Measuring the Directional Ocean Spectrum from Simulated Bistatic HF Radar Data

Round 1

Reviewer 1 Report

This paper derives the radar cross section (RCS) of HF radio wave scattering
from the sea surfaces for bistatic case, and this paper shows that
inverted wave parameters from Doppler spectra are sensitive to
noise (or error) as larger bistatic angle.
It would be better to discuss on the matrix of the linearized integral equation (e.g., condition number), but this is not required in this stage.
The comments are as follows:
(1) Line 75: It has already shown that the RCS retaining scattering angle
and incident angle reduces to the RCS by Barrick in the monostatic and grazing
incidence case exactly (including combination of hydrodynamic and
electromagnetic effect) in previous studies.
(2) The right hand side of equation (8) is not [.......]_{z=0} but
[.......]_{z=f}, and expanded by kf to the second-order.
Equations (5)-(9) do not hold in sea water (z=0<f).
In some previous studies, this (z=f) is considered.
If the result of the RCS is identical, the reason should be stated.
(3) Table 1: T_p (spectral peak period ?) is longer than 100 sec
for buoy data.
Is it true or low frequency noise ?
(4) Table 2: Bistatic angle and angle between Braggs are the almost same
but there are differences (1-mono,1-bistatic).
Should be they the same or not ?
(5) equation 3: pm 1 (plus minus 1) 1 is omitted in the subscript of sum.

Author Response

Review 1

Our responses below are in red and are marked in the revised manuscript in red.

This paper derives the radar cross section (RCS) of HF radio wave scattering
from the sea surfaces for bistatic case, and this paper shows that
inverted wave parameters from Doppler spectra are sensitive to 
noise (or error) as larger bistatic angle. 
It would be better to discuss on the matrix of the linearized integral equation (e.g., condition number), but this is not required in this stage.
The comments are as follows:
(1) Line 75: It has already shown that the RCS retaining scattering angle 
and incident angle reduces to the RCS by Barrick in the monostatic and grazing
incidence case exactly (including combination of hydrodynamic and 
electromagnetic effect) in previous studies.

We are not aware of any published results that reduce exactly to the Barrick monostatic RCS. The Gill result does not although differences are very small. If the reviewer wishes us to revise this statement perhaps they could point us to the relevant work.

(2) The right hand side of equation (8) is not [.......]_{z=0} but
[.......]_{z=f}, and expanded by kf to the second-order. 
Equations (5)-(9) do not hold in sea water (z=0<f).
In some previous studies, this (z=f) is considered.
If the result of the RCS is identical, the reason should be stated.

The Stratton Chu integral can be evaluated on any plane and z=0 is chosen for convenience. An sentence to this effect has been added.

(3) Table 1: T_p (spectral peak period ?) is longer than 100 sec 
for buoy data. 
Is it true or low frequency noise ?

Thank you for this. The peak periods and directions had been placed in the wrong columns for the buoy simulation cases. This has now been corrected.

(4) Table 2: Bistatic angle and angle between Braggs are the almost same
but there are differences (1-mono,1-bistatic). 
Should be they the same or not ?

They should not. They are similar because of the geometry and the fact that one of the radar is monostatic.

(5) equation 3: pm 1 (plus minus 1) 1 is omitted in the subscript of sum.

Thank you for this. It has been corrected.

Author Response File: Author Response.pdf

Reviewer 2 Report

General Comments

This manuscript gives the theoretical basis for bistatic wave measurement with a phased- array HF radar. Simulations of monostatic and bistatic radar data have been made using wave models and wave spectral data. The Seaview monostatic inversion method for waves, currents and winds has been modified to allow for a bistatic configuration and has been applied to the simulated data for two receive sites. The work concerning bistatic HFSWR is of great interest to HF radar community. However, some main issues and modifications should be addressed considering the organization of this manuscript. I recommend a minor modification for this manuscript.

First, what’s the main contribution of the work? The derivation of the first- and second-order cross section for bistatic HF radar or the inversion the directional ocean spectra from the simulated data? The title of manuscript is “Measuring the Directional Ocean Spectrum from Simulated Bistatic HF Radar Data”. But the description of the inversion method is very limited (just one page, no details can be found). In contrast, I find that the contents from page 5-17 gives the theoretical basis for bistatic HF radar. Such an organization of this manuscript makes me confused.

Second, I strongly recommend the authors to separate the manuscript into two parts (or manuscripts). The first part gives the theoretical contribution of the first- and second-order cross section for bistatic HF radar. The second part gives more details on the inversion method and results. That maybe more appropriate.

Specific Comments

Page 3 line 44 “... is shown in figure 2 where we good agreement is shown”. This sentence should be modified. 

Page 4 line 63, change “equation 28” to “the second order bistatic radar cross section”.

Figure 10, some noises at the wings of the spectrum. Why? 

Line 77 and 80, “the scattering angle can vary between 0° and 90°”, I think the word “scattering angle” should be changed to “bistatic angle”.

The introduction may add two references:

1) The first work concerns shore-to-air bistatic HF radar “Chen et al. The Scattering Coefficient for Shore-to-Air Bistatic High Frequency (HF) Radar Configurations as Applied to Ocean Observations. Remote Sens. 2019, 11, 2978; doi:10.3390/rs11242978”.

2) The second reference contains a completed work on ocean wave spectrum extraction from bistatic HF radar data using a nonlinear method, “Murilo Teixeira Silva, A Nonlinear Approach to Ocean Wave Spectrum Extraction From Bistatic HF-Radar Data, Master degree thesis, Memorial University of Newfoundland, 2017, October.” No.19 reference in the manuscript may be replaced by this one.

Author Response

Review 2

Our responses below are in red and are marked in the revised manuscript in red.

This manuscript gives the theoretical basis for bistatic wave measurement with a phased- array HF radar. Simulations of monostatic and bistatic radar data have been made using wave models and wave spectral data. The Seaview monostatic inversion method for waves, currents and winds has been modified to allow for a bistatic configuration and has been applied to the simulated data for two receive sites. The work concerning bistatic HFSWR is of great interest to HF radar community. However, some main issues and modifications should be addressed considering the organization of this manuscript. I recommend a minor modification for this manuscript.

First, what’s the main contribution of the work? The derivation of the first- and second-order cross section for bistatic HF radar or the inversion the directional ocean spectra from the simulated data? The title of manuscript is “Measuring the Directional Ocean Spectrum from Simulated Bistatic HF Radar Data”. But the description of the inversion method is very limited (just one page, no details can be found). In contrast, I find that the contents from page 5-17 gives the theoretical basis for bistatic HF radar. Such an organization of this manuscript makes me confused.

Second, I strongly recommend the authors to separate the manuscript into two parts (or manuscripts). The first part gives the theoretical contribution of the first- and second-order cross section for bistatic HF radar. The second part gives more details on the inversion method and results. That maybe more appropriate.

In order to modify the Seaview inversion we needed to have bistatic cross-sections that we were confident about and thus the first part of the paper is important for the main contribution: the simulations and inversions. To clarify this we have added a few extra words in the last paragraph of the introduction and have put the, now enhanced, inversion description including references to more detailed descriptions into a separate section (3). We hope this is clearer and acceptable to the reviewer.

Specific Comments

Page 3 line 44 “... is shown in figure 2 where we good agreement is shown”. This sentence should be modified. 

Thank you for this. It has been modified.

Page 4 line 63, change “equation 28” to “the second order bistatic radar cross section”.

Thank you for this. We have made the change.

Figure 10, some noises at the wings of the spectrum. Why? 

A Gaussian noise floor has been added to the simulations. This is now noted in the paragraph preceding Fig 10.

Line 77 and 80, “the scattering angle can vary between 0° and 90°”, I think the word “scattering angle” should be changed to “bistatic angle”.

Thank you for this. We have made the change.

The introduction may add two references:

Thank you for drawing our attention to these which are now referenced in the introduction.

1) The first work concerns shore-to-air bistatic HF radar “Chen et al. The Scattering Coefficient for Shore-to-Air Bistatic High Frequency (HF) Radar Configurations as Applied to Ocean Observations. Remote Sens. 2019, 11, 2978; doi:10.3390/rs11242978”.

2) The second reference contains a completed work on ocean wave spectrum extraction from bistatic HF radar data using a nonlinear method, “Murilo Teixeira Silva, A Nonlinear Approach to Ocean Wave Spectrum Extraction From Bistatic HF-Radar Data, Master degree thesis, Memorial University of Newfoundland, 2017, October.” No.19 reference in the manuscript may be replaced by this one.

 

Author Response File: Author Response.pdf