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
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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?
Citation: https://doi.org/10.5194/egusphere-2025-2563-RC1
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