the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
The azimuth observation by GNSS: A case study at Kakioka
Abstract. For the azimuth observation to be made at its magnetic observatories routinely, Japan Meteorological Agency (JMA) has adopted a traditional method based on Polaris sighting. Due to its difficulty to implement under overcast weather conditions and to its demand on observers, for overtime work into the evening, we are motivated to seek for an alternative method based on the GNSS observation that might potentially relieve those two disadvantages. An experiment is made at Kakioka to assess the eligibility and effectiveness of the GNSS method for JMA's unmanned observatories, Memambetsu and Kanoya. The GNSS observations themselves achieve as high a precision as approximately 1 arcsecond, as far as they are analyzed with Static mode. Derived from the results of GNSS observation and some supplementary horizontal angle measurements, the azimuth of the azimuth mark for the absolute measurement is determined with a precision of a few arcsecond, which is comparable to the azimuth precision achieved by the Polaris sighting. However, we end up with their significant difference by about 10 arcseconds. We discuss this discrepancy to be possibly due to a local geoid gradient. The Polaris observation is made with a theodolite tilted in the gravitational direction, also known as the vertical line deviation, whereas the GNSS observations are based on the azimuth of the compliant ellipsoid plane.
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RC1: 'Comment on egusphere-2025-2563', Anonymous Referee #1, 03 Jul 2025
- AC1: 'Reply on RC1', Hiroki Matsushita, 01 Aug 2025
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CC1: 'Comment on egusphere-2025-2563', Thomas Martyn, 29 Aug 2025
The rationale for the paper's study is set out clearly in the Introduction – particularly the difficulties in obtaining regular Polaris measurements. Attaching numerical values to this would improve context e.g. number of visits / measurements annually, number not possible do to cloud cover etc. This could be included in the form of a table and would give improved background
The description of the process used to calculate the GNSS derived baseline is clear, however as how mentioned by a prior commentor, the schematic could be made to more closely represent the observatory layout.
The fundamental aspect of the study is the ability of GNSS data to accurately determine the observatory azimuth. Much of the discussion, however, focuses on the precision of the measurements obtained with little discussion given to the accuracy of the measurements. To this end, further details of the post processing could be added and their effects / importance discussed. For instance: the effect of using local Continuously Operating Reference Station to constrain positions (Line 86: Because of the short baseline length, neither the atmospheric delay correction nor the ionospheric delay correction is made)
Citation: https://doi.org/10.5194/egusphere-2025-2563-CC1 -
RC2: 'Comment on egusphere-2025-2563', Jan Wittke, 02 Sep 2025
The paper by Matsushita et al, “The azimuth observation by GNSS: A case study at Kakioka” describes how to perform GNSS measurements for geomagnetic observatory work.
Please see my comments below:
The paper is well written and the methods as well as the procedures are presented in a concise way. After reading the whole paper, I was wondering about the scientific significance of the approach. Even this might be a relative new way for some geomagnetic observatories, GNSS observations is a common practice in surveying and a well established method. To improve the paper I would suggest following:
- The introduction should cover more the pro and cons of different GNSS survey modes with respect geomagnetic observatory work.
- The allover critical value is βGSNN. There should be a serious discussion how this angle is derived out of the GPS measurements. How does the distance between between the points influence the accuracy? How does the calculation is affected with errors?
- As the angles β1 and β2 are measured with a theodolite are they also affected by DoV? When yes, how does this influence affect the overall angle θ2 ?
- In the comparison between Polaris observations and GNSS observations, there should be a summary on the Polaris observation method and how uncertainties are compared to the GNSS method. As the authors identify a potential derivation between both methods it remains unclear which one is more absolute accurate. Is it possible to correct the more inaccurate value with the other method?
Citation: https://doi.org/10.5194/egusphere-2025-2563-RC2
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The paper by Matsushita et al, “The azimuth observation by GNSS: A case study at Kakioka” is an interesting read. The text is generally easy to follow, and the language is clear. I do not have a background in magnetic observatories, so I can not adequately comment on the novelty of the research. My recommendation is publication following a minor revision.
In the title, I suggest that the authors spell out GNSS, so it is immediately clear to all readers, including those outside the magnetic observatory community, what this paper is about by reading the title.
The introduction is a bit short, and the manuscript contains a very limited number of references. As such, this is not an issue, but I suggest that the author consider expanding the introduction with one or two more paragraphs with background, e.g., the issues that led to this research must likely also be issues at other laboratories.
It’s a minor issue, but the schematic diagram in figure 1 is not really matched to the actual layout in figure 1. Could the schematic diagram be “twisted” a bit to make the resemblance more intuitive?
Figure 4 contains some clear outliers where the signal systematically jumps several arcseconds back and forth. It would be appropriate to include a discussion of the origin of these outliers and how they are handled, e.g., personally, I would be tempted to cull outliers.
The authors find a systematic difference between Polaris observations and GNSS observations. It is suggested that Deflection of the Vertical (DoV) is the main reason for this, and convincing arguments are made. However, it would be appropriate to include a short discussion of any other potential sources for a systematic difference.
In the summary of the manuscript, it is not really clear, but still implicitly suggested that JMA is moving towards GNSS based azimuth observations. It would be interesting for the readers to learn a bit more here. Has a decision been made and if so, how will the systematic difference be handled?