Analysis of Ionospheric TEC and DCB Using GPS/Galileo Observations from a Moving Navy Training Ship

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article presents the results of an interesting experiment that presents TEC data on a ship in the Pacific Ocean and compares the data with GIM TEC data. Placement of GNSS instruments on mobile marine platforms, such as research ships on commercial marine vessels and ocean buoys. Ship-based GNSS receivers can collect GNSS data over vast ocean areas. They can also contribute to a better understanding of TEC ionosphere monitoring over the seas. In addition, the authors considered the influence of geomagnetic factors that could potentially affect the DCB receiver response.

While working on this article, I have a few comments and suggestions that I would like to make.

1. In lines 237-239 and 296-298, the authors mention anomalies that are related to certain processes, but they do not discuss what the underlying factors might be. It would be helpful to see more discussion about the potential causes of these anomalies. It would be beneficial to provide a physical interpretation of the observed phenomena.

2.  I also noted that there is a significant difference between the VTEC values for the HANS and the CODE GIM on days 261-264. However, the physical mechanisms behind this difference are not discussed. On these days, there is no significant geomagnetic activity.

3. in 4.3. The authors of the article use the TEC time series from DOY 249 to 300, 2024. The black and red lines represent the VTEC values for the HANS and the CODE GIM, respectively. However, the authors do not specify the coordinates in which the CODE GIM TEC data were taken. Please specify this in the text.

4. Figure 4 seemed very interesting. If possible, add a graph of geomagnetic activity (for example, the Dst index) to this figure, which will allow you to analyze the results along the trajectory in the most effective way.

 

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors    

It is recommended that the paper be rejected in its present form. To resubmit, the authors should substantially revise the manuscript, taking into account the following comments.

1. The main results focus on the behavior of the DCB (and its effect on TEC determination), but the discussion of this parameter is not sufficient. The discussion should begin with an analysis of all error sources that can affect DCB estimation, in particular the ionospheric modeling error, which can be considered a fundamental source of DCB errors. It should be clarified that DCBs are, by definition, satellite and receiver biases that can be regarded as fixed and independent of external factors such as, among others, the evolution of observation geometry, the local receiver environment, and ionospheric modeling errors. These factors are responsible for the changes observed in the estimated DCBs, not for changes in the true hardware DCB values.

2. The ionospheric model adopted is a spherical harmonic expansion (SHE), but the limitations of this model in oceanic areas, where experimental TEC data are nonexistent or very sparse, should be discussed. In addition, the ship route from North America to South America, the presence of large horizontal gradients, and the strong variability associated with the Equatorial Ionospheric Anomaly can significantly affect the performance of the ionospheric model used. The limitations of the SHE should be described and related to the results obtained.

3. If the DCB results discussed in the paper are interpreted in terms of estimation errors, it should be noted that there is no significant correlation with solar and geomagnetic indices (the correlation with the Dst index, with R = 0.35, cannot be considered relevant). In the opinion of the present reviewer, the effects of these indices on the DCB error estimates are, in fact, embedded in the overall impact of ionospheric mis‑modeling. An analysis of the effects of such mis‑modeling should be presented.

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

This study investigates possible causes of instabilities associated to TEC calculations using moblie platforms receivers of satellite signals. This is closer to a technical report than to a scientific article.

Below you can find some comments

Major comments: The actual contribution of the study, compared to other studies, is not clear and this must be clearly added. The claimed correlations are not supported by their corresponding coefficients and lack significance analysis.

Specific comments:

L164: Please give a reference for the constant K in eqs (1) and (2).

Figure 4 a and b: it is not clear for me how to "see" IPP in Figure 4 a/b. I gather that colored lines show the VTEC, thus where are IPPs? 

What is the time resolution for each TEC result?

Lines 215-220. Actually, it is not at all obvious that IPP in Galileo and GPS are similar. A separate figure or, better, simply a figure with the two series of both IPPs would show whether these two are so similar.

There is no direct comparison between VTEC in Galileo and GPS (to see, e.g., whether these are consistent). I suggest adding this as well (besides the comparison with CODE).

It is not clear for me whether all values used for correlation are diurnal daily averages? If yes, one should investigate cases when high TEC anomalies to the south and low TEC anomalies to the north of the trajectory are cancelling each other, when averaged. 

Lines 376 – 400: In order to actually evaluate the correlation an analysis of the significance has to be included, and this is missing. As plots and numbers look now, there is no actual correlation between DCV and any of the geomagnetic/solar indices, or other ionospheric or technical parameters. Moreover, the correlation with F10.7 should take into account the fact that solar perturbation effects on ionosphere may be delayed by 1-2 days, thus this should be taken into account (a time lagged correlation). No conclusion can be drawn at this point about any link between DCB and solar/magnetic activity or VTEC.

Lines 405-…: The same as above is valid for DCB-VTEC correlation.

Lines 25-430: No, there is no clear conclusion about a correlation of DBC with VTEC or Dst. 

Conclusions:

Lines 457-460. This is simply a false statement, it should be deleted.  

In general, after reading the whole manuscript, I cannot say what is the actual contribution of the study, compared to other studies, and this must be clearly added.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

The article is well-presented and the figures and calculations appear to be accurate. The authors have accomplished a significant task that will aid in more accurate calculations of seagoing vessel coordinates.

I feel this article is suitable for publication as it stands.

Author Response

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Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thanks to the authors for the revision of the article. I have no questions, I believe that the article can be published.

Reviewer 2 Report

Comments and Suggestions for Authors

After the revision done by the authors in the new version of the manuscript, the paper can be accepted for publication in present form.

Reviewer 4 Report

Comments and Suggestions for Authors

The article may be accepted for publication.