The whole concept of GPS relies on the
idea that a GPS signal flies straight from the satellite to the receiver.
Unfortunately, in the real world the signal will also bounce around on just about
everything in the local environment and get to the receiver that way too.
The result is a barrage of signals arriving at the receiver: first the direct one, then
a bunch of delayed reflected ones. This creates a messy signal.
If the bounced signals are strong enough they can confuse the receiver and cause
Sophisticated receivers use a variety of signal processing tricks to make sure that
they only consider the earliest arriving signals (which are the direct ones).