GLOBAL POSITIONING SYSTEMS

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GLOBAL POSITIONINGSYSTEMS

• A Space Age Navigational Tool

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What Is GPS?

• GPS can pinpoint your position anywhere on the
  globe, usually within a few yards or meters.
• GPS is available to everyone, everywhere, day and
  night, and best of all, at no cost for use of the
  navigational data.

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GPS USES
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Uses of GPS

• GPS operates 24 hours a day,
• in all weather conditions,
• on land,
• on water
• and even in the air.
• Current applications include
• boating, fishing, hunting, scouting on land or
  from the air,
• hiking, camping, biking, rafting, pack trips by
  horseback,
• hot air ballooning, general aviation,
  snowmobiling and
• skiing, search and rescue, emergency vehicle
  tracking,
• 4 wheeling, highway driving and a host of other
  outdoor
• activities where accurate positioning is
  required.

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History of Development

• Developed by the Department of Defense
• to simplify accurate navigation

• Conceived in 1960 and
• became fully operational
• in 1995

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Three Components of GPS

• Satellite
• Clock (time)
• Receiver

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How GPS Determines Your Position ?
GPS uses 24 satellites orbiting at 11,000
nautical miles above earth to TRIANGULATE your
position.

• Continuously monitored by ground stations located
  worldwide.
• The satellites transmit signals that can be
  detected by anyone
• with a GPS receiver.
• Using the receiver, you can determine your
  location with great precision.

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What is Triangulation?

• You are TOTALLY lost
• You find a friendly-looking person and ask,
  "Where am I?"
• The person says to you,"You are 625 miles from
  Boise, Idaho."

If you know you are 625 miles from Boise, you
could be anywhere on this circle.
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So you ask another person! She says, "You are
690 miles away from Minneapolis, Minnesota."
This is helpful -- if you combine this
information with the Boise information, you have
two circles that intersect.
If you know you are 625 miles from Boise and 690
miles from Minneapolis, then you know you must
be at one of two points.
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If a third person tells you that you are 615
miles from Tucson, Arizona, You can figure out
which of the two points you are at!
With three known points, you can determine that
your exact location is somewhere near Denver,
Colorado!
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Trilateration is a basic geometric principle that
allows you to find one location If you know its
distance from other, already known locations. The
geometry behind this is very easy to understand
in two dimensional space.
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Where are you?
Satellite 3
Satellite 1
Satellite 2
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One value will correct error and make all four
spheres meet perfectly
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GPS SATELLITES
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HOW GPS WORKSIn 5 easy steps
4. You need to know where the satellite is In
space
3. To measure time GPS needs an accurate clock
5. As the GPS signal Travels through the
ionosphere And the earths atmosphere, it gets
delayed
2. GPS measures distance using travel time of a
radio message
IONOSPHERE
X
1. Triangulation from Satellites Is the basis of
the system
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SOUNDS EASY, HUH!
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Geometric Dilution of Precision (GDOP)
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THIS IS SELECTIVE AVAILABILITY!
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Selective Availability (SA)

• A built in variable error that the US military
  requires
• the GPS satellites to transmit.

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Why?
Theoretically to keep Saddam from buying a 100
GPS unit from America, strapping it to an ICBM
and sending it our way with great accuracy. SA
error is changing all the time, every second.
Spot readings in the same place with the same GPS
will change slightly every reading. 95 of the
time the position shown on your GPS receiver is
supposed to be off 100 meters or less, (like an
atom bomb is going to matter much if it's 100
meters off target!). The other 5 of the time the
error can be more, but won't necessarily be. 100
meter error means you are within a circle 200
meters in diameter.
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Selective Availability (SA) was discontinued on
May 1, 2000

• The accuracy of GPS for civilian users before its
  removal was about 100 meters (about 300 feet)
• After removal, accuracy increased to within 20
  meters (about 60 feet) and in many cases, it is
  much, much better.

Before
After
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Any Questions?