Global Positioning Systems (GPS)

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Global Positioning Systems (GPS)
Finding your position Historical Efforts

• Greek philosopher, Erotoshenes (276 194 B.C)
  had dug wells in a place called Syene to observe
  the suns rays and tried to measure the size of
  the earth. He is credited with devising a system
  of latitudes and longitudes.
• Ptolemy, had made use of latitudes and longitudes
  in his conception of the world.
• As late as the 18th Century there was no
  international standard for an accepted latitude
  and longitude network.
• The British has their own system, with the prime
  meridian running through the Greenwich
  Observatory
• The French, as influenced by the Cassini Family,
  erected its own Prime Meridian running through
  Paris.
• So, pinpointing a position had no real standards
  to base from.

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Erosthosthenes method of measuring the size of
the earth at Syene
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The documentation of the French Prime Meridian
that passes through Paris.
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Global Positioning Systems (GPS)
Finding your position Historical Efforts

• Determining latitude at sea presents no great
  problem.
• The use of a sextant helps. It is an instrument
  for measuring the angle of stars above the
  horizon. When the angle and the time are
  measured, they can be used to calculate a
  positional line on a sea chart.
• The sextant can also be used to take a sighting
  of the noon sun to determine the degree of
  latitude.
• Since longitudes are not parallel lines, and they
  converge on the poles, measuring longitude will
  need a different approach.
• Determining longitude over land was not a problem
  if you have a chronometer (the forerunner of a
  clock), but determining longitude on a ship was
  much more difficult because earlier chronometers
  will not work at sea because of the motion of the
  ship.

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Global Positioning Systems (GPS)
Finding your position Historical Efforts

• Measuring longitude over land would simply be
  reading off the difference in time from two
  different east west locations because we do know
  that the globe is made up of 360 degrees of
  longitude and it takes 24 hours for the globe to
  rotate 360 degrees.
• So, the difference in time can proportionally
  represent the number of degrees.
• The problem was that chronometers do not work
  well at sea.
• Thus, sailors had no way of measuring which
  longitude they are located at sea.
• To solve this problem, the British Parliament
  offered 20,000 pounds to anyone who can
  accurately measure longitudes while at sea.
• A British chronometer maker, by the name of John
  Harrison, invented a chronometer by using a
  counter-balancing dual pendulum to compensate for
  the wave motion at sea.
• It worked, but Harrison was not given the award
  and died without it ultimately, it was awarded
  to his daughter.

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John Harrisons first chronometer with its dual
pendulum mechanism to counterbalance wave
actions. This chronometer is now kept at the
Greenwich Observatory.
John Harrison 1693-1776
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John Harrisons second chronometer. Again, the
dual pendulum can still be seen.
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John Harrisons third chronometer.
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John Harrisons 4th chronometer, which is more
compact and resemble the modern day pocket watch.
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Fast forward to today and how we measure latitude
and longitude with GPS.
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Global Positioning System (GPS)

• How GPS work
• Global Positioning Systems (GPS) is a navigation
  system based on signals received from satellites
  initially launched by the Department of Defense
  in 1978.
• It was originally called NAVSTAR (Navigation
  System with Timing and Ranging).
• It was initially designed for use by the US
  military.
• It grew to a full constellation of 24 satellites
  in 1994.
• But in 1983 when President Reagan mentioned GPS
  at the slip of his tongue, civilian mapmakers
  demanded the use of these signals for mapping
  purposes.
• It was then GPS was opened up for use by civilian
  mapmakers.
• The initial impact was that a better and more
  accurate surveying and mapping datum was
  developed (this is the North American Datum of
  1983, or NAD83)
• As the technology of utilizing GPS signals
  developed and matured, todays society is reaping
  the endless list of benefits that GPS has made
  possible.

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Global Positioning Systems (GPS)

• How GPS work
• The 24 GPS satellites circle the earth twice a
  day in three groups of two very near orbits
  (totals 6) that have been precisely determined to
  ensure total coverage globally at any one time.

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Global Positioning Systems (GPS)

• How GPS work
• Each satellite is built to last 10 years
  replacement satellites are constantly built and
  launched into orbit
• Each satellite has a wing-span of solar panels
  and operates with solar energy. But there are
  back-up batteries on board to ensure continuous
  operation should there be no solar power, as in
  solar eclipses or on the shadow side.
• Very accurate atomic clocks are carried on board
  these satellites (estimated to be one second off
  in a million years).
• GPS signals are broadcasted every half second 24
  hours a day, under any weather conditions,
  covering the entire surface of the earth, and
  with no subscription fee. (compare with OnStar!)
• GPS receivers on the ground compares the time
  signal sent by the satellites with the time
  signals received by the GPS receiver. This
  difference is calculated by the GPS receiver unit
  to determine how far away is the GPS satellite.

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Global Positioning Systems (GPS)

• How GPS work
• The GPS receiver must lock into position with at
  least three satellites to be able to calculate
  2-dimensional position, and at least four
  satellites to calculate 3-D (including elevation)
  position. This process, based on trigonometry,
  is called trilateration.

http//electronics.howstuffworks.com/gps.htm http
//www.trimble.com/gps/index.shtml
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YouTube video explaining how GPS works (335
min.) http//www.youtube.com/watch?vwi_3XwkA8cQf
eaturerelated
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Global Positioning Systems (GPS)

• Accuracy Aspects of GPS
• Early civilian GPS accuracy were good to 50
  meters. While this accuracy is not good enough
  for mapping, it is good enough for several
  applications, e.g. accidents with airbag
  deployment and when ambulances arrive, they can
  certainly see the accident within 50 meters (or
  165 feet).
• During the Clinton administration, Selective
  Availability was taken off, and GPS accuracy was
  then active to about 3 meters (slightly over 10
  feet).
• In the meantime, the European Union launched its
  own GPS satellites, called Galileo, and the
  Russians has its own GPS satellites.
• Today, most GPS units have an accuracy of 1
  meter.
• Survey grade GPS base stations, with built in
  differential calculations can be as accurate as 1
  cm.
• As accurate as these data can be, there is also
  error due to atmospheric scatters and delay of
  time signals.

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International collaboration in the development of
GPS signal plan.
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Global Positioning Systems (GPS)

• Accuracy Aspects of GPS
• These errors can be corrected and may be done by
  professionals who needed superior accuracy.
• Garmin, a GPS receiver manufacturer, also has the
  option of WAAS (Wide Area Augmentation System),
  an automatic self-correction system built into
  some high end receivers.
• For most of the uses by the average citizen, I
  meter resolution is accurate enough.
• For mapmakers, the process of Differential GPS is
  important.
• Differential GPS is based on the use of a base
  station that has a known and well-established
  position data will be broadcasted from the base
  station to be used by a GPS receiver to verify
  the accuracy of signals received from the
  satellites somewhat like a dual process just to
  verify the signal accuracies.

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Global Positioning Systems (GPS)

• Uses of GPS
• One can certainly think of many different uses of
  GPS.
• It ranges from the most common use in automobile,
  aerial, and marine navigation to keeping track of
  hiking trails to locating restaurants and gas
  stations.
• But there are many more innovative uses of GPS
  simply because GPS units are portable and works
  in real time.
• Today, GPS units can be integrated with hand-held
  computers, blackberries, and even cell phones.
• The portability of GPS units makes it ideal for
  fieldwork applications, e.g. fighting brush
  fires, ecological mapping, archaeological finds,
  etc.
• But there are also advanced scientific
  developments that are possible simply because of
  GPS.

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Experiments with autonomous (no drivers)
navigation. The team of winners of an annual
competition successfully designed an automobile
that traveled over 130 miles of an obstacle
course in a desert terrain, without a driver.
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Some Resources http//www.navcen.uscg.gov/gps/ ht
tp//www.colorado.edu/geography/gcraft/notes/gps/g
ps_f.html http//www.gpsy.com/gpsinfo/ (GPS
Resource Library) Using GPS with Google Earth
A User Guide http//earth.google.com/userguide/v4
/ug_gps.html GPS Visulaizer http//www.gpsvisualiz
er.com/ NASA GPS applications exchange
http//gpshome.ssc.nasa.gov/
VW google-based GPS http//www.cnet.com/gps.ht
ml