A couple of experts demonstrated how to wirelessly change the time on NTP servers over long distances using an inexpensive equipment. A remote attacker can shift time on a stratum 1 NTP server by sending forged radio time signals. The attack is very dangerous, an attacker can launch a GPS attack from a distance up to 100 meters, the same distance can be increased by amplifying the signals and reaching a distance of up to 2 km. The attacks work against NTP systems that are widely deployed in Europe, North America, and China.”]
Source: https://securityaffairs.co/wordpress/47803/hacking/hacking-ntp-servers.html