First, one of the basic assumptions we've been
using throughout this tutorial is not exactly true. We've been saying that you calculate
distance to a satellite by multiplying a signal's travel time by the speed of light. But
the speed of light is only constant in a vacuum.
As a GPS signal passes through the charged particles of the ionosphere
and then through the water vapor in the troposphere it gets
slowed down a bit, and this creates the same kind of error as bad clocks.
There are a couple of ways to minimize this kind of error. For one thing we can predict
what a typical delay might be on a typical day. This is called modeling and it helps but, of course, atmospheric conditions
are rarely exactly typical.
Another way to get a handle on these atmosphere-induced errors is to compare the
relative speeds of two different signals. This "dual
frequency" measurement is very sophisticated and is only possible with advanced
receivers.
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