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How GPS* Works

*Global Positioning System

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How GPS Works The Global Positioning System (GPS)

consists of 24 Earth-orbiting satellites.

These satellites allow any person who owns

a GPS receiver to determine his or her

precise longitude, latitude and altitude

anywhere on the planet

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GPS Constellation

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Triangulation

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Triangulation (continued)

Ifyou know you are 625 miles from Boise, youcould be anywhere on this circle.

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Triangulation (continued)

Ifyou know you are 625 miles from Boiseand 690 miles from Minneapolis, then you

know you must be at one of two points.

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Triangulation (continued)

With three known points, you can determine that yourexact location is somewhere near Denver, Colorado!

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3-D Triangulation

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3-D Triangulation

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3-D Triangulation

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3-D Triangulation

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3-D Triangulation

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3-D Triangulation

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Measuring Distance GPS satellites send out radio

signals that your GPSreceiver can detect.

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Measuring Time

Measuring the time would be easy if you

knew exactly what time the signal left the

satellite and exactly what time it arrived atyour receiver, and solving this problem is

key to the Global Positioning System.

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Measuring Time (continued)

The only way to implement a system like

this would require a level of accuracy only

found in atomic clocks.

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Measuring Time (continued)

The Global Positioning System has a very

effective solution to this problem -- a GPS

receiver contains no atomic clock at all. Ithas a normal quartz clock.

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Finding the Satellites

The other crucial component of GPS

calculations is the knowledge of where the

satellites are.

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Cool GPSF

acts The first GPS satellite was launched in

1978.

The current system is composed of second

generation GPS satellites, called Block II.

The first Block II satellite was launched in

1989.

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Cool GPSF

acts (continued)

The Defense Department declared GPS fully

operational in 1995.

When the system was first introduced,miscalculations were programmed into GPS

transmissions to limit the accuracy of non-military

GPS receivers. This operation was cancelled in

May 2000.

There are 24 GPS satellites in orbit at this moment

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GPS Accuracy Before and After SA

Removal

May 1, 2000 May 3, 2000

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Cool GPSF

acts(continued)

The 24 satellites cost an estimated $12

billion to build and launch.

Each satellite weighs about 1,735 lb (787

kg).

The satellites are in orbit about 12,500 mi

(20,000 km) above the Earth.

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Cool GPSF

acts(continued)

A satellite takes 12 hours to orbit the Earth

once.

The Russians have a system identical to the

U.S. system called GLONASS.

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What GPS Can Do The Global Positioning System, a collection

of 24 Earth-orbiting satellites, has a number

of possible applications, spanning acrossseveral areas of society.

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What GPS Can Do (continued)

The basic function of a GPS receiver is to

figure out its location on Earth.

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GPS Basics One of the basic characteristics of GPS

receivers is that they find your location only

when you are outside.

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GPS Basics(continued)

The simplest GPS receiver would give you

just the coordinates of your location on

Earth in latitude, longitude and altitude.

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GPS Basics(continued)

Even low-end receivers have some sort of

electronic map stored in memory.

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GPS inM

otion A standard GPS receiver will not only place

you on a map at any particular location, but

will also trace your path across a map asyou move.

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GPS inM

otion(continued)

y How far you've traveled (odometer)

y How long you've been traveling

y Your current speed (speedometer)

y Your average speed

y A "breadcrumb" trail showing you exactly where

you have traveled on the map

y The estimated time of arrival at your destination

if you maintained your current speed

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GPS User Input Most receivers have a certain amount of

memory available for you to store your own

navigation data.

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GPS User Input(continued)

The basic unit of user input is the waypoint.

A waypoint is simply the coordinates for a

particular location. You can save this in

your receiver's memory in two ways:y You can tell the receiver to record its coordinates when

you are at that location.y You can find the location on a map (the internal map or

another one) and enter its coordinates as a waypoint.

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GPS User Input(continued)

Examples ofwaypoints:

y Good camp sites

y Favorite road-side shops

y Excellent fishing spots

y Scenic overlooks

y Where you left your car

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GPS User Input(continued)

You can also combine a series of different

waypoints to form a route.

One way to use this function is to

periodically record waypoints as you make

a trip so that you can backtrack, or follow

the same route again on another trip.

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Computer Connections Receivers with route capabilities will let you save

a certain number of waypoints to memory so that

you can use them again and again. If the receiver has a data port, you can also

download your routes to a computer, which has

much more storage memory, and then upload them

again when you plan to follow those routes.

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Computer Connections(continued)

Some recent receivers let you download

detailed maps of an area into the GPS, or

supply detailed maps with plug-in mapcartridges.

These maps can give you street-level detail

in cities and the receiver may even providedriving directions as you drive!

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Important Features for a

Hiking GPS "Map" screen

Water proofness

Long battery life

Built in Maps

Route capability

Waypoints Multiple Datum capability is pretty standard

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Important Features for a

Hiking GPS (continued)

Fit nicely in your pocket and lightweight

Bearing to next waypoint

Screen Size

12 channel parallel receiver system

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Non-Essential Features for a

Hiking GPS

A minimum of8 megs ofmap memory

Rotatable screen

UTM readout may be a consideration for

some

Topo Maps uploaded into your GPS

External amplified antenna

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See This Web Site forM

ore Info

For Hiking GPS Recommendations:

http://gpsinformation.us/main/gpshiking.htm

For Auto GPS Recommendations:

http://gpsinformation.us/main/gpsauto.htm

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Wide Area Augmentation System

WAAS

WAAS is based on a network o

f

approximately 25 ground

reference stations that covers a

very large service area.

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WAAS

wide area ground reference stations (WRSs)

wide area master station (WMS)

ground uplink system (GUS)

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WAAS

TheWAAS will improve basic GPS

accuracy to approximately 7 meters

vertically and horizontally