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Letter
Peer-Review Record

First Assessment of Geophysical Sensitivities from Spaceborne Galileo and BeiDou GNSS-Reflectometry Data Collected by the UK TechDemoSat-1 Mission

Remote Sens. 2020, 12(18), 2927; https://doi.org/10.3390/rs12182927
by Matthew L. Hammond 1,*, Giuseppe Foti 1, Jonathan Rawlinson 2, Christine Gommenginger 1, Meric Srokosz 1, Lucinda King 2,3, Martin Unwin 2 and Josep Roselló 4
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Remote Sens. 2020, 12(18), 2927; https://doi.org/10.3390/rs12182927
Submission received: 14 August 2020 / Revised: 5 September 2020 / Accepted: 8 September 2020 / Published: 10 September 2020
(This article belongs to the Section Ocean Remote Sensing)

Round 1

Reviewer 1 Report

Comments on “First Assessment of Geophysical Sensitivities from Spaceborne Galileo and BeiDou GNSS-Reflectometry Data Collected by the UK TechDemoSat-1 Mission” (remotesensing-916685)

The submitted paper presents the analysis of spaceborne observation of the Earth‐reflected signal from Galileo and BeiDou global satellite navigation system. The sensitivities of Galileo and BeiDou GNSS-R observation on ocean surface wind speed and sea ice occurrence have been demonstrated. The results presented in this paper give new strength to the prospect of GNSS-R concept in the multi-GNSS era, and suggest the potential of building spaceborne GNSS-R missions with improved effectiveness. In general the paper is well written, the methods are well described and the results presented in a logical, convincing manner. I only have a few minor comments, when addressed, will make the manuscript more clear.

1)The authors claim that it is the “first assessment of geophysical sensitivities from spaceborne Galileo and BeiDou GNSS-Reflectometry”. However, there have been other works on such exploration. For example, CYGNSS raw IF data have been processed, from which reflected signal from Galileo and BeiDou-3 were used for altimetry over lake on open ocean. It would be more rigorous to remove “first” from the title or put specific applications in the title. Note that this reviewer appreciates the contributions of the manuscript on spaceborne multi-GNSS reflectometry, whether it is the “first” demonstration.


2)It would be more clear to use BeiDou-3 instead of BeiDou, as only the BeiDou-3 B1C signal can be recorded by the receiver.


3) Line 35-36: I would suggest to put references for all those spaceborne GNSS-R missions (UK DMC, TDS-1, CYGNSS, Bufeng-1).


4) Line 57-58: CYGNSS mission has been also collected a large amount of raw IF data. Taking into account the time frames of the TDS-1 (2014-2019) and CYGNSS mission (2016-present), and the evolution of the Galileo system, it may be risky to have such statement.


5) Fig 1: It seems that the SNRs of the Galileo and BeiDou-3 DDMs are not as high as the GPS one. Could the authors please comment on that? Is it due to the transmitted power, or the antenna gain towards the specular point?


6) In addition to the sensitivity study, it would be also useful to do a kind of “link budget” study, i.e. to understand if the current instrumental configurations, e.g. antenna gain, can guarantee similar geophysical retrieval performance using reflected Galileo and BeiDou-3 signals. I understand that such study may be limited by the amount of available raw IF data. It would be helpful if the authors can comment on that.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper presents and conducts an initial analysis of the signal-to-noise ratio using TDS-1 collections of GPS, Galileo and Beidou. The sensitivity of SNR to surface parameters, model reanalysis wind-speed and observational sea-ice concentration fields are assessed.

Line 78: DDM should be defined when appearing at the first time here.

Figure 1: label of color-bar and axes is hard to read, and needs to be fixed.

Figure 2 does not show (a) and (b), which should be added or use (left) and (right). The wind speed U10 (ERA-5 U10) needs to be explained here.

Line147: A bit more information about “full TDS-1 Galileo data catalogue” and proper references should be given. This will help readers to understand Figure 3.

Figure 3: What is meant by Galileo Satellite (i.e. E11 etc.) in color bar? This is not clear and needs to be explained in figure caption (or text).

Figure 5: It may be clear to show the direction of satellites (from open ocean to sea-ice) using an arrow bar, or simply say “approximately from north to south”.

Lines 224-226: Conclusion in Section 4 related to Beidou needs to be more clear, as it is only analyzed in section 3.1 and appears in just one figure (Fig.1).  

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

see attached file

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

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