1. Introduction to GPS Global Positioning System
Understanding GPS
Buying a GPS Unit
Using a GPS Unit
Chad Sperry
WIU GIS Center

2. Understanding GPS What does GPS do?
Answers the question -“Where am I?”
How? – by computing a 3D coordinate

3. Understanding GPS Satellite Based Navigation Implemented by DoD
24 Geosynchronous Satellites in Orbit
20,200 kilometer altitude
11 hours 58 minutes orbital period
6 to 10 satellites always in view
Implemented by DoD
24/7/365
World-Wide Coverage
Operates Under “All” Weather Conditions
Prefers Line-of-Site
No Cost to End Users (FREE!!)

4. How is a GPS Signal Measured?
Understanding GPS
How is a GPS Signal Measured?
Station A
* If a train leaves Station A at 0600 and is traveling at 60 miles per hour. It arrives at Station B at How far did it travel?
Station B
Answer: Change in Time x Speed = Distance
( ) x 60 = 180 miles

5. Understanding GPS Three Main Types of GPS Consumer Grade Mapping Grade
$100-$500
Can record waypoints
Sufficient for many tasks
10-20 foot accuracy
Mapping Grade
$3-$5k
Designed for data collection
Sub-Meter accuracy
Post-Processed
Survey Grade
$30-$50k
Centimeter level accuracy

6. GPS Range Measurement Accuracy
Error Source SPS PPS
Selective Availability
Atmospheric Error
Ionospheric
Tropospheric
Clock & Ephemeris Error
Receiver Noise
Multipath
Total User Equivalent Range Error, 1drms
Typical Horizontal DOP
Total Stand-Alone Horizontal Accuracy, 2 drms
*Note: All Measurements in Meters
Source: Charting the Future, NAPA/NRC, May, 1995.

7. Understanding GPS
Consumer Grade Units
What do they do?

8. Understanding GPS Track log Waypoints Routes
Breadcrumb trail of where you’ve been
Waypoints
Allows a user to store a location and return at a future date – Prize Trees!!
Routes
Navigate between multiple waypoints