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Why the GT-100 Time-synchronization GNSS Receiver Module is Like Fragrant Soup Curry

Exploring the Japanese Comfort Food Curry and Rice

An educational website on culinary culture and food created by Japan's Ministry of Agriculture, Forestry and Fisheries introduces various popular Japanese cuisines and dishes, such as osechi (cuisine traditionally served for the New Year holiday) and miso soup. Alongside these, it also features curry and rice, which ranks as the most popular school-lunch dish among students. In fact, one might call curry and rice Japan's top comfort food.

A number of different texts describe the rich history behind Japanese curry, which originally came from India via the United Kingdom in the Meiji Period (1868–1912), developed into high-end cuisine during the Taisho Period (1912–26), and then became popularized as a common household dish during the mid–Showa Period (1946–70). In this article, rather than focusing on curry and rice as a culinary dish itself, I want to concentrate on the common Japanese perception that it's unusual to eat curry sauce alone without rice.

Do you or anybody you know ever eat curry that way?　And if so, don't you think that such people have exceedingly rare tastes, and should probably keep those unique and unusual tastes to themselves? (That's my impression, at least).

Eating Curry Without Rice?!

When I first heard the phrase "L5-band fully independent search," the idea of a person who eats curry sauce on its own came to mind. But why do such a thing?!　Dual-band GNSS receivers use both L1-band signals (the rice) and L5-band signals (the curry sauce); why would one want to use the L5 curry alone?　However, after talking to an engineer involved in this technology's development, I came to better understand the reasons behind the inclusion of independent L5-band search functionality and the circumstances surrounding it.
The L5-band fully independent search reminds me of a local dish in Hokkaido: soup curry, which can be eaten as soup on its own and contains dashi broth and various ingredients simmered together to enrich the flavor. Rather than dwelling endlessly on curry, let's take a look at FURUNO ELECTRIC's latest GNSS receiver module, the GT-100.

A Rising Trend in Dual-band Technology

As I discussed in a previous article, mulitiband GNSS, using signals on multiple frequency bands to compensate for signal error originating in the ionosphere, and also to eliminate the effects of multipath propagation in built-up urban areas, is highly beneficial. One rapidly rising trend in communications and positioning is the inclusion of dual-band GNSS functionality in a wider range of devices, even smartphones. This feature was previously found only in certain high-priced, multiband-compatible receivers used for survey and other professional operations.

In September 2022, FURUNO released the GT-100, their first dual-band-compatible time-synchronization GNSS receiver module. By offering compatibility with signal receipt in both the L1 (1575.42 MHz) and L5 (1176.45 MHz) bands, this product achieves high-level performance.

I talked with Shinya Kowada (Development Department, System Products Division, FURUNO), who was involved in the GT-100's development. He told me the following:

"Urban areas are exposed to a wide range of factors that degrade precision in operations such as positioning and time synchronization. These include multipath propagation, as well as intentional interference such as jamming and spoofing. We pursued development [of the GT-100] based on the goals of avoiding or minimizing the effects of such interferences while ensuring certain precision levels and reliable product operation over the long term. By doing so, we hoped to reduced labor and costs related to maintenance, malfunction responses and so forth, making 'reliability' itself a product function in the wider sense of the word."

Unique FURUNO Dual Band

Regarding the major benefits offered by dual-band compatibility, Kowada states resolutely, "As part of FURUNO, we felt a responsibility to tackle this challenge.

Dual band means compatibility with both L1- and L5-band signals. Using multiple frequency bands enables compensation for ionospheric effects on signals, and this boosts overall performance. However, some might be confused as to why we chose to also include L5-band fully independent search functionality when dual-band compatibility is already included. Well, we did this to further improve the receiver's reliability and robustness."

As the term implies, L5-band fully independent search entails completely independent signal searching without relying on L1 signals. To those who know about the workings of GNSS in particular, going out of one's way to devote resources specifically to this L5 band function may seem baffling.

"Normally, GNSS positioning starts with initial acquisition of the L1-band positioning signal—L1 C/A—then proceeds to acquisition of other positioning signals based on retrieved satellite information, and finally to calculations," says Kowada. "In vehicular applications, a strong focus is placed on the TTFF [time to first fix], so algorithms are used to facilitate acquisition of L1-band signals first followed by L5-band signals in order to shorten the TTFF. However, when attempting to acquire L5 signals from the outset without first acquiring L1 signals, the chip rate [coding frequency for positioning signals] is 10 times that of L1, meaning that roughly 10 times the amount of time is required. Simultaneously searching for both L1 and L5 signals would require even more time. In the field of mobility products, where shaving even 0.1 seconds off of a TTFF matters, such an approach is out of the question.

"The process of serving up the rice, or L1 band, first before adding the curry sauce, or L5 band, is something most people in the field simply accept unquestioningly as the established norm. Buy it's not a specific rule set in stone.
However, in timing-related operations where sustained operations over the long term is the primary focus, there are fewer limits on TTFFs, which makes this type of parallel search approach more useful. When some environmental condition or circumstance prevents the receiver from acquiring L1 signals, if the product being used is a GT-100, then it will be able to carry out a fully independent L5-band search, obtain appropriate positioning information, and preserve precise timing results. Of course, when outside factors no longer interfere with L1 receipt, the receiver automatically reverts back to dual-band mode using both L1 and L5. Therefore, rather than simply waiting idly for L1 signals to come in as other receivers might do in this type of situation, the GT-100 provides a means of continuing operations."

Japanese people often say curry tastes best after sitting for a night and then being reheated. The above example is similar to a situation wherein someone heats up their leftover curry sauce only to realize that they forgot to cook the rice. In this case, they could take the less-orthodox approach of enjoying that curry sauce as Hokkaido-style soup curry—in other words, using the L5-band fully independent search.

L5-Band Fully Independent Search Improves Jamming Robustness

When thinking about environmental conditions or circumstances that prevent L1 signal receipt, one thing that immediately springs to mind are the jamming and spoofing issues which have become a serious problem in Europe.
Kowada elaborates: "Using L5-band fully independent search is one effective way to improve receiver's robustness. Countless reports of L1-band jamming and spoofing have come in, but no example of intentional interference with L5-band signals has been reported thus far."
In other words, the L5 band is still safe, which is why FURUNO ELECTRIC went out of their way to include the L5-band fully independent search function in the GT-100 receiver—a product intended not for vehicular use, but for timing applications. However, as with computer viruses and other forms of cyberattack and interference, perpetrators come up with new ways of interfering with signals all the time. This cannot be avoided, and new countermeasures must be developed each time. For the moment, though, we are a step ahead of the hackers and attackers in regard to the L5 band.

Kowada does add, however, that it is important to avoid raising one's expectations too high or developing misunderstandings when it comes to L5-band fully independent search.
"Take, for example, a twin-engine passenger airliner. Even if one engine fails, it is still possible to continue flying for several hours and make a safe landing. Regardless, no pilot would ever turn off one engine intentionally and fly that way on purpose when both engines are perfectly functional. With the GT-100, if dual-band receipt using both L1 and L5 are possible, that method provides the best possible performance and thus is used. The L5-band fully independent search function is only an emergency measure to be called upon when necessary if the situation arises, just like a plane flying on one of its two engines. It was probably a bold and eccentric idea to include this functionality in a receiver intended fully for timing-related applications, but if anyone is capable of pulling that off, it's FURUNO [laughs]."

It seems that Kowada still has a lot he wants to say on the topic—regarding their achievement of sufficient precision using just single band, for example, or their realization of the world's lowest signal jitter. I hope to find more time to sit down and talk with him about these things as well his ideas for future developments.
For more information on the current state of the GT-100, I encourage you, the reader, to take a look at its 24-page technical white paper written by Kowada himself: Dual Band GNSS Receivers: Safeguarding Critical Infrastructure (download form). And in addition, Kowada shared with me that he preferd soup curry with rice. So much for my analogy!