Shawn, the picket fencing is caused by multi-path. You receive the signal directly from the transmitter as well as the signal reflected from the airplane. As the airplane moves, the distance from the plane to the transmitter as well as the plane's distance from you changes. Because the path lengths are not the same, the phase relationship between the two signals are different. As the distance between the plane, the transmitter and you change, the phase relationship changes as well. If the two signals are less than 90 degrees out of phase, the two signals will reinforce each other and you will receive a slightly stronger signal. If they are more than 90 degrees out then the signals will tend to cancel each other and you receive less signal. It is this constant change in received signal strength, caused by the changing phase relationship, that causes the picket fencing. The passive radar referred to in the Wikipedea article uses the time difference between the two signals, the direction of the reflected signal and the Doppler shift of the reflected signal to measure the location, distance and speed of the object, not the picket fencing caused by the object.
