1. How Does GPS Work ?

2. Objectives To Describe: The 3 components of the Global Positioning System How position is obtaining from a radio timing signal Obtaining instantaneous positions High precision GPS and how it works The method of operation Errors and data quality issues with GPS

3. The Global Positioning System 24+ satellites 20,200 km altitude 55 degrees inclination 12 hour orbital period 5 ground control stations Each satellite passes over a ground monitoring station every 12 hours 3 Components = a) Ground Control Segment; b) Space Segment; and c) User Segment

4. Obtaining a Position From a Radio Timing Signal Precise atomic clocks are needed to measure travel time Getting perfect timing means synchronizing the satellite with the receiver The travel time for a satellite directly overhead is about 0.06 seconds The difference in sync of the receiver time minus the satellite time is the travel time a GPS receiver determines the travel time of a signal from a satellite by comparing the "pseudo random code" it's generating, with an identical code in the signal from the satellite

5. Instantaneous positioning with GPS Accuracy of +/- 10 m (30 ft) error (horizontal) +/- 15 m (45 ft) error (vertical) Your location is: 37 o 23.323’ N 122 o 02.162’ W

6. High-precision GPS D -Use the carrier phase, not pseudorange -Dual-frequency receivers ( to remove ionosphere ) -High-precision orbital information -Good monuments -Multiple stations ( to remove satellite clock variations ) -Sophisticated processing software -Collect lots of data

7. High-Precision Systems for Field Surveying Trimble 5700 geodetic receiver with flat antenna Movable tripod with choke-ring antenna Monument stability and centering and leveling the antenna are critical !

8. Method of Operation Global positioning system (GPS) Receiver determines location, speed, direction, and time 3 satellite signals are necessary to locate the receiver in 3D space 4 th satellite is used for time accuracy Position calculated within sub-centimeter scale

9. Atmospheric Delays  Ionosphere (use dual frequency receivers)  Troposphere (estimate troposphere) Troposphere Ionosphere

10. Multipath ‘echoes’ Similar issue to ‘ghost’ images on TV Causes inaccurate measurements If we were outside right now, what would cause potential multipath errors?

11. Summary 3 components to GPS are: 1) Ground Control Segment; 2) Space Segment; and 3) User Segment Radio signals are matched in satellite and receiver to obtain travel times used to calculate distance and position High Precision GPS can obtain millimeter level accuracy (but at a cost) Atmospheric delays and multipath can cause GPS inaccuracies