GPS uses a *constellation of satellites* that send out GPS signals, which are received by a compatible GPS receiver. While the U.S. GPS system is most frequently referred to, other systems, including the Russian **GLONASS** system and smaller regional systems, also exist. GPS navigation can help position devices to within a *foot of their actual position*, allowing highly accurate placement for geofencing and other GPS uses. GPS also provides a consistent time signal, meaning that *GPS receivers may be integrated into network time systems*. Like other radio frequency–based systems, *GPS signals can be jammed or spoofed*, although attacks against GPS are uncommon in normal use. GPS jamming is illegal in the United States, but claims have been made that GPS spoofing has been used to target military drones, causing them to crash, and real-world proof-of-concept efforts have been demonstrated. GPS technology is a **major part of geolocation** capabilities used to determine where a device is. Geolocation is used for location-aware authentication, geofencing, and many other functions. GPS is *often combined with other location-centric data like Wi-Fi network names and Bluetooth connections*. This can provide rich data about the location of devices and is increasingly leveraged by device manufacturers.