An Introduction to the Global Positioning System

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Introduction To GPS
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Earth

• Circumference
• 24,900 Miles
• Rotation
• 1,038 Miles/Hour
• 1 Day Cycle

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Moon

• 1/4 Size of the Earth
• Distance from Earth
• 239,000 Miles

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The History of GPS

• Feasibility studies begun in 1960s.
• Pentagon appropriates funding in 1973.
• First satellite launched in 1978.
• System declared fully operational in April, 1995.
• Open to the public, 2000.

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How does GPS work?

• Stations on earth, and a GPS receiver, the
  distances between each of these points can be
  calculated.
• The distance is calculated based on the amount of
  time it takes for a radio signal to travel
  between these points.
• Using satellites in the sky, ground allows the
  GPS receiver to know where you are, in terms of
  latitude and longitude, on the earth.

• The more satellites the GPSr can see, the more
  accurate your reading.
• The GPSr must see the satellites, so it does
  not work well in dense forests, inside caves,
  underwater, or inside buildings.

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Components of the System

• User segment
• GPS antennas receiver/processors
• Position
• Velocity
• Precise timing
• Used by
• Aircraft
• Ground vehicles
• Ships
• Individuals

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Handheld GPS Receivers

• Garmin eTrex
• 100
• Garmin-12
• 150
• Casio GPS wristwatch
• 300
• The GPS Store

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GPS Satellite Vehicle

• Four atomic clocks
• Three nickel-cadmium batteries
• Two solar panels
• Battery charging
• Power generation
• 1136 watts
• S band antennasatellite control
• 12 element L band antennauser communication
• Block IIF satellite vehicle (fourth generation)

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GPS Satellites
Distance from Earth 12,000 Miles 2 Orbits in 24
Hours Travel 7,000 MPH Powered by Solar Energy
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Satellites
GPS satellites are controlled and operated by the
Dept. of Defense. 24 satellites in orbit
dedicated to GPS. 6 satellites are within view
of any location at one time, provided that
physical terrain, or structures do not block
them. Satellites constantly transmit their
locational information and time data.
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Space

• 24 satellites
• 12,000 miles above Earth
• Orbit twice daily
• At least 4 satellites in view
• Distance from Earth to Moon - 256,000 miles

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User Segment

• Military.
• Search and rescue.
• Disaster relief.
• Surveying.
• Marine, aeronautical and terrestrial navigation.
• Remote controlled vehicle and robot guidance.
• Satellite positioning and tracking.
• Shipping.
• Geographic Information Systems (GIS).
• Recreation.

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What does a GPS receiver do?

• GPS receiver allows you to pinpoint your
  location, anywhere in the world, based on
  latitude and longitude coordinates.
• It can tell you in what direction you are
  heading.
• It can show you
• how fast you are going
• your altitude
• a map to help you arrive at a destination

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Signal From One Satellite
The receiver is somewhere on this sphere.
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Signals From Two Satellites
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Three Satellites (2D Positioning)
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Three Dimensional (3D) Positioning
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System Performance

• Standard Positioning System
• 100 meters horizontal accuracy
• 156 meters vertical accuracy
• Designed for civilian use
• No user fee or restrictions
• Precise Positioning System
• 22 meters horizontal accuracy
• 27.7 meters vertical accuracy
• Designed for military use

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Real Time Differential GPS
True coordinates x0, y0 Correction x-5,
y3
DGPS correction x(30-5) and y(603) True
coordinates x25, y63
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How GPS Works
Okay, youre lost, you have amnesia, and you have
absolutely no idea where you are. You go up to
someone, and like a good amnesiac you ask, Where
Am I? This person tells you that you are 625
miles from Boise, Idaho. You say, sarcastically,
Wow, thats some great information. Thanks
Buddy.
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Were Getting Closer
You still have absolutely no idea where you
are, so once again you go up to someone and ask,
Where Am I? This person tells you that youre
690 miles from Minneapolis. You just scratch your
head, but then you start to remember what you
learned in GPS class, and, with the US map in
your pocket to guide you, you proceed.
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Now We Know!
You see a another smiling face and ask where
am I one more time. This time you get the reply
you are 615 miles from Tucson. The light pops on
in your brain. Its all clear now. You draw three
circles extending in the radii of 625 miles from
Boise, 690 from Minneapolis, and 615 miles from
Tucson. The circles intersect in Denver! Arent
you glad that you took this class?
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GPS by Trilateration
GPS Distance D Speed of Light x Time or D
CT
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Sources of Signal Interference
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Obstruction
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Waypoint

• A waypoint is based on coordinates entered into a
  GPS receivers memory.
• It can be either a saved position fix, or user
  entered coordinates.
• It can be created for any remote point on earth.
• It must have a receiver designated code or
  number, or a user supplied name.
• Once entered and saved, a waypoint remains
  unchanged in the receivers memory until edited
  or deleted.

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Planning a Navigation Route
Waypoint
Start
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How A Receiver Sees Your Route
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What is Geocaching?

• All around the world, people have hidden caches
  and posted the coordinates on a website.
• Anyone can go on the site and write down or
  download the coordinates of the cache to their
  GPS and go find it.
• Once you have found a cache, you can go on the
  site and report your findings
• This activity started in May of 2000, when the
  government lifted the Selective Availability of
  the GPS signals so that consumer GPS devices
  where as accurate at the military devices.
• There are over 300,00 active caches in 222
  countries.

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How do I start to Geocache?

• Go to www.geocaching.com (or other geocache site)
• Click on Hide and Seek
• Enter a zip code, city and state or coordinates
  near where you want to hunt.
• Note the level of difficulty, the type of cache
  and when it was last found.
• Once you have selected a cache to hunt for, you
  can
• Download or manually enter the waypoint to your
  GPS
• Print out the page with the coordinates, hint and
  logs

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Types of Caches

• Traditional caches- can be any size
• usually have a log book or paper and a pencil for
  you to sign
• trinkets or items to trade with items you have
  brought
• May contain a Travel Bug
• Micro caches- small containers, such as film
  canisters or magnetic key cases usually only
  have a log in them for you to sign
• Virtual Caches- The spot is the cache itself.
  May be a historic site, a place with a unique
  activity, or scenic sights.
• Multi-tier caches- have clues inside for you to
  follow to get to the next cache which may lead to
  another cache
• Benchmarks- these are permanent markers placed
  around the US government for surveying purposes.
  Some people enjoy seeing how many of these
  markers they can find.

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Geocaching Sites

• Geocaching.com http//www.geocaching.com -
  Number 1 site for geocaching the one that
  started it all
• Navicache.com http//www.navicache.com - Number 2
  web site for geocaching
• Geocachers Creed http//www.geocreed.info/
• Todays Cacher http//www.todayscacher.com -
  online magazine for geocachers
• Geocacher University http//www.geocacher-u.com
• Markwells FAQs http//members.aol.com/_ht_a/mark
  lent60544/myhomepage/Geocaching/markwellgcfaq.htm
• Buxley's Geocaching Waypoint http//brillig.com/ge
  ocaching
• KeenPeople.com http//www.keenpeople.com
• The First 100 Geocaches http//members.cox.net/pk
  public/index.html
• Geocaching with Kids http//www.eduscapes.com/geoc
  aching/kids.htm

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Educational Resources

• From a Distance- NASA site with Information and
  Lesson Plans http//wwwedu.ssc.nasa.gov/fad/defaul
  t.asp
• All About GPS (tutorial) http//www.trimble.com/gp
  s/index.html
• GIS and GPS Lesson Plan Resources
  http//www.gis2gps.com/GIS/lessons/lessons.html
• What is Latitude and Longitude http//www.rain.org
  /campinternet/gis/gis-lat-long-whatis.html
• How GPS Receivers Work http//electronics.howstuff
  works.com/gps.htm
• GPS Guide for Beginners (PDF download)
  http//www.garmin.com/aboutGPS/manual.html

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Sites of similar activities

• Letterboxing http//www.letterboxing.org/
• Bookcrossing http//www.bookcrossing.com/
• GPS Drawing http//www.gpsdrawing.com/info.htm
• Benchmarking or Benchmark hunting
  http//www.ngs.noaa.gov
• Degree Confluence Project http//www.confluence.or
  g/index.php
• GPS Games http//www.gpsgames.org/

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Citations

• Dixon, Conrad. Using GPS. 2nd ed. Dobbs Ferry
  Sheridan House, 1999.
• Letham, Lawrence. GPS Made Easy. Seattle The
  Mountaineers, 1996.
• McNamara, Joel. Geocaching for Dummies. Hoboken
  Wiley Publishing, 2004.