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36. Global Positioning System
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36. Introduction to the Global Positioning System (GPS)
Why do we need GPS?
Position: a basic needsafe sea travel, crowed skies, resource management, legal questions
Positioning: a challenging joblocal navigation is relatively easy, but at sea there are no landmarks
Navigation and Positioning SystemsLandmarks – subject to change, only works in local areasDead reckoning – complicated, errors accumulate quicklyCelestial – complicated, only works on clear nights, limited precisionLORAN – limited coverage, limited and variable accuracyOMEGA – radio direction beacons, limited accuracy and subject to radio interferenceSatNav – doppler radar-based, few satellites, infrequent updatesGlobal Positioning System – this is the one!
What is GPS? Where did it come from?
A very accurate system designed and operated by the U.D Dept of Defense
originally designed for positioning nuclear submarines
A large investment ($12 billion) has been made in its development over the past 25 yearsCongress approved because of other potential applicationsOnly possible with today’s computer, clock and satellite technology
NAVSTAR GPS System
Control Segment
Space Segment
User Segment
Monitor Stations
GroundAntennas
Master Station
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28 satellites in 6 orbital planes20,200 km altitude
Space Segment: Constellation of GPS Satellites
NAVSTAR SatellitesAltitude: 10,900 miles
very high orbit for accuracy, coverage, and survivability
Size: 17 feetWeight: 1900 poundsOrbital period: 12 hoursOrbital plane: 55 degrees to equatorial plane28 satellites
24 active, 4 spares
Control Segment:User Segment: GPS Receiver
X, Y, Z, T
Latitude, longitude and elevation to tens of feet in real time
Satellite Receiver
ComponentsAntennaElectronics to receive satellite signalsMicroprocessor to process the data that determines the antenna position Controls to enable user input to the receiverDisplay screen
How Does GPS Work? 5 Steps…
1. Basis of system is triangulation from satellites
2. Measures distance using travel time of radio signals
3. Depends on accurate timing -- good clocks
4. Must know exact locations of satellites
5. Correct for atmospheric and ionsphericdelays
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Triangulation from Satellites
By measuring the distance to several satellites we can determine our location
Satellite
11,000 miles
One measurement puts us somewhere on the surface of a sphere
A second measurement narrows our location to the intersection of two spheres
Intersection of two spheres is a circle
12,000 miles
11,000 miles
A third measurement narrows the possible locations to two points
Intersection of three spheres is two points
The fourth measurement only intersects one of the two points
Estimated ranges to each satellite intersect within a small area when corrected for the receiver clock bias
Measure how long it takes the GPS signal to reach the receiver
Radio waves travel at the speed of lightTime (seconds) X 186,000 miles/sec = milesIf satellite is overhead the time is 0.06 seconds
Measuring the Distance to the Satellites
With good clocks, all we need to know is exactly when the signal left the satellite
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How do we know when the signal left the satellite?
Use the same code at satellite and receiverSynchronize satellites and receivers so they generate the same code at the same timeThen look at the incoming code from the satellite and see how long ago our receiver generated the same code
Measure time difference between same part of the coded signal
Receiver
Satellite
Time Difference
ΔT1
ΔT2
ΔT3
ΔT4
Synchronous Satellite Outputs
Differing Times Signals Received
Receiver
1
2
3
4
System depends on very accurate clocks to measure travel time
Satellites have atomic clocksaccurate to 1 billionth of a secondalso very expensive
However, ground receivers only need to have consistent clocks
How do we know the location of the satellites?
High orbits are very stable, symmetric and there is no atmospheric dragCorrections are transmitted by Defense Dept to the satellitesCorrections are transmitted by satellites to ground receivers
Solving the Distance Equation
R
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Atmospheric and Ionospheric Corrections
The ionosphere and atmosphere slow down the signalsModels are used to correct for these effects
Selective Availability
In the past the government sometimes introduced artificial clock and ephemeris errors to prevent hostile forces from using it
unfortunately the errors were also in the data of “friendly” forces – us – and SA was the greatest source of error
On May 2, 2001 selective availability was turned off and accuracy of most receivers should be at least 20 meters
Differential GPS
Uses two GPS receivers together, one stationary, one “moving”Stationary receiver is at a known locationStationary receiver compares calculated position with known location to determine amount of errorThen transmits the error to moving receivers
Differential GPS
Garmin Corp.
How Accurate is GPS?
Depends onReceiver designTime spent on measurementsRelative positions of satellites
Survey systems can provide sub-centimeter accuracyOther good systems can deliver 10 - 20 meter accuracy, 1 to 3 meters with differential GPS
GPS Errors
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0.6
1.5
0.6
10.5
Multipath
Total
0.30.3Receiver Noise
0.20.5Troposphere
0.45.0Ionosphere
02.5Orbit Errors
01.5Satellite Clocks
Differential GPS
Standard GPS
Typical Error in Meters(per satellite)
Error Budget Applications of GPS
Aircraft, ship and vehicle navigationAerial photography acquisitionLocation of features and boundaries for input to GIS and digital image classification and accuracy assessmentMapping topography, soils, forests, geology, wetlands, utilities,….Survey and legal land descriptionCivil engineering, construction of highways, bridges, dams,….
Applications of GPS, cont.
Resource inventory (plot location)Precision farmingVehicle tracking (trucks, buses, taxis,…)Emergency (police, fire, ambulance) and rescueTiming (precise to a billionth of a second)
Eventually everyone will have a GPS address
Some GPS sites on the Internet
GPS Overviewhttp://www.nasm.si.edu/galleries/gps/
Commercial homepages with background information as well as product descriptions
http://www.trimble.com/gpshas a very nice tutorial
http://www.garmin.com/aboutGPS/
Combining GIS and GPS Capability with Satellite Imagery for In-Field Forestry Applications
Rick Kerns
Tom Burk
Marvin Bauer
Outline
Hardware Software ImageryField Applications
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HardwareCompaq iPAQ with Magellan GPS315 for early field prototyping due to software support, cost and easy availability
HardwareFujitsu Stylistic 3500R with Magellan GPS ReceiverIncreased functionality and power with Windows 2000
Software
ESRI ArcPad 5.0.1Runs on WinCE or WindowsFamiliar “ArcView like”interfaceSupports MrSID compressed images
Software
StarPal HGIS+ GPS Mapping Software
Runs on WinCE or WindowsSimple user interfaceCustomizable wizardsSupports various sensor input besides GPS
Imagery
Imagery depends on user’s applicationWe have focused on
Digitized aerial photographsIKONOS Geo product (1 meter resolution)
Imagery must be compressed to be stored and viewed on small platform computers
Possible with LizardTech and ER Mapper products
Specific Field Applications
Regeneration SurveysImagery allows evaluations of spatial homogeneity
Corner Location and Line Running
Effort, as measured by amount of time, reduced
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Regeneration Surveys
Survey traverse and sample plot locations are mapped in GIS prior to entry into the fieldGPS is used to navigate through the traverseGIS is used in the field to collect data at the sample plots
Regeneration Surveys
The digital satellite data is used to create a “greenness” map (NDVI)Suspected “holes” in the vegetation are mapped on the desktop GISWhile traversing the survey the holes are verified, mapped and attributed.
Corner Location and Line Running
Approximate corner locations are mapped using a PLS layer at the 40 level (¼ of ¼ PLS section)A roads layer is used as a visual reference during navigationGPS is used for navigating to the corners and running the boundary lines
ArcPad’s Custom Data Entry Forms
StarPal’s Custom GridBack-up data in the field
Data collected in the field can be downloaded via a standard serial connection to a laptop computer
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Updated data in ArcViewUrban Forest Health Field Data Collection
“The field computer with ArcPadand GPS allowed us to view DOQs and parcel data from the county surveyor departments while running the real time tracking option in the field. Additionally, we had the capability to call up information from the parcel file data table. The aerial view of the study sites from the DOQs along with the information from the parcel data table, combined with the guidance of the real time tracking option, allowed us to know exactly where we were and better acquire our field data.”
ArcPad displaying parcels and DOQ data
Summary: How important is it to know where you are?
GPS, like GIS, is very complementary to remote sensing, providing a cost effective way to acquire precise location information, including elevation.It is an integral part of aerial and satellite data acquisition, as well as field data collection and mapping.