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It used to be the case that the signal used by civilian GPS units were intentionally degraded, so that they would not have the precision of the military GPS frequency and thus couldn't be used to threaten the US military establishment. This was termed in a somewhat more politically correct manner as “selective availability." Intentional degradation was standard practice by the government for a number of years, until President Clinton decided to discontinue intentional degradation in 2000 so that civilians could enjoy the same GPS accuracy as the military.
Another source of jamming, is somewhat less endemic, is GPS signal jamming, which basically functions to degrade the signal to the point that it just can't be used within a certain area. While this hasn't been made commonplace yet, its use is on the rise, whether in the war zone to prevent the enemy taking advantage of their superior technology or to just prevent tracking by a third party such as an employer, a government investigator or even a spouse. The legality of this practice has been met with some dispute.
In our increasingly electromagnetically noisy world, degradation may occur just by merit of being near lots of signal-happy devices. Malfunctioning devices may also create noise that can degrade GPS signals, such as on a mass scale. An example of this happened in Moss Landing, California, where malfunctioning TV signal amplifiers knocked out GPS signals for an entire harbor.
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Your surrounding environment can have a big impact on signal degradation as well. If you don't have a clear view of the sky, chances are, the signal is not going to be getting to you in an undegraded form. This may happen most conspicuously if you are either indoors, underground or underwater, where the signal may not be able to get to you at all.
Scrambling of the signal may also occur. For instance, in an urban environment, you are often surrounded by a maze of shiny, reflective surfaces, which can bounce the signal and degrade it terrifically. A forest is another place where this may happen, just as being in the bottom of a valley. This type of degradation is known as signal multipathing.
Even being near metallic objects, such as the defrosters or tinted windshields in your car, can create a sort of Faraday Cage that will further degrade the signal.
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Clouds and precipitation in general, as seemingly unsubstantial as such a scrambling effect may be, may have a subtle effect on possible degradation, however, it is not typically noticeable on civilian-grade GPS units and so should not be of concern.
Space weather, however, does matter. When the particles from solar flares hit the Earth's protective magnetic field, the event which also causes beautiful auroras can also create magnetic storm conditions that can degrade a GPS signal, a process known as scintillation.
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Other Sources of Signal Error
Of course, there are other things that may be preventing you from getting the most accurate read that you can. Your GPS receiver may not be of particularly high quality, or the internal clock may not be as precisely timed as it needs to be. The positioning of the satellites also matters, as if they are clumped together or in a line relative to you, signal triangulation simply will not be effective.