Satellite positioning (also known as GPS or GNSS) works by using timestamps broadcast by satellites.
GNSS receivers use timestamps from at least 4 satellites, combined with the known satellite position in space, to calculate the receiver’s position on earth.
This works perfectly when the satellite signal is unobstructed - in direct Line Of Sight (LOS).
However, most GNSS-enabled devices these days are used in more developed areas, where satellite signals can be reflected or obstructed by objects such as buildings, vehicles, landmasses and even vegetation. When receivers pick up a reflected or Non Line of Sight (NLOS) signal, the timing message will be older than it should be (echos always arrive late) and so the subsequent estimation of distance to the satellite will be too long, resulting in an inaccurate positioning fix. Even if the line of sight signal is available, any reflected copies can interfere with it when they are all picked up by the receiver together, corrupting the true measurement, making it look either early or late through distortion of the signal. This problem, known as multipath interference, is especially problematic in cities and towns.
Multipath interference and NLOS signals can result in positioning fixes in cities being wrong by over one hundred metres.