Ideal conditions for GPS usage are a clear view of the sky with no obstructions from about 5 degrees elevation and up. Any obstructions in the area of the GPS antenna can cause a very significant reduction in accuracy. Examples of interfering obstructions include: buildings, trees, fences, cables etc. It is usually impossible to get a GPS signal when indoors due to the signal being absorbed by the building. Mobile phones with GPS which seems to receive a GPS signal indoors get their position by triangulating the signals from the cellular network and not from the GPS receiver.

Obstructions may have the following effects thereby reducing accuracy:

• Reduced number of satellites seen by the receiver.
• Reduced strength of satellite geometry.
• Satellite signal multipath.
• Corruption of GPS measurements.

Satellite signal multipath is caused by GPS signals being reflected from surfaces near the GPS antenna that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. In order to get an accurate measurement from a GPS satellite, it is necessary that the signal from the GPS satellite travels directly from the satellite to the GPS antenna. If the signal has been reflected off of another surface prior to being received at the antenna, its length will be greater than was anticipated and will result in positioning error. Multipath is difficult to detect and sometimes hard to avoid.