GCP and PPK Utilization Plan to Deal with RTK Signal Interruption in RTK-UAV Photogrammetry

Round 1

Reviewer 1 Report

The paper provides a method to survey a large area with a kinematic unmanned aerial vehicle in real time taking into account data disconnection and urban environment. The work is well researched based on a systematic protocol and attention. There is definitely something interesting to find for many researchers interested in applications of this type mainly of GPS precision augmentation.

Advantages:

The work is considered interesting and relevant to the scope of the journal. Researchers understand the rationale as well as the methods and results.

Disadvantages.

1.         The introduction should better present the objective of the paper in a more precise and summarized manner.

2.         There are problems with the graphs, the way of presenting the results should be drastically improved.

3.         They should expand the conclusions; they could go deeper in terms of challenges or better future implementation of these techniques in urban environments.

Problems needed to be addressed on the graphs:

- Fig. (2,3,4). These are maps and do not meet any cartographic criteria, they have no scale, title, north, coordinates or legend. They should elaborate the maps with a GIS or cartography program, these images are constructed from Google maps which is not correct for the presentation of a research work.

- Fig. (6). This figure is not understood. What does the letters (a and b) refer to, if it is the capture of the program, it does not contribute anything, besides the table box is not adequate for this figure, I recommend eliminating it.

- Fig. (7). This figure is not understood in the same way as the previous one. What the letters (b,c,d,e) refer to is the capture of the program, it does not contribute anything, this is a scientific article, not a manual or step by step tutorial, I recommend to eliminate it.

- Fig. (8 and 9): Increase the size of the figures, differentiate better the colors are not adequate, increase the size of the letters.

- Fig. (10 and 11): Increase the size and improve the definition, it is blurry, to make this graph more understandable.

- All graphs should be explained at the bottom of the figure in as much detail as possible. Eliminate the black boxes in the table in the figures.

In conclusion, the authors should improve the paper in several aspects to make it more understandable and suitable for publication.

Author Response

Dear reviewer,

  Comment 1:  The introduction should better present the objective of the paper in a more precise and summarized manner
Response: We have modified the wording of the Introduction to be more explicit. In the process of revising the introduction, we deleted sentences that did not fit the clearer purpose of the research.
In addition, sentences that are difficult to understand have been modified as follows.
We have modified the wording of the Introduction to be more explicit. In the process of revising the introduction, we deleted sentences that did not fit the clearer purpose of the research.
In addition, we have corrected sentences that are difficult to understand. Many sentences were deleted in the process. Therefore, I would like you to confirm it through the revised manuscript.
Word - Review - Tracking - I would like you to check it by setting it to "All comments and changes".

Comment 2:  There are problems with the graphs, the way of presenting the results should be drastically improved.
Response:  We changed the pictures and graphs.
Scale, North, Coordinates, and Legend were added to Figure 2 and produced using QGIS.
Figures 6 and 7 have been deleted.  Figures 8 and 9 increased the symbol size (0.07 to 0.20).  In Figures 10 and 11, blurry areas are blacked out to make them easier to understand.
The black borders of all graphs and pictures have been removed.

Comment 3:  They should expand the conclusions; they could go deeper in terms of challenges or better future implementation of these techniques in urban environments.
Response:  We have extended the conclusions by adding the third and fourth conclusions as follows.
Third, it was found that, regardless of whether the GCP is not positioned in the center of the target area, the vertical bias that may arise during ppk processing can be effectively removed using 1GCP.
Fourth, with the growing popularity of UAVs and urban air mobility (UAM), the demand for highly accurate data sets in urban areas will increase. Even though RTK reception is poor in places like urban canyon areas, it is expected that high-precision 3D data will be successfully generated using the experimental results of this study.

Author Response File: Author Response.docx

Reviewer 2 Report

1. This paper describes a problem encountered while surveying large areas using a unmanned aerial vehicle and presents a solution using PPK and GCP. The authors conducted a study to test the effectiveness of this solution and concludes that it can solve the RTK signal disconnection problem and achieve high accuracy within a radius of 1 km.

2. This manuscript contains too many abbreviations, making it difficult to read. It would be best to create a list and define all the abbreviations clearly.

Author Response

Dear reviewer,

Comment 1:  This manuscript contains too many abbreviations, making it difficult to read. It would be best to create a list and define all the abbreviations clearly.

Author Response File: Author Response.docx

Reviewer 3 Report

Overall, the abstract provides a clear and concise summary of the study's objectives, methods, and findings. However, there are a few areas where the wording could be improved for clarity and precision:

Line 1: "Real-time kinematic unmanned aerial vehicle" could be abbreviated as "RTK UAV" to save space.

Line 3: "Accuracy of the aerial triangulation (AT) results is reduced" could be more specific. For example, you could mention that the accuracy is reduced due to the lack of RTK signal or that the RTK signal is not available in certain areas.

Line 6: "PPK" should be spelled out on first use as "post-processing kinematics" for clarity.

Line 7: "Received RTK signal" could be more precisely described as "the percentage of time during which the RTK signal was received."

Line 11: It might be helpful to clarify what "these problems" refer to. For example, you could state that you are trying to address the accuracy reduction caused by the lack of RTK signal in certain areas.

Line 12: "Before and after PPK were performed" could be rephrased to "ATs were conducted both with and without PPK using GCPs located at the four corners and center."

Line 13: "Root mean square error (RMSE) trend" could be more clearly described as "the trend of RMSE values."

Line 14: "According to the distance from each GCP used and the 41 check points (CPs)" could be rephrased to "Based on the distance between the GCP used and the 41 check points (CPs)."

Line 16: "Up to 10 cm for H and up to 20 cm for Z up to a distance of 1 km" could be rephrased to "Up to 10 cm horizontal error and up to 20 cm vertical error were observed within a 1 km radius of the GCP."

The article describes a study that aimed to improve the accuracy of aerial triangulation (AT) when surveying a large target area using a real-time kinematic unmanned aerial vehicle (RTK-UAV) by using post-processing kinematic (PPK) instead of relying solely on RTK signals. The authors conducted experiments with a data set that had varying levels of RTK signal reception and analyzed the root mean square error (RMSE) of the AT results before and after PPK.

The results show that PPK significantly increased the positional accuracy of the captured images, even when the reception rate of the existing RTK was poor. The GPS navigation file and GLONASS navigation file used in PPK effectively obtained centimeter-level positioning accuracy even when the UAV and controller were disconnected. The authors used Pix4d mapper for AT, which performs block bundle adjustment (BBA) and removes many errors such as film distortion, lens aberration, and atmospheric refraction. The precision of relative orientation was within 0.5 pixel error, and the authors utilized the existing relative orientations after BBA.

Overall, the article presents a well-structured and clear description of the study methodology and results. The authors effectively explain the rationale for using PPK and the challenges associated with performing AT using UAVs. The use of Pix4d mapper for AT is also well-justified, and the authors provide a good explanation of how they ensured consistent relative orientation work. One limitation of the study is that the experiments were conducted in a controlled environment, and the results may not be generalizable to other settings. Nonetheless, the article provides valuable insights into the potential benefits of using PPK for AT in UAV surveys.

The accuracy of the survey results obtained by RTK-UAVs depends on the quality of the RTK signal. When the RTK signal is good, the 3D location accuracy of the captured images is good, and the accuracy of the AT (Absolute Time) is improved, resulting in improved 3D location accuracy of the spatial data constructed using such an image. On the other hand, when the RTK signal is poor or disconnected, errors occur, and the accuracy of the 3D data that can be obtained is lowered. In such cases, additional shooting or multiple Ground Control Point (GCP) surveys may be required to produce high-quality 3D spatial information. The study also found that the use of PPK (Post-Processed Kinematics) is essential in an environment where only a small number of GCPs can be used, and the placement of GCPs affects accuracy differently under 1GCP+Non-PPK and 1GCP+PPK conditions.

Author Response

Dear reviewer,

Comment 1:  a few areas where the wording could be improved for clarity and precision:
Response:  We revised the summary to reflect what reviewer #2 pointed out.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Suggested changes were made