Performance Analysis of a Portable Low-Cost SDR-Based Ionosonde
Round 1
Reviewer 1 Report
Review comments on manuscript of Oleksandr Koloskov entitled ‘Performance analysis of a portable low-cost SDR-based ionosonde’ submitted to Atmosphere.
The software defined radio ionosonde is a very useful tool for its easy assembly, low cost and capability in network observation. It’s very impressive to know that the ISDR has operated at UAS station for more than 5 years. I found the manuscript is generally significant, except flaws existing in some aspect. I suggest returning the manuscript to authors for major revision.
1.The title of the paper is Performance analysis of a portable low cost SDR based ionosonde. The manuscript introduced the hardware of ISDR standard configuration at UAS, passive ionosonde at UAS, and mobile receiving ionosonde onboard the research vessel. However, in the results section, the authors mainly presented the comparison results between ISDR and IPS42 at UAS station, while the results of passive ionosonde at UAS was absent and result onboard research vessel was just only within one case. I suggest the authors modify the content, by either adding more results on UAS passive ionsonde and onboard research vessel, or deleting the content of UAS passive ionosonde (including figure 3) and onboard research vessel in the equipment introduction section.
2.It was claimed that one year data were used. But the maximum data points is only 5490. What is the detection cadence of the two ionosondes? 15min, 30min or 1hour? For an ionosonde in regular observation mode, the data point in one year should exceed 30,000. In addition, Why are the data points of foEs and h’Es different? In my opinion, the data points of foEs and h’Es should be the same. This is also the case of foE and h’E.
3.How was the manual scaling done? What scaling software was used? I suggest to display an example of manual scaling results super-imposed on the ionogram.
4.Line 260-272. I suggest to give a schematic figure to show the antenna configurations of ISDR and IPS42.
5.It was strange the ISDR was developed at ICTP but none of the authors is from this institute.
6.The parameters of ionosonde are inconsistent or unclear. L362-363. 320 frequencies in 1-16MHz. What is the frequency step? 942 heights from 85-825km, but is was said the height resolution is 4.5km at L159.
7.Figure 9. It was strange that the foF2 show obvious opposite variation in different months. For example, the foF2 reaches its maximum at 12LT in April and September, while it reaches its minimum in November, December and January. I checked the foF2 results from another Antarctic station and didn’t see such trend. I suggest the authors check the data time carefully.
Some minor correction:
8.L51 DPS 4D should be DPS4D.
9.L54 DGS 256 should be DGS256.
10.L93 resent should be recent.
11.Coordinates of UAS should be presented
12.L344 withing should be within
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The article is very interesting, it presents important contributions in the development of low-cost technologies for the study of the ionosphere, it is well documented and the experiments generate good results, compared with good sources of validation.
The graphics are well presented and the discussion is on point.
Developments based on USRP components have been used for this type of applications for some time, however, some contributions such as the use of Doppler for data analysis are valuable.
It is recommended to deepen the use of GNSS devices that exist on the market and that can further improve the sampling rate and resolution when measuring phase differences.
Author Response
We appreciate the time and effort that reviewer has dedicated to analysis of our manuscript.
Reviewer 3 Report
The manuscript presents a brief information on the working ionosonde ISDR located at Antarctica. The general configuration of the software defined radio ionosonde is provided along with comparisonds of critical frequency and virtual height with respect to IPS-42 ionosonde already operating on the site. Although the effort for building a low cost and high performance portable ionosonde is worthwhile, there are certain uncertainties in the given details about the instrument and the data analysis:
1) The ionosonde is reported to be operational for the last five years, yet, the comparisons are provided for only one year (2021). Why?
2) The IC-718 unit is reported to be not sufficient, yet, it is kept in operation for more than 5 years. Why?
3) The information given on the transmitting and receiving configurations is highly confusing. SDR is known for its flexibility in use. What are the possible modes of operations are defined? What are the different coding and modulation schemes that are implemented? Why 16 bit code is selected as a default? The authors mention the long sounding time compared to IPS. If shorter code lengths are used, surely this time will be shortened. Considering the lower limits of ionosphere in high latitude regions, perhaps this would be a better option.
4) There is no information on the user interface in both transmission and reception (for the ionograms). The frequency sweep range is different and shorter compared to IPS. Is there a specific reason for this?
5) The frequency step size of both ionosondes are not provided.
6) The preprocessing of the ionograms are achieved by a correlation receiver. It is a well known fact that matched filter or its correlation receiver implementation perform very poorly in cases of moving targets introducing Doppler effect. The Doppler effect must be severe in high latitude cases. This can be observed as unresolved frequency spreads on the ionograms. Although the authors claim that they have a distinct advantage to identify and resolve Doppler shifts in received signals, there is no justification provided in this manuscript. The references to this issue generally comes from a Ukrainian Antarctic Journal which cannot be reached over internet.
7) The two orthogonal rhombic receiving antennas do pick up cross polarized signals yet it is a common mistake to dominate them as O and X modes.
8) The receiving and processing information given in the manuscript is also highly confusing. Both the ionosondes ‘operate every 15 minutes’ yet they are ‘saved every hour’ and ‘operate around the clock’?
9) Why does IPS use an LF antenna for transmission?
10) There is an automatic scaler, UACides, referenced to [29,46,47]. These references are unreachable from the internet. There is no information on the capabilities of UACides. The manual scaling is mentioned but how it is applied is also not clear. What kind of scaling is used for IPS ionograms? Are the comparisons based on ionograms processed under the same kind of scaling?
11) The spread of the ionogram traces are mainly due to the windowing in FFT operation. The authors are not aware of this and the information given on page 10 is wrong.
12) The atypical traces in Figure 6 a and b are not interpreted. What kind of ionosphere causes such traces?
13) The comparison given on page 11 is very adhoc. The bias of height comparisons is concluded with the underestimation of IPS. Perhaps it is ISDR that is overestimating the heights?
14) Why does IPS not providing ‘proper’ results in November and December? What is wrong? Is it the season, the hour, hardware or the software? The evidence provided in Figure 9 is not convincing.
15) What is the reason for the inverted foF2 hourly distribution in Figure 9 for November, December, January and February? It is the summer time in the Southern Hemisphere so the maximum should be expected at local noon.
16) What is the ‘median’ mentioned on page 14? Is it the hourly medians of critical frequency obtain every 15 minutes for each month?
17) The discussion given on page 15 between lines 530 and 551 is very problematic. It is mentioned that ‘the performed comparison cannot be considered comprehensive’ and this statement is highly contradictory to the overall conclusion of the manuscript that ISDR is a reliable accurate and cost effective instrument.
18) The ISDR is mentioned to have a longer sounding time which is presented as a disadvantage for studying fast ionospheric processes. Yet, the claim is that this instrument is a reliable device for observing Doppler phenomena in various ionospheric disturbances such as TIDs and it ‘can track highly dynamic ionospheric processes’.
19) Black and white images loses the signal strength information.
20) Since the collection of signals and processing takes time along with sounding, none of the operations are in real time! It is better to mention near-real time.
Format concerns:
1) Rephrase the sentences that contain ‘etc’. It is better to use ‘such as’
2) There are certain misspelt words and errors such as resent, manually scaling ionogram, must be considering, withing.
3) Do not use It’s, doesn’t. Use formal versions.
4) There is a general lack of comma throughout the manuscript: After conjuctions, the initial phrases in the beginning of a sentence, before ‘respectively’. Please use a spell checker and use proper form of punctuation marks.
5) Separate units from numbers.
6) Rephrase lines 96 to 100; 363 to 366; 413-416
7) Use greater instead of bigger
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Though SDR ionosonde were developed at several institutes and experiments were done at many locations. I think this is the first report of long term convetional observation completed with SDR ionosonde. The authors has made extensive explanation or revision according to my concerns. So I suggest to accept the paper in current form.
Author Response
We are extremely grateful to the Reviewer for taking the necessary time and effort to review the manuscript. Your constructive comments and valuable suggestions were very helpful to improve the paper.
