During WWII, the Germans were using radio beacons (the "Knickebein" system) to guide their bombers into English territory. They set up two beacons, one in Kleve, a city in western Germany, and one at Stollberg Hill (North Frisia). The two radio beams intersected over Derby. Source: https://de.wikipedia.org/wiki/Knickebein_(Funkfeuer) A Flat Earther has claimed that this would be impossible on a globe earth (see here, timestamp: 6:46). This is his main argument: the knickebein beacons relied upon line-of-sight propagation of radio beams. The distance between Kleve and Derby or Stollberg and Derby however is simply too great to allow for line-of-sight propagation on a globe earth. So this is his central claim: (timestamp: 7:54). I can't find any solution to this. According to William CY Lee, the radio horizon (i.e. the service range of the beacons) can be calculated as R= √2ha + √2hb where R is the distance in miles, ha is the aircraft altitude and hb is the ground-station antenna height in feet. If we plug in the numbers (say, the height of the beacon in Kleve (239ft) and the flight height of the German bombers), we get a maximum beam range of 217,8 miles. This means that the German bombers would have lost the transmission signal of the Kleve Knickebein tower after traveling ~218 miles. However, most British cities were more than 300 miles away. So how could these beams have reached the German bombers over Derby? Is the Earth flat afterall? What are your thoughts?