Impact of Ocean Waves on Guanlan’s IRA Measurement Error

Round 1

Reviewer 1 Report

Review of Impact of Ocean Waves on Guanlan's IRA Measurement Error, by Yining Bai, Yunhua Wang , Yanmin Zhang, Chaofang Zhao and Ge Chen

Remote sensing of the sea state by satellite instruments has yielded much information over the last many decades, although there are many challenges to properly process and understand the signals recorded. So, the paper by Bai et al. can be helpful to add to the body of knowledge needed to improve processing and understanding of remote sensing data. The paper is well written for the most part, with little confusion about what is intended to be conveyed, although there are some grammar errors and problems with the wording in some places. The main challenge facing the paper at the moment, from my perspective, is simply that the Introduction needs some serious work, and that there are times where the explanations are not so easy to follow. Below, I have outlined my concerns. I think with major revisions, this paper could be suitable for publication.

The Introduction by far needs the most work, with the rest of the paper mostly needing some clarifying text here and there and some grammar corrections. The problem with the Introduction is the references to many studies in the literature, with no explanation as to what the findings were, and worse, no discussion of how the findings in these studies informs the story being told in this paper. Examples include references throughout much of page 2 (lines 46-96), such as: lines 63-65, He et al. (2003); lines 70-72, Bao (1999); lines 72-74, Schulz-stellenfleth and Lehner (2001). This isn't every instance, just to provide some concrete examples. If you are going to reference a paper because it is relevant to your work, you should explain what the study found and set up a story line about how your work will build upon it. If you are going to reference a paper just to cite some of the previous developments in the field (which I urge you not to do simply to teach the reader, but to set the context for the work in this paper), you at least should briefly outline the important findings from that study. For example, "Bao (1999) studied the nonlinear integral transform between ocean wave spectra and phase image spectra of a XT-InSAR, considering the effect of velocity bunching [34]." does not explain what Bao (1999) found, it just states what they examined. This is not enough of a reason for a citation, and makes reading the Introduction less informative. Please address all instance of this.

Most or all discussions regarding correlation are simply too unclear, and because of this, I'm not entirely certain what is meant. I think that you mean, maybe in every case, the spatial correlation between the master and slave images from the IRA instrument. Nevertheless, in all cases individually, it needs to be more clearly stated what is exactly meant by correlation coefficients, if this is autocorrelation or decorrelation of a single property, or if not what properties the correlation is measured between. If this is clarified at least once per section, or per theme, this is enough. Also, if the point is generally that the effects waves have on the measurements with the IRA instrument affect the magnitudes and not the correlations in Section 3 but affect both in Section 4, please state this explicitly.

Is the simulation of the SSH field just one simulation? Why not a Monte Carlo simulation of SSH fields. If it's just one simulated field, specify that and address why you expect that a Monte Carlo simulation of many fields would produce the same results. Usually, doing a series of simulations yields a stronger result.

Here is a list of requested text corrections, with line numbers.

Line 34: This scheme provides higher accuracy ...

Line 43: and altimeter sensors; the SSH measurement (comma to semi-colon)

Line 64: spatial derivative (?), or is it the temporal derivative? Specify

Line 143: Removing the flat phase...

Also, please define 'flat phase' before, or within, this first reference.

Line 162: Run-on sentences. Split here "direction angle. When the ..."

Lines 174 to 179: Some short, one-sentence summary of this would be nice to have in the Introduction so that readers will have some idea of what this phenomenon (velocity bunching) is before this point in the paper, particularly because this phenomenon is a specific focus of the paper.

Line 178-179: I am not clear on what you mean by 'whether the waves are close to or away from the platform'. Do you really mean position, which would influence the angle 'beta', or do you mean velocity towards or away from the platform (the V and V_r in Eq. 11)? Can you rewrite this to be a little clearer, with a simple example just written in text?

Line 198: I suggest 'we try' or 'one tries'. Otherwise it's an imperative, which seems odd in a journal article.

Line 199: 'filed' -> 'field'

Line 204: 'average' -> 'averaging'

Line 214: in Section 5

Line 215: swells have been chosen in

Line 222: when the swell propagation direction is 90° to the range direction. [I know this is stated earlier, but it doesn't hurt to restate small details for clarity a bit. I would also suggest adding this 'to the range direction' text to the Fig. 3 caption.]

Figure 3 caption: please include the units in the caption, like (in m).

Lines 228-233: The correlation coefficient between which two properties, exactly? The master and slave images? Please state this clearly in the text. Also, if you have chosen two properties which are invariant to significant wave height propagation direction in the ocean (not to mention that a correlation of 1 is a bit suspicious as well), and thus have no changes in correlation coefficient, what about choosing different properties so that you can track how the signal degrades with SWH in correlation as well as in magnitude deviation from actual SSH? This comment on correlation coefficient can be applied to the next two paragraphs discussing those properties as well.

Line 295: swells have also been chosen in different

Figure 6 caption: Include units. Also, I think that the propagation direction is relative to the range direction. Please state this explicitly in the caption.

Lines 308-309: on the order of centimeters

Line 309: attributed to layover making

Line 320: and even with the 3 m SWH

Line 373: owing to ocean waves being essentially the superposition of many sine waves with different

Lines 385-397: I would like to see it restated again what the propagation directions of 0°, 90°, etc. are relative to. The range direction (in the direction of 'r', from Fig. 1) is my understanding, but it's good to make it painfully clear to the reader.

 

Line 434: sensitivities

Line 450: their wavelengths are

Line 451-454: wave smoothing window should be a rectangular window, on the order of kilometers, and needs to be large enough to filter out high frequency signals that influence mesoscale and submesoscale low frequency signals.

Line 456: 'is not' -> 'does not'

Line 459: of a rectangular ... of a unidirectional

Line 464: on the order of millimeters

Line 467: is an integer

Line 473: two directions (and drop the words 'only' in this sentence)

Line 477: same corrections as for Line 473

Line 481: 'is not' -> 'also does not' (delete ', too' at the end)

Lines 492, 496 and 500: same as for Lines 473 and 477

Line 504: 'should choose the' -> 'should be a'

Line 517: smoothing is along

Line 519: smoothing is along

Line 525: should be chosen to be parallel ... and fewer smoothing pixels should be chosen

Line 548-549: when the smoothing window

Lines 553, 557, 561, 573, 577, and 579: same corrections as for Line 473

Lines 569-570: 1) The sentence is not grammatically correct. 2) I'm not clear that you have explained why the there is an error in your tables due to data processing. Please say more about this, or refer to an earlier explanation again, if there was one I missed.

Line 609: 'determinate region' -> 'region of study'

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper investigated the various effects on SSH measurement error
using IRA by the simulation, which is useful.
However, the description of the simulation procedure is not organized,
it is difficult to follow it.

I wonder validity of equation (2) only by generating normal random
numbers . The auto-correlation of the generated random number may be zero
(<E(x,y)E(x+dx,E+dy)>=0 for (dx,dy) \ne 0), which is not realistic.

There are many mathematical symbols which are not defined
or duplicated. I pointed out some of this but not exhaustive.

Specific comments:
(1) eq.(1): What is R ? Is it same as that in (11) (Line188) ?
(2) eq.(3):E(x,y,z): Is it E(x,y) ? (eq.(2)) What is the integration range with respect to z ?
(3) What is R in eq. (4) ? Is it same as that in eq. (1) ?
(4) What is X in eq. (4) ?
(5)What is relationship of S_2 in eq.(12) with S_1 in eq.(3) or (4) ?
(6)Eq. (12): What is E ? It seems to be duplicated with E in eq.(3)
(7) Is Delta h (eq 7) and h (eq 10) the same meaning ? If not, what
 is the relationship between them ? SWH is also defined in Line 284,
which seems to be valid.
(8) How eq. (11) is incorporated in the simulation ?
There are some explanations in Line 190, but how eq.(3) or
 is modified ?
(9) Line 202: How the phase phi_{ln} is evaluated ?
(10) How the phase difference (as eq.(6)) is evaluated ? Is it
from eq. (12) or (13) ?

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper investigates the effects of the velocity bunching and the layover induced by sea waves on retrieved sea surface height (SSH) by interferometric radar altimeter (IRA).  The results suggested that the layover primarily affects the correlation of master-slave images and the effect of interference imaging, though the velocity bunching has more impact on the mean SSH inversion. The authors also concluded that layover and velocity bunching can have different effects for different sea conditions and system parameters.

The authors have conducted the extensive analysis and comparisons using different parameters including SWH values, Wavelengths, directions, etc. I found the paper is very interesting and of importance to the altimetry community. I recommend the paper to be accepted for the publication after minor amendments. 

Some minor comments:

Page 1, line 12-24, I think it is better to include some of numeric results in the abstract, e.g., RMSe value, etc.

Page 2, line 46-99, it is very long paragraph, it is better to break this into 2-3 paragraphs. This will make the paper more readable.

Page 2, throughout the document, I found a few incorrect referencing format (line 62,70, 72,74, etc.). Please double check all references and make sure you follow the journal referencing style.

Page 3, line 110-112, include the wavelengths for the Ku and Ka bands.

Page 6, line 215-223, check the line spacing, and make it consistent with the rest of document.

Page 10, line 300-301, “This also agreed with the theoretical analysis.” Which theoretical analysis? You have to provide reference for this statement.

Page 11, line 334, Table 6,move it to the next page completely.

Page 14, line 402, Figure 7, I think it is better to use the same y-axis limit (RMSe, 0-14 cm) for figure (a) and (b) as it will better show the differences.

Page 19, line 560, Table 16, move it to the next page.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thank you for improving the readability of the text.  I think it is better now, and I recommend the paper be accepted for publication after some cleaning up of English grammar in the text.  One thing: there should be a year after "Schulz-stellenfleth et al." in line 71.

 

Author Response

Response to Reviewer 1 Comments

Point 1:I think it is better now, and I recommend the paper be accepted for publication after some cleaning up of English grammar in the text.  One thing: there should be a year after "Schulz-stellenfleth et al." in line 71.

 

Response 1: Thanks for your comment.According to the requirements of the journal, the references are marked with square brackets and numbers, without the need to add a year.The year of the citation is indicated in the references section.Some English grammar problems have been corrected.

Reviewer 2 Report

The replies and revised version are satisfactory, and it is
possible to publish.

Minor comment.
The imaginary unit (sqrt(-1)) is 'i' in Eqs .(3) and (4),
while it is 'j' in Eqs.(16),(20),(21). Please unify them.

 

Author Response

Response to Reviewer 2 Comments

Point 1: The imaginary unit (sqrt(-1)) is 'i' in Eqs .(3) and (4),
while it is 'j' in Eqs.(16),(20),(21). Please unify them.


 

Response 1: Thanks for your comment.In this paper, i in eqs. (3) and (4) has been modified to j, which is consistent with the following text.