How Does GPS Work ?

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

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

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

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?

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