This picture is easy to understand. The tangent of the circle (earth) made by the ground point (radar) intersects with the radius of the extension line of the earth in the air 10km. This intersection is the farthest distance for ground radar to find enemy planes. The geocentric angle formed by this point, geocentric point and radar is a, similarly, the included angle between the early warning aircraft and the farthest enemy aircraft is 2a, 2a-a=a, corresponding to (63765438.