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

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

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!!)

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 0900. How far did it travel? Station B Answer: Change in Time x Speed = Distance (0600-0900) x 60 = 180 miles

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

GPS Range Measurement Accuracy Error Source SPS PPS Selective Availability 0.0 0.0 Atmospheric Error Ionospheric 7.0 0.01 Tropospheric 0.7 0.7 Clock & Ephemeris Error 3.6 3.6 Receiver Noise 1.5 0.6 Multipath 1.2 1.8 Total User Equivalent Range Error, 1drms 8.1 4.1 Typical Horizontal DOP 2.0 2.0 Total Stand-Alone Horizontal 32.4 16.4 Accuracy, 2 drms *Note: All Measurements in Meters Source: Charting the Future, NAPA/NRC, May, 1995.

Understanding GPS Consumer Grade Units What do they do?

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