An Assessment of CyGNSS v3.0 Level 1 Observables over the Ocean
Round 1
Reviewer 1 Report
Minor comments:
1- Need spacing between text and figures (Fig 3 and Fig 7).
2- Line 98 "the use of verification data from ECMWF ERA-5 model output", please show that ERA-5 and the CyGNSS data have the same averaging time.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript provides an assessment of CyGNSS v3.0 Level 1 Observables over global Oceans. The methods are clear and give good support to the conclusions. The manuscript is structured in good form as well, I suggest publication of this manuscript as it is.
Please remember to give the exact definition of the azimuth, which is used in Figure 5.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Summary
In this technical note, the authors assess the latest version of L1 products from the CyGNSS satellite constellation. The products involve Normalized Bistatic Radar Cross Section (NBRCS) and Leading Edge Slope (LES) from individual pairs of CyGNNS units and GPS transmitters, and are relevant for near-surface windspeed retrievals. To ascertain overall goodness of the intercalibration and stability of L1 products, the authors take NBRCS and LES data since 2016, coregister each individual data point with coinciding ERA-5 10m windspeed, and select a subset (all observations associated with windspeeds between 6 and 8 m/s). Based on the data subset, the authors identify geographical biases in L1 products that are associated with the bistatic geometry of individual antennas.
The technical note is generally well-written and addresses interesting shortcomings in L1 products. However, the authors should resolve a few major and minor issues before publication. Solving these issues should help to better understand this work as well as its importance and to reach a wider audience.
Major Points
Reading ll. 69-75, a mathematical framework for NBRCS and LES seems missing that allows to understand how bistatic antenna geometry may affect these products. Please provide such a mathematical framework. Ideally, this could be complemented by a sketch that shows the GNNS measurements principle and the identified shortcomings (perhaps in a separate sketch near the discussion).
The authors found deficiencies in both LES and NBRCS. Are these differences correlated across products and would that leave any impact on windspeed retrievals? Please discuss this in Section 5.
Reading ll. 57-59, it appears more (and perhaps similar) studies have been performed in the past. Please expand this section to better explain and cite earlier efforts. With respect to your findings, studies that address bistatic antenna geometry should be introduced and cited here, too.
The study is based on a specific set of near-surface windspeed conditions (i.e. u10 between 6 and 8 m/s). Please provide an explanation for this choice. Has the study been repeated for other windspeed bins (perhaps bins associated with hurricanes) and what are the outcomes?
This study relies heavily on ERA-5 reanalysis data. It seems important to cite previous work that validated ERA-5 (in particular wind speed).
The assessed products form the basis for windspeed retrievals. Perhaps the authors should spend a few sentences in the introduction to explain how windspeed retrievals work.
Minor Points
l. 9 Please define CyGNNS upon first use and make sure to spell it consistently.
l. 11 Please briefly list employed wavelengths and typical footprint size of each observation.
ll. 27-29 Please rephrase this sentence as it is identical to first sentence in the abstract.
l. 32 Please spell out UK-DMC and UK-TDS upon first use.
l. 73 DDM is already defined earlier.
l. 77 Please define SVN upon first use.
ll. 113-118 Please explain the dynamic range of this product across typical windspeeds to understand if 0.2 dB is small or large variation.
ll. 194-197 I have trouble following this. Perhaps this could appear in the discussion, instead, and be further elaborated there.
Fig. 6 Please explain how this figure is related to Fig. 4. And why are there several lines of the same color?
Fig. 4/Fig. 5/Fig. 6/Fig. A2/Fig. A3: Connected an earlier major point: Is it possible to quantify the (maximum) impact on derived windspeeds given these deviations?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
In their revised version of the technical note, the authors improved upon all major and minor issues that I raised. I recommend prompt publication of this version.
