Multipath interference is caused by reflected radio signals from surfaces near the
GPS receiver that can either interfere with or be mistaken for the true signal that
follows an uninterrupted path from a satellite. An example of multipath is the
ghosting image that appears on a TV equipped with rabbit ear antennas. Multipath is
difficult to detect and sometimes impossible for the user to avoid, or for the receiver
to correct. Common sources of multipath include car bodies, buildings, power lines
and water. When using GPS in a vehicle, placing an external antenna on the roof of
the vehicle will eliminate most signal interference caused by the vehicle. Using a
GPS receiver placed on the dashboard will always have some multipath
interference.