User Manual

1
Overview
8
CrossCheck CDPD Mobile Unit Manual
1.4
The Global Positioning System
The Global Positioning System (GPS) is a satellite-based navigation
system operated and maintained by the U.S. Department of Defense.
GPS consists of a constellation of 24 satellites providing world-wide,
24-hour, three-dimensional (3D) coverage. Although originally
conceived for military needs, GPS has a broad array of civilian
applications including timing, surveying, fleet management, marine,
land, aviation, and vehicle navigation.
GPS is the most accurate technology available for navigation. As a
satellite-based system, GPS is immune from the limitations of land-
based systems, which have limited coverage and whose accuracy
varies with geographic location and, even under ideal conditions,
cannot compare with GPS.
By computing the distance to GPS satellites orbiting the earth, a GPS
receiver can calculate an accurate position. This process is called
satellite ranging. GPS receivers can also provide precise time, speed,
and course measurements which are important for vehicle mobile
positioning and communications applications.
1.5
GPS Receiver
The CrossCheck CDPD includes an advanced GPS receiver, which
provides the position, course, speed and time information required for
AVL and fleet management applications. A brief overview of the GPS
receiver’s architecture and operation is provided in the next paragraph.
The CrossCheck CDPD’s GPS receiver features an eight-channel
digital signal processor (DSP) which operates at the GPS L1
frequency (1575.42 MHz) and processes the Coarse/Acquisition (C/A)
code portion of the GPS signal. The RF and digital signal-processing
components of the GPS module are custom integrated circuits
designed by Trimble.