Development of Global Tropospheric Empirical Correction Model with High Temporal Resolution

Round 1

Reviewer 1 Report

Please remove shortcuts from the abstract. The abbreviation should be expanded upon the first occurrence in the text. I do not understand the idea of Figures 1-9, which do not contribute much to the whole work. Consider remove it. Authors created ITG model (Yao, Y., Xu, C., Shi, J. et al.ITG: A New Global GNSS Tropospheric Correction Model. Sci Rep 5, 10273 (2015)). Why aren't they working on improving it and creating a new one? In particular, pointing out the imperfections of the ITG model. What influence has this method on the determination of the position accuracy, especially on the PPP method? Please cite the application of this method in practice: Wang, Guo Quan. 2013. “Millimeter-Accuracy GPS Landslide Monitoring Using Precise Point Positioning with Single Receiver Phase Ambiguity (PPP-SRPA) Resolution: A Case Study in Puerto Rico.” Journal of Geodetic Science 3 (1): 22–31. https://doi.org/10.2478/jogs-2013-0001. Ge, Yulong, Peipei Dai, Weijin Qin, Xuhai Yang, Feng Zhou, Shengli Wang, and Xingwang Zhao. 2019. “Performance of Multi-GNSS Precise Point Positioning Time and Frequency Transfer with Clock Modeling.” Remote Sensing 11 (3): 347. https://doi.org/10.3390/rs11030347. Maciuk, Kamil. 2018. “GPS-Only, GLONASS-Only and Combined GPS+GLONASS Absolute Positioning under Different Sky View Conditions.” Tehnicki Vjesnik - Technical Gazette 25 (3). https://doi.org/10.17559/TV-20170411124329. Rabbou, Mahmoud Abd, and Ahmed El-Rabbany. 2015. “PPP Accuracy Enhancement Using GPS/GLONASS Observations in Kinematic Mode.” Positioning 06 (01): 1–6. https://doi.org/10.4236/pos.2015.61001. Wu, Qiong, Mengfei Sun, Changjie Zhou, and Peng Zhang. 2019. “Precise Point Positioning Using Dual-Frequency GNSS Observations on Smartphone.” Sensors 19 (9): 2189. https://doi.org/10.3390/s19092189. Figures 1-9 – What is DOY and year? What height it concerns? Does the temperature scale is correct (300 ° C on the equator - Figure 1)? „Figure 1 shows that the annual mean temperature values decrease with ascending latitude and elevation, with the minimum value apparent in Antarctica and the maximum near the equator” – pleonasm Figures 4-6 – air pressure in °C? The impact of the troposphere is at least an order of magnitude smaller than the ionosphere, it is assumed that it is 2-3 m at the zenith. What will be the impact of the created model on improving accuracy, since it differs from existing models by a few percent? Table 1 – bias is the same for all models. Delete it. The same Table 2, Figures 13,15 – only first two models (NOAA, GPT2) are clearly visible. The other two are not. The conclusions are very general and looks like a abstract of the text. Please show what's novelty of this text against existing research

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The submitted manuscript proposes a model for tropospheric condition, The interest for such a problem is due - in addition to the obvious quest for a better knowledge of the atmosphere - to the effects troposphere generates on satellite-based navigation signals, indeed affecting the accuracy of positioning system. With respect to such an issue, an important comment to the paper is that it lacks of a precise indication about the number of data to be used to compute the model, and therefore its usefulness for a possible implementation in receivers, if any, cannot be judged.

In case the last option is not of interest for the Authors, the paper could still have some interest for off-line modeling. However, in the opinion of this reviewer, a number of issues should be analysed and fixed by the Authors in view of a possible publication:

1) in relation (2) meaning of a (without index) should be explained, and in the following relation (3) the symbol c_pi is not clear at all;

2) line 124: meaning of peak and grains should be explained;

3) [very important]: according to defining relations (6) , the role of annual variations in WGTEM is not clear, as the terms c_1 and c_2 affecting the part related to the day of the year are (apparently) depending only in the hour of the day. In such a case these terms should be constant during the year, and the cosine function would have an argument monotonically increasing during the year, which is clearly not matching seasonal variations. This aspect need to be clarified by Authors and, if the relations are actually correct, should be explained in the text. Otherwise there would no chance for the readers to understand why the proposed model should improve TropGird2 model (line 123).

4) along the same line, it is not clear if the findings presented (lines 163 and Figures 1-9) refer to a specific day or to some kind of annual average; 

5) to insist even more with this time-dependency of the model, the receipt reported in lines 133-138 would - in the opinion of this reviewer - not be a workable one if the day dependency should not be considered: assuming as an example a database of 4 measurements per day (00, 06,12,18 UTC) the coefficients for t-3, t+1, t+5 would be equal. Probably this reviewer failed to understand the process, that could mean that description could not be clear enough for all perspective readers.

As a general comment, while the idea to correct problems in available models by modifying the coefficients involved in the expansion is valide, and the comparison with other models is appreciated, the description of the proposed model should be improved.

Minor typos:

line 38 "enomical" should be economical;

line 118: consider using "altitude" instead of "height";

line 138 "T" should not be in capital letter in order to agree with the symbol used in previous lines;

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I accept the corrected text in its current form.

Reviewer 2 Report

This reviewer acknowledges the work performed by Authors to address the comments. Specifically, description of relation (6) - while not perfect - looks now more understandable by readers, and several improvements have been applied to the manuscript.