Pointing Calibration for Spaceborne Doppler Scatterometers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The Doppler scatterometer will introduce greater errors when measuring ocean currents on a space platform compared to a ground platform. This manuscript proposes a new calibration method to estimate the pointing deviation. This method can eliminate the main pointing errors and allows for the acquisition of global ocean current data with lower system stability requirements. Many simulation analyses were also conducted in the results, and some conclusions were obtained.. But I also have a number of concerns regarding this manuscript, which are detailed below:

1. The prior data set of ocean currents is of great significance in the manuscript. But how can one consider the prior data as the true data? Is it necessary to explain the relationship between the data measured by the instrument and the actual data of the ocean current? Also, where can we obtain the actual data of ocean currents? If the actual data cannot be obtained, will it introduce new errors to the results? So, not only the errors mentioned in the text, but there might be other reasons as well? Therefore, I suggest that we first ensure the authenticity, accuracy and validity of the prior data set, and present it in the manuscript.
2. In Figures 5 and 6, no detailed explanations were provided for PSD and STD.
3. From Fig.5 to Fig. 8, the black dotted line is approximately 7.4 cm/s. However, this data is not mentioned in the manuscript. So is it really the 7 cm/s that was mentioned in the manuscript? The annotations and the explanations in the manuscriptare different. Could this also be a source of error?
4. Some English expressions in the article are not standard, and it is recommended tomodify the manuscript.

Author Response

1. The prior data set of ocean currents is of great significance in the manuscript. But how can one consider the prior data as the true data? Is it necessary to explain the relationship between the data measured by the instrument and the actual data of the ocean current? Also, where can we obtain the actual data of ocean currents? If the actual data cannot be obtained, will it introduce new errors to the results? So, not only the errors mentioned in the text, but there might be other reasons as well? Therefore, I suggest that we first ensure the authenticity, accuracy and validity of the prior data set, and present it in the manuscript.

Response: We have inserted the following paragraph to discuss this issue in the discussion section:

“Given that we have no real measurements at the small spatial scales used in this study, we used model results to estimate the prior data that might be available when the mission flies. To mitigate this limitation, we used two independent ocean circulation models (LLC-4320 and COAS) with different ocean-atmosphere coupling and with independent runs to assess the robustness of our conclusions. We found that both models had very similar impact on the ocean leakage and prior corrections. To further differentiate the prior data from the model data, we applied substantial spatial and temporal averaging, to achieve spectra similar to spectra obtained from the real ocean data products, AVISO and OSCAR. Finally, we found that using a very simple wind-driven circulation model based on wind-driven Ekman flow, or a climatology based on the simulated instrument data itself, achieved significant improvements on the algorithm performance. Although using model data may be optimistic, we emphasize that prior data is not required to achieve the performance required by the potential ODYSEA mission.”

2. In Figures 5 and 6, no detailed explanations were provided for PSD and STD.

Response: The captions of Figures 5 and 6, as well as the text referring to these figures, was modified to define PSD and STD more clearly.

3. From Fig.5 to Fig. 8, the black dotted line is approximately 7.4 cm/s. However, this data is not mentioned in the manuscript. So is it really the 7 cm/s that was mentioned in the manuscript? The annotations and the explanations in the manuscriptare different. Could this also be a source of error?

Response: The correct number is 7.4cm/s and has been made consistent in text and figures.

4. Some English expressions in the article are not standard, and it is recommended to modify the manuscript.

Response: we have revised the manuscript for non-standard English expressions both by rereading and using two grammar checkers.

Reviewer 2 Report

Comments and Suggestions for Authors

This article explores various methods of improving the pointing accuracy of spaceborne Doppler scatterometers by providing prior information. The methods are clearly explained and well argued, and the article is a valuable reference.
However, the article could be improved in a few minor ways:
1. The abstract is too vague in its description of the methods. It should clearly state the specific methods used by the authors to enhance readability.
2. The formula (6) lacks a description of ρ.

Author Response

1. The abstract is too vague in its description of the methods. It should clearly state the specific methods used by the authors to enhance readability.

Response: the reviewer is right. The abstract has been modified to give greater insight of the proposed calibration method.

2. The formula (6) lacks a description of ρ.

Response: ρ has now been defined in the text after the formula.

 

Reviewer 3 Report

Comments and Suggestions for Authors

After reading the manuscript, I believe that overall, the text is well articulated and organised and well written. The presentation of the algorithm is quite clear, and the authors make clear in the narrative, the potential and limitations of this proof-of-concept approach to calibrating pointing error, although there is still work to be done and the proposal needs to be validated. Therefore, I consider that the manuscript is ready to be accepted. Below, authors will find some minor comments that may improve the presentation.

The introduction seems somewhat repetitive, - first two or three paragraphs are very similar-. 

Lines 36-40: the authors could introduce references.

In Materials and Methods: could the authors give an idea of how much the quickly varying errors impact the total error, or at least a range of magnitude?

Line 102-104: it would be convenient to include a reference.

Figure 2 has two  b) in its description

Figure 4 has  low resolution, or at least it is difficult to see. It might be useful to improve it.

 

Author Response

1. The introduction seems somewhat repetitive, - first two or three paragraphs are very similar-. 

Response: The first 3 paragraphs have been collapsed into two paragraphs, avoiding unnecessary information.

2. Lines 36-40: the authors could introduce references.

Response: added reference to Paduan and Washburn who review turning radial velocities into currents.

3. In Materials and Methods: could the authors give an idea of how much the quickly varying errors impact the total error, or at least a range of magnitude?

Response: we have given our current best estimate of 2-3 arcsec, or ~5 cm/s, in the paragraph in Section 2 where we discuss the quickly varying errors.

4. Line 102-104: it would be convenient to include a reference.

Response: added reference to Marsh & van Sebille and added two sentences to clarify.

5. Figure 2 has two  b) in its description

Response: the repeated b) description has been removed.

6. Figure 4 has  low resolution, or at least it is difficult to see. It might be useful to improve it.

Response: Figure 4 has been redone at 300 dpi resolution and with better color tables and limits on the colorbars to improve readability.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors I recommend publication.