The US Federal Aviation Authority is telling aircraft to steer a few hundred miles clear of the site of a US military base in California, where secretive weapon tests being conducted will cause GPS outages for much of the West Coast.

An advisory issued by the FAA on Saturday notifies pilots that GPS systems will be unreliable or nonexistent within a 253 nautical mile radius from Naval Air Weapons Center at China Lake in the Mojave Desert, between 9:30 a.m. and 3:30 p.m. Pacific Time on six dates in June.

More than half of California is affected for all aircraft flying above 50 feet, including the very busy airspace of Los Angeles and the San Francisco Bay Area, as well as the southern two-thirds of Nevada. Planes that fly higher will be affected at longer range, with aircraft that reach 40,000 feet above sea level experiencing disruptions as far away as Oregon.

The US military didn’t give any details on exactly what kind of test could be causing such a far-reaching disruption.

“We’re aware of the flight advisory,” Deidre Patin, spokesperson for Naval Air Warfare Center Weapons Division, told Gizmodo. “I can’t go into the details of the testing. It’s general testing for our ranges.”

The FAA is also warning pilots flying Embraer Phenom 300, a popular private jet, that testing could interfere with the craft’s “flight stability controls” in addition to GPS systems.

The UK issued its own signal-jamming advisory, warning that their tests will affect the Scottish Hebrides intermittently during the month of July.

GPS jamming technology has existed for years, and can even be obtained by consumers for relatively little cost. The sheer size of the area affected by whatever the Navy is testing, however, is perhaps without public precedent.