessment, as the uncertainty of the quatlity of integer values obtained by hand is not clear. The authors may refer to https://link.springer.com/article/10.1007/s10291-015-0445-5 as a resource for ambiguity resolution uncertainty.
The authors are recommended to discuss a previous paper on lake ice remote sensing using GNSS reflectometry in which phase altimetry has been mentioned (https://www.mdpi.com/2072-4292/12/17/2721).
Lines in figures 8 to 10 are presented in different colours, however, one is not able to distinguish different lines due to, probably, the width of lines, or the similiarities of some of the colours. The figures should graphically be improved.
The effect of Tropospheric delay on phase measurement is not clear. Some additional sentences are required. The authors may benefit from this paper https://www.sciencedirect.com/science/article/pii/S0263224120306114.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The authors present an interesting method for lake altimetry and discuss a real-life example. The paper provides the methodology with great details and the step-by-step description of the data processing helps understanding all the aspects. The manuscript is generally well written and reads fluently. No major changes needed in my opinion, but the following points should be addressed though before publication.
Equation (2): subscript “J” should be “i” for consistency with the description. The same applies to eq (3).
Line 165: the term “apodization window” is used here, while it is called “Hamming window” in section 3.1. Please use consistent naming. I’d suggest “Hamming window” as this term is more widely known in the GNSS community than “apodization window”.
Equation (5): it is unclear why “tcfk” is not just “tk”. What is the difference? The link between tcfk and tk should be clarified.
Fig 5: it is confusing that the reflected signal has a larger amplitude than the direct signal. To be explained.
Line 195-196: what is the “RTK measurement”? Is it the double-difference carrier phase? I’d delete this sentence as it is more confusing than helpful.
Eq (10): is “\DeltaL” not the same as “\Phi_r” of equation (9)? If yes, please use consistent symbols. If not, please explain the difference.
Section 2.3.3.1: how was the antenna phase center variation (PCV) taken into account? A few words on this would be helpful.
Eq (12): shouldn’t it be 10^-6 instead of 10^6?
Line 268: Ax=B instead of Ax=b
Fig 10 (b): according to the legend, ambiguities are fixed using the LAMBDA method. This has never been introduced in the text, where a manual method is proposed. This needs to be aligned. Either the use of the LAMBDA method should be explained in the text, or the figure legend should be changed.
Line 348: “elongation is less than 1 cycle and is therefore not significant”. Can you elaborate a bit? With a target centimeter-level accuracy, a wavelength of 19cm seems quite significant to me…
Line 348: Figure 15 does not exist.
Line 340-357: this whole paragraph needs to be rewritten. Some sentences are repeated twice. To be reviewed.
Figure 14: the legend is unclear, with (b) described twice with different description.
Section 4: the conclusion should not introduce any new element. The first half of the text should be put in a separate “Discussion” section. The conclusion should be rewritten to only summarize the work done and the main results.
Some typos:
Line 34: use –> uses
Line 40: use à uses
Line 210 and 216: “arm lever” should be “lever arm”
Line 266: adding or subtracting à add or subtract
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
