Revealing the Land Subsidence Deceleration in Beijing (China) by Gaofen-3 Time Series Interferometry

Round 1

Reviewer 1 Report

In this study, the Gaofen-3 SAR phase data were used to study the ground subsidence of Beijing in recent years. Whereas the interferometric coherence and orbital errors of the Gaofen-3 were analyzed and corrected. This study is innovative in using the GF-3 SAR data, which can provide a new idea for the Gaofen-3 data in the land subsidence research.
There are several questions as follows:

1.       The coverage of SAR images in Figure 1 is not exactly illustrated, i.e. the real SAR swaths should be shown rather than the cropped ones.

2.       As for the equation explanation, the symbols should be in the same paragraph rather than in another paragraph, e.g. “Where ?? is the critical baseline of the InSAR system,” for eq. (4), similar problems should be corrected.

3.       Is Figure 3 the average coherence maps? Is it before or after filtering?

4.       Is SRTM used in this paper? How is this considered for Gaofen-3 SAR data, where DEM errors cause large pseudo-phases due to the large spatial baseline? What special measures have been taken for the DEM error with respect to long perpendicular baseline?

5.       Please correct the caption of Figure 7. “The wrapped interferograms with inaccuracy orbit information of Gaofen-3 SAR images.” Inaccurate rather than inaccuracy.

6.       As for the orbital error correction, some interferograms still have obvious orbital phase as shown in Figure 8, please give some comments on it.

7.       It’s better to add the difference map between Sentinel-1 and Gaofen-3 deformation map within the similar period.

8.       Whether can you provide time series deformation based on Gaofen-3 data?

Author Response

Thank you very much for the insightful comments on this manuscript! In the revised manuscript, we tried our best to improve the manuscript based on your constructive suggestions. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors.

First of all, I would like to thank the authors for their work and determination in carrying out this study. The manuscript entitled "Revealing the land subsidence deceleration in Beijing (China) by Gaofen-3 time series interferometry" contains:

- Introduction to the manuscript.

- Study area and datasets

- Methodology

- Gaofen-3 interferometry analysis and refinement

- Gaofen-3 time series Results analysis and comparison

- Conclusions.

The manuscript presents the case study, which presents the results of InSAR time series analysis with Gaofen-3 SAR images for the extraction of land subsidence in Beijing in 2020-2021. The authors also report that long perpendicular baselines seriously reduce coherence in the study area, and artifacts induced by imprecise orbit information reduce the quality of Gaofen-3 interferograms.

I have comments on the following manuscript:

Page 2, Table 1 - I recommend adding the maximum declines determined by these studies - to show the trend of declines.

Page 3, Figure 1 – Where are the blue lines which delineate the boundary of Beijing City? Provide a clearer localization of the Beijing district.

Page 3, Table 2 – 1st line – senor or sensor? Please edit the date to a clearer format: 2020.06.09-etc.

Page 4, line 121 – Insert the equation SNR using PS and PN.

Page 9, Line 283 - Is starting a chapter with a picture ok? Insert text and then a picture. It is the same with Figure 6, which first is a picture, and then is a text with a link to the picture in question.

Page 10, Figure 9, 10 – I assume that the blue color in figures is stable territory. Adding some values for the color scale between 35 mm to -98 mm would be more appropriate. To clarify that the blue color represents, e.g., zero drop. Add the values to the color scales in the other images.

Page 10, line 299 – please add: "area of" red dash lines in Figure 9.

Page 12, line 345 – decrease during 2020 and 2021 was max -80 mm/a, but in pictures 9,10, the color scale is up to -98 mm.

Page 13, Figure 12 – What were the total decreases for the observed four years? Since every year in the subsidence areas, the decrease was up to 12 cm per year, in 4-5 years, with this trend, the decrease can reach up to 0.5 m. Is it possible to record such decreases by repeated GNSS measurements at the same ground control point with annual periods?

Are the subsidence areas located near any pumping stations of the underground water?

Discussions - I miss a comparative debate on the results achieved with previous scientific papers, e.g. from Table 1. The manuscript has further formal mistakes or deficiencies, which will remove in the eventual process of the check English and final editing of the paper by the editorial office according to the valid journal standards.

Best regards

Author Response

Thank you very much for the insightful comments on this manuscript! In the revised manuscript, we tried our best to improve the manuscript based on your constructive suggestions. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors performed time series InSAR analysis using GaoFen-3 SAR images and propose a method for detecting land subsidence with the same level as the commonly used Sentinel-1 satellite, which is considered to be a useful paper.

The paper would be better if you could add a description of what advantages and problems are solved by using Gaofen-3 compared to conventional methods.

The following are minor points.

[LNE117]

The formula is left-leaning.

[LINE121]

Ps and Pn are explained but not written in the formula.

Author Response

 Thank you very much for the insightful comments on this manuscript! In the revised manuscript, we tried our best to improve the manuscript based on your constructive suggestions. Please see the attachment.

Author Response File: Author Response.docx