ATLAS: Latest Advancements and First Observations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper depicts initial experiments employing ATLAS, where the observations were utilized to recalibrate the radar equation for real-world effectiveness.The number of measurements are limited and a sophesticated signal processing chain has to be adapted to increase the performance of the radar. Nevertheless, the obtained results exhibit encouraging indications toward the application of ATLAS in SSA experiments. Here are some comments on the manuscript:

1. Providing a description of a mathematical signal model illustrating the signal processing chain enhances the readability of the manuscript.
2. The primary objective of the paper is to address the calibration procedure. Including figures illustrating several iterations in beam convergence enhances the manuscript's readability.
3. A more detailed explaination on the intra and inter pulse motion compensation.
4. Enhance image visibility by adjusting the contrast of the images.
5. Ensure consistency in the use of parentheses for SI units in all figures. For example, in Figure 5, indicate Doppler velocity as [m/s], not Doppler velocity (m/s).

Comments for author File: Comments.pdf

Author Response

Dear reviewer. Your feedback has been immensely valuable in improving the quality and clarity of our research. We truly appreciate the thoroughness of your review and the insightful comments you provided.

We have carefully considered each of your suggestions and have implemented many of them in the revised version of the paper.

1.Providing a description of a mathematical signal model illustrating the signal processing chain enhances the readability of the manuscript.

• Thank you for this suggestion. The updated version of the manuscript now contains a dedicated subchapter to the signal model. More detail on the mathematical description of the second branch of signal processing is also provided.

2.The primary objective of the paper is to address the calibration procedure. Including figures illustrating several iterations in beam convergence enhances the manuscript's readability.

• Thank you for this suggestion. A new figure illustrating several beam convergence iterations was added to the calibration results and a detailed explanation of the cross-scanning procedure was added to the Materials and Methods. We hope that by reading the procedure and comparing with the new image it will increase the understanding of the calibration method by the readers.

3.A more detailed explanation on the intra and inter pulse motion compensation.

• The Signal model mathematical description that is added shows the range migration term and the Doppler term that need compensation. A mathematical explanation of how the radon transform is used for compensation was added.

4.Enhance image visibility by adjusting the contrast of the images.

• Thank you for this suggestion. The figure with the signal processor was readjusted and the contrast of the small plots inside was adjustedto enhance the readability of the images.

5.Ensure consistency in the use of parentheses for SI units in all figures. For example, in Figure 5, indicate Doppler velocity as [m/s], not Doppler velocity (m/s).

• Thank you, all the images now use curved parentheses.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents the advancements achieved in the development of the ATLAS radar system, dedicated to tracking low-orbit objects. The authors present the calibration procedure and the corrections applied to the radar equation terms to better describe the equipment at hand.

 

The paper is very well written, the procedures are clearly described/detailed and the results indicate that the system is able to attain its goal of observing space objects. One aspect that could be improved is related to the descriptions of future work: will there be observations of other objects (possibly smaller than the ISS) performed in order to test the capabilities of the system?, what are the new techniques invoked in the Discussion section related to detector design and Doppler velocity extraction?, when are these activities supposed to happen?

Author Response

Dear reviewer. Thank you for taking the time to read and review our article. We appreciate the comments you provided.

1. Will there be observations of other objects (possibly smaller than the ISS) performed in order to test the capabilities of the system?

• Thank you for the question. Yes, we aim to detect objects of different sizes to better characterize the system. We included a recent observation of the Chinese space station in the article to ensure the readers we aim to detect other objects.

2. What are the new techniques invoked in the Discussion section related to detector design and Doppler velocity extraction?

• Regarding detector design, we intend to explore subspace detector design, multi-stage adaptive detectors and machine learning-based detectors. Regarding Doppler velocity extraction we intend on solving the ambiguity by using multiple PRF. We added this information to the conclusion as well as references to contributions on these ideas.

3. When are these activities supposed to happen?

• Response: All of these ideas are ongoing work and will be shown to the community during the next 2 years.

Reviewer 3 Report

Comments and Suggestions for Authors

1、In Section 2.1, the ATLAS system used can be divided into a transmitting part and a receiving part to be introduced in detail. The parameters can be introduced in more detail. The working principle diagram of ATLAS can also be drawn to facilitate an in-depth understanding.

 

2、Lines 113-114: Proposed introduction to the well-defined and continuously adjusted orbit followed by the International Space Station, with a detailed analysis of how it minimizes errors associated with predicted orbital paths. Then list these related errors in detail and briefly analyze their causes.

 

3、In Section 2.3: This article proposes a method of using the sun to calibrate radar performance and pointing system, but the description is too concise. It is recommended to add a schematic diagram of the calibration process and introduce the coordinate system used for calibration to facilitate readers' understanding of this calibration method.

 

4、It is recommended that the article briefly introduce the concept of the far-field region of the antenna and why the sun can be considered a uniform disk with a diameter of 0.5º.

 

5、In Section 4.1, the concepts of main beam and sidelobes are mentioned, but they are not marked in the actual Figure 3,which may easily lead to misunderstanding.

 

6、t is recommended to add 2 to 3 sets of experiments to observe space targets with different  Radar Cross Sections and different distances, and use real observation results to verify the accuracy of the SNRp-Range fitting curve obtained by the non-linear least squares method.

Author Response

Dear reviewer. Your feedback has been immensely valuable in improving the quality and clarity of our research. We truly appreciate the thoroughness of your review and the insightful comments you provided. We have carefully considered each of your suggestions and have implemented many of them in the revised version of the paper.

1. In Section 2.1, the ATLAS system used can be divided into a transmitting part and a receiving part to be introduced in detail. The parameters can be introduced in more detail. The working principle diagram of ATLAS can also be drawn to facilitate an in-depth understanding.

• A new section was added where a technical summary of the transmitting and receiving chains is provided. For a more detailed description, a comprehensive technical overview of ATLAS is presented in a previous contribution (reference 5 of the article).

2. Lines 113-114: Proposed introduction to the well-defined and continuously adjusted orbit followed by the International Space Station, with a detailed analysis of how it minimizes errors associated with predicted orbital paths. Then list these related errors in detail and briefly analyze their causes.

• Thank you for this consideration. We agree that stating "the ISS follows a well-defined and continuously adjusted orbit, minimizing errors associated with predicting its orbital path" without providing any further explanation and/or references is incorrect. From our experience, the ISS has been an easy target to follow, appearing in the majority of the observations we make of it. Unlike other satellites, the ISS trajectory is constantly corrected and NASA provides constantly updated state vectors, the mass of the ISS, drag area, drag coefficient, solar radiation area, and solar radiation coefficient, unlike other satellites. This allows the usage of special perturbation models reducing orbit propagation errors and ultimately increasing the chance of detecting the ISS. We updated the text for this to be more clear and added references to support it.

3. In Section 2.3: This article proposes a method of using the sun to calibrate radar performance and pointing system, but the description is too concise. It is recommended to add a schematic diagram of the calibration process and introduce the coordinate system used for calibration to facilitate readers' understanding of this calibration method.

• Thank you for this suggestion. A paragraph introducing the Az/El coordinate system was added. A new figure illustrating several beam convergence iterations was added to the calibration results and a detailed explanation of the cross-scanning procedure was added to the Materials and Methods.

4. It is recommended that the article briefly introduce the concept of the far-field region of the antenna and why the sun can be considered a uniform disk with a diameter of 0.5º.

• Thank you for the suggestion. The definition of far field region as well as the value for ATLAS was added. Regarding the diameter of the sun, there was a mistake and for ATLAS frequency the apparent diameter is approximately 0.57º. We corrected the text and added references that support this value of the diameter as well as the approximation as a homogeneous disk.

5. In Section 4.1, the concepts of main beam and sidelobes are mentioned, but they are not marked in the actual Figure 3,which may easily lead to misunderstanding.

• Thank you for this suggestion. The caption in Figure 3 (now Figure 10) is more descriptive and identifies the regions in the plots that correspond to the main lobes and sidelobes.

6. It is recommended to add 2 to 3 sets of experiments to observe space targets with different Radar Cross Sections and different distances and use real observation results to verify the accuracy of the SNRp-Range fitting curve obtained by the non-linear least squares method.

• We strongly agree with you. We are currently working on making new observation campaigns where we cover objects at different RCS and distances to cover a broader range of the SNRp-Range plane.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript has been carefully revised and there is no other problem.

Line 312, one of the equations in the manuscript is missing the number.