Underground Imaging by Sub-Terahertz Radiation

Round 1

Reviewer 1 Report

Dear Authors,

it seems a good experimental work concerning swept FM radars for underground imaging in W and F bands.

I think that some minor changes should be done in order to improve the paper.

1- It is wise to add a short resume of swept-FM radar theory i.e. how the relevant information are extracted from echo signals (at least target range).
2- Fig.2 it could be useful to tune the picture using standard device symbol (directional couplers, schottky diode detector, LP/HP filters )
3- lines 111-130: we have peaks at specific frequencies...which are the relationships with the described ranges (short formula with actual values)?
6- fig.6: The picture is perhaps more clear using dB instead of linear scale. Please, complete the picture adding the units of measure also.

Best Regards,

Author Response

Dear Reviewer:

Thanks for your comments.

Reply to your question has been attached.

Bests

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper describes experimental demonstration of underground imaging using sub-terahertz receivers at W and F bands.The contents are understandable but need clarification of experimental setups, analysis and discussions.

The major difficulties are the following two points:

1. Azimuthal and range resolutions are important parameters for this experiments but setup and analysis details are missing.
Ground illumination should be determined by the beam pattern of the horns, but their parameters are not given as well as the lens attached to W-band Tx horn. The measured azimuthal resolution should be discussed together with the illumination pattern.
For range resolution should be clarifies. An equation of peak frequency as a function of target range and sweep frequency will help, together with number of FFT points and apodization. Measured range resolution should be discussed as well.

2. The discussion in section 4 do not refer the experimental results in section 3. 
Millimeter powers were not explained in the experiment section and imaging depth is different from the measurements. Measurement of non metallic objects are not described in the experiment section.

Followings are additional comments to the paper:
Abstract
3. F-band was not used for measurement of buried objects. The target was buried in mixed soils, as explained in section 3. Sweep time is not always 10ms. Beam-widths are not described in the paper.

Section 2
4. Power of transmitters should be given for each experiment.

5. There are different expression for the targets; 'aluminum rod', 'metal cylinder' and 'metallic test cylinder'.

6. In figure 3, how are the transmitted power is terminated ?
Are absorbing materials used ?

7. Explanation for oscilloscope, LabVIEW and Sweep Out signal are made twice.

8. The high frequency component may not be multiple reflections, since frequency shifts are too small.

9.I think the experiment with wooden board is better compared with an experiment without wooden board.

10. The measured peak frequency is better compared with setup parameters referring to the equation.

11. 'imagine system' should be 'imaging system'

12. What is the absorbing material used in figure 8 and 9.

Section 4
13. The discussion do not properly refer the experimental results as in my former comments in 2.

14. Development of Klystron mean sense only when power limitation of current system is discussed. In think detailed description of Klystron 
do not fit in this discussion. Readers can get puzzled. What is 'PCM-QM'? What is the content of the high frequency circuit? Where radiation is emitted from? What is the purpose of the multi-stage depressed collector? What is the content of SoC?
I think improvement of performance using high power source should be discussed.

15. Figure 14 only show (a).

Author Response

Dear Reviewer:

Thanks for your comments and suggestions. All your questions have been answered in the attached file.

Bests

Yours Sincerely  

Author Response File: Author Response.pdf

Reviewer 3 Report

Authors presented impressive work using sub-Terahertz Radiation for underground imaging.  From my point of view, the work is worth to be published. 

Author Response

Dear Reviewer:

We thank you for the comment and recommendation.

Bests

Yours Sincerely 

Reviewer 4 Report

Section 4 needs more details, not everything was clear.

I also suggest another proofread of the entire paper - minor wording issues and typos.

Author Response

Dear Reviewer:

We thank you for the comments. Here is the reply.

Point 1: Section 4 needs more details, not everything was clear.

Response 1: We thank the Reviewer for pointing out this deficiency. The entire section 4 has been re-written to provide more detail making this paper clearer.

The attenuation testing has been added in this section to explain why there is a need for a higher power source and more sensitive receiver components.

The un-related klystron information has been removed from this section thereby avoiding confusing the reader.

Point 2: I also suggest another proofread of the entire paper - minor wording issues and typos.

Response 2: We thank the Reviewer for this comment. We have refined the sentences and tried to eliminate the typos throughout the paper.

Bests

Yours Sincerely