What is the signal wrong sometimes?
 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
Defence. 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.
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