Evolution of Single Photon Lidar: From Satellite Laser Ranging to Airborne Experiments to ICESat-2

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript aims to describe the complex but rewarding journey leading to the current Satellite Single Photon LiDAR system, as narrated by one of the field's pioneering figures. The author has undoubtedly made significant contributions to the field, and this manuscript is of considerable value for advancing single-photon technology and single-photon LiDAR technology. However, before the manuscript can be accepted, there are several points that I suggest to be addressed to further improve its quality.

1.     The manuscript primarily consists of technical reviews and the author’s personal research experiences, rather than a structured academic paper. I encourage the author to provide more details, algorithmic formulas, or examples for each stage of the system's evolution, provided there are no confidentiality concerns.

2.     Precision is crucial in ranging systems. Given that the reflected beam power is generally stronger in airborne systems than in satellite systems, the sensitivity of the SPAD and the corresponding imaging algorithms must be improved accordingly. These aspects are equally important. I suggest that the author discuss the enhancements made to the device and algorithms over the years.

3.     The extremely low count of reflected photons leads to a trade-off between photon detection efficiency and the dark count rate of the SPADs. I recommend that the author include more details on how the field has addressed this challenge.

4.     In line 314 of section 5, the author cites reference number 18, which is not included in the reference list. Please double check.

5.     In Table 2, the contrast between the black text and the dark blue or green background makes it difficult to read. I suggest improving the readability by adjusting the color contrast.

 

Author Response

Please see the attached.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

A. Methods and Technical Detail

 

The methods section needs more detailed explanations of the experimental setup and procedures. Specifically, the description of the Single Photon Lidar (SPL) development could include more granular details about the data collection process, calibration methods, and error analysis. This would provide readers with a more comprehensive understanding of the methodology.

 

There is a lack of discussion regarding the limitations of the methods used. Addressing potential weaknesses or challenges encountered during the experiments would strengthen the paper by demonstrating a critical and reflective approach to the research conducted.

 

 

B. Motivation and Relevance

The motivation behind the study should be more clearly articulated at the beginning of the paper. While the historical context is well-covered, it’s essential to link this to the current research gaps and explain why this study is timely and important. 

Comments on the Quality of English Language

The paper would benefit from improved clarity and conciseness in writing. Some sections contain overly complex sentences and technical jargon that may obscure the main ideas. Simplifying the language and breaking down complex sentences could make the content more accessible to a broader audience.

 

Some figures looks very informal. Table 2 and the page 11. 

Author Response

Please see the attached.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

In this paper, the author shares their experience of the complicated path which ultimately led NASA researchers and their contractors to the highly successful ATLAS Single Photon Lidar. This topic will be of great interest to the readers. There are a few minor issues that I hope the author can address.

1. The abstract is somewhat verbose; it would benefit from a more concise description.

2. In the introduction section, please include a discussion on the significance and objectives of this paper.

3. “The Project team decided to replace the 16 weak beams of the Sigma Space design with 3 weak beams and 3 strong beams.” What was the rationale behind increasing the 3 strong beams, and could the strong beams potentially cause saturation in the detectors?

4. In line 207, “a much lower pulse energy, transmitted optimally through a common transmit/receive telescope and combined with a single photon sensitive receiver, can be used to obtain an adequate return signal.” This is a remarkable technology. In its application to ICESat-2 (ATLAS), what do you consider to be the most challenging aspect, and how was it addressed?

5. In Section 5, the title includes "SINGLE PHOTON LIDA8." What does LIDA8 mean? It might be a typographical error.

6. MMLA is the first Airborne Single Photon Lidar. While examining early SLR2000 data, a plot of measured satellite range vs predicted range exhibited a linear slope. Does this issue also occur in ATLAS? Have any other problems been successfully resolved and applied to ICESat-2?

Author Response

Please see the attached.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript provides a comprehensive and insightful historical review of the evolution of Single Photon Lidar (SPL) technology, tracing its origins from early satellite laser ranging experiments to its current applications in missions such as ICESat-2. This paper is a valuable contribution to the field, offering detailed background information, including personal experiences that are particularly valuable for the community. The inclusion of these experiences enriches the paper, making it a significant resource for both current researchers and future developments in SPL technology. Below are some minor comments and suggestions:

 

1. There are some typo errors;

1) Line 43, mis typo

"later forced NASA to choose between them. ,"

2) Line 44, intentional use of bold font?

“of the Sigma Space design with 3 weak beams and 3 strong beams, each of the latter containing”

3) Line 174, double punctuation marks.

"escope.. The same holds true for large meter-class astronomical telescopes which, to re-"

4) Line 188, intentional underlining?

“the optimized transmitting telescope when pointed correctly at the target is given by the”

5) Line 200, missing equal sign

“where nt is the number of photons transmitted per laser pulse, η_t = η_r 0.66 are estimated”

 “where nt is the number of photons transmitted per laser pulse, η_t = η_r=0.66 are estimated”

6) Line 203,

“any), gT =1.12 ₂and gR =1 are the optimized geometric telescope gains described previously,”

 “any), g_T =1.122 and g_R =1 are the optimized geometric telescope gains described previously,”

 

2. Figures, tables and their legends should be aligned accordingly.

3. Several symbols in Eq. (2) and (3) are explained redundantly in the following paragraphs.

 

Comments for author File: Comments.pdf

Author Response

Please see the attached.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the issues. 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all my comments, and the revised manuscript is ready for publication.