From GPS Receiver to GNSS Reflectometry Payload Development for the Triton Satellite Mission

Round 1

Reviewer 1 Report

This paper details the design and testing of a GNSS-R payload module. The paper describes first the specifications and accuracy of the core GPSR which processes the GPS signals to obtain the receiver PVT. Then the paper describes the GNSS-R payload (now it is a multi-GNSS receiver) based on the GPSR mentioned earlier. The paper is overall good, and here are my comments:

* Almost all the figures need proper description within the text, for example:

              Fig3: describe the data distribution so that the reader can get this information directly without checking the source/reference.  Is this multi-GNSS? How many signals are tracked? Is this a simulation of all the visible GPS/GAL/GLO satellites?

Fig6 the x-axis is not visible and the figure resolution is not enough.

Fig7: describe the axis, the sub-figures, the colors, etc.

Fig9: describe the contents of this figure in the context of the ground test mentioned.

Fig10: not readable. SP=Specular point? The number corresponds to PRNS? The DDM color/value interpretation?

Fig12: not readble. Explain the contents of the figure in the text.

 

L44-45: explain why “such a civil GPSR module/chipset is not able to be applied to the satellite missions”?

 

L61: Why “The generated data volume from the FS-7 constellation will be three times that of the FS-3.”? Is it the number of satellites in the constellation? More channels in the receiver? Different orbit choice? Etc

 

L112: Be more specific instead of using “and so on”

 

L114: Is this “one key feature of the space-grade GPSR” a key feature of the NSPO space-grade receiver compared to terrestrial receivers or it is a requirement for space graded GNSS receivers  in general? Be more specific.

 

L119: “It is faster than most of the space-grade GPS receivers”. Provide reference and give some examples for other cold start time for other satellites in the literature

 

Fig7 the upper panel: the error seems to increase toward the end of the plot, show more seconds of data to check for this. How long did the overall test last?

 

L161-163: “By examining the needs and referring to … etc” what are these needs exactly?

 

L206: “However, there are still some distinguishable features …” List the exact distinguishable features.

 

L207-208: “such as reflected QZSS signal processing and the capability of more DDM generation (8 DDMs per second).” in the requirements it is mentioned “Potential QZSS” and “at least 4 channels”. A paragraph to compare the requirements and the actual implementation/specifications is needed.

 

L276-277: “However, the average error of the comparison is about 0.5 dB, the reason is that … etc”. here, the reflections are close to land, can the presence of rocks, dry land, moist surfaces, etc affect the estimation of the reflected signal SNR?

 

L286-287: “in order to satisfy the requirements of the NSPO future missions and university CubeSats, …etc”, the cubesats were not mentioned in the text. This need to be added to the introduction/motivation. Also, illustrate how this reflectometry mission based on the 1.8Kg GPSR receiver is foreseen to be used on X Unit cubesats.

 

 

 

 

 

 

 

Author Response

The authors would like to thank the reviewer for the technical suggestions to make the paper complete. Replies to reviewer’s specific suggestions are given as the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The presented paper is an attempt to describe the GNSS Reflectometry payload development for the Triton Satellite Mission. So far, it is hardly possible to make a correct assessment of the paper since there are many questions and remarks. The paper needs a serious revamp. 

I have jotted down some flaws and drawbacks that have to be corrected before the second submission:

Where did the authors take Fig. 3?

Section 2, "2. GPS Receiver (GPSR) Development" contains the GPSR block diagram and the results of the experiments. Unfortunately, nothing about development itself. Whereas it is not clear who has carried out the experiments?

Ibid. What do authors mean under position and velocity errors? That is quite hard to grasp what does it mean error? Whether they were systematic or random, whether they were absolute or root mean square, there are so many alternatives?

Subsection 3.1, entitled GNSS-R Payload Development, contains information about GNSS-R payload but has no information about the development.

Figure 10 needs more explanation, especially its bottom part.

In Subsection 3.2, there is some confusion, insofar as at the beginning it is pointed out that the test has been performed by airplane, but in what follows, we are reading about UAV, and then again about the airplane. These are different things, are not these? It is quite hard to understand where the one test has finished and where the other test commenced.

rows 255-256 "According to the result, it’s a well negative relation between Hs and maxDDM." It is not obvious how authors have come to this conclusion. It is evident that the data from Fig. 14, 15 are not enough for the inference. 

The description and explanation of the "Link budget analysis flowchart" are pretty modest.

The paper is structured not in a good manner. Most figures do not follow their first mention in the text, which makes reading complicated. The titles of the sections and subsections partially do not match with their content very often.

 

 

Author Response

The authors would like to thank the reviewer for the technical suggestions to make the paper complete. Replies to reviewer’s specific suggestions are given as attachment. Replies to reviewer’s specific suggestions are given as the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have accounted for and clarified all of my previous major concerns about the paper. 
The paper may be published in its current form.