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09/25/02 GPS_field_guide 8:19 A9/P9
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There may also be sources of signal multipath (i.e on Figure 8) occurring when the GPS
signal is reflected off these objects before reaching the receiver. This increases the travel
time of the signal and creates errors of distance estimation between the satellite and the
receiver.
·
Figure 6 Examples of signal multipath
Figure 8 - Example of multipath GPS signal connected to buildings or mountains
This underlines the importance for the users to be located in the most open area as
possible before taking the measurement.
There are other sources of error over which the user does not have control, including:
· Atmosphere delays — The satellite signal slows as it passes through the
atmosphere. The GPS system uses a model that calculates an average amount of
delay to correct for this type of error.
· 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.
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