When Subterranean Earth Meets Outer Space

  • For Timing

The first thing that comes to my mind when thinking about time-synchronized scientific observation networks is the seismic observation network. In a previous article, I explored the ways in which seismic observation has served as an important application for GPS technology since its early years. This time, I want to delve deeper into how seismic observation and GPS became so intertwined.

A Report Announcing Easy Clock Completion

"We have completed our verification tests on the precision of a consumer-use GPS receiver board and created a seismic observation clock using this board. Furthermore, we are proud to announce that this has enabled the easy creation of a seismic observation clock at low cost that realizes high precision and low power consumption."
This is an excerpt from the article "Building a High-precision Clock for Seismic Observation Using a GPS Receiver Board" (Jishin ["Earthquakes"], vol. 46, 1993, pp. 67–77) by Yuichi Morita and Takeshi Nishimura of the Tohoku University Faculty of Science.

The excerpt was taken from Morita's and Nishimura's academic report, published in 1993. While stating that the oven-controlled crystal oscillator (OCXO), whose calibration is automated by using signals from Japan Broadcasting Corporation (NHK) or JJY stations emitting Japan standard time "has seen widespread usage in Japan and exhibits high reliability," the two authors also point out problems, including "the inability to utilize JJY and similar outside of Japan" and "high power consumption demands preclude the use of battery power [for such a system]." To overcome these issues, the two attempted to build their own clock using a GPS receiver module. The following is a circuit diagram for that clock.

The diagram is taken from the abovementioned academic report; a FURUNO ELECTRIC GN-72 GPS receiver can be seen on the right-hand side.

The GPS receiver unit shown in the diagram above is the GN-72, FURUNO's first model released when car navigation systems were just beginning to gain widespread popularity. According to FURUNO's GPS Development History, the GN-72 was "a compact, single-circuit-board GPS receiver for widespread use, and an example of revolutionary technology at its time." In addition to the ability to output at one pulse per second (1 PPS), the receiver was designed to be compatible with leap second introduction and provided serial output of high-precision time information. Nishimura and Morita, both of whom are seismologists, took note of the GN-72's ability to output high-precision time information. In the report, they state that the feature can be utilized in automated time calibration via a temperature compensated crystal oscillator (TCXO) to easily obtain absolute time with error of 0.1 microsecond or less.
They also stated, "We are using this GPS clock in a broadband seismograph in Africa's Republic of Zaire, where it continues to operate reliably to this day." They praise the technology highly, writing, "A self-calibrating maintenance-free-clock, eliminating the need for any initial setup by the user, is extremely useful." While their report presents a clear and concise account of the facts and logical conclusions, the reality behind this accomplishment is a tireless series of trial-and-error attempts — a testament to their passion, dedication and D.I.Y. minds. To me, it represents a vivid account of the moment that seismology meets GPS technology. In other words, it could represents subterranean earth meets outer space.

Talking with a Seismologist

I had some help discovering the abovementioned report which discusses the moment at which subterranean earth and outer space meet. I talked with Associate Professor Shigeki Nakagawa of The University of Tokyo's Earthquake Research Institute regarding the relationships between the volcanology and seismology fields and GPS-based time synchronization. He sent over various materials for me to study, among which the above report was the oldest. The descriptions therein of the tough challenges with electrical work really drew me in.
According to Nakagawa, GPS-based time synchronization showed its true power during investigations of subsurface structures. Said investigations entailed (1) deployment of at least several dozen observational devices throughout the investigation site; (2) measurement of sounds and vibrations at numerous locations in response to gunpowder explosions, vibration generators and the like; (3) unification and analysis of the resulting observational data; and (4) development of an in-depth understanding regarding subsurface structures which are normally not visible to us on the surface.
During such observation and analysis operations, great amounts of time and labor are required to ensure time calibration for each device and make time corrections for the resulting observational data. Failure to do these makes it impossible to obtain meaningful data, so they are important parts of the process. However, if researchers have access to GPS-based automated time calibration to enable completely reliable time information records from each device, the entire process becomes much easier. The above subsurface structure investigation efforts were followed up by experimental collections of seismographic data via satellite channels from numerous locations throughout Japan, which was made possible thanks to GPS calibration at each seismograph location.

Says Nakagawa, "Time synchronization is extremely important in natural earthquake observation. The observational data is collected from locations that are dozens of kilometers apart, and time accuracy is vital for these distant measurement points. With seismic observation, accurate time information is necessary whether the measurement locations are near each other or far apart, making GNSS utilization a prerequisite in today's world. It's become so commonplace that most people consider GNSS a given, and don't give it much thought anymore."

Perhaps you, the reader, can now understand why it is apt to say that seismology has only advanced into new frontiers thanks to its encounter with outer space—regardless of how romanticized such a statement may sound.

Time-Synchronization GPS Receiver Technology Development in Response to Seismologists and Earthquakes

Photograph: The GT-77 model from 1997; taken from FURUNO's GPS Development History.
This model is described as "still playing an active role in seismograph network synchronization operations today."

After talking with Nakagawa and reading the materials he provided, I came to better understand how seismologists have not only utilized GNSS as a tool, but have developed a deep-reaching understanding of the framework as well as its advantages and disadvantages, and made full and effective use of the system to leverage the full potential of those advantages.
Take the report mentioned at the beginning of this article, for example. It mentions, among its various needs and requirements, "solving the trade-off between power consumption and time accuracy through intermittent time calibration by turning the GPS receiver device on and off at prescribed intervals" and "ways to ensure sustained time information output even when trackable satellite numbers are small."
In order to address such time-synchronization needs, FURUNO created its GT product series, which was originally based on the navigation-use GN-series products and custom-designed for time synchronization applications through repeated improvements and changes.
Ever since the Great Hanshin Earthquake in 1995, Japan's seismic observation network has seen rapid expansions and improvements. Many of the seismographs in dense seismographic measurement networks at the Japan Meteorological Agency's seismic observation locations, within the National Research Institute for Earth Science and Disaster Resilience's observation network for strong seismic events, and at universities, local government organizations, private enterprises and other organizations involved in seismic observation, are GT-series products.
GNSS time synchronization products serve as fundamental elements in telecommunication and broadcasting infrastructure throughout society today, and these products came about and advanced thanks in large part to the hard work of seismologists against earthquakes.

Writer introduction

Mr. Mitsunari Kita

Mr. Mitsunari Kita   Science and technology writer

Born in Ishikawa Prefecture in 1964. Based on his experience in covering industrial technology, cutting-edge technology, and space development, he is passionate about unraveling and conveying difficult topics in an interesting way to people of all ages, from children to senior citizens. From 2009 to 2014, he was a member of the editorial board of "JAXA's," the official magazine of the Japan Aerospace Exploration Agency. Author and co-author of the following books: 『あなたにもミエル化? ~世間のなりたちを工学の視点から~』(幻冬舎mc)、『私たちの「はやぶさ」その時管制室で、彼らは何を思い、どう動いたか』(毎日新聞社)、『東京大学第二工学部70周年記念誌 工学の曙を支えた技術者達』(東京大学生産技術研究所) etc.,

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Column by Mr. Mitsunari Kita  (Science and technology writer)