A Comparative Study on Radio Frequency Interference Suppression and foF2 Scaling for Ionograms

Round 1

Reviewer 1 Report

Thank you for your article. I divided my comments into more technical and more language related.

TECNHICAL COMMENTS

Title should be simplified: "A Comparative Study on Radio Frequency Interference Suppression and foF2 scaling for Ionogram". "High Frequency Radio Frequency" sounds quite awful.

The acronym foF2 is never defined.

line 89: What are A and B in equation (!)?

line 90: Where is f_B used?

lines 110-111: X_R is a (n-1)xm matrix in the absence of X(l,:): this sentence makes no sense. And please write "(n-1) x m", not "(n-1)xm".


lines 111-113: " It is expected that for the intense RFI the correlation between X(l, :) and XR is high. However, the cross correlation between RFI and other types of radar returns, such as ionospheric echoes and background noise, are expected to be low.": please explain why.

lines 151-152: "the clutter is named to represent the RFI majorly occurred at specific frequencies": This sentence makes no sense.

I am not very convinced that the comparision using CC, RMSE and Mean Bias is meaningful. It really only indicated the level of similarities between results obtained usiing RFIM/FACS and original algorithms, not whether the results are better or not. In addition, some of the conclusions are rather obvious.

lines 410-411: "these features are not distinct for the manually scaled foF2 and autoscaled foF2 values". I don't understand this, please better explain it.

lines 481-483: "On the whole, FACS algorithm can result in about 5-12% improvement in uncertainty and 9-80% improvement in accuracy, and the RFIM algorithm will make 0.9-10% improvement in uncertainty and 11-90% in accuracy." Please explain on how you obtain these numbers as it it not clear from the text.


WORDING COMMENTS AND SUGGESTIONS

"clutter" is normally only singular, please change all "clutters" to "clutter"

line 70: "o RFIM" should be "of RFIM"

line 172: "RFIs occurred at specific frequencies are clearly seen" -> "RFI events at specific frequencies are clearly noticeable"

line 208: "In order to realize the difference" -> "In order to investigate the difference"

line 247: "it" should be capitalized.

caption of Figure 3: clutter, not cluttr


lines 282-284: "the SNRs of the FACS ionograms, which are decreased with increasing neighboring range cells, are smaller than those of original and RFIM ionograms." Please avoid writing "decrease/incease with increase, etc.". Suggestion for here: "the SNR of the FACS ionograms, which decreases with the number of neighboring range cells, is smaller than for the original and RFIM ionograms."

lines 308-309: "increase with increasing the neighboring range cells" -> "increase with the number of neighboring range cells"

lines 358-360: "increase with increasing the threshold of standard deviation" -> "increase with higher thresholds of standard deviation" (same for the similar sentence that follows).

line 412: "as present in Tables 3 and 4" -> "as shown in Tables 3 and 4"

line 428: "the more the neighboring range cells are selected" ->  "the more neighboring range cells are selected"

lines 430-431: "the corresponding SNR are all decreased with increasing neighboring range cells" -> "the corresponding SNR aall decrease with the neighboring range cells"

line 464: "an effectively" -> "can effectively"

lines 466-467: "the more the neighboring range cells are selected, the smaller the SNR is resulted" -> "the more neighboring range cells are selected, the smaller the resulting SNR is"

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The author compared the mitigation effects of two suppression schemes on RFI noise in ionospheric observation data, the mitigation ability of the two schemes for RFI noise is fully verified. Through parameter sensitivity tests, the optimal range of key parameters for the two schemes was also discussed. The impact characteristics of the two schemes on observation errors were also analyzed by comparing data after mitigation and an independent data source. The research results have a good reference value for the practical application of the two methods. The structure of the paper is reasonable, the discussion is clear and concentrated. A minor revision is recommended.

 

Special comments:

Introduction: Why do authors choose the two methods? It is recommended that the author provide their reasons. In fact, although the two methods are different, they both essentially rely on statistical relationships to eliminate RFI. Why not choose the method based on orthogonal projection filtering mentioned in the introduction?

 

Figure 5: It is obvious that when the FACS algorithm selects more adjacent data, this method not only eliminates RFI, but also has an obvious smoothing effect on observation data. How can we objectively select adjacent data in applications to weaken this smoothing effect?

 

Line 215-216：“RFIM and FACS can both effectively suppress narrow band RFI with very large intensities, namely, the RFIs at 9.6MHz, 15.5MHz, 28.4MHz.”

There are also RFIs in the microwave imager onboard polar-orbiting meteorological satellites, but it's particularly variable with frequency. Why is the RFI here basically unchanged with frequency? Is there any research on the source of this RFI?

 

Line 294-296：“ Although RFIM can effectively identify and suppress RFIs during daytime, the its capability of RFI suppression during nighttime is worse than that during daytime”

The author needs to explain what causes the poor effect of RFI elimination at night, which will help readers correctly understand the principle of this method.

 

Figure 9： From a quantitative perspective, the similarity between these results is significant, but the similarity probably comes from the large signal-to-noise ratio. By calculating the difference between the two noise filtered data, the authors can better identify the difference between the two methods.

 

Line 356-368： “It is clear that their 356 correlations are all very high (0.9-0.96), RMSEs are low (0.67-0.74 MHz), and absolute 357 mean biases are also small (0.0019-0.058 MHz).”

The "low/small" here requires a benchmark, and the standard deviation of the variable itself should be given to better measure the effectiveness of the method.

 

Line 458-459：“accordingly. Therefore, from the results presented in Figures 6 and 7 458 and Tables 1-4, it suggests that the use of 5-20 neighboring range cells for FACS algorithm”

The threshold value 20 here is understandable, but where does the threshold value 5 come from? Is there a relatively objective method to determine the threshold value in such a large range (5-20)? Some discussions should be added about this.

Author Response

Please see the attachment.

Author Response File: Author Response.docx