How GPS Works Global Positioning System

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How GPS WorksGlobal 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)
If you know you are 625 miles from Boise, you
could be anywhere on this circle.
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Triangulation (continued)
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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Triangulation (continued)
With three known points, you can determine that
your exact 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
  GPS receiver 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 at your
  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. It has
  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 GPS Facts

• 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 GPS Facts (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 GPS Facts (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 GPS Facts (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 across several
  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 in Motion

• 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 as you move.

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GPS in Motion (continued)

• How far you've traveled (odometer)
• How long you've been traveling
• Your current speed (speedometer)
• Your average speed
• A "breadcrumb" trail showing you exactly where
  you have traveled on the map
• 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
• You can tell the receiver to record its
  coordinates when you are at that location.
• 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 of waypoints
• Good camp sites
• Favorite road-side shops
• Excellent fishing spots
• Scenic overlooks
• 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 map cartridges.
• These maps can give you street-level detail in
  cities and the receiver may even provide driving
  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 of 8 megs of map 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 for More 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 SystemWAAS

• WAAS is based on a network of
• 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

• The WAAS will improve basic GPS accuracy to
  approximately 7 meters vertically and
  horizontally