Figure 1-12.—Satellite ranging.
Navigation Set Clock Error
GPS navigation sets determine distance to a
satellite by accurately measuring the time difference
between satellite signal transmission and when the
navigation set receives this signal. This difference in
time is directly proportional to the distance between
the satellite and the receiver. Therefore, the same
time reference must be used by both the receiver and
the satellite.
The clock in the GPS receiver in not nearly as
accurate as the atomic clock in the satellite. This
causes the receiver and satellite clocks to be slightly
out of sync,
which in turn causes the time
measurements to be inaccurate. The error is further
compounded by the distance calculation, so the
position of the navigation set cannot be accurately
determined.
The navigation set compensates for these errors
by using the distance measurement from a fourth
satellite to calculate the clock error common to all
four satellites. The navigation set then removes the
clock error from the distance measurements, and then
determines the correct navigation set position.
Signal Delay and Multipath Reception
Two types of atmospheric delay can affect the
accuracy of navigation set signal measurements. The
first is tropospheric delay. Tropospheric delay can be
accurately predicted; the prediction is included in the
almanac data.
The second type of delay is caused when the
satellite signal passes through the ionosphere. This
type of signal delay is caused by the ionosphere being
thicker in some areas and by satellite signals received
from nearer the horizon having to pass through more
of the ionosphere than those received from directly
overhead. Ionospheric delay will phase shift the
lower satellite transmission frequency, L2-RF, more
than the higher frequency, L1-RF. The navigation set
measures ionospheric delay by measuring the phase
shift between these two signals and then uses this
computation to compensate for the ionospheric delay.
Multipath reception is caused by a satellite signal
reflecting off of one or more objects. This causes the
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