A radar station, Located at the origin of x y plane, as shown in (Figure 1). detects an airplane coming straight at the station from the east. At first observation (point A). the position of the airplane relative to the origin is RA. The position vector RA has a magnitude of 360 m and is located at exactly 40° above the horizon. The airplane is tracked for another 123° in the vertical east-west plane for 5.0 s, until it has passed directly over the station and reached point B. The position of point B relative to the origin is RB (the magnitude of RD is 880 m . The contact points are shown in the diagram, where the $x$ axis represents the ground and the positive y direction is upward.