Some of the balloons floated by North Korea to dump trash across South Korea have been equipped with GPS transmitters, South Korea’s military said Sunday.

According to South Korea’s Joint Chiefs of Staff, GPS transmitters were found in some of the North Korean balloons, and Pyongyang may be using them to look into the balloons’ flight data, Yonhap News Agency reported.

A South Korean military official said the military is “tracking North Korea’s possible development of balloon flotation technology and preparing for contingencies.” So far this year, North Korea has sent about 6,000 trash balloons to South Korea, according to the South Korean military.

North Korea is estimated to have spent 600 million won ($444,148) on the balloon campaign, according to the Joint Chiefs of Staff.

Source: Qatar News Agency