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Sources of GPS ErrorSources of GPS Error

• Clock Errors

• Ionospheric and Tropospheric Delays

• Atmospheric Delays

• Multipath

• Ephemeris Errors

• Clock Errors

• Ionospheric and Tropospheric Delays

• Atmospheric Delays

• Multipath

• Ephemeris Errors

• Satellite Geometry

• Selective Availability

• Satellite Geometry

• Selective Availability

Ionospheric DelaysIonospheric Delays

• Ionosphere - blanket of charged particles 80-120 miles above the earth

– Charged particles affect the speed of light

– Charged particles slow down radio signals

– Charged particles also refract radio waves

• Ionosphere - blanket of charged particles 80-120 miles above the earth

– Charged particles affect the speed of light

– Charged particles slow down radio signals

– Charged particles also refract radio waves

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Tropospheric DelaysTropospheric Delays

• Troposphere – we usually think of this as the atmosphere – from earth surface to about 10 miles above earth

– Troposphere refract radio waves

– Water vapor slows down radio waves (but at different rate than ionosphere)

• Troposphere – we usually think of this as the atmosphere – from earth surface to about 10 miles above earth

– Troposphere refract radio waves

– Water vapor slows down radio waves (but at different rate than ionosphere)

Ionospheric DelaysIonospheric Delays

• Effects on radio signals

– when electromagnetic radiation travels through ionosphere, it slows down at a rate inversely proportional to (1/freq)2

– so lower frequencies slow down more

• Effects on radio signals

– when electromagnetic radiation travels through ionosphere, it slows down at a rate inversely proportional to (1/freq)2

– so lower frequencies slow down more

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Ionospheric DelaysIonospheric Delays

• Dual frequency receivers (survey grade) receive both L1 and L2 frequencies

• If we compare the arrival times of signals from both frequencies we can compute the reduction in speed of light (and signal speed)

• By knowing this speed reduction, the dual frequency receivers are able to calculate a more accurate position

• Dual frequency receivers (survey grade) receive both L1 and L2 frequencies

• If we compare the arrival times of signals from both frequencies we can compute the reduction in speed of light (and signal speed)

• By knowing this speed reduction, the dual frequency receivers are able to calculate a more accurate position

Tropospheric DelaysTropospheric Delays

• Although they are similar to ionoshperic delays, the water vapor delay is not frequency dependant– Dual frequency receivers can’t correct for

tropospheric delays

• Although they are similar to ionoshperic delays, the water vapor delay is not frequency dependant– Dual frequency receivers can’t correct for

tropospheric delays

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Ionospheric / TroposphericDelays

Ionospheric / TroposphericDelays

• One way to limit ionospheric and tropospheric errors is to set a mask angle– Mask angle is a setting in the GPS receiver that

tells it to not use satellites that are low on the horizon

– Satellites low on the horizon means that their signal will travel through a larger portion of the troposphere / ionosphere – therefore more errors

– 15 degree is a typical mask angle setting

• One way to limit ionospheric and tropospheric errors is to set a mask angle– Mask angle is a setting in the GPS receiver that

tells it to not use satellites that are low on the horizon

– Satellites low on the horizon means that their signal will travel through a larger portion of the troposphere / ionosphere – therefore more errors

– 15 degree is a typical mask angle setting

MultipathMultipath• Satellite signals can be

reflected off buildings, water, trees, etc. and then travel to the receiver antenna

– this reflected travel path is longer than it should have been -and gives a longer travel time

– longer travel times yield incorrect positions

• Satellite signals can be reflected off buildings, water, trees, etc. and then travel to the receiver antenna

– this reflected travel path is longer than it should have been -and gives a longer travel time

– longer travel times yield incorrect positions

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Ephemeris ErrorsEphemeris Errors

• Ephemeris errors are due to slight deviations in the orbital paths of the satellites from their predicted paths

• Ephemeris errors are due to slight deviations in the orbital paths of the satellites from their predicted paths

Satellite GeometrySatellite Geometry

• Satellite geometry affects accuracy of GPS positions

– Geometric Dilution of Precision (GDOP)

– GDOP refers to where the satellites are in relation to each other

• Satellite geometry affects accuracy of GPS positions

– Geometric Dilution of Precision (GDOP)

– GDOP refers to where the satellites are in relation to each other

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Satellite Geometry EffectsSatellite Geometry Effects• Satellites spread far apart

– bands of uncertainty shown

• Satellites spread far apart

– bands of uncertainty shown

Possible LocationsPossible Locations

Satellite Geometry EffectsSatellite Geometry Effects

• Satellites clustered close together– our position will

be in a larger “box”

• i.e., greater uncertainty in position

• Satellites clustered close together– our position will

be in a larger “box”

• i.e., greater uncertainty in position

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Satellite GeometrySatellite Geometry• Satellite geometry classified by GDOP

• We usually only quantify PDOP– Position Dilution of Precision (PDOP)

– PDOP includes • VDOP (Vertical DOP)

• HDOP (Horizontal DOP)

• GDOP also includes TDOP (Time DOP) which quantifies clock offset errors

• Satellite geometry classified by GDOP

• We usually only quantify PDOP– Position Dilution of Precision (PDOP)

– PDOP includes • VDOP (Vertical DOP)

• HDOP (Horizontal DOP)

• GDOP also includes TDOP (Time DOP) which quantifies clock offset errors

Satellite Geometry EffectsSatellite Geometry Effects• Satellites spread far apart

– LOW PDOP

• Satellites spread far apart

– LOW PDOP

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Satellite Geometry EffectsSatellite Geometry Effects

• Satellites clustered close together

– HIGH PDOP

• Satellites clustered close together

– HIGH PDOP

Satellite GeometrySatellite Geometry• Most receivers allow you to set a PDOP mask

– PDOP Mask determines a level of PDOP above which the receiver will not collect data

– low PDOP (less than 4) means low position error

– high PDOP (greater than 8) means high error

• PDOP is predictable - mission planning allows you to select times when you can work with less error

• Most receivers allow you to set a PDOP mask– PDOP Mask determines a level of PDOP above

which the receiver will not collect data

– low PDOP (less than 4) means low position error

– high PDOP (greater than 8) means high error

• PDOP is predictable - mission planning allows you to select times when you can work with less error

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GPS Elevation AccuracyGPS Elevation Accuracy• GPS estimates of elevation typically have

errors that are 2 to 5 times worse than the horizontal position error

– strictly based on geometry

– VDOP (Vertical DOP) is lower if more satellites are overhead

• we usually don’t have that many satellites directly overhead

• GPS estimates of elevation typically have errors that are 2 to 5 times worse than the horizontal position error

– strictly based on geometry

– VDOP (Vertical DOP) is lower if more satellites are overhead

• we usually don’t have that many satellites directly overhead

Lower Vertical DOP (smaller elevation error)Lower Vertical DOP (smaller elevation error)

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High Vertical DOP (greater elevation error)High Vertical DOP

(greater elevation error)

Satellite GeometrySatellite Geometry• Most receivers allow you to set an elevation

angle mask

– Elevation Angle is the angle above the horizon

• when satellites are close to horizon, there is more atmosphere, troposphere, and ionosphere for the signals to go through - ie. more noise

• when satellites are close to the horizon, the VDOP is greater

• Most receivers allow you to set an elevation angle mask

– Elevation Angle is the angle above the horizon

• when satellites are close to horizon, there is more atmosphere, troposphere, and ionosphere for the signals to go through - ie. more noise

• when satellites are close to the horizon, the VDOP is greater

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Satellite GeometrySatellite Geometry• Most receivers allow you to set an elevation

angle mask

– Elevation Angle Mask determines the minimum elevation angle below which the receiver will no longer use a satellite in its computations

• a typical value for Elevation Angle Mask is 15o

• the lower limit for Elevation Angle Mask is 5o

• Most receivers allow you to set an elevation angle mask

– Elevation Angle Mask determines the minimum elevation angle below which the receiver will no longer use a satellite in its computations

• a typical value for Elevation Angle Mask is 15o

• the lower limit for Elevation Angle Mask is 5o

Selective AvailabilitySelective Availability• The US Department of Defense has the ability

to degrade the accuracy of GPS SPS for civilian users– Selective Availability (SA)

– SA is the process of introducing artificial clock errors or ephemeris errors

– typical GPS errors with SA are 100 meters horizontal, up to 300 meters vertical

• The US Department of Defense has the ability to degrade the accuracy of GPS SPS for civilian users– Selective Availability (SA)

– SA is the process of introducing artificial clock errors or ephemeris errors

– typical GPS errors with SA are 100 meters horizontal, up to 300 meters vertical

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Selective AvailabilitySelective Availability

• SA was active throughout most of the time GPS has been operating

• SA was turned off temporarily during the Gulf War

• SA was turned off May 1, 2000 (hopefully permanently)

• SA was active throughout most of the time GPS has been operating

• SA was turned off temporarily during the Gulf War

• SA was turned off May 1, 2000 (hopefully permanently)

Other Error SourcesOther Error Sources• Sunspots distort GPS signals

• Electronic noise distorts signals– when we configure the GPS receiver, we can set a

mask for Signal-to-Noise Ratio

– we want the SNR to be as high as possible

– a value of SNR for relatively high accuracy data collection is 6.0

– a lower limit value for SNR mask is 4

• Sunspots distort GPS signals

• Electronic noise distorts signals– when we configure the GPS receiver, we can set a

mask for Signal-to-Noise Ratio

– we want the SNR to be as high as possible

– a value of SNR for relatively high accuracy data collection is 6.0

– a lower limit value for SNR mask is 4