Sources of GPS signal errors

Sources of GPS signal errors
Factors that can degrade the GPS signal and thus
affect accuracy include the following:
Ionosphere and troposphere delays - The
satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in
model that calculates an average amount of
delay to partially correct for this type of error.
Signal multipath - This occurs when the GPS
signal is reflected off objects such as tall
buildings or large rock surfaces before it reaches
the receiver. This increases the travel time of the
signal, thereby causing errors.
Receiver clock errors - A receiver's built-in clock
is not as accurate as the atomic clocks onboard
the GPS satellites. Therefore, it may have very
slight timing errors.
Orbital errors - Also known as ephemeris errors,
these are inaccuracies of the satellite's reported
location.
Number of satellites visible - The more satellites
a GPS receiver can "see," the better the
accuracy. Buildings, terrain, electronic
interference, or sometimes even dense foliage
can block signal reception, causing position
errors or possibly no position reading at all. GPS
units typically will not work indoors, underwater
or underground.
Satellite geometry/shading - This refers to the
relative position of the satellites at any given
time. Ideal satellite geometry exists when the
satellites are located at wide angles relative to
each other. Poor geometry results when the
satellites are located in a line or in a tight
grouping.
Intentional degradation of the satellite signal -
Selective Availability (SA) is an intentional
degradation of the signal once imposed by the
U.S. Department of Defense. SA was intended to
prevent military adversaries from using the
highly accurate GPS signals. The government
turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.