Impact of Attitude Model, Phase Wind-Up and Phase Center Variation on Precise Orbit and Clock Offset Determination of GRACE-FO and CentiSpace-1

Round 1

Reviewer 1 Report

please check the attached report

Comments for author File: Comments.pdf

Author Response

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

For the LEO-based navigation system, not only the high precision LEO orbit but also the high precision clock offset should be determination. The manuscript analyzes the results of CS-1 and GRACE-FO orbit determination and clock offset from three aspects: attitude, phase wind-up, and PCV. It proves that all three Influencing factors are considered necessarily. It is an interesting work in the field of LEO-based navigation system.

suggestions for a minor modification:

• Table 3, Table 4 and Table 5 use the exact same title.
• line 136, The full stop is missing.
• line 254, as the figure shown, the average PWU errors for CS-1 is larger than GRCC. The conclusion that begins with “Obviously” seems to be wrong.
• line 316, 1.88mm, 1.72mm should be 1.88cm, 1.72cm.
• Currently, the POD accuracy evaluation is based on comparison with JPL. It would be better if the results of SLR validation can be added.

Author Response

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Capacity enhancement for communication and navigation augmentation applications using Low Earth Orbit (LEO) satellite is a hot topic recent years, and there have been a huge demand for precise orbit determination (POD) LEO satellites. This paper aimed to discuss the impact of attitude model, phase wind-pp and PCV on precise orbit and clock offset determination, and this is certainly a very useful investigation for GNSS community. Nevertheless some recommendations are given in the following:

• Figure 1 and Table 1 are not necessary and there is not affected to understand for reader if they are removed.
• Line 195-Line 221, the differences of different POD and clock offset schemes used nominal and measured attitude models is small, and the slight improvement results can be ignored and is not statistically significant because only six-day results are provided in the paper.
• Line 254-255, “Figure 8 shows the PWU errors for GRCC and CS-1 in DOY 273, 2018. Obviously, the average PWU errors for GRCC is larger than CS-1”. In Figure 8, it is not clear that the average PWU errors for GRCC is larger than CS-1.

Author Response

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

The submitted manuscript presents a quantitative assessment of some satellite-dependent effects on the orbit reconstruction of LEO satellites using high-quality dual-frequency GPS measurements, in post-processing.
In particular, the effects of attitude motion, phase-windup (PWU), phase center variation (PCV) on GRACE-C and CentiSpace-1 were analyzed.

The topic is relevant to the journal and of interest to the reader. The results can be of use for the post-processing of GNSS data of LEO satellite, especially if the highest level of accuracy is required, as in the case of navigation enhancement service satellites, as pointed out by the authors.

The manuscript is, in general, well written, with some sentences that need to be improved in terms of clarity.

The objectives of the manuscript are clearly identified.

The main point to be improved is the method used to quantify the various effects studied in this paper.
The errors are currently quantified using the difference between overlapping orbits of two solutions produced using the same model, but with different data sets.
This difference is a good indicator of self-consistency, but does not strictly represent an indicator of modeling accuracy.
Instead, I suggest to use the difference between the orbital solutions produced using different models, as done for the clock offset.
In fact, the authors are comparing a simpler model with one more accurate, that represents the reference.
Then, the overlapping differences of the best model can be used as a reference to compare the errors under analysis.
For GRACE, the difference with the JPL solution is also useful and should be kept.

Finally, the authors say that the difference in the estimated clock offset is significant for some applications. This point should be justified, introducing a quantitative discussion and/or references.

The literature review can be improved, providing information on previous similar studies, if present.

Some minor points are also provided in the attached annotated PDF.

Comments for author File: Comments.pdf

Author Response

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 5 Report

It is wotth to investigate the LEO POD under the framework of GNSS with LEO enhancement, although these aspects have been already studied and well known. Especially the impact of attitude biases, as high precision attitude system might not be available for very small satellites because of the payload restriction. The paper is well organised and written in Egnlish.  However the PWU correction must be taken into consideration as for most of the precise processing. Instead of comparison of with and without PWU, it should be the comparison PWU with nominal and observed attitude. By the way, the small difference of few cm in orbit and clock products is very crucial or negligible for LEO enhancement? There are a few comments annotated in the upload file. 

Comments for author File: Comments.pdf

Author Response

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The research question of the paper is clearly defined and appropriately answered. The methods used are sound and repeatable.