GPS requires a direct line of sight between the receiver and the satellite. When an object lies within the direct path, accuracy suffers due to reflections and weakening of signals. This is particularly problematic in urban environments, within valleys and on mountain slopes. In all three situations, the objects (buildings and the Earth itself) are substantial enough to completely block the GPS signals. When weak signals are received, they may have been reflected off buildings and the surrounding landscape. Reflections generate
multi-path signals arriving with a small time delay at the receiver. This results in inaccurately calculated position.
Even when the object is less substantial (tree cover, car roof, your body), reflection and weakening of signals may still occur. This can sometimes be observed when viewing your recorded GPS track logs on top of aerial imagery. In the image on the left, the true position of the footpath follows the shadowy area in the forest. However, as the GPS receiver enters the forest (walking from east to west), it can be observed that reflections cause the recorded track to incorrectly shift slightly to the south.
When carrying a GPS device, generally, the higher the antenna is fixed, the better the reception. Good positions include the shoulder strap or the top pocket of a backpack, mounted on top of a cycle helmet, or a roof antenna on a car.
