Fearing cyber attacks targeted at GPS and satellite navigation systems, ships are shifting back to World War II-era technology for their backup systems, including radio technology, Reuters reported.

Most ships currently operate without a backup navigation system and rely solely on GPS, but new risks of cyber attacks, including attempts to jam devices and disrupt communication, has shipping companies looking for reliable alternatives.

Read: GPS Terrorism: Hackers Could Exploit Location Technology To Hijack Ships, Airplanes

South Korea is reportedly developing a new system that would use an earth-based navigation technology known as enhanced long range navigation, or eLoran. The low-frequency system modernizes technology first deployed in the 1940s to provide a stable and secure navigation standard.

The United States has also taken interest in eLoran. While the U.S. government has toyed with the idea of the navigation system for more than a decade, the push for it has piqued given new cyber threats that may disrupt GPS.

Earlier this year, the Director of National Intelligence Daniel Coats warned the Senate Select Committee on Intelligence that countries including Russia, China, Iran and North Korea have been attempting to interfere with GPS signals.

“The global threat of electronic warfare (EW) attacks against space systems will expand in the coming years in both number and types of weapons. Development will very likely focus on jamming capabilities against dedicated military satellite communications (SATCOM), Synthetic Aperture Radar (SAR) imaging satellites, and enhanced capabilities against Global Navigation Satellite Systems (GNSS), such as the US Global Positioning System (GPS),” Coats said in an unclassified report.

Read: What Is Elsa? CIA Can Track Location Of Wi-Fi-Enabled Devices

The number of incidents involving GP interruptions has been on the rise in recent years. In 2016, South Korea reported hundreds of fishing ships had to return to port early after hackers jammed their GPS signals. North Korea was blamed but denied its involvement.

Earlier this year, ships operating in the Black Sea reported to the U.S. Coast Guard Navigation Center that their GPS navigation was disrupted. More than 20 ships issued similar complaints over the disruption.

Part of the problem with GPS and Global Navigation Satellite Systems (GNSS) stems from the weak signals they transmit. The systems typically transmit from 12,500 above the Earth and can be disrupted with relatively inexpensive equipment.

By contrast, eLoran is much more difficult to interrupt because of its strong signal. The average signal of the eLoran system is estimated to be 1.3 million times stronger than the average GPS signal. The trade-off is the eLoran system requires a powerful transmitter and large antenna, which would be easier to detect.

Still, the necessity of a backup system for GPS remains. About 90 percent of world trade is transported via ships, and operating without an alternative in case of GPS disruption puts those vessels at risk of veering off course or colliding with other ships.