Teletrac Prism TM, Информация по установке

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Teletrac, Inc. - Prism TM Information and Installation Guide
1622-0300 B1 3/18/04 47
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.
R
OUGH
T
RIP ON THE
G
ROUND
Trouble for the GPS signal doesn't end when it gets down to the ground. The signal may bounce
off various local obstructions before it gets to our receiver.
This is called multipath error and is similar to the ghosting you might see on a TV. Good receivers
use sophisticated signal rejection techniques to minimize this problem.
P
ROBLEMS AT THE
S
ATELLITE
Even though the satellites are very sophisticated they do account for some tiny errors in the
system.
The atomic clocks they use are very, very precise but they're not perfect. Minute discrepancies
can occur, and these translate into travel time measurement errors.
And even though the satellites positions are constantly monitored, they can't be watched every
second. So slight position or "ephemeris" errors can sneak in between monitoring times.
S
OME
A
NGLES ARE
B
ETTER
T
HAN
O
THERS
Basic geometry itself can magnify these other errors with a principle called "Geometric Dilution of
Precision" or GDOP. It sounds complicated but the principle is quite simple.
There are usually more satellites available than a receiver needs to fix a position, so the receiver
picks a few and ignores the rest.
If it picks satellites that are close together in the sky the intersecting circles that define a position
will cross at very shallow angles. That increases the gray area or error margin around a position.