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Open AccessArticlePost Publication Peer ReviewVersion 2, Revised

Lasers for Satellite Uplinks and Downlinks (Version 2, Revised)

1
Space Systems Academic Group, Naval Postgraduate School, Monterey, CA 93940, USA
2
Department of Mechanical Engineering (CVN), Columbia University, New York, NY 10027, USA
*
Author to whom correspondence should be addressed.
Received: 7 March 2020 / Accepted: 7 March 2020 / Published: 12 June 2020
Peer review status: 2nd round review Read review reports

Reviewer 1 Stefano Speretta Delft University of Technology Reviewer 2 Jakub Mielczarek Institute of Physics, Jagiellonian University, ul. Lojasiewicza 11, 30-348 Krakow, Poland Reviewer 3 Davide Conte Assistant Professor at Embry-Riddle Aeronautical University,Prescott, Arizona
Version 1
Original
Approved with revisions
Authors' response
Approved with revisions
Authors' response
Approved with revisions
Authors' response
Version 2
Revised
Approved with revisions
Authors' response
Approved
Authors' response
Approved with revisions
Authors' response
Version 2, Revised
Published: 12 June 2020
DOI: 10.3390/sci2020044
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Version 1, Original
Published: 18 March 2020
DOI: 10.3390/sci2010016
Download Full-text PDF
The use of Light Amplification by Stimulated Emission of Radiation (i.e., LASERs or lasers) by the U.S. Department of Defense is not new and includes laser weapons guidance, laser-aided measurements, even lasers as weapons (e.g., Airborne Laser). Lasers in support of telecommunications is also not new. The use of laser light in fiber optics shattered thoughts on communications bandwidth and throughput. Even the use of lasers in space is no longer new. Lasers are being used for satellite-to-satellite crosslinking. Laser communication can transmit orders-of-magnitude more data using orders-of-magnitude less power and can do so with minimal risk of exposure to the sending and receiving terminals. What is new is using lasers as the uplink and downlink between the terrestrial segment and the space segment of satellite systems. More so, the use of lasers to transmit and receive data between moving terrestrial segments (e.g., ships at sea, airplanes in flight) and geosynchronous satellites is burgeoning. This manuscript examines the technological maturation of employing lasers as the signal carrier for satellite communications linking terrestrial and space systems. The purpose of the manuscript is to develop key performance parameters (KPPs) to inform U.S. Department of Defense initial capabilities documents (ICDs) for near-future satellite acquisition and development. By appreciating the history and technological challenges of employing lasers rather than traditional radio frequency sources for satellite uplink and downlink signal carrier, this manuscript recommends ways for the U.S. Department of Defense to employ lasers to transmit and receive high bandwidth, large-throughput data from moving platforms that need to retain low probabilities of detection, intercept, and exploitation (e.g., carrier battle group transiting to a hostile area of operations, unmanned aerial vehicle collecting over adversary areas). The manuscript also intends to identify commercial sector early-adopter fields and those fields likely to adapt to laser employment for transmission and receipt. View Full-Text
Keywords: telescopes; lightweight telescope mirrors; adaptive optics; better resolution; increased accuracy; more bandwidth; cluster of satellites; innovative platform; more capabilities into smaller packages; far-shorter time from click to customer telescopes; lightweight telescope mirrors; adaptive optics; better resolution; increased accuracy; more bandwidth; cluster of satellites; innovative platform; more capabilities into smaller packages; far-shorter time from click to customer
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Dmytryszyn, M.; Crook, M.; Sands, T. Lasers for Satellite Uplinks and Downlinks. Sci 2020, 2, 44.

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1

Reviewer 1

Sent on 11 May 2020 by Stefano Speretta | Approved with revisions
Delft University of Technology

The paper shows a lot of work in collecting and assembling different references together (for a total of 143 references). Typically, when this is done in survey papers, most of the papers referenced are explained or mentioned in the text showing that the reference has a clear reason to be present in the paper. In this paper, there are a lot of references that are only mentioned in a list (page 4, references 61-64 for example) which raises the question is all these references are really needed or they are simply used to build up a big volume f references. I would recommend citing papers that are useful for the topic presented rather than inflating the number of references and not needing them in the story.

The English level is definitely at the mother-language level so no comments on that.

What I would improve in the paper is the style: there is a lot of text quoted which is unclear if it helps in the story, also the sentences in the different paragraphs sound very disconnected between each other (an example is the last paragraph os section 4 on page 11: the whole section is discussing the advantages and it is unclear why mentioning that the European Space Agency has a dedicated program on optical communications would benefit the section and sounds instead out of place). I would also mention that acronyms are used in a very mixed way: several of them are always expanded (National Aeronautics and Space Administration) and some others are explained several times: I would keep a consistent approach and show the expanded version only the first time. Some acronyms are also very well known (such as NASA) so expanding them every time does not benefit the text.

I would also consider the KPP: their discussion is extremely qualitative and I think it would be hard to use such descriptions to steer future policies. I would see a clear comparison of performances attainable with lasers and RF as a more useful comparison (and thus justification the potential investment). Currently, all advantages are clearly generic and do not touch potential problems like performances under fog/rain/dust/sand.  Some comparisons look a bit stretched and sometimes based only on presentations at conferences or white papers (this might also endanger the objectiveness of the results presented).

Response to Reviewer 1

Sent on 04 Jul 2020 by Mark Dmytryszyn, Matthew Crook, Timothy Sands

We thank all three reviewers for their time and efforts, and we value all the suggestions to improve the manuscript. It was ubiquitously agreed the paper was extremely qualitative, and that was done on purpose considering the intended audience of readership (namely scientifically-minded decision makers). Nonetheless, we have spent contemplative time considering where quantitative amplification could be done without turning-off the intended reader and we judge the proper place to be in section 3 with comparisons of lasers and RF in addition to beam propagation to amplify the importance of ATP and lastly more detailed elucidation of atmospheric attenuating factors. We stipulate that our intended audience of decision makers generally simply put down papers upon first sight of equations, and we accepted the enormous burden of trying to make the paper accessible to those readers. Having said that, we deem section 3 is an area the intended audience would skip, continuing straight to section 4….and we judge the paper’s utility would be retained under that assumption. We have taken another opportunity to “scan” the manuscript for numbers, equations, and acronyms that typically tend to cause intended readers to simply not read the article judging it to be poorly written. We have not eliminated the quoted texts, since that mechanism of understanding exists in some of the readership and we deem inclusion does not distract from the benefits of other readers who don’t prefer to see such. This premise also applies to the use of references. The intended audience will have been “sold to” most days of their career and tend to mistrust advocacy. We seek to help placate that mistrust by the use of extensive references aiding readers’ attempts to validate and/or refute the assertions in the manuscript. One reviewer found the manuscript incoherent, while a second found the manuscript “coherent and undoubtedly accessible to non-specialist readers”, which was precisely our goal. The third reviewer judged the coherence as “good and informative narrative”, so major efforts were not expended changing the flow of the manuscript.

Reviewer 2

Sent on 20 May 2020 by Jakub Mielczarek | Approved with revisions
Institute of Physics, Jagiellonian University, ul. Lojasiewicza 11, 30-348 Krakow, Poland

The article provides a short review of the current status of the free-space laser communication, with inclination on possible military applications. The discussion is coherent and undoubtedly accessible to non-specialist readers. However, this is achieved at a cost of qualitative nature of the analysis. In my opinion, supplementary quantitative discussion and bringing some technical details would strengthen the article. Specifically, the following improvements are worth considering:

  1. Differences in beam losses for RF and laser communication deserve to be quantified. E.g. how many dB/km loss at characteristic frequencies?
  2. It is relevant to comment on possible waveforms of the optical signal. There are different possibilities: amplitude modulation, phase modulation, modulation via polarization, …
  3. Worth to mention different choices of coding and resulting values of bit error rate (BER).
  4. Can we quantify worst vs. best case for BER depending on atmospheric and enlightening conditions for a typical Earth-LEO setup?
  5. It should be clarified under which atmospheric and enlightening conditions the systems will be operational. How much (value of BER) the Sun light is affecting the operation? What are the atmospheric frequency windows for optical communication?
  6. Due to the atmospheric effects, beam spreading in the uplink and downlink channels are very different. Nature of this difference and relevant consequences should be clarified, e.g. differences in wavelengths and channel capacity.   
  7. Since the beam pointing plays a crucial role in the free-space communication, more information about obstacles and current status of the relevant technology should be provided. E.g. how much the currently available MEMS-based beam stirring systems meet accuracy requirements of ~1 microradian?
  8. An important extension of classical laser communication is the quantum communication with light. Specifically, the so-called quantum key distribution (QKD) has been successfully conducted using Micius satellite. While the quantum communication is mentioned in the article, some more comments on feasibility and importance of implementation of the approach should be made. This is especially relevant because of potential military applications of QKD.
  9. It would be good to comment on a considered configuration with modulating retro-reflector, which do not require to place a laser on the orbit.
  10. Due to the military context of the article, the issue of satellite laser communication with submarines should be clarified. Can the communication be conducted with underwater segments? If yes, then please discuss at which wavelength and what is the maximal depth.
  11. Is the free-space laser communication potentially harmful to humans, e.g. via vision impairment? Consider e.g. operation of a downlink in (or over) an urban area, and „loosing” a beam between satellite and moving terrestrial segment.
  12. While access to optical spectrum for the purpose of communication is unregulated at the moment, this may change together with dissemination of the technology. Can we (based on our current knowledge) forecast risks and opportunities related to this issue?

Response to Reviewer 2

Sent on 04 Jul 2020 by Mark Dmytryszyn, Matthew Crook, Timothy Sands

We thank all three reviewers for their time and efforts, and we value all the suggestions to improve the manuscript. It was ubiquitously agreed the paper was extremely qualitative, and that was done on purpose considering the intended audience of readership (namely scientifically-minded decision makers). Nonetheless, we have spent contemplative time considering where quantitative amplification could be done without turning-off the intended reader and we judge the proper place to be in section 3 with comparisons of lasers and RF in addition to beam propagation to amplify the importance of ATP and lastly more detailed elucidation of atmospheric attenuating factors. We stipulate that our intended audience of decision makers generally simply put down papers upon first sight of equations, and we accepted the enormous burden of trying to make the paper accessible to those readers. Having said that, we deem section 3 is an area the intended audience would skip, continuing straight to section 4….and we judge the paper’s utility would be retained under that assumption. We have taken another opportunity to “scan” the manuscript for numbers, equations, and acronyms that typically tend to cause intended readers to simply not read the article judging it to be poorly written. We have not eliminated the quoted texts, since that mechanism of understanding exists in some of the readership and we deem inclusion does not distract from the benefits of other readers who don’t prefer to see such. This premise also applies to the use of references. The intended audience will have been “sold to” most days of their career and tend to mistrust advocacy. We seek to help placate that mistrust by the use of extensive references aiding readers’ attempts to validate and/or refute the assertions in the manuscript. One reviewer found the manuscript incoherent, while a second found the manuscript “coherent and undoubtedly accessible to non-specialist readers”, which was precisely our goal. The third reviewer judged the coherence as “good and informative narrative”, so major efforts were not expended changing the flow of the manuscript.

Reviewer 3

Sent on 09 May 2020 by Davide Conte | Approved with revisions
Assistant Professor at Embry-Riddle Aeronautical University,Prescott, Arizona

Good and informative narrative. However, the paper would benefit from more quantitative visuals, such as graphs, figures, and/or tables comparing the efficiency, power requirements, and other important factors of laser communications with "traditional" communication techniques. 

Response to Reviewer 3

Sent on 04 Jul 2020 by Mark Dmytryszyn, Matthew Crook, Timothy Sands

We thank all three reviewers for their time and efforts, and we value all the suggestions to improve the manuscript. It was ubiquitously agreed the paper was extremely qualitative, and that was done on purpose considering the intended audience of readership (namely scientifically-minded decision makers). Nonetheless, we have spent contemplative time considering where quantitative amplification could be done without turning-off the intended reader and we judge the proper place to be in section 3 with comparisons of lasers and RF in addition to beam propagation to amplify the importance of ATP and lastly more detailed elucidation of atmospheric attenuating factors. We stipulate that our intended audience of decision makers generally simply put down papers upon first sight of equations, and we accepted the enormous burden of trying to make the paper accessible to those readers. Having said that, we deem section 3 is an area the intended audience would skip, continuing straight to section 4….and we judge the paper’s utility would be retained under that assumption. We have taken another opportunity to “scan” the manuscript for numbers, equations, and acronyms that typically tend to cause intended readers to simply not read the article judging it to be poorly written. We have not eliminated the quoted texts, since that mechanism of understanding exists in some of the readership and we deem inclusion does not distract from the benefits of other readers who don’t prefer to see such. This premise also applies to the use of references. The intended audience will have been “sold to” most days of their career and tend to mistrust advocacy. We seek to help placate that mistrust by the use of extensive references aiding readers’ attempts to validate and/or refute the assertions in the manuscript. One reviewer found the manuscript incoherent, while a second found the manuscript “coherent and undoubtedly accessible to non-specialist readers”, which was precisely our goal. The third reviewer judged the coherence as “good and informative narrative”, so major efforts were not expended changing the flow of the manuscript.

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