Leica Geosystems GPS Basics - Manual (Page 15)

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GPS Basics -1.0.0en How GPS works
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a. Satellite elevation. Signals from low
elevation satellites will be affected more
than signals from higher elevation
satellites. This is due to the increased
distance that the signal passes through
the atmosphere.
b. The density of the ionosphere is
affected by the sun. At night, there is
very little ionospheric influence. In the
day, the sun increases the effect of the
ionosphere and slows down the signal.
The amount by which the density of the
ionosphere is increased varies with
solar cycles (sunspot activity).
Sunspot activity peaks approximately
every 11 years. At the time of writing, the
next peak (solar
max) will be
around the year
2000.
In addition to this,
solar flares can
also randomly
occur and also
have an effect on
the ionosphere.
Ionospheric errors
can be mitigated
by using one of
two methods:
- The first method
involves taking an
average of the
effect of the reduction in velocity of light
caused by the ionosphere. This correc-
tion factor can then be applied to the
range calculations. However, this relies
on an average and obviously this
average condition does not occur all of
the time. This method is therefore not
the optimum solution to Ionospheric
Error mitigation.
- The second method involves using
“dual-frequency” GPS receivers. Such
receivers measure the L1 and the L2
frequencies of the GPS signal. It is
known that when a radio signal travels
through the ionosphere it slows down at
a rate inversely proportional to it’s
frequency. Hence, if the arrival times of
the two signals are compared, an
accurate estimation of the delay can be
made. Note that this is only possible
with dual frequency GPS receivers. Most
receivers built for navigation are single
frequency.
c. Water Vapour also affects the GPS
signal. Water vapor contained in the
atmosphere can also affect the GPS
signal. This effect, which can result in a
position degradation can be reduced by
using atmospheric models.