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Satellites

There are quite anumber of satellitesout there in space.

They are used for awide range ofpurposes: satelliteTV, cellular phones,military purposesand etc. Satellitescan also be used by

GPS receivers.

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GPS Satellites

The GPS OperationalConstellation

consists of 24satellites that orbitthe Earth in veryprecise orbits twice a

day. GPS satellitesemit continuousnavigation signals.

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Receivers and Satellites

GPS units are madeto communicate with

GPS satellites(which have a muchbetter view of theEarth) to find out

exactly where theyare on the globalscale of things.

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GPS Signals

Each GPS satellitetransmits data thatindicates its location

and the current time.All GPS satellitessynchronizeoperations so that

these repeatingsignals aretransmitted at the

same instant.

Physically the signal is

just a complicateddigital code, or in other

words, a complicated

sequence of on and

off pulses.

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Time Difference

The GPS receiver

compares the time a

signal was transmittedby a satellite with the

time it was received.

The time difference

tells the GPS receiver

how far away the

satellite is.

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Calculating Distance

Velocity x Time = Distance

Radio waves travel at the speed of light, roughly 186,000miles per second (mps)

If it took 0.06 seconds to receive a signaltransmitted by a satellite floating directly

overhead, use this formula to find your distance

from the satellite.

186,000 mps x 0.06 seconds = 11,160 miles

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Triangulation

GeometricPrinciple:

You can find one

location if youknow its distancefrom other,already-known

locations.

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Triangulation

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Where in the U.S. am I?

To pinpoint your location better, you ask somebody else

where you are.

She says, "You are 427 miles from Minneapolis,

Minnesota. If you combine this information with the

Boise information, you have two circles that intersect.

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Where in the U.S. am I?

If a third person tells you that you are 216 miles from

Tucson, Arizona, you can eliminate one of the

possibilities, because the third circle will only intersect

with one of these points. You now know exactly where

you are

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Where in the U.S. am I?

You are in Denver, CO!

This same concept works in three-dimensional space, as

well, but you're dealing with spheres instead of circles.

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Atomic Clocks

GPS satellites use Atomic Clocks

for accuracy, but because of the

expense, most GPS receivers do

not.

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Line of Sight Transmissions

Line of sight is the

ability to draw a

straight line between

two objects without anyother objects getting in

the way. GPS

transmission are line-of-sight transmissions.

Obstructions such as trees, buildings, or naturalformations may prevent clear line of sight.

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Light Refraction

Sometimes the GPSsignal from thesatellite doesnt

follow a straight line.

Refraction is the

bending of light asit travels throughone media toanother.

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Signal Refraction

Signals from satellites can be like light.

When they hit some interference (air patterns

in the atmosphere, uneven geography, etc.)

they sometimes bend a little.

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Signal Interference

Sometimes the

signals bounce off

things before theyhit the receivers.

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PDOP

All of this

combines to makethe signal less

accurate, and

gives it what we

call a high PDOP.

PDOP = Positional Dilution of Precision

11,000 miles

11,000 miles

11,000 miles

11,000 miles

A PDOP of 8 is poor

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Differential Correction

Differential correction isa technique that greatlyincreases the accuracyof the collected GPSdata. It involves using areceiver at a knownlocation - the "basestation- and comparing

that data with GPSpositions collected fromunknown locations with"roving receivers."

ISU Base Station - http://134.50.65.125/

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Latitude and Longitude

Latitude and Longitudeare spher icalcoord inateson the

surface of the earth.Latitude is measuredNorth or South of theEquator. Longitude is

measured East or Westof Greenwich. GPS usesLatitudes andLongitudes to reference

locations.

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Waypoints

Waypoints are locations or landmarks that can

be stored in your GPS. Waypoints may be

defined and stored in the unit manually by

inputting latitude and longitude from a map orother reference.

Latitude and

Longitude

Your

location

Direction

ofwaypoint

Date

and

Tim

e

Waypoint

Or more usually,waypoints may be

entered directly bytaking a reading with theunit at the location itself,giving it a name, and

then saving the point.

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GPS Uses

Police and

Emergency

MedicalServices

Firefighters

Map makers Science

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ABOUT GPS

QUESTIONS??