Millimeter-Wave SAR Imaging for Sub-Millimeter Defect Detection with Non-Destructive Testing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript discusses the issue of millimeter-wave imaging for non-destructive testing. An experimental platform is constructed using the FMCW radar sensor AWR1642, which operates in the 77-81 GHz frequency range.

1. The imaging technique, including both the algorithm and the scanning scheme, has been researched in previous studies (e.g. [1-2] in the following). So, what is the unique contribution of this work? Please include more detailed discussions or comparisons in the manuscript.

[1] M. T. Ghasr, S. Kharkovsky, R. Bohnert, B. Hirst and R. Zoughi, "30 GHz Linear High-Resolution and Rapid Millimeter Wave Imaging System for NDE," in IEEE Transactions on Antennas and Propagation, vol. 61, no. 9, pp. 4733-4740, Sept. 2013, doi: 10.1109/TAP.2013.2270174.

[2] S. Li, G. Zhao, H. Sun and M. Amin, "Compressive Sensing Imaging of 3-D Object by a Holographic Algorithm," in IEEE Transactions on Antennas and Propagation, vol. 66, no. 12, pp. 7295-7304, Dec. 2018, doi: 10.1109/TAP.2018.2869660.

 

2. As discussed in Section 2.4, the manuscript focuses only on 2-D imaging although a wideband radar sensor is used. So, it would be beneficial to extend the discussion to 3-D imaging, which can detect defects that may exist at varying down-range distances.

Minors:

1. The semicolon is not suggested before introducing a formula, e.g. before eqs. (1) to (3).

2. On page6, line 210: the word ‘where’ should not be indented.

Author Response

Comments and Suggestions for Authors

1. The imaging technique, including both the algorithm and the scanning scheme, has been researched in previous studies (e.g. [1-2] in the following). So, what is the unique contribution of this work? Please include more detailed discussions or comparisons in the manuscript.

[1] M. T. Ghasr, S. Kharkovsky, R. Bohnert, B. Hirst and R. Zoughi, "30 GHz Linear High-Resolution and Rapid Millimeter Wave Imaging System for NDE," in IEEE Transactions on Antennas and Propagation, vol. 61, no. 9, pp. 4733-4740, Sept. 2013, doi: 10.1109/TAP.2013.2270174.

[2] S. Li, G. Zhao, H. Sun and M. Amin, "Compressive Sensing Imaging of 3-D Object by a Holographic Algorithm," in IEEE Transactions on Antennas and Propagation, vol. 66, no. 12, pp. 7295-7304, Dec. 2018, doi: 10.1109/TAP.2018.2869660.

Authors’ response: We thank the reviewer for suggesting these valuable references that enhance our paper's quality. We have tried our best to uncover the novelty of our paper in regards to previous works, especially those referenced in [1] and [2]. Accordingly, we made revisions in abstract as well as introduction sections to emphasise our contributions in frontside. Finally, we have also added [1] into Table 2 to improve the comparison.

2. As discussed in Section 2.4, the manuscript focuses only on 2-D imaging although a wideband radar sensor is used. So, it would be beneficial to extend the discussion to 3-D imaging, which can detect defects that may exist at varying down-range distances.

Authors’ response: Thank you for your fruitful comments. We certainly agree with the reviewer that 3-D imaging can detect defects at down-range distances and improve the detection capability. However, we focus on the imaging methodology toward low-cost and low complexity designs within the scope of this study. We will definitely evaluate this insightful comments and consider in future studies. We agree to that the advancement to 3-D imaging signifies a promising potential for future investigation.  

Minors:

1. The semicolon is not suggested before introducing a formula, e.g. before eqs. (1) to (3).

Authors’ response: We thank the reviewer for his/her careful review. The errors have been corrected.

2. On page6, line 210: the word ‘where’ should not be indented.

 Authors’ response: We thank the reviewer for his/her careful review. The error has been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper was presented a cost-effective and high-resolution 77-81 GHz mmWave Synthetic Aperture Radar (SAR) imaging system designed specifically for NDT, optimized to detect minimal defects in materials, including low-reflectivity ones. By optimizing key system parameters, including frequency slope, sampling interval, and scanning aperture, high-resolution SAR images are achieved with reduced computational complexity and storage requirements.

The article is interesting and contains a lot of interesting data. The quality of the drawings presented is very good.

I have few suggestions:

1. In the abstract, the statement "Millimeter-Wave (mmWave) radar imaging hold significant potential for non-destructive testing (NDT) applications, offering high resolution and deep penetration capabilities. Existing structural, experimental, and algorithmic approaches to achieve high-resolution imaging typically increase cost ,system complexity, and computational load." they are not needed. Such sentences may appear in the introduction.

2. The most important numerical results should be in the abstract. Please provide results confirming the validity of the theses put forward.

3. The abstract should state the novelty in an unambiguous manner. What distinguishes this work from other similar works.

4. In the second chapter the Figure with the methodology should be shown.

5. Figure 5 is blurry. Is it supposed to be like that?

6. The conclusion lacks numerical results. Numerical data confirming the validity of the research conducted and the theses put forward should be provided.

Author Response

Comments and Suggestions for Authors

In this paper was presented a cost-effective and high-resolution 77-81 GHz mmWave Synthetic Aperture Radar (SAR) imaging system designed specifically for NDT, optimized to detect minimal defects in materials, including low-reflectivity ones. By optimizing key system parameters, including frequency slope, sampling interval, and scanning aperture, high-resolution SAR images are achieved with reduced computational complexity and storage requirements.

The article is interesting and contains a lot of interesting data. The quality of the drawings presented is very good.

I have few suggestions:

1. In the abstract, the statement "Millimeter-Wave (mmWave) radar imaging hold significant potential for non-destructive testing (NDT) applications, offering high resolution and deep penetration capabilities. Existing structural, experimental, and algorithmic approaches to achieve high-resolution imaging typically increase cost ,system complexity, and computational load." they are not needed. Such sentences may appear in the introduction.

Authors’ response: We thank the reviewer for pointing this out. We have removed relevant sentences based on the reviewer’s comments, and revised the paper accordingly.

2. The most important numerical results should be in the abstract. Please provide results confirming the validity of the theses put forward.

Authors’ response: Thank you for your comments. We revised the abstract accordingly.

3. The abstract should state the novelty in an unambiguous manner. What distinguishes this work from other similar works.

Authors’ response: Thank you for your comments. We re-organised the abstract, and focusing on the aspects that make the contribution clear.

4. In the second chapter the Figure with the methodology should be shown.

Authors’ response: Thank you for fruitful comment. We have added Figure 1 to present a high-level workflow in the Methodology section.

5. Figure 5 is blurry. Is it supposed to be like that?

Authors’ response: Thank you for sharing your concern. The figures present originally reconstructed images. No additional enhancement was intentionally made. As we have shown in the paper, image quality increases as spatial sampling intervals decrease.

6. The conclusion lacks numerical results. Numerical data confirming the validity of the research conducted and the theses put forward should be provided.

Authors’ response: Thank you for fruitful comment. We have revised the conclusion accordingly, and added numerical results.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for the revisions. I have only the following minor concerns. 

In the text, most of the numbers use commas instead of periods to separate the integer and decimal parts. Please use periods instead. 

The writing should be carefully checked before publication.

Author Response

Comment:

Thank you for the revisions. I have only the following minor concerns. 

In the text, most of the numbers use commas instead of periods to separate the integer and decimal parts. Please use periods instead. 

The writing should be carefully checked before publication.

Authors’ reply: Thank you. We have carefully revised the paper by addressing the points the reviewer noted.

Author Response File: Author Response.docx