A Calibration Method for Large-Footprint Full-Waveform Airborne Laser Altimeter without a Calibration Field
Round 1
Reviewer 1 Report
This paper gave the novel calibration method without the calibration field for aiming the airborne large-footprint laser altimeter. The detection of laser spots using the step surface boundary was proposed to obtain the geometric measurement parameters of the laser altimeter, and the validation of the feasibility of the method was conducted.
This study provided a foundation for the next large scale forestry surveys with airborne large-footprint laser altimeter system and provides a new idea for the on-orbit calibration and verification of the subsequent carbon satellite laser altimeter system.
Author Response
Thank you very much for your recognition and support of this manuscript. We sincerely hope that this revised version of the manuscript will meet your requirement.
Reviewer 2 Report
The authors present a method to calibrate large-footprint airborne laser altimeters without calibration field.
There is something unclear to me in the explanation of the mathematical method. What exactly it comes to achieve in comparison to the 'calibration field' technique. You did not compare your results with the real terrain topography, so how can one validate these achievements.
According to my understanding, Eq. 5 and 6 are the most important parts of the analysis. What do these two equations accomplish in the analysis? (make this clear in the text).
Do you avoid the additive information streams from the earlier back-reflections? if so, why? How would your method affect the user's interest in forest canopy information?
Please make an effort to clarify the above questions to the reader (I would be happy to following your improved version to understand better)
Some more specific comments:
1. Define “POS” abbreviation
2. Table 1: What kind of detector is used in your laser altimeter (it should be ‘fast responder’ at a level of 1 GHz)? , it wasn’t clear to me at the beginning that the ‘echo waveform obtained by your sensor’ (L162) is a single gaussian waveform in the time domain. (maybe worth noting earlier that ‘you are processing laser waveform signals’ in the time domain only).
3. L148: correct “Galibration” to “Calibration”
4. L171: correct “the signals emitted to the surface” to “the signals emitted from the surface”
5. Fig11: I guess the laser waveform signal decays with R2 in the range 0-20000 ns due to atmospheric backscattering (just like a LIDAR), how do you handle the data in this sense? Please make it clear to the reader.
6. L284: change “were shown” to “are shown”
7. Fig13: what are the x/y units?? The Fig. does not contribute much to understanding, as there is no visual representation of comparing wavefront/laser-spot results.
8. L334-335: "The feasibility of the method in this paper was verified by experiments of the air-borne large-footprint laser altimetry" - How does it verified?
Author Response
Thank you very much for your recognition and support of this manuscript. We sincerely hope that this revised version of the manuscript will meet your requirement. The responses to the modification suggestion has been attached.
Author Response File: 
Author Response.docx
Reviewer 3 Report
In this manuscript, a series of experiments were carried out using the data of large-footprint full-waveform airborne laser altimeter. A new calibration method without the calibration field was proposed in this research, aiming at the airborne large-footprint laser altimeter system. The geometric parameters of the laser spot were calibrated through the laser echo waveforms in the specific terrain, including the divergence angle and energy distribution. It is innovative and practical, but there are still some deficiencies. Below you can find some minor comments.
1. Line 199, "All frames of the satellite-borne laser echo waveform data are not all valid signals.", the authors deal with airborne large-spot LiDAR data in the paper, but here it is expressed as satellite-borne laser echo waveform data, please explain.
2. Figure 10 shows the surface image and spot distribution in the experimental areas. The authors have selected the laser data for each experimental area in Figure 11, could the approximate location be marked in Figure 10 for the reader's understanding.
3. There is no legend in the fitting results of airborne laser spots in Figure 13.
4. Line 199, what does "POS system" mean? Please explain the narrower terms that appear for the first time in the paper.
5. Please supplement the "P point" to indicate which position coordinates in Figure 3, and explain the corresponding symbols in Formula 1 in Figure 3, such as ?
6. What is the reference datum corresponding to the coordinates of the point position in Formula 1? Please explain.
7. Line 303, “The reflected signal through the laser spot will inevitably lead to a sharp reduction in energy, and the definition of the spot edge is still calculated according to the 1/e2 of the energy distribution as the basis for the solution”. Why is “1/e2 of the energy distribution as the basis”? Please supplement the references.
8. There is a lack of comparative experiments in the paper, please add the validation data and relevant graphs of the calculated results of the method in this paper.
Minor editing of English language required
Author Response
Thank you very much for your recognition and support of this manuscript. We sincerely hope that this revised version of the manuscript will meet your requirement. The responses to the modification suggestion has been attached.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Good luck!
Author Response
Thank you for your comments and suggestions that helped us to improve the paper. The structure of the manuscript has been adjusted that the Section 4 "Results and discussion" has been separated into 2 parts (4. Results and 5. Discussion). We have carefully polished the language and the grammar in the revised manuscript.
