Global Positioning Systems and GIS An Introduction

Department of Geography | Kansas State University | 118 Seaton Hall | Manhattan, Kansas 66506 GEOG 302 Cartography and Thematic Mapping

Global Positioning Systems Global Positioning Systems and GISand GIS

An IntroductionAn Introduction

Tom Vought and Katie FrankeTom Vought and Katie Franke

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GPS and GIS

GPS = Global Positioning System

Geocaching = an entertaining adventure game for GPS users.

Individuals and organizations set up caches all over the world and share the locations of these caches on the internet. GPS users can then use the location coordinates to find the caches.

GIS = Geographic Information System

Any system for capturing, storing, analyzing, managing and presenting data and associated attributes which are spatially referenced to Earth.

Data capture = entering information into a GIS

Needs to be done well [garbage in garbage out]

Tends to be very time consuming

Positions from a GPS can also be directly entered into a GIS

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GIS Defined

A geographic information system is a framework for understanding our world and applying geographic knowledge to solve problems and guide human behavior.

A GIS can produce information that answers specific questions and allows you to share that information with others. By visualizing relationships, connections, and patterns in data, you can make informed decisions and increase efficiency throughout your organization.

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GIS Defined Tool for visualizing and analyzing the

interaction of spatial phenomena by overalying and displaying geodata layers

Interaction of Software (ArcGIS)

Data (shapefiles, rasters)

Users (you)

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GIS – John Snow’s map of cholera in London [1854]

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What is a GIS – 3 views

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The Earth’s GraticuleSystem for locating places on Earth

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Where is Manhattan?

39o 12’ N96o 35’ W2,705 miles north of the Equator

5,126 miles west of the Prime Meridian

4,480 miles east of the International Date Line

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What is GPS?

GPS = Global Positioning System

Space-based (satellite) positioning system for navigation, tracking, and mapping

Designed and maintained by the U.S. Department of Defense

Global and available “24-7-365”

Your location is:37o 23.323’ N

122o 02.162’ W

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Evolution of GPS – 1970s

Grew out of two US military projects that were combined into the Defense Defense Navigation Satellite SystemNavigation Satellite System (DNSS):

US Navy Timation Project

US Air Force 621B Project

DNSS led to Navstar (Navigation System with Timing and RangingNavigation System with Timing and Ranging) – later referred to as Global Positioning System or “GPS”.

First generation of GPS satellites (Phases I and II) deployed.

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Evolution of GPS – 1980s

1984: No-cost civilian access to the GPS guaranteed by the US government.

1989: First Phase III (Block II) satellites launched.

1993: IOC (Initial Operation Capability) achieved with 24 fully functional satellites.

July 1995: FOC (Full Operational Capability) achieved, 24 fully functional Block II satellites.

Name: NAVSTAR

Manufacturer: Rockwell International

Altitude: 12,544 Miles

Weight: 1900 lbs.

Size: 17 ft. (solar panels extended).

Orbital Period: 12 hours

Orbital Plane: 55° to equatorial plane

Planned Lifespan: 7.5 years

Thrusters for orbit adjustments

Current Constellation: 29 Block II satellites

Block II NAVSTAR GPS Satellite

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GPS ConstellationNot all satellites shown…

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Other Systems

BeidouBeidou – China’s regional system; proposal to expand to global system called COMPASSCOMPASS

GalileoGalileo – global system proposed by EU for 2011-12

GLONASSGLONASS – Russia global system being restored with India

IRNSSIRNSS – proposed regional system by India

QZSSQZSS – Japanese proposed regional system

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How Does GPS Work?

Velocity of GPS radio signal is constant = 186,000 mi/sec

Need to measure travel time of signal from satellite to ground receiver.

GPS satellites have 4 onboard atomic clocks: 2 rubidium and 2 cesium.

Distance = Velocity x TimeDistance = Velocity x Time

600 miles

Three people are driving to the same destination, but we don’t know what the destination is.

The first person starts at the red dot and drives in a straight line at 60 mph for 10 hours (600 miles), so they would be somewhere on the edge of this circle.

Introducing TrilaterationWith one data source…

The second person starts at the green dot and also drives in a straight line at 60 mph for 10 hours (600 miles), so they would be somewhere on the edge of their circle.

This means that the destination must be on one of the two yellow dots.

600 miles 600 miles

Introducing TrilaterationAdd a second data source…

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600 miles 600 miles

Finally the third person starts at the purple dot and also drives in a straight line at 60 mph for 10 hours (600 miles), so they would be somewhere on the edge of their circle.

This means that the destination must be on the yellow dot.

600 miles

Introducing Trilaterationand a third data source…

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Even if the speed of and distance traveled by each vehicle was different – the process for identifying the destination would be the same.

What is this distance?

Making Trilateration Harder!

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GPS TrilaterationSame as previous example, but backwards. “Destination” is known, but where were the three starting points?= Velocity x Time

How do we measure the distance between person on the ground and each of three satellites?

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How Does GPS Work?

Velocity of GPS radio signal is constant = 186,000 mi/sec

Need to measure travel time of radio signal from satellite to ground receiver.

Distance from three satellites mandatory, four gives a more accurate position fix.

Distance = Velocity x Time

50 milliseconds 40 milliseconds

90 m

illis

econ

ds

One-Way Satellite RangingDistance = Velocity x Time

0.050 x 186,000 = 9,300 miles0.090 x 186,000 = 16,740 miles0.040 x 186,000 = 7,440 miles

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Sources of GPS Error

Slight errors in time measurement introduce large errors in distance – very precise clocks needed to measure time in billionths of seconds.

1 millisecond mistake = 186 miles!

Major Sources of Positional Error:

Satellite/Receiver Clock Error

Receiver Noise

Atmospheric Delay

Multipath Error

Selective Availability

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GPS signals are delayed as they pass through the atmosphere

Troposphere

Ionosphere

Less Error More Error

Atmospheric Delay

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Selective Availability (S/A)

Government introduces artificial errors to reduce GPS position accuracy

With S/A on – no better than 100 meter accuracy

Turned off in 2000, but could be turned back on at any time…

100 meters

100 meters

40 meters

20 meters

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Current GPS Status

U.S. Coast Guard Navigation Center

http://www.navcen.uscg.gov/gps/status_and_outage_info.htm

Latest GPS Status Message

http://www.navcen.uscg.gov/ftp/GPS/status.txt

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GPS and GIS

GPS = Global Positioning System

Geocaching = an entertaining adventure game for GPS users.

Individuals and organizations set up caches all over the world and share the locations of these caches on the internet. GPS users can then use the location coordinates to find the caches.

GIS = Geographic Information System

Any system for capturing, storing, analyzing, managing and presenting data and associated attributes which are spatially referenced to Earth.

Data capture = entering information into a GIS

Needs to be done well [garbage in garbage out]

Tends to be very time consuming

Positions from a GPS can also be directly entered into a GIS