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Even in South Korea, it is damaged by such jamming radio waves.

It was a day after noon in San Diego, California. When the airport controller tried to check on the monitor for planes entering the airport, he noticed that the system was not working properly. Around the same time, pagers used for emergency calls to doctors stopped working at the Naval Medical Center. Port traffic management systems also became unavailable. In town, people trying to use their cellphones noticed that the radio waves weren't working, and banks couldn't withdraw cash from ATMs. The turmoil lasted for two hours.

The mysterious incident took place in January 2007 and it took three days to find out the cause. The reason is the Navy's training in the Port of San Diego. The Navy is blocking radio signals in an attempt to test what to do if communications are lost. At the time, I inadvertently blocked the radio signals sent by GPS satellites to certain areas of the city.

So why is there such confusion when the signal from GPS is blocked?

In fact, GPS satellites no longer just send location information to car navigation systems. GPS has become an invisible utility that we unknowingly rely heavily on. Mobile phone companies use GPS time information to connect mobile phones to their cell towers. Power companies also use GPS to synchronize multiple grids. Banks and stock exchanges use it as a timestamp to prevent fraud. Society's dependence on GPS is increasing year by year.

Some worry that they may be overly reliant on vulnerable technology. You don't need to do anything special, like naval training, to trigger what's happening in San Diego. All you need is a GPS jammer.

A GPS jammer is a plastic device sized to fit on a car dashboard. It can be purchased online, and can be used, for example, by truck drivers who don't want their bosses to monitor their whereabouts. The machine has caused problems at airports, and in some cities, cell phone signals have been blocked. A single jammer can block GPS for several kilometers. As a result, researchers around the world are digging their wits to prevent GPS jamming.

・"Weak signal" is a weakness

GPS is powered by radio signals from satellites. It is transmitted by the NavStar network, a group of US military satellites. At least 24 satellites are always in operation, and at least 4 are visible from anywhere on Earth.

Each satellite always transmits its position and the time measured by the onboard atomic clock. The GPS receiver compares it to its own clock and calculates the distance to each satellite. The GPS receiver locks onto at least four satellites and calculates the time difference to find the exact position. Currently, many receivers use GPS as an inexpensive and accurate time mechanism.

"The problem is that the GPS signal is very weak, like the headlights of a car 20,000km away," said David Last, former director and adviser to the Royal Academy of Navigation. However, with the satellite's limited energy supply, it is difficult to make the signal stronger.

Finally, experience first-hand how easy it is to block GPS signals and how it affects current technology. In 2010, he carried out experiments aboard a 500-ton ship, the THV Galathea, in the North Sea. Galathea is the pride of the fleet with the latest navigation equipment. Finally, a simple jammer emits noise at the same frequency as the GPS satellites to see what Galathea can do without GPS.

As soon as Mr. Rust activated the jammer, the boat went crazy. Looking at the display of the navigation system, the Galassia suddenly appeared, flying over northern Europe and Ireland at Mach speed. The alarm goes off, the ship's navigation backs up, and the gyrocompass crashes. This is because the gyrocompass is calibrated using GPS. The same goes for radar. Even communication with the ship's satellites was lost. This is because GPS is also used to point the antenna in the right direction. "The staff were well trained and explained so I knew exactly what was going on," Last said. "But like us, they were surprised."

・The driver's trick is an accident

Finally tried a simple jammer on the market. The US, UK and other countries also ban their use, but these low-tech devices can be bought online for as little as $30. The seller claims it is to protect privacy. It blocks devices that record the car's location, making it popular with truck drivers who don't want to know where they are. You can also use this jammer to block radio waves from in-vehicle units for road tolling purposes. It can also be used to disable trackers attached to cars stolen by criminals. "Originally, I thought the jammer might have been secretly assembled by a young man in his bedroom," Rust said, "but now it's being made in a factory in China."

Last worries that GPS jammers will cause the same damage on the ground as Galathea, and he's not the only one worried. In November 2010, NASA's "Space-Based Positioning, Navigation, and Timing" executive committee warned that if a jammer was activated in a city, it could be catastrophic. I don't know how much there is in the market, but committee members pointed out that the risk is growing. In the future, a device called a "spoofer" could subtly alter the information received by GPS receivers, which could make the problem worse.

In an incident last year at Newark International Airport in New Jersey, just one jammer proved to be enough to cause chaos. Airport controllers have just installed a new GPS-based landing system to allow aircraft to approach in poor visibility. However, the system would shut down once or twice a day, and it would take months to find out why. The reason was a portable gps signal jammer that a driver used to spoof a highway toll booth near the New Jersey Turnpike. The driver passes by the place twice a day, and each time he shuts down the airport system.

Future airport control systems won't work without GPS, but so will railways. The Federal Railroad Administration is trying to use GPS as the centerpiece of its railroad management program. GPS is also becoming increasingly important as police and fire departments arrive on the scene.

・GPS is an invisible utility

Not only is it impossible to navigate when GPS is not available. "We're unknowingly relying on GPS," said Donald Jewel, editor of GPS World magazine, who worked on building the U.S. Air Force's GPS system. According to him, there are more than 1 billion GPS receivers in operation today, but more than 90% of them are only used to receive accurate time data from satellites...

Cell phones are a prime example of this usage. In a radio tower, as the user moves, it is handed over to another tower and therefore needs to be synchronized with each other. GPS time signals are an inexpensive and reliable method. The time adjustment of each tower is also used to identify each other. In fact, many wireless communication devices use GPS time adjustment for synchronization. Maybe that's what caused the turmoil in San Diego in 2007.

·time is money

GPS time adjustments are also used as time stamps for financial transactions such as stocks. ATM can also communicate wirelessly. This is because it uses a time-based password that requires synchronization. The cause of the ATM outage during the San Diego accident is still unknown, but it may be related.

Electric utilities also use GPS time to manage their grids. If the frequency periods of multiple supplies do not match, the supplies will partially cancel each other, reducing efficiency. Having an accurate time signal allows you to pinpoint the start of each cycle. For example, the U.S. grid needs to synchronize more than 5,000 power sources. However, in 2006, sunspot activity temporarily disabled GPS, making it impossible to know the destination of the power supply, resulting in billing errors. This means that there may be power outages due to GPS errors.

Given the possibility of these problems, the law is working to crack down on GPS jammers. In February of this year, the FCC (Federal Communications Commission) announced that it would impose fines on sellers and owners of jammers. The problem for authorities, however, is that most sellers are in East Asia, and the law only covers the use of jammers, often not even possession.

・ Towards a safety net

Against this backdrop, navigation researchers are demanding GPS backups, and the UK's National Physical Laboratory and others are taking steps to respond.

Fortunately, backups are already with us, and the idea has been around since the 1940s. Like GPS, it provides navigation and accurate time notification, Enhanced LORAN (eLORAN).

Basic LORAN (LORAN stands for Long Range Navigation) is similar to GPS but uses terrestrial radio signals instead of satellites. It doesn't have global coverage, but it can be better than GPS. LORAN is much stronger in wavelength than GPS signals and almost impossible to interfere.

Its new version of eLORAN uses a more reliable transmitter and an improved cesium atomic clock. The software has also been modified, the error is about 10 meters, and the time is almost as accurate as GPS. In future receivers, users may even switch to eLORAN without realizing it, Last said.

In Europe, a team from the UK Lighthouse Directorate is testing eLORAN and recommending its expansion to the UK government. But across the Atlantic in the US, the current LORAN service is about to cease. So far, the U.S. government has rejected all proposals to invest in eLORAN. It costs about $20 million a year, which is less than launching a single GPS satellite. "We want people in government with vision and technical knowledge to understand it," Jewel said.

Also, GPS signals may not be needed in the next 20 to 30 years. If atomic clocks were cheaper, they could be embedded in the device itself, which requires accurate time. As for the navigation function, the IMU (Inertial Measurement Unit) eventually tracks the movement from the starting point without any external signal. Currently, the IMU uses a gyroscope to measure orientation and an accelerometer to measure velocity. By using this information with time information, the acceleration is converted to velocity and distance, and the relative position is calculated.

Currently, IMUs are bulky and expensive, with a deviation of 1.5 kilometers per hour of travel. But DARPA plans to use microchip-sized atomic clocks and similar small, precise accelerometers to boost performance.

While such safety nets have been rolled out, the number of generations living without GPS unaware is increasing, as is the number of jammers. Beyond that, confusion caused by GPS outages like San Diego's is likely to be more frequent.

GPS looks fragile, but unknowingly becomes a living infrastructure. It seems necessary to take urgent legal and technical measures.