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7/29/2019 GPSLecture Burgess
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ESSC 541-542 Lecture 4.14.05
INTRODUCTION TO GPS:
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Presentation OutlineI. Learning Format
II. GPS Basics
III. GPS Under the Hood
IV.Datums and Coordinate Systems
V. Mobile Mapping Technology
VI.Hands on with GPS
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Learning Format Lecture
One hour presentation and in class
orientation to GeoXT
Lab
Field trip (when?) to practice using theGPS in the field
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What is GPS?The Global Positioning System (GPS)
A Constellation of Earth-OrbitingSatellites Maintained by the United
States Government for the Purpose ofDefining Geographic Positions On andAbove the Surface of the Earth. Itconsists of Three Segments:
Control Segment
Space Segment
User Segment
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GPS Satellites (Satellite Vehicles(SVs)) First GPS satellite launched in
1978
Full constellation achieved in 1994
Satellites built to last about 10years
Approximately 2,000 pounds,17feet across
Transmitter power is only 50 wattsor less
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GPS Lineage Phase 1: 1973-1979
CONCEPT VALIDATION
1978- First Launch of Block 1 SV
Phase 2: 1979-1985
FULL DEVELOPMENT AND TESTS
Phase 3: 1985-Present
PRODUCTION AND DEPLOYMENT
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Precise Positioning System (PPS) Authorized users ONLY
U. S. and Allied military
Requires cryptographicequipment, specially
equipped receivers
Accurate to 21 meters 95%of time
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Standard Positioning Service (SPS)
Available to all users
Accuracy degraded bySelective Availability until2 May 2000
Horizontal Accuracy: 100m
Now has roughly sameaccuracy as PPS
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Space Segment 24+ satellites
6 planes with 55
inclination Each plane has 4-5
satellites
Broadcasting position
and time info on 2frequencies
Constellation hasspares
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Space Segment Very high orbit
20,200 km
1 revolution inapproximately 12 hrs
Travel approx. 7,000mph
Considerations
Accuracy
Survivability
Coverage
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Control Segment
Master Control StationMonitor StationGround Antenna
Colorado
Springs
Hawaii AscensionIslands
DiegoGarcia
Kwajalein
Monitor and Control
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Control Segment: Maintaining the System
(5) Monitor Stations
Correct Orbitand clockerrors Create newnavigation message
Observeephemerisand clock
Falcon AFB
Upload Station
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User Segment
Over $19 Billion invested by DoD
Dual Use System Since 1985
(civil & military) Civilian community was quick to take
advantage of the system Hundreds of receivers on the market
3 billion in sales, double in 2 years 95% of current users
DoD/DoT Executive Board sets GPSpolicy
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Common Uses for GPS
Land, Sea and AirNavigation and
Tracking
Surveying/ Mapping
Military Applications
Recreational Uses
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How the system works
Space Segment24+ Satellites
The CurrentEphemeris is
Transmitted toUsersMonitor
Stations Diego Garcia Ascension Island Kwajalein
Hawaii Colorado Springs
GPS ControlColorado Springs
EndUser
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Triangulation
Satellite 1 Satellite 2
Satellite 3 Satellite 4
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Distance Measuring
Rate = 186,000 miles persecond (Speed of Light)
Time = time it takessignal to travel from theSV to GPS receiver
Distance = Rate x Time
Each satellite carriesaround four atomicclocks
Uses the oscillation of cesium andrubidium atoms to measure time
Accuracy?plus/minus a second over more
than 30,000 years!!
The whole systemrevolves around
time!!!
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SV and Receiver Clocks
SV Clocks 2 Cesium & 2 Rubidium in each SV $100,000-$500,000 each
Receiver Clocks Clocks similar to quartz watch Always an error between satellite
and receiver clocks ( t)
4 satellites required to solve forx, y, z, and t
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PROBLEM
Cant use atomic
clocks in receiver
SOLUTION
Receiver clocks
accurate over shortperiods of time
Reset often
4th
SV used torecalibrate receiverclock
Cesium Clock =$$$$$$$!!!
Size of PC
4
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Breaking the Code
Transmission Time
Receiver
The Carrier Signal...
combined with
The PRN code...
produces the
Modulated carrier signal
which is transmitted...demodulated...
And detected by receiver,Locked-on, butWith a time delay...
Time delay
Satellite
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Accuracy and Precision in GPS
Accuracy
The nearness of a measurement to the
standard or true value Precision
The degree to which several
measurements provide answers very closeto each other.
What affects accuracy andprecision in GPS?
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Sources of Error
Selective Availability
Intentional degradation of
GPS accuracy 100m in horizontal and
160m in vertical
Accounted for most error instandard GPS
Turned off May 2, 2000
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Sources of Error
Geometric Dilution ofPrecision (GDOP)
Describes sensitivity of receiver tochanges in the geometricpositioning of the SVs
The higher the DOP value, thepoorer the measurement
QUALITY DOP
Very Good 1-3Good 4-5Fair 6Suspect >6
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Sources of Error
Clock Error Differences between satellite
clock and receiver clock
Ionosphere Delays Delay of GPS signals as they
pass through the layer ofcharged ions and free electronsknown as the ionosphere.
Multipath Error Caused by local reflections ofthe GPS signal that mix withthe desired signal GPS
Antenna
Hard Surface
Satellite
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Differential GPS Method of removing errors that affect GPS
measurements
A base station receiver is set up on a locationwhere the coordinates are known
Signal time at reference location is compared totime at remote location
Time difference represents error in satellitessignal
Real-time corrections transmitted to remotereceiver Single frequency (1-5 m) Dual frequency (sub-meter)
Post-Processing DGPS involves correcting at a
later time
Referencelocation
Remotelocation
= Error
www.ngs.noaa.gov/OPUS
Online post-processing
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Wide Area Augmentation System (WAAS)
System of satellites and groundstations that provide GPS signalcorrections
25 ground reference stations acrossUS
Master stations create GPScorrection message
Corrected differential messagebroadcast through geostationarysatellites to receiver
5 Times the accuracy (3m) 95% oftime
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Datums and Coordinate Systems
Many datums and coordinatesystems in use today
Incorrect referencing ofcoordinates to the wrong datumcan result in position errors ofhundreds of meters
With, sub-meter accuracyavailable with todays GPS,careful datum selection andconversion is critical!
Why should I worry about datums andcoordinate systems when using GPS?
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Geodetic Datums: What are they?
Define the size and shapeof the earth
Used as basis forcoordinate systems
Variety of models:
Flat earth
Spherical
Ellipsoidal
WGS 84 defines geoid
heights for the entire earth
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Coordinate Systems: What are they?
Based on Geodetic Datums
Describe locations in two or three
dimensions (ie. X,Y,Z or X,Y)
Local and Global
Common systems Geodetic Lat, Long (global) UTM (local) State Plane (local)
Variety of transformation methodswww.uwgb.edu/dutchs/UsefulData/UTMFormulas.HTM
Online conversion tool
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World UTM Zones
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Geodetic Latitude, Longitude
Prime Meridian and Equator are referenceplanes used to define latitude and longitude
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Which is the correct location?
Same location can have many reference positions,
depending on coordinate system used
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Mobile Mapping
Integrates GPS technologyand GIS software
Makes GIS data directlyaccessible in the field
Can be augmented withwireless technology
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Mobile Mapping Pros and Cons
Pros More efficient data entry
Ready access to GIS data
Less transcription error
Possible real-timeupload/download throughwireless
Cons Cost
Data storage limit
Digital data can be
lost/corrupted
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Hands on with GPS
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Paul BurgessUniversity of Redlands
Redlands [email protected]
Questions?
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OBJECTID Northing Easting GPSDate GPSDateTime
1 3982854.745 656956.881 09-Apr-04 09-Apr-04
2 3982854.748 656956.885 09-Apr-04 09-Apr-043 3982854.84 656956.953 09-Apr-04 09-Apr-04
4 3982854.697 656957.42 09-Apr-04 09-Apr-04
5 3982854.55 656957.393 09-Apr-04 09-Apr-04
6 3982854.539 656957.365 09-Apr-04 09-Apr-04
7 3982854.443 656957.304 09-Apr-04 09-Apr-04
8 3982854.37 656957.461 09-Apr-04 09-Apr-04
9 3982854.303 656957.619 09-Apr-04 09-Apr-04
10 3982854.089 656957.571 09-Apr-04 09-Apr-04
11 3982854.062 656957.672 09-Apr-04 09-Apr-04
12 3982854.02 656957.581 09-Apr-04 09-Apr-04
13 3982853.882 656957.36 09-Apr-04 09-Apr-04
14 3982853.776 656957.314 09-Apr-04 09-Apr-04
15 3982853.821 656957.262 09-Apr-04 09-Apr-04
16 3982853.9 656957.222 09-Apr-04 09-Apr-04
17 3982853.865 656956.896 09-Apr-04 09-Apr-04
18 3982853.825 656956.811 09-Apr-04 09-Apr-04
19 3982854.382 656954.229 09-Apr-04 09-Apr-04
20 3982857.014 656958.041 09-Apr-04 09-Apr-04
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References
Bertorelli, Paul. GPS Explained. Downloaded April 2005: http://www.eaa1000.av.org/technicl/gps/gps.htm
Henstridge, Fred N and Bob Nelson. Introduction to GPS. Presentation for the ICAO/FAA WGS-84 Seminarand Workshop, November 9, 1999, San Salvador. Accessible on the International OceanographicCommissions website at : Http--ioc.unesco.org-oceanteacher-OceanTeacher2-02_InfTchSciCmm-01_CmpTch-10_enavsys-gps_0008.ppt.url
Peter H. Dana. Coordinate Systems Overview. Accessible at:
http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html Peter H. Dana. Geodetic Datum Overview. Accessible at:
http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html
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http://www.eaa1000.av.org/technicl/gps/gps.htmhttp://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/datum/datum_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/datum/datum_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.htmlhttp://www.eaa1000.av.org/technicl/gps/gps.htm