Download - GE1 GPS Lecture 3[1]
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8/8/2019 GE1 GPS Lecture 3[1]
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To beam or not to beam
POSITIONING ACCURACIESPOSITIONING ACCURACIESWITH GPSWITH GPS
GE 1 Earth Trek
Lecturer: Ariel C. Blanco
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Ranging TechniquesRanging Techniques
Two-way ranging: Active
Electronic distance measuring devices (EDMs)Radar, Sonar, Lidar
One-way ranging: PassiveGPS
Light beam
Range
Range = C x Time/2
reflector
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OneOne--way Ranging with GPSway Ranging with GPS
Range
Radio Signal
Range = C x Time
1 microsecond error = ~ 300 meters1 nanosecond error = ~ 1 foot
z
Sphere ofposition
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How GPS Works in 6 StepsHow GPS Works in 6 Steps
Trilateration from satellites is
basis of system. Satellite
geometry expressed in DOP
values is important
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To trilaterate, GPS
measures distance
using speed of light2
To measure the distance you
need good clocks and a fourth
SV
Once you know the distance,
you need to know SV's
position
5Then correct for
atmospheric delays
Selective availability
and differential
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TrilaterationTrilaterationOne measurement narrows downour position to the surface of a sphere
Were somewhere on
the surface of this
sphere
19,000km
TrilaterationSecond measurement narrows itdown to intersection of two spheres
20,000km
19,000km
Intersection of
two spheres is a
circle
TrilaterationThird measurement narrows tojust two points
20,000
km
19,000kmIntersection of
three spheres isonly two points
21,000km
Trilateration
In practice 3 measurements areenough
One point will be a ridiculous answer
Out in space
Or moving at high speed
4th measurement required
To cancel out receiver clock errors
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Satellite RangingRadio signals travel at the speedof light (300,000 km/sec) :
Done by measuring travel
time of radio signals
Time (sec) x 300,000= km travelled
Distance known as pseudorange
Pseudorange
Use the same code at the receiver and satellite
Synchronize the satellites and receivers so they aregenerating the same code at the same time
Then we look at the incoming code from the satelliteand see how long ago our receiver generated the same
code
from satellite
from ground receiver
measure time
difference between
same part of code
When did the Signal leave the Satellite?
Need Accurate Clocks
Accurate clocks necessary tomeasure travel time
Satellites have atomic clocks
Ground receivers need consistent clocks
Adding a fourth satellite eliminates receiver clockerrors
Accurate ClocksThe ideal situation: in 2D for sakeof drawing
This is where we really are
4 secs 6 secs
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Accurate ClocksAdding a third measurement
Third measurement would go
through our position if correct
4 secs 6 secs
5 secs
Accurate Clocks
With fast clocksBad position because clock is off by
one second
5 secswrong
time
7 secswrong
time
Accurate ClocksThird measurement with fast clocks
6 secs
Wont go through the other two
5 secs
wrong
time
7 secs
wrong
time
Differential corrections will remove
this error
Knowing Where the Satellites Are
Monitor Stations(Control Segment)
Keep SV positions
updated
SV positiontransmitted to user -
ephemeris
High orbit(Space Segment)
Very stable orbit
No atmospheric
drag
Survivability
Earth coverage
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(User
Segment)
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Reference Ellipsoid
a
b
a = semi-major axisb = semi-minor axis
Flattening f(a b)
a=
b
H
heightlellipsoidaH
longitude
latitude
WGS-84 Ellipsoid
a = 6378137.000000 m
b = 6356752.314245 m
1/f = 298.2572235630
GPS Heights vs. Elevations
e = Orthometric Height
H = Ellipsoid Height
N = Geoid Height
e = H - N
NN
N
e ee
H H
H
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Determining Orthometric Heights
Ortho. HeightOrtho. Height == H.A.E.H.A.E.Geoid HeightGeoid Height
Earth Surface
Ellipsoid GeoidHeight
Height above MSL(Orthometric height)
H.A.E.
Geoid height= H.A.E.
Geoid
Atmospheric CorrectionsAtmospheric Corrections
Ionosphere is a band of
charged particles
Troposphere is ourweather
Differential correctionsremove this error
Ionosphere
Troposphere
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Selective Availability
U.S. Government can introduce errors
When they do its the largest source of error
Differential corrections remove this error
Post process RTCM (Real Time Corrections)
Selective Availability (SA) - Term given to the act ofpurposefully limiting the accuracy of GPS : Error Budget
Metres
S/A
Atmospheric
Receivers
Ephemeris
Satellite Clocks
0 20 40 60 80 100
The Integer Ambiguity = First Partial Wavelength
N = Integer Ambiguity
Solving for theInteger
Ambiguity yields
centimeter precision
Carrier Phase Results
Baseline or Vector
(cm precision)
Azi = 212o 42 49.8244
Dist = 557.05307 m
Elev = 4 .8751 m
X = -408.251 m
Y = -84.830 m
Z = -369.413 mOR
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GPS AccuracyGPS Accuracy
Discussion of factors influencing GPS
Position accuracy
How accurate is GPS ?How accurate is GPS ?
The answer:
IT DEPENDS !
Range of GPS Accuracy
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Accuracy of Code vs. Phase Solutions
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GPS Accuracy Issues
GPS position accuracy depends upon a number ofissues:
Available satellite geometry
Atmospheric error
Receiver errorsMultiPath error
Satellite position error
Clock error
Satellite Geometry
The more separated thesatellites are - the betterthe geometry, and thebetter the position
solution
The closer the satellitesare in space, the worsethe geometry, and thepoorer the solution
Good Satellite Geometry Poor Satellite Geometry
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Dilution of Precision (DOP)
Point representing position is
really a square
4 secs
6 secs
Uncertainty
Uncertainty
Indicator of position quality from satellite geometry Dilution of Precision (DOP)Even worse at some angles
Box gets bigger if satellites
close together
Atmospheric Effects
< 10 km > 10 km
Multipath
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How Accurate is It?
Depends on some variables
Time spent on measurements
Design of receiver/software
Relative positions of satellites
cm to mm accuracies from survey products
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Receiver attributes# of Channels
One channelrequired for each
frequency (L1, +/-
L2)
8 minimum (4 SVs);12 or more desirable
Antenna Remote, fixed
Power source
Internal, external
Data Storage
Way-points vs. datalogging
Positions vs. raw
data
Data upload &download
Data dictionary
upload for storing
positions by
attributes (pt., line,area)
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Receiver attributesIonosphere Correction or model
Dual channel vs. single channel receiver
Troposphere model?
DGPS capable
Beacon antenna for real-time DGPS
Download and post-process
WAAS capable
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WAAS
How good is WAAS?How good is WAAS?
+ -3 meters
+-15
meters
With Selective Availability set to
zero, and under ideal conditions,
a GPS receiver without WAAS
can achieve fifteen meter
accuracy most of the time.*
Under ideal conditions a
WAAS equipped GPS receiver
can achieve three meter
accuracy 95% of the time.*
* Precision depends on good satellite geometry, open sky view, and no user induced errors.
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GPS EquipmentHandHand--helds $100helds $100--$450$450 navigation instrumentsnavigation instruments
Garmin
Magellan
GPS forPDAs
Way Points collection
Manual entry into GIS,no attribute info. stored
Differential ready butno post-processing
For survey apps.:
+/- ~15 meters
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GPS EquipmentSub Meter (x, y) Accuracy, HandSub Meter (x, y) Accuracy, Hand--helds: $1000helds: $1000--$5000$5000
Trimble
Ashtech
Topcon
Sokkia
others
Datalogging with datadictionary to upload/store
attribute info.
GPS signal data collection for
later post-processingCustom RTK and beacon
antennae feasible
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GPS EquipmentGeodeticGeodetic--quality Instrumentsquality Instruments
TrimbleTrimble
LeicaLeica
AshtechAshtech
SokkiaSokkia
OthersOthers
Cm mm in x and y; 2 cm in z
Stationary AntennaLarge memory for continuous
data collection
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Why test GPS accuracy?Manufacturers specifications are for open sites and long data sets
Open Sites:What the GPS salesman said. 1 meter
13 meters
Forested Sites:What can really happen in forested sites with short data sets.
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Thanks!
49GPS+