How Does GPS Work ?

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

Method of Operation What is needed to locate a place accurately? 3 satellite signals are necessary to locate the receiver in 3D space Receiver determines location, speed, direction, and time 4 th satellite is used for time accuracy Position calculated within sub-centimeter scale

High-precision GPS D -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, ‘cross referencing’

Atmospheric Delays  The Ionosphere & Troposphere can cause signal delay because of varying density of air masses, pollution, etc.  So multiple signals are needed to calibrate/triangulate Troposphere Ionosphere

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 !

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

Multipath ‘echoes’ What can cause inaccurate measurements? Natural & Man-made barriers or reflectors can distort measurements If we were outside right now, what would cause potential multipath errors?