Sources of GPS Error Ionospheric Delays

Sources of GPS Error Ionospheric Delays

Sources of GPS Error
• Clock Errors
• Satellite Geometry
• Ionospheric and
Tropospheric Delays
• Selective
Availability
• Atmospheric Delays
• Multipath
• Ephemeris Errors
Ionospheric 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
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Tropospheric 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)
Ionospheric Delays
• Effects on radio signals
– when electromagnetic radiation travels
through ionosphere, it slows down at a rate
inversely proportional to (1/freq)22
– so lower frequencies slow down more
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Ionospheric 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
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 / Tropospheric
Delays
• 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
Multipath
• 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 Errors
• Ephemeris errors are due to slight deviations
in the orbital paths of the satellites from their
predicted paths
Satellite 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
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Satellite Geometry Effects
• Satellites spread far apart
– bands of uncertainty shown
Possible Locations
Satellite Geometry Effects
• Satellites
clustered close
together
– our position will
be in a larger
“box”
• i.e., greater
uncertainty in
position
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Satellite 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 Effects
• Satellites spread far apart
– LOW PDOP
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Satellite Geometry Effects
• Satellites
clustered close
together
– HIGH PDOP
Satellite 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
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GPS 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
Lower Vertical DOP
(smaller elevation error)
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High Vertical DOP
(greater elevation error)
Satellite 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
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Satellite 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 15oo
• the lower limit for Elevation Angle Mask is 5oo
Selective 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
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Selective 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)
Other 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
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