User's Guide

Teletrac, Inc. - Prism TM Information and Installation Guide
1622-0300 B1 3/18/04 13
ROUGH TRIP ON THE GROUND
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.
PROBLEMS AT THE SATELLITE
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.
SOME ANGLES ARE BETTER THAN OTHERS
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.
Commonly refered to as HDOP.
If it picks satellites that are widely separated, the circles intersect at almost right angles and that
minimizes the error region.