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.