User's Guide

Teletrac, Inc. - Prism TM Information and Installation Guide
12 1622-0300 B1 3/18/04
GETTING THE MESSAGE OUT
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.
Step 5: Correcting Errors
ROUGH TRIP THROUGH THE ATMOSPHERE
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.