the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Mar 2024
Optimization of a direct detection UV wind lidar architecture for 3D wind reconstruction at high altitude
Abstract. An architecture for a UV wind lidar dedicated to measuring vertical and lateral wind in front of an aircraft for gust load alleviation is presented. To optimize performance and robustness, it includes a fiber laser architecture and a Quadri Mach-Zehnder (QMZ) interferometer with a robust design to spectrally analyze the backscattered light. Different lidar parameters have been selected to minimize the standard deviation of wind speed measurement projected onto the laser axis, calculated through end-to-end simulations of the instrument. The optimization involves selecting an emission/reception telescope to maximize the amount of collected photons backscattered between 100 m and 300 m, a background filter to reduce noise from the scene, and photo-multiplier tubes (PMT) to minimize detection noise. Simulations were performed to evaluate lidar performance as a function of laser parameters. This study led to the selection of three laser architectures: a commercial solid-state laser, a design of a fiber laser, and a hybrid fiber laser resulting in standard deviations of 0.18 m/s, 0.17 m/s, and 0.09 m/s, respectively, at 10 km of altitude. To reconstruct the vertical and lateral wind on the flight path, the lidar is addressed to four different directions to measure four different projections of the wind. We calculate analytically (and validate through simulations) the addressing angle with respect to the flight direction that minimizes the root mean squared error (RMSE) between the reconstructed vertical and lateral wind components and the actual ones, assuming turbulence that follows the Von Karman turbulence model. We found that the optimum angle for an estimation at 100 m is about 50°, resulting in an improvement of about 50 % compared to an angle of 15°–20° typically used in current studies.
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RC1: 'Comment on amt-2024-41', Anonymous Referee #1, 14 May 2024
This paper focuses on two issues associated with application of a direct-detection Doppler lidar for measuring winds ahead of an aircraft to feed forward for gust load alleviation. First, it describes development of a design to maximize wind measurement precision, focusing on telescope, background filter, and detector specifications. It then utilizes an atmospheric model of molecular and aerosol backscatter to predict performance for three different laser transmitters. The second issue addressed is optimization of the angle of deviation for a lidar that utilizes a four-direction concept, assuming turbulence from a commonly employed turbulence model.
I find the paper quite well done and certainly worthy of publication. Although the two main issues could have been addressed separately, they fit together acceptably into a single article. The figures are appropriate and illustrate the main points and conclusions.
In reading the paper, I would have liked to have seen a bit more discussion of the QMZ interferometer and explanation of how the SNR lead to errors in wind speed. This is more of a personal preference – the paper is very well referenced and the Appendix provides the necessary details for estimating wind error from photon count. Perhaps a figure that illustrates how the output from the detectors changes as function of wind speed (or phase) would be tutorial and useful in illustrating the concept.
Although speckle noise is an important component in velocity measurement uncertainty estimate, there is very little discussion of the basis for speckle noise and which system and laser transmitter parameters affect it. For example, line 223 on page 8 says that “ the speckle noise decreases because the backscattering is predominantly molecular, which is less coherent than particulate backscattering. I may have missed it, but I didn’t see in the text or the appendix that discusses the speckle relative to the signal coherence and how this is incorporated into the simulation.
Figure 3 is quite informative and sums up the discussion on wind speed standard deviation nicely.
The angle optimization part of the study produces a nice and useful result. While reviewing the paper, I thought that thus problem had to have been addressed earlier in slider studies of wind energy, but I perused the literature a bit and didn’t find it. Consequently, this result should be of significant interest to the community.
As with all simulation studies, this work begs for follow-up research to demonstrate the concept and validate the simulation. The authors should add some text at the end on anticipated future work and tell the reader how they intend to use the results of the study.
Citation: https://doi.org/10.5194/amt-2024-41-RC1 - AC1: 'Reply on RC1', Thibault Boulant, 04 Jul 2024
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RC2: 'Comment on amt-2024-41', Anonymous Referee #2, 21 May 2024
The manuscript by Boulant al. reports on the development of a UV wind lidar system based on a fiber laser and Quadri Mach-Zehnder (QMZ) interferometer to measure vertical and lateral wind ahead of an aircraft for gust load alleviation. Key instrument parameters of the telescope, background filter and detector were optimized through simulations to minimize wind speed errors. The end-to-end simulator was then used to evaluate the lidar performance for three different laser designs. In addition, the scanning angle of the instrument, employed in a four-axis scheme to reconstruct the 3D wind vector, was optimized assuming turbulence according to the von Karman model.
General comments:
The work is certainly of interest to readers working in the field of active remote sensing, and particularly to the wind lidar community. The manuscript is well-structured and the figures support the intriguing findings derived from the simulations. However, given the results reported in the paper, I am wondering a) to what extent the developed end-to-end simulator is applicable to other wind lidar instruments based on a QMZ (not necessarily for GLA), b) what the major limitations of the simulator are, c) what refinements of the simulator are planned in the future, and d) if (or when) experimental validation of the simulation results with the described instrument is foreseen. In my opinion, these four relevant questions are currently unaddressed and should be discussed in the conclusions section of the manuscript. Addressing these points will strengthen the importance of the paper and potentially increase its impact on a wider audience. Aside from this general remark, there is a number of specific issues that should be considered prior to publication, as detailed below.
Specific comments:
- Given the multitude of system parameters discussed in the text, I strongly suggest to add a table that summarizes the specifications of the lidar instrument including both the given (or fixed) parameters, such as telescope diameter, primary mirror focal length, detector gain, as well as the derived optimized parameters such as focusing distances, solar filter bandwidth, laser pulse energy, pulse repetition frequency, etc.
- Line 71: The impact on micro-vibrations on the frequency of the Aeolus laser is discussed in a more recent publication (Lux et al., AMT, 14, 6305–6333, 2021). Please add this reference to the one already provided (Mondin et al., 2017).
- Line 198: Why did the authors assume a solar filter bandwidth of 1 nm, although the spectral width of the Rayleigh signal considering Doppler shifts of +/- 100 m/s accounts for only 1 pm (line 186)? What is the actual limitation for the lower bound of the spectral bandwidth (transmission, price)?
- Lines 233ff.: The maximum solar filter bandwidths of >10 nm, calculated for the three different laser sources, are much broader than what is typically used in such systems. I am not sure if these values are realistic. Given that 1 nm bandwidth corresponds to a minimum pulse energy of 88 µJ (line 195), the fiber laser parameters (PRF: 40 kHz, average power: 10 W hence pulse energy: 250 µJ) suggest a maximum bandwidth of 2.3 nm. For the hybrid fiber laser, the maximum bandwidth is then 6.8 nm. Please check the values given in the text.
- Line 200: What is the influence of the particle backscattering on the QMZ interferometer output signals? I am wondering if the accuracy of the wind speed retrieval suffers from Mie contamination which is significant at lower altitudes.
- Fig. 4: I suggest to mark the two scanning angles (15°, 51°) that are discussed in the text to better visualize the improvement in the RMSE.
- Perhaps one could also add a 2D color plot, similar to those in Fig. 3, which depicts the RMSE vs. angle and altitude. This would illustrate the influence of scanning angle and altitude on the wind error in a more general manner with the two plots shown in Fig. 3a) and b) representing two intersection curves.
- Can the authors please check the numbers in the parentheses in lines 295f.? I calculate a different value of 2r for θ = 15° when using the equation given in line 279: 2r = 54 m instead of 36 m.
- Line 300: I think it would be helpful for the reader when referring the statement to the equation that contains the term 1/((2sin(θ))². I suppose Eq. (3) is meant here.
- Line 328: Please mention the respective RMSE values obtained for the two angles in the text to quantify the improvement at the optimized scanning angle.
- Although the example shown in Fig. 5 is illustrative, I am missing information about its representativity for different turbulence scenarios. How does the RMSE at the two different angles vary for multiple runs of the simulation? How does it scale with the variance of turbulence σ²?
- The three terms “scan angle”, “scanning angle” and “lidar angle” are used synonymously in the text and should be harmonized to avoid confusion.
Technical corrections:
- Lines 31, 88: The acronym GLA was already introduced in line 21 and can thus be used here.
- Line 109: Word missing: “… and to focus it into a multimode fiber”.
- Line 119: The acronym OPD was introduced before.
- Line 124: The symbol D_0 should be clarified.
- Line 139: Change (Liméry,2008) to Liméry (2018).
- Line 188: Correct to “This leads to:”.
- Line 190: The symbol γ is not described in the text. Also, please check the equation for correctness.
- Fig. 3: The label of the right y-axis should read “Standard deviation on wind speed (m/s)”.
- Fig. 3: The PRF in the label “Meron C” should be changed from 40 kHz to 400 Hz.
- Lines 237, 240: The commas have to be replaced by dots.
- Line 239: Correct to “can be reached”.
- Caption of Fig. 4 can be shortened by writing “[…] for wind measurement at (a) high and (b) low altitude”.
- Lines 255ff.: The formatting of the symbols in the text should be corrected, e.g. d, r and z should be printed in italics. Conversely, Eqs. (2)-(4), the terms “sin”, “cos” and “tan” should be printed upright. This comment also applies to the Appendix sections where upright letters and italics are not used consistently.
- Line 275: Replace “that is homogeneous and isotropic” with “which is homogeneous and isotropic”.
- Line 317: The symbol L_0 should be introduced.
- Line 325: Remove “)” after “Figure 5”.
- Fig. 5a) Either remove the tick labels from the x-axis or add an axis description (x position in m?). Clarify “plan xOz” in the caption.
In conclusion, I recommend major revision of the manuscript to improve the quality of the work before its publication in AMT.
Citation: https://doi.org/10.5194/amt-2024-41-RC2 - AC2: 'Reply on RC2', Thibault Boulant, 04 Jul 2024
-
RC3: 'Comment on amt-2024-41', Anonymous Referee #3, 21 May 2024
- AC2: 'Reply on RC2', Thibault Boulant, 04 Jul 2024
- AC3: 'Reply on RC3', Thibault Boulant, 10 Jul 2024
Status: closed
-
RC1: 'Comment on amt-2024-41', Anonymous Referee #1, 14 May 2024
This paper focuses on two issues associated with application of a direct-detection Doppler lidar for measuring winds ahead of an aircraft to feed forward for gust load alleviation. First, it describes development of a design to maximize wind measurement precision, focusing on telescope, background filter, and detector specifications. It then utilizes an atmospheric model of molecular and aerosol backscatter to predict performance for three different laser transmitters. The second issue addressed is optimization of the angle of deviation for a lidar that utilizes a four-direction concept, assuming turbulence from a commonly employed turbulence model.
I find the paper quite well done and certainly worthy of publication. Although the two main issues could have been addressed separately, they fit together acceptably into a single article. The figures are appropriate and illustrate the main points and conclusions.
In reading the paper, I would have liked to have seen a bit more discussion of the QMZ interferometer and explanation of how the SNR lead to errors in wind speed. This is more of a personal preference – the paper is very well referenced and the Appendix provides the necessary details for estimating wind error from photon count. Perhaps a figure that illustrates how the output from the detectors changes as function of wind speed (or phase) would be tutorial and useful in illustrating the concept.
Although speckle noise is an important component in velocity measurement uncertainty estimate, there is very little discussion of the basis for speckle noise and which system and laser transmitter parameters affect it. For example, line 223 on page 8 says that “ the speckle noise decreases because the backscattering is predominantly molecular, which is less coherent than particulate backscattering. I may have missed it, but I didn’t see in the text or the appendix that discusses the speckle relative to the signal coherence and how this is incorporated into the simulation.
Figure 3 is quite informative and sums up the discussion on wind speed standard deviation nicely.
The angle optimization part of the study produces a nice and useful result. While reviewing the paper, I thought that thus problem had to have been addressed earlier in slider studies of wind energy, but I perused the literature a bit and didn’t find it. Consequently, this result should be of significant interest to the community.
As with all simulation studies, this work begs for follow-up research to demonstrate the concept and validate the simulation. The authors should add some text at the end on anticipated future work and tell the reader how they intend to use the results of the study.
Citation: https://doi.org/10.5194/amt-2024-41-RC1 - AC1: 'Reply on RC1', Thibault Boulant, 04 Jul 2024
-
RC2: 'Comment on amt-2024-41', Anonymous Referee #2, 21 May 2024
The manuscript by Boulant al. reports on the development of a UV wind lidar system based on a fiber laser and Quadri Mach-Zehnder (QMZ) interferometer to measure vertical and lateral wind ahead of an aircraft for gust load alleviation. Key instrument parameters of the telescope, background filter and detector were optimized through simulations to minimize wind speed errors. The end-to-end simulator was then used to evaluate the lidar performance for three different laser designs. In addition, the scanning angle of the instrument, employed in a four-axis scheme to reconstruct the 3D wind vector, was optimized assuming turbulence according to the von Karman model.
General comments:
The work is certainly of interest to readers working in the field of active remote sensing, and particularly to the wind lidar community. The manuscript is well-structured and the figures support the intriguing findings derived from the simulations. However, given the results reported in the paper, I am wondering a) to what extent the developed end-to-end simulator is applicable to other wind lidar instruments based on a QMZ (not necessarily for GLA), b) what the major limitations of the simulator are, c) what refinements of the simulator are planned in the future, and d) if (or when) experimental validation of the simulation results with the described instrument is foreseen. In my opinion, these four relevant questions are currently unaddressed and should be discussed in the conclusions section of the manuscript. Addressing these points will strengthen the importance of the paper and potentially increase its impact on a wider audience. Aside from this general remark, there is a number of specific issues that should be considered prior to publication, as detailed below.
Specific comments:
- Given the multitude of system parameters discussed in the text, I strongly suggest to add a table that summarizes the specifications of the lidar instrument including both the given (or fixed) parameters, such as telescope diameter, primary mirror focal length, detector gain, as well as the derived optimized parameters such as focusing distances, solar filter bandwidth, laser pulse energy, pulse repetition frequency, etc.
- Line 71: The impact on micro-vibrations on the frequency of the Aeolus laser is discussed in a more recent publication (Lux et al., AMT, 14, 6305–6333, 2021). Please add this reference to the one already provided (Mondin et al., 2017).
- Line 198: Why did the authors assume a solar filter bandwidth of 1 nm, although the spectral width of the Rayleigh signal considering Doppler shifts of +/- 100 m/s accounts for only 1 pm (line 186)? What is the actual limitation for the lower bound of the spectral bandwidth (transmission, price)?
- Lines 233ff.: The maximum solar filter bandwidths of >10 nm, calculated for the three different laser sources, are much broader than what is typically used in such systems. I am not sure if these values are realistic. Given that 1 nm bandwidth corresponds to a minimum pulse energy of 88 µJ (line 195), the fiber laser parameters (PRF: 40 kHz, average power: 10 W hence pulse energy: 250 µJ) suggest a maximum bandwidth of 2.3 nm. For the hybrid fiber laser, the maximum bandwidth is then 6.8 nm. Please check the values given in the text.
- Line 200: What is the influence of the particle backscattering on the QMZ interferometer output signals? I am wondering if the accuracy of the wind speed retrieval suffers from Mie contamination which is significant at lower altitudes.
- Fig. 4: I suggest to mark the two scanning angles (15°, 51°) that are discussed in the text to better visualize the improvement in the RMSE.
- Perhaps one could also add a 2D color plot, similar to those in Fig. 3, which depicts the RMSE vs. angle and altitude. This would illustrate the influence of scanning angle and altitude on the wind error in a more general manner with the two plots shown in Fig. 3a) and b) representing two intersection curves.
- Can the authors please check the numbers in the parentheses in lines 295f.? I calculate a different value of 2r for θ = 15° when using the equation given in line 279: 2r = 54 m instead of 36 m.
- Line 300: I think it would be helpful for the reader when referring the statement to the equation that contains the term 1/((2sin(θ))². I suppose Eq. (3) is meant here.
- Line 328: Please mention the respective RMSE values obtained for the two angles in the text to quantify the improvement at the optimized scanning angle.
- Although the example shown in Fig. 5 is illustrative, I am missing information about its representativity for different turbulence scenarios. How does the RMSE at the two different angles vary for multiple runs of the simulation? How does it scale with the variance of turbulence σ²?
- The three terms “scan angle”, “scanning angle” and “lidar angle” are used synonymously in the text and should be harmonized to avoid confusion.
Technical corrections:
- Lines 31, 88: The acronym GLA was already introduced in line 21 and can thus be used here.
- Line 109: Word missing: “… and to focus it into a multimode fiber”.
- Line 119: The acronym OPD was introduced before.
- Line 124: The symbol D_0 should be clarified.
- Line 139: Change (Liméry,2008) to Liméry (2018).
- Line 188: Correct to “This leads to:”.
- Line 190: The symbol γ is not described in the text. Also, please check the equation for correctness.
- Fig. 3: The label of the right y-axis should read “Standard deviation on wind speed (m/s)”.
- Fig. 3: The PRF in the label “Meron C” should be changed from 40 kHz to 400 Hz.
- Lines 237, 240: The commas have to be replaced by dots.
- Line 239: Correct to “can be reached”.
- Caption of Fig. 4 can be shortened by writing “[…] for wind measurement at (a) high and (b) low altitude”.
- Lines 255ff.: The formatting of the symbols in the text should be corrected, e.g. d, r and z should be printed in italics. Conversely, Eqs. (2)-(4), the terms “sin”, “cos” and “tan” should be printed upright. This comment also applies to the Appendix sections where upright letters and italics are not used consistently.
- Line 275: Replace “that is homogeneous and isotropic” with “which is homogeneous and isotropic”.
- Line 317: The symbol L_0 should be introduced.
- Line 325: Remove “)” after “Figure 5”.
- Fig. 5a) Either remove the tick labels from the x-axis or add an axis description (x position in m?). Clarify “plan xOz” in the caption.
In conclusion, I recommend major revision of the manuscript to improve the quality of the work before its publication in AMT.
Citation: https://doi.org/10.5194/amt-2024-41-RC2 - AC2: 'Reply on RC2', Thibault Boulant, 04 Jul 2024
-
RC3: 'Comment on amt-2024-41', Anonymous Referee #3, 21 May 2024
- AC2: 'Reply on RC2', Thibault Boulant, 04 Jul 2024
- AC3: 'Reply on RC3', Thibault Boulant, 10 Jul 2024
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