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

Instantaneous positioning with GPS

Accuracy of• +/- 10 m (30 ft) error (horizontal)• +/- 15 m (45 ft) error (vertical)

Your location is:37o 23.323’ N

122o 02.162’ W

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

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 !

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

• 4th satellite is used for time accuracy

• Position calculated within sub-centimeter scale

Atmospheric Delays

Ionosphere (use dual frequency receivers) Troposphere (estimate troposphere)

Troposphere

Ionosphere

Multipath ‘echoes’

• Similar issue to ‘ghost’ images on TV• Causes inaccurate measurements• If we were outside right now, what would cause

potential multipath errors?