**7. GPS errors**

GPS signals and measurements are prone to many disturbance factors commonly known as GPs errors. The first error source is due to the drift of both the satellite and receiver clocks. Despite their high level of accuracy, satellite clocks still drift slightly from GPS time. For affordability reasons and size, receiver clocks are usually much cheaper; consequently, they drift from GPS time rapidly. This drift translates into significant range errors in receiver measurements.

Once it departs the satellite antenna, the GPS signal needs to travel thousands of kilometers to reach to the receiver antenna and then the receiver circuitry. The first and longer part of this trip is by space where the signal maintains its characteristics. However, when the signal enters the atmosphere, this medium causes some unwanted effects. The two primary layers of the atmosphere, namely, ionosphere and troposphere, respectively, will add delays to the signal transit time and, hence, cause some errors in the measurements.

Once it nears the receiver antenna, the signal usually experiences reflections and echoes, i.e., it often bounces off objects near the receiver causing it to hit the antenna from different directions—a phenomenon known as multipath. Multipath is one of the major sources of errors, which harms GPS signals [6]. All aforementioned disturbances are a result of the nature of the signal or the propagation medium and are considered unintentional. Intentional signal degradation or replacement is, in many cases, a more problematic source of GPS errors. One major type of intentional errors is signal jamming. Signal jamming is deliberate interference caused by broadcasts of radio frequency (RF) signals around the receiver neighborhood with the aim of preventing the tracking of true GNSS signals.
