The new method is important because the balloons are extremely difficult to track without it – they are slow-moving and can fly extremely high, in some cases as high as much as 60 miles up, and can evade the more traditional radars that are oriented toward detecting fast-moving missiles and counterterrorism threats, a lawmaker familiar with the intelligence told CNN.

Source: China spy balloon: US developed method to track balloon fleet within the last year, sources say | CNN Politics