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Peer-Review Record

Micro-Motion Estimation of Maritime Targets Using Pixel Tracking in Cosmo-Skymed Synthetic Aperture Radar Data—An Operative Assessment

Remote Sens. 2019, 11(14), 1637; https://doi.org/10.3390/rs11141637
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Remote Sens. 2019, 11(14), 1637; https://doi.org/10.3390/rs11141637
Received: 20 May 2019 / Revised: 2 July 2019 / Accepted: 4 July 2019 / Published: 10 July 2019
(This article belongs to the Special Issue Exploitation of Synthetic Aperture Radar Targets Velocities)

Round 1

Reviewer 1 Report

A very clear and well-written paper describing the estimation of micro-motion of ships from SAR data. The paper includes enough details about the methodology used and a sufficient results from relevant real-life scenarios.


Minor problems:

page 2: "Compresses Sensing" --> "Compressive" or "Compressed"

page 2: "Several methods has" --> "have"

Table 2: "shearch" --> "search"

Page 4: "... through time. and ..." --> ". And"

Page 4: "-5dB" --> this is a dash, a minus sign is longer; and there must be a space between the number and the unit --> "-- 5 dB"

Conclusions: "This paper is proposed ..." --> "This paper proposed"

Materials and Methods: First sentence is grammatically weird

Materials and Methods: "has a too" --> "has a tool"

Author Response

Reply to Reviewers' comments on “Pixel Tracking for Micro-Motion Estimation of Maritime Targets by COSMO-SkyMed Synthetic Aperture Radar Data - An Operative Assessment”



We would like to thank the Associate Editor and the Reviewers for their useful suggestions towards the improvement of the paper.

In the sequel, we present our itemized response (Reviewers’ comments are reported in black plain text, while our answers are in italic blue text).

We hope that the revised version of our manuscript conforms all the requirements.



Reviewer_1

A very clear and well-written paper describing the estimation of micro-motion of ships from SAR data. The paper includes enough details about the methodology used and sufficient results from relevant real-life scenarios.

We thank the Reviewer for his/her kind words. We are pleased to read that the manuscript has been appreciated.


Minor problems:

page 2: "Compresses Sensing" --> "Compressive" or "Compressed"

page 2: "Several methods has" --> "have"

Table 2: "shearch" --> "search"

Page 4: "... through time. and ..." --> ". And"

Page 4: "-5dB" --> this is a dash, a minus sign is longer; and there must be a space between the number and the unit --> "-- 5 dB"

Conclusions: "This paper is proposed ..." --> "This paper proposed"

Materials and Methods: First sentence is grammatically weird

Materials and Methods: "has a too" --> "has a tool"


We are grateful to the Reviewer for his/her careful reading. The revised paper has been examined in detail in order to correct all the typos.

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Author Response File: Author Response.pdf

Reviewer 2 Report

I would like to comment on three issues:


Language usage: There are numerous English mistakes throughout the paper, which should be corrected so that a reader can understand the manuscript correctly. Examples of  sentences which should be corrected are

"The vector of shifts is complex in that below the coregistration process the displacements are estimated both in the range direction and in the azimuth direction."

"This paper is proposed a new approach where the m-m estimation of ships, occupying thousands of pixels, is measured processing the information given by sub-pixel tracking generated during the coregistration process of several re-synthesized time-domain and not-overlapped sub-apertures."

"This software has a228 too, that is able to estimate the coregistration shifts."

The usage of English language in the Abstract should also be improved. 

Contents of the theory part: a) The paper need to be significantly expanded, e.g. system model should be given, defining the axes, motion types, etc. b) The presentation is a bit confusing. Is the author using an interferometric stack, or is the author creating multiple sub-apertures? c) Correlation may be able to extract the motion, but I believe the motion has to be greater than a certain limit. That limit should be analyzed. d) How is the proposed technique different from LRSD? e) It is also possible that the ships have greater motion due to wave motion. This motion can be detected by the proposed technique, but very small motion may not be detected. Please comment.

Results part: More details should be given about the data. Comparison with existing methods should be included. Why is the author's method better than the existing methods? What are the limitations of the existing methods and the author's method?  

Author Response

Reply to Reviewers' comments on “Pixel Tracking for Micro-Motion Estimation of Maritime Targets by COSMO-SkyMed Synthetic Aperture Radar Data - An Operative Assessment”



We would like to thank the Associate Editor and the Reviewers for their useful suggestions towards the improvement of the paper.

In the sequel, we present our itemized response (Reviewers’ comments are reported in black plain text, while our answers are in italic blue text).

We hope that the revised version of our manuscript conforms all the requirements.

Reviewer_2


I would like to comment on three issues:



1) Language usage: There are numerous English mistakes throughout the paper, which should be corrected so that a reader can understand the manuscript correctly. Examples of sentences which should be corrected are:

- "The vector of shifts is complex in that below the coregistration process the displacements are estimated both in the range direction and in the azimuth direction."

- "This paper is proposed a new approach where the m-m estimation of ships, occupying thousands of pixels, is measured processing the information given by sub-pixel tracking generated during the coregistration process of several re-synthesized time-domain and not-overlapped sub-apertures."

- "This software has a too, that is able to estimate the coregistration shifts."

The usage of English language in the Abstract should also be improved.

The Reviewer is right. In the revised paper, English usage has been heavily improved (including the abstract) and we have rephrased the quoted sentences.



2) Contents of the theory part:

a) The paper need to be significantly expanded, e.g. system model should be given, defining the axes, motion types, etc.;

We would to thank the Reviewer for his/her comment. We have added more details in the estimation procedure and in the computational scheme sections. All the performed modifications in the revised manuscript are highlighted in red. Specifically, we have added a subsection entitled “Computational model”.

b) The presentation is a bit confusing. Is the author using an interferometric stack, or is the author creating multiple sub-apertures?;

We thank the Reviewer to underline this aspect: multiple sub-apertures are used in this work. This issue has been clarified on subsection 2.2 “Computational Architecture Description”.



c) Correlation may be able to extract the motion, but I believe the motion has to be greater than a certain limit. That limit should be analyzed;

We thank the Reviewer for this remark. We have listed in the revised paper the limits of the proposed algorithm which are essentially related to the following parameters.

- The maximum observable frequency:

it is directly proportional to the sampling frequency, in fact, the maximum observable frequency is equal to the sampling frequency divided by two. In this paper, we have used 200 samples (time sub-apertures) spanned along 12 s which correspond to the time baseline of 0.06 s. In this case, the maximum observable frequency is approximately 33 Hz that means that the vibrational modes generated by an engine rotating at a maximum angular speed of 33*60 rpm can be observed;

- The minimum sensitivity in observing vibrational amplitudes:

it is strictly related to the sensor resolution. In this paper, we used Spotlight data having approximately 1 m of resolution and we have set an oversampling factor of 200 (into the coregistration parameters); this means that the range-azimuth resolution cell of 1 m is theoretically split into 200 sub-pixels, so the minimum estimable displacement is equal to 5 mm.

d) How is the proposed technique different from LRSD?

We have explicitly clarified that, this paper, is not using the Low-Rank Plus Sparse Decomposition (LRSD) method which has largely used in [1] to estimate the inclination of ships. In the present work the vibrational fingerprint of ships is estimated, that the proposed work represents an operational assessment for the problem at hand, aiming at evaluating the operational effectiveness and the operational suitability of LRSD for micro-motion estimation of maritime targets. For this purpose, an exhaustive experimental analysis, based on COSMO-SkyMed SAR data, is presented.

e) It is also possible that the ships have greater motion due to wave motion. This motion can be detected by the proposed technique, but very small motion may not be detected. Please comment.

We thank the Reviewer for this interesting aspect. We have further elaborated on it.

We definitely agree with the Reviewer that the ship might move together to the sea waves, but, this phenomenon is observed only over very low frequencies and very high oscillation amplitudes.

Thus, it is worth to notice that this study is concentrated to perform vibrational estimation over higher frequencies with respect to displacement variations due to the sea currents. In this case, the vibrational spectrum generated by the engines can be easily separated from the spectrum generated by the sea.

Furthermore, a mask to the correlation amplitude has been applied while estimating the shifts using the normalized cross-correlation. This strategy permits the extrapolation of the ship from the background in order to concentrate the calculation efforts only on the target.


3) Results part: More details should be given about the data.

In the revised paper, we have added further details about the used data set.

Particularly, the following information has been included:

The following data has been processed (which characteristics has been reported the new table number three):

1 (one) Spotlight SAR image observed by the CSK satellite system in the following configuration:

Location:

Taranto (Italy);

Satellite Height:

627863.775618 m

Satellite ID:

CSKS1 (First satellite of the constellation)

Scene Centre Geodetic Coordinates:

40.463857°N 17.233015°E

Scene Sensing Start UTC:

(yy-mm-dd) 2012-07-12

(hh-mm-ss) 16:47:10.074928684

Scene Sensing Stop UTC:

(yy-mm-dd) 2012-07-12

(hh-mm-ss) 16:47:17.643165988

Azimuth Focusing Bandwidth:

23131.019234

Range Focusing Bandwidth:

247705078.125000 Hz

Radar Frequency:

9600000000.00 Hz

Radar Wavelength:

0.031228 m

Reference Incidence Angle:

40.000000°

Ground Range Instrument Geometric Resolution:

0.890077 m



Comparison with existing methods should be included.

We thank the Reviewer for this remark. In the revised paper, we have added a discussion about the existing methods.

The following sentences have been included in the revised manuscript:

“The estimation of the velocities to be carried out with the radar is done using the investigation technique called along-track interferometry (ATI). Unfortunately, this configuration requires the use of two radars spaced from each other of a small baseline with a length varying from a few meters up to a few dozen meters, however extended in the along-track direction. The longer the baseline, the greater the sensitivity of the Doppler variations of the SAR bistatic system. It is clear that in order to achieve a high sensitivity to low frequency vibrations, it is necessary to dimension the ATI geometry on a very high baseline. Unfortunately, there are no satellite systems configured in this way and therefore it is necessary to investigate on other aspects [1]. Sectioning the along-track orbit into multiple ATI observations is not even a winning solution as the raw data of satellites for Earth observation are not over-sampled [1]. Figure 5 (a) and (b) are the magnitude and phase of the infra-chromatic coherence. The author is proposing these results to demonstrate that the phase information for this type of SAR acquisition is useful because of the not oversampled characteristic characterizing the CSK raw data. The micro-motion can be observed by extrapolating the information of the zero-Doppler at the time of SAR focusing. Unfortunately, even this technique is not considered a winning solution because it makes use of raw images that do not lend themselves to displaying vibration maps. Considering only the perturbations of the Doppler centroid on pixels occupied by moving targets allows the estimation of the displacement in the azimuth direction only. The proposed technique allows the estimation of shifts, even if in lower intensity, also in the range direction, thanks to the highly precision shifts estimation performed by the correlation during the coregistration computational stage.”

[1] Filippo, Biondi. "COSMO-SkyMed Staring Spotlight SAR Data for Micro-Motion and Inclination Angle Estimation of Ships by Pixel Tracking and Convex Optimization." Rem. Sens. 11.7 (2019): 766.



Why is the author's method better than the existing methods?

We thank the Reviewer for this remark. In the revised paper, we have highlighted that the proposed method represents the best solution to the problem under investigation. In fact, the following sentences have been added:

“The proposed investigation technique is considered to be the best one as is applied on the magnitude of the data and without considering the phase information. Unfortunately, in order to obtain reliable results by applying the ATI technique, it is necessary to design multi-antenna observation systems. The proposed technique uses the information as a displacement assumed by the pixels of the focused SAR image while perturbing the Doppler spectrum. This technique of investigation, compared to the others, is easier to apply and to implement because the information provided by the amplitude is generally more stable and therefore much more immune to unwanted phenomena of spatial decorrelation.”

What are the limitations of the existing methods and the author's method?

We thank the Reviewer for this remark. As mentioned above, the main limitation of ATI is that suitable data that meet the requirements for estimating ship vibrations is not easily available. On the contrary, the proposed method, whose principal limitations are due to the frequency and the amplitude of the vibrations, represent the best solution with its intrinsic immunity to spatial decorrelation.



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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for the revisions.


In the approach,the author talks about "temporally small baseline". However, my understanding is that the author is basically making sub-apertures. Later on in the paper, the authors mention 

"Specifically, the single observation is divided into Doppler sub-apertures in order to investigate the fastest displacements of moving targets"


and


"The computational block 2 consists of a temporal n-stage splitter that is responsible for generating n temporal sub-apertures". As the authors are making sub-apertures, why do they not mention that in the Abstract as it is confusing?


The author also mentions sub-aperture several times in the paper. Therefore, this ambiguity between the Abstract and rest of the paper should be removed.


Editing changes are needed, e.g., Figure 2 is mentioned before Figure 1, sometimes Fig. is used and sometimes Figure is used, a few example sentences that should be either improved or corrected are:


1. To this end, observe that the problem of motion and m-m detection of targets is usually solved using synthetic aperture radar (SAR) along-track interferometry through two radars spatially separated by a baseline along the azimuth direction


2.  In [2] the effects of rotation and vibration in millimeter-waveforms SAR processing simulated and real data has been assessed


3. strategies for ships and ship wake detection has been consolidated using also Compressed Sensing an


4. In order to make this approach operating


5.  Doppler sub-apertures is not even a winning solution because

Author Response

See attached file

Author Response File: Author Response.pdf

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