This June, the military is conducting experiments at the Naval Air Weapons Center in the Mojave Desert that could affect planes' GPS, the FAA warned pilots. What kind of tests? Well, we don't exactly know. The Notice to Airmen (NOTAM) only described it as "GPS Interference Testing" and didn't go into details -- the Weapons Center also refused to reveal anything to Gizmodo, telling the publication that it's "general testing for [their] ranges."
They could be trying out defensive measures against enemy planes and drones... or it could be something more mysterious that we'll likely never be able to confirm. Whatever the military is doing already began on June 7th and is centered around the Navy's 1.1 million acre installation in China Lake. The jammers they're taking for a spin could affect anything 50 feet and above, so you won't feel their effects on the ground. As you can see below, the area they cover is shaped like an ice cream cone: the higher you go, the wider it is. At 40,000 feet above sea level, the devices can jam GPS equipment up to the California-Oregon border.
While the Navy warned all pilots of possible outages, it's specifically requesting Embraer Phenom 300 business jets to avoid affected locations for six days this month. The next tests after June 7th will take place on June 9th, 21st, 23rd, 28th and 30th, starting from 9:30AM up until 3:30PM Pacific time.