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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??