radar horizon
distance at which the radar beam rises enough above the Earth's surface to make detection of a target at low level impossible
en-GB/ˈɹeɪdɑː həˈɹaɪzn/
en-US/ˈɹeɪˌdɑɹ hɚˈaɪzən/
distance at which the radar beam rises enough above the Earth's surface to make detection of a target at low level impossible