Teletrac Prism TM, Информация по установке

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Teletrac, Inc. - Prism TM Information and Installation Guide
46 1622-0300 B1 3/18/04
G
ETTING THE
M
ESSAGE
O
UT
Once the DoD has measured a satellite's exact position, they relay that information back up to the
satellite itself. The satellite then includes this new corrected position information in the timing
signals it's broadcasting.
So a GPS signal is more than just pseudo-random code for timing purposes. It also contains a
navigation message with ephemeris information as well.
With perfect timing and the satellite's exact position you'd think we'd be ready to make perfect
position calculations. But there's trouble afoot. Check out the next section to see what's up.
Step 5: Correcting Errors
Up to now we've been treating the calculations that go into GPS very abstractly, as if the whole
thing were happening in a vacuum. But in the real world there are lots of things that can happen
to a GPS signal that will make its life less than mathematically perfect.
To get the most out of the system, a good GPS receiver needs to take a wide variety of possible
errors into account. Here's what they've got to deal with.
R
OUGH
T
RIP THROUGH THE
A
TMOSPHERE
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.