Teletrac Prism TM, Информация по установке

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Teletrac, Inc. - Prism TM Information and Installation Guide
48 1622-0300 B1 3/18/04
If it picks satellites that are widely separated the circles intersect at almost right angles and that
minimizes the error region.
Good receivers determine which satellites will give the lowest GDOP.
I
NTENTIONAL
E
RRORS
!
As hard as it may be to believe, the same government that spent $12 billion to develop the most
accurate navigation system in the world is intentionally degrading its accuracy. The policy is
called "Selective Availability" or "SA" and the idea behind it is to make sure that no hostile force
or terrorist group can use GPS to make accurate weapons.
Basically the DoD introduces some "noise" into the satellite's clock data which, in turn, adds noise
(or inaccuracy) into position calculations. The DoD may also be sending slightly erroneous orbital
data to the satellites which they transmit back to receivers on the ground as part of a status
message.
Together these factors make SA the biggest single source of inaccuracy in the system. Military
receivers use a decryption key to remove the SA errors and so they're much more accurate.
The Bottom Line
Fortunately all of these inaccuracies still don't add up to much of an error. And a form of GPS
called "Differential GPS" can significantly reduce these problems.