GEO%20241:%20GPS%20Global%20Positioning%20System

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GEO 241 GPSGlobal Positioning System
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What is GPS?

• Satellite based navigation system
• Developed by the Dept. of Defense
• 12 billion system
• Began construction in mid 70s
• Developed for military operations, but with a
  provision for civilian use
• Consists of 24 (28) high orbit satellites sending
  out coded radio signals that are picked up by
  receivers that calculate position

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How does GPS Work?

• Space Segment
• Control Segment
• User Segment

Image courtesy of http//www.aero.org/publication
s/GPSPRIMER/GPSElements.html
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Space Segment

• 24 NAVSTAR (NAVigation Satellite Timing And
  Ranging)
• Orbit is 20,200 km
• 1 revolution in 12 hours
• Satellite spec
• 1900lbs
• 12 ft with solar panels extened
• 12 hour orbital periods

Image courtesy of http//www.nmt.edu/mreece/gps/
whatisgps.html
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Control Segment

• Receives/Transmits Information to SV
• Master Control Station Colorado Springs
• Monitoring stations distributed throughout the
  world.

Image courtesy of http//www.nmt.edu/mreece/gps/
whatisgps.html
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Control Segment cont.

• Receives/Transmits Information to SV
• Master Control Station Colorado Springs
• Monitoring stations distributed throughout the
  world.

Image courtesy of http//www.colorado.edu/geograp
hy/gcraft/
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User Segment

• Applications
• Location
• Navigation
• Mapping
• Tracking

Image courtesy of http//www.nmt.edu/mreece/gps/
whatisgps.html
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Satellite Signal

• GPS satellites transmit information via radio
  signals on 2 frequencies
• Radio waves are electromagnetic waves, like
  light, and travel at 186,000 miles per second
• A satellite overhead will transmit its signal to
  us in 6/100ths of a second
• Most receivers measure in nanoseconds
  (0.000000001 second)

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Satellite Signal cont.

• Signals cannot penetrate water, soil, buildings
  and other obstacles.

Image courtesy of students.washington.edu/
ruby42/me.htm
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Satellite Almanac

• Sent along with timing and position information.
• Prediction of all satellite orbit.
• Needed to run mission planning using certain
  software.
• Some GPS receivers will receive and store the
  almanac.
• Valid for about 30 days.

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Satellite Ranging

• Speed of light x time distance
• EX. Car traveling at 30 mi/hr
• In 1 hr, what is the distance the care traveled?
• 30mi/hr 1 hr 30mi
• Speed of the radio signal
• Equals the speed of light (300,000 km per second
  or 186,000 miles)

50 millisecond
40 millisecond
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Measuring the distance from a satellite

• Speed of light x time distance
• Radio waves are electromagnetic waves, like
  light, and travel at 186,000 miles per second
• Each satellite is equipped with an atomic clock
• Most receivers measure in nanoseconds
  (0.000000001 second)

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Measuring the distance from a satellite
Why is it so important to know the distance of
the satellite?
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Triangulation
50 millisecond
40 millisecond
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Triangulation
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Triangulation
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Triangulation
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Triangulation
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• ERRORS

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GPS Errors (typical)

• Satellite clock error 2 feet
• Ephemeris error 2 feet
• Receiver error 4 feet
• Atmospheric delay 12 feet
• Selective Availability (if on) 25 feet
• Multipath errors and obstructions

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Differential GPS

• A receiver placed at a known location calculates
  the combined error in the satellite range data
• That correction can be applied to all other
  receivers in the same locale, to eliminate
  virtually all error in their measurements.
• This can be done in real time or by
  post-processing the data after collection

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Differential GPS
Figure Courtesy of Trimble Navigation Limited
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Differential GPS Methods

• Postprocessed Differential
• Error are corrected after data has been collected
• Real-time Differential
• Errors are corrected in the field
• WAAS (Wide Area Augmentation Signal)
• Errors are corrected in the field

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WAAS (Wide Area Augmentation Signal)

• Developed by Federal Aviation Administration
• Free differential correction
• 1 - 3 meter accuracy
• Will allow aircraft navigation with GPS
• There are 2 geostationary WAAS satellites (east
  west coasts of U.S.)

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Accuracy

• High accuracy requires more rigorous data
  collection parameters.
• Logging Interval
• the frequency position data is collected
• Elevation Mast
• The amount of sky viewable
• Signal-To-Noise Ratio
• Strength satellite signal compare to noise
• Position of Dilution of Precision
• Satellite Geometry

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Logging Interval

• Logging interval defines the frequency at which a
  position is stored.
• Point features Set at 1 second
• Line/Area features should match the base
  station logging interval. Logging intervals may
  also depend on speed of travel
• If walking 5 seconds
• If driving 1 second

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Elevation Mask

• Elevation masks restrict your receiver to using
  only those satellites above a certain elevation
  in the sky.
• This ensures that a base station can always see
  all the satellites used by the rover.
• The default elevation mask for a rover is 15
• Lowing masks increase of satellites, increases
  atmospheric delay and increases effects of
  multipath.

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Signal to Noise Ratio Mask

• As the proportion between the signal and the
  noise decreases, data is distorted by the noise.
• The higher the value of the SNR, the better.
• Quality is degraded if it falls below 6.0
• Typical SNRs range between 10 and 25