GPSGlobal Positioning

SystemLecture 11

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)

AntennaThere 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)

AntennaThere are 24 active satellites in orbit3 extra satellites in orbit that are not in use

SatelliteSatellite

Atomic Clock9.2 billion oscillations per second

Each satellite transmits on two L Band frequencies

Every satellite can transmit its own unique code

The satellites can be repositioned by the DoD

Atomic Clock9.2 billion oscillations per second

Each satellite transmits on two L Band frequencies

Every satellite can transmit its own unique code

The satellites can be repositioned by the DoD

GPS SatelliteGPS Satellite

GPS Satellite OrbitsGPS Satellite Orbits

4 satellites in each of 6 orbits

Pass over a monitoring station once every 12 hours

12,600 mile high orbitThere are always more than 4 satellites visible anywhere on the planet

4 satellites in each of 6 orbits

Pass over a monitoring station once every 12 hours

12,600 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

Each satellite signal radiates outward in a sphere2 satellite signals overlap to produce a circle of probable location

3 satellite signals overlap to produce 2 points of probable locationOne can usually be discarded as not relevant

4 satellite signals overlap to produce 1 point of location with elevation

Each satellite signal radiates outward in a sphere2 satellite signals overlap to produce a circle of probable location

3 satellite signals overlap to produce 2 points of probable locationOne can usually be discarded as not relevant

4 satellite signals overlap to produce 1 point of location with elevation

TrilaterationTrilateration

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

http://www.gsm-modem.de

Possible Sources of Error

Possible 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 signal

Accuracy off by up to 100 mMilitary 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 signal

Accuracy off by up to 100 mMilitary 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 SystemCurrently only in North America

Asia - MSAS, Europe - EGNOSIssues 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 SystemCurrently only in North America

Asia - MSAS, Europe - EGNOSIssues local correction factors based on local conditions

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

Signal CorrectionSignal Correction

Tricks that are not correcting errors:Map-matching

Estimated location is snapped to a known path on the map

Interpolated positionGPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost

Instant averagingGPS unit collects more than one position at each waypoint and averages them

Tricks that are not correcting errors:Map-matching

Estimated location is snapped to a known path on the map

Interpolated positionGPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost

Instant averagingGPS unit collects more than one position at each waypoint and averages them