GPSGlobal Positioning

System

What is GPS?What is GPS?

The Global Positioning System.A system designed to accurately

determining positions on the earth

The Global Positioning System.A system designed to accurately

determining positions on the earth

What are the parts of GPS?

What are the parts of GPS?

Space SegmentControl SegmentUser Segment

Space SegmentControl SegmentUser Segment

GPS ComponentsGPS Components

Developed by the DoDDepartment of Defense

Funded in 1976 by Congress

Became fully functional in the early 1980’s

Developed by the DoDDepartment of Defense

Funded in 1976 by Congress

Became fully functional in the early 1980’s

Space SegmentSpace Segment

SatellitesAtomic ClockSolar PanelsTransmitter / Receiver (High frequency,

low energy radio waves)Antenna

There are 24 active satellites in orbit3 extra satellites in orbit that are not in

use

SatellitesAtomic ClockSolar PanelsTransmitter / Receiver (High frequency,

low energy radio waves)Antenna

There are 24 active satellites in orbit3 extra satellites in orbit that are not in

use

GPS SatelliteGPS Satellite

GPS Satellite OrbitsGPS Satellite Orbits

4 satellites in each of 6 orbitsPass over a monitoring station

once every 12 hours12,600 mile high orbitThere are always more than 4

satellites visible anywhere on the planet

4 satellites in each of 6 orbitsPass over a monitoring station

once every 12 hours12,600 mile high orbitThere are always more than 4

satellites visible anywhere on the planet

GPS Satellite OrbitsGPS Satellite Orbits

Control SegmentControl Segment

5 monitoring stationsGround station at Schreiver AFB in Colorado

Coordinates time with the Space SegmentMaster control station

HawaiiAscension IslandDiego GarciaKwajalein

Correction factors issued twice per dayTimingOrbital adjustments

5 monitoring stationsGround station at Schreiver AFB in Colorado

Coordinates time with the Space SegmentMaster control station

HawaiiAscension IslandDiego GarciaKwajalein

Correction factors issued twice per dayTimingOrbital adjustments

User SegmentUser Segment

1984 first commercial GPS receivers on the market

Receivers that interpret the signals received from the space segment and ground based transmitters.

Quartz clocks in the GPS receivers are corrected by the satellite signals

GPS unitsAccuracy ~10m - sub centimeter

1984 first commercial GPS receivers on the market

Receivers that interpret the signals received from the space segment and ground based transmitters.

Quartz clocks in the GPS receivers are corrected by the satellite signals

GPS unitsAccuracy ~10m - sub centimeter

GPS UnitsGPS Units

www.geodatasolutions.co.uk/ landsurvey.html

http://www.garmin.com http://www.bruceair.com/Multimedia_presentations.htm

ResultResult

Segments work together to establish a box where your location is

Segments work together to establish a box where your location is

TrilaterationTrilateration

TrilaterationTrilateration

Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.

The time difference between the same part of the code indicates how far the satellite is from the receiverDistance = time * velocity

Velocity = 186,000 miles/second~.06 seconds travel time

Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.

The time difference between the same part of the code indicates how far the satellite is from the receiverDistance = time * velocity

Velocity = 186,000 miles/second~.06 seconds travel time

TrilaterationTrilateration

http://www.digit-life.com/articles/gpssystem/index.html

http://www.gsm-modem.de

Possible Sources of ErrorPossible Sources of Error

InterferenceAtmosphere, Solid Objects

EphemerisDisturbances of satellite orbit:

Sun and moon gravitationPressure of solar radiation

Clock ProblemAtomic clocks are nearly perfect so even

very small errors can be significant

InterferenceAtmosphere, Solid Objects

EphemerisDisturbances of satellite orbit:

Sun and moon gravitationPressure of solar radiation

Clock ProblemAtomic clocks are nearly perfect so even

very small errors can be significant

Sources of ErrorSources of Error

ReceiversWhat the receiver is designed forKind of GPS chipCan the unit reject errors

Multi PathReflection of signals off of objects so that the

same signal reaches the receiver with different offsets

1/100 second discrepancy Misread of 1,860 miles

ReceiversWhat the receiver is designed forKind of GPS chipCan the unit reject errors

Multi PathReflection of signals off of objects so that the

same signal reaches the receiver with different offsets

1/100 second discrepancy Misread of 1,860 miles

MultipathMultipath

http://www.kowoma.de

http://earthmeasurement.ca

Selective Availability (S/A)Selective Availability (S/A)

A disruption to the signal to make commercial units less accurateA random timing introduced into the satellite

signalAccuracy off by up to 100 m

Military units filter out S/A2000 - S/A was shut off so that commercial

units received an unaltered signal10 - 15 m accuracy without correction

The government can turn on S/A or turn off the signal if needed.

A disruption to the signal to make commercial units less accurateA random timing introduced into the satellite

signalAccuracy off by up to 100 m

Military units filter out S/A2000 - S/A was shut off so that commercial

units received an unaltered signal10 - 15 m accuracy without correction

The government can turn on S/A or turn off the signal if needed.

Signal CorrectionSignal CorrectionDifferential GPS

Base stations are used to receive the same satellite signal (Base Station)

Signals to the base station and the mobile GPS receiver are compared to eliminate some sources of error

3 - 5 m accuracyWAAS - Wide Area Augmentation System

Currently only in North AmericaAsia - MSAS, Europe - EGNOS

Issues local correction factors based on local conditions

Improves accuracy of GPS location< 3 m accuracy (to subcentimeter)

Differential GPSBase stations are used to receive the same

satellite signal (Base Station)Signals to the base station and the mobile

GPS receiver are compared to eliminate some sources of error

3 - 5 m accuracyWAAS - Wide Area Augmentation System

Currently only in North AmericaAsia - MSAS, Europe - EGNOS

Issues local correction factors based on local conditions

Improves accuracy of GPS location< 3 m accuracy (to subcentimeter)