Justme-- don't discount completely multipath propagation as a cause for signal interference for digital transmissions. It is a huge problem in WIFI applications and your digital camera is operating near the same 2ghz frequencies. I forgot to mention that as another reason for signal degradation. The solutions get back to antenna design and implementation. Mutltipath signal propagation is enhanced while moving at highway speeds. Signal reflections off buildings, trucks, cars and other metal surfaces can be significant and cause severe interference.