What is the meaning of the term “picket fencing”?

"Picket fencing" refers to the rapid, rhythmic fluttering sound sometimes heard on VHF or UHF signals when either the transmitting or receiving station is moving in a vehicle. It gets its name because the audio effect sounds similar to dragging a stick along a wooden picket fence while running. This phenomenon occurs because of multipath propagation. As a vehicle moves, the radio signal bounces off buildings, hills, and other nearby objects. These multiple signal paths combine at the receiving antenna, constantly shifting in and out of phase, which causes the signal strength to rapidly fluctuate. Distractors like "alternating transmissions" or "a type of ground system" sound superficially plausible but are completely unrelated to the actual acoustic flutter described by this unique term.