GPS Technology

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GPS Technology
Global Positioning Systems

• Jackie Van Ryzin, John Hinner,
• Ryan Maier, and Adam Kabat

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• Global Positioning Systems
• Technology involves
• a complex network of
• global satellites that utilizes radio
• signals and mathematic
• calculations to determine the location
• of the GPS receiver.

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History and Development

• 1964, prior to GPS, there was the Transit system
• Transit had no timing devices and took 15 minutes
  to calculate a position
• US Department of Defense wanted a more precise
  method so they spent 12 million on what resulted
  in GPS

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History and Development

• GPS was originally known as the Navstar Global
  Positioning System
• started in 1973 to reduce the need for other
  forms of navigational aid
• Overcame many navigational obstacles
• Assisted in many navigational applications

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History and Development

• Original use of GPS was for military positioning,
  navigation, and weapons aiming system to replace
  Transit.
• It had higher accuracy and stable timing devices
  on board to achieve precise time transfer
• 1978-1st GPS satellites we launched
• the first products for civilian use were
  available in the mid 80s

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History and Development

• met the requirement for Full Operational
  Capability (FOC) as of April 27, 1995
• the first of the currently in-use satellites were
  launched in February 1989
• the most recent satellite was launched on March
  20, 2004

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Satellites

• Satellite constellation
• First of GPS satellites launched
• in 1978
• 16 years later,
• system up to full power
• 24 Earth-orbiting
• satellites

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Satellites

• Back-up Satellites
• at least 3 at all
• times in case of failures
• constantly developing and launching
• too expensive to fix, just replace
• Satellites orbit so that at any time, 4
• satellites are visible at any place
• on Earth

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Satellites

• Solar-powered
• Rocket boosters keep
• them on track/orbit

• complete 2 full rotations
• within 24 hours
• 7,000 miles per hour
• 12,000 miles above
• the Earth

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Radio Waves

• Transmit 2 low power signals (L1 L2)
• frequency of 1575.42 MHz in the UHF band
• Travel at speed of light
• pseudorandom code, ephemeris data, and almanac
  data
• Ephemeris data tells satellite status
  (healthy/unhealthy, current date and time)
• Almanac tells where it should be

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Measuring Distance
Pseudo-random Code

• Satellite transmits long digital pattern
• Individual satellite ID
• Receiver begins running same pattern at same time
• When signal received, the lag or delay notes the
  time traveled
• Multiply by speed of light to get distance
• this assumes straight line
• no interference

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Measuring Distance
Synchronization

• Satellites and receiver must be in-sync
• Synchronize to the nanosecond
• Atomic clocks in satellites
• 50,000-100,000
• Quartz clocks in receivers
• Affordable
• Constantly resetting to maintain accuracy
• Look to four satellites to gauge own inaccuracy

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Trilateration / Triangulation3-D / 2-D

• 2-D Example

If you are unsure of your location but you know
you are 25 miles from point A, you would know
that you are somewhere on a circle with radius of
25 miles.
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• 2-D Example

You learn from someone else that you are also 20
miles from point B. You now know that you are at
one of two locations, the intersections of the
two circles.
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• 2-D Example

To determine your exact location, you ask a third
person who responds that you are 15 miles from
point C. This leaves you with one point at the
intersection of the circles.
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3-D Translation

• Spheres in place of circles
• Two spheres give a perfect circle of
  possibilities
• Third sphere intersects this circle at two points
• Only one of these will be on Earth (feasible)
• Eliminate the one in space (infeasible)

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• Generally receivers will look at four satellites
  rather than the minimum three
• This offers greater accuracy
• Also helps in determining altitude

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Differential GPS

• Four spheres will not intersect at same point if
  measured incorrectly
• Distances are proportionally incorrect
• GPS receiver adjusts
• Adjusts distances proportionally to intersect
• Resets clock to be in-sync based on proportional
  inaccuracy
• Always adjusting so accuracy is near that of the
  atomic clocks in the satellites
• Department of Defense monitors any changes and
  transmits this to all receivers as part of
  satellite signal

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Errors that Occur

• Earths atmosphere slows the signal
• Receiver uses built-in model to partially adjust
  for this type of error
• Signal multi-path
• Large objects such as skyscrapers cause signal to
  bounce and reflect, taking a longer path
• Receiver clock errors
• Orbital or
• ephemeris errors
• Misreported location
• or orbital data
• Number of satellites
• visible

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Error Budget
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• To correct using differential GPS
• Gauge inaccuracy at a stationary receiver that
  knows its location
• Broadcast that inaccuracy to local DGPS receivers
• With DGPS
• accuracy up
• to 1-3 meters
• Provides
• accuracy to
• 10 meters

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Protocol and PC Connections

• Basic link protocol
• All data is transferred from the GPS unit
  (transmitter) to the PC (host) in byte-oriented
  packets
• Each packet contains a three-byte header, a
  number of data bytes, and a three-byte trailer

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Packet Format
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Protocol and PC Connections

• Any device that receives a data packet must send
  an ACK or NAK packet in turn
• The GPS unit recognizes an ACK from its packet ID
  of 6 and NAK packet ID of 21
• The checksum protocol in the trailer of every
  packet allows the receiver to send a NAK if the
  sum of the data it received is the same as the
  sum of the data that was sent
• Additional basic packet IDs are Product Request,
  which is sent from a host to request product
  information about the GPS unit that is being
  currently communicated with, and Product Data,
  which is sent to a host in return

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Application Protocol

• Packets sent to the host are often grouped
  together during transfers to allow better
  surveillance of the transfer
• These transfers include standard beginning and
  end packets of type records and Xfer Complete

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typedef struct char ident6 /
identifier / semicircle_Type posn /
position / longword unused /
should be set to zero / char cmnt40 /
comment / byte smbl / symbol id
/ byte dspl / display option
/ D103_Wpt_Type
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Experimentations

• Accuracy
• Recorded points at 10 minute intervals from one
  place to determine the change in the data due to
  error or inaccuracy
• G\PANKDC\cs225\projects\gps\Webpage\gps1.rtf
• Unit comparison
• We used two different units and compared both of
  their tracks to the road map to determine
  differences between them and accuracy to the
  actual map.
• G\PANKDC\cs225\projects\gps\Webpage\gps track.bmp

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Experimentations

• Number of Satellites
• Recorded data from single place when more
  satellite signals were achieved to determine the
  difference in values.
• G\PANKDC\cs225\projects\gps\Webpage\gps4.bmp
• G\PANKDC\cs225\projects\gps\Webpage\gps3.bmp
• WAAS
• WAAS is Wide Area Augmentation System
• 25 ground stations, 2 geostationary satellites
• Still developing, not available in all areas
• Unable to complete experiment, no WAAS satellite
  signal received

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What GPS can tell you

• Odometer
• Time traveled
• Speedometer
• Average speed
• Trace your path
• Estimate time of arrival at current speed

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How to use a GPS

• Each unit or model is
• different but the basics
• are the same
• Start at the main menu
• Some general options
• Mark
• Find
• Satellite
• Routes
• Tracks
• Setup
• Accessories

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Mark

• Shows location longitude and latitude
• Displays elevation in feet
• Shows the distance you have traveled
• Displays direction you are going (bearing)

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Find

• Options to find are
• waypoints (that you have marked)
• Favorites (that you have saved or marked as
  favorite stops)
• Cities (saved internally in map, detail depends
  on unit and map downloaded)
• Exits (services, rest stops, etc.)
• Can find by
• Nearest to current location
• Name that was given to that point

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Satellite

• Shows view of satellites
• looking up at the sky
• from location
• Satellites are numbered,
• outer ring is horizon
• center is directly above location
• middle circle is at a 45 angle from the vertical
• Bars above numbers denote signal strength
• D means it has a WAAS signal
• Shows elevation and location

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Other Options

• Routes Tracks
• Used to look up past paths
• Allows to retrace old travels
• Or return to original location by same path

Some views that can be used for tracking and
navigation
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• Setup
• Time
• Units
• Display
• interface (for PC
• communication)
• system (GPS on/off,
• WAAS on/off, language)
• Others dependent on the unit
• Accessories
• Sun Moon, Calendar, Hunt Fish, Area
  Calculations, Calculator
• Dependent on the model and its original purpose,
  these are just a few of the accessories Garmin
  offers

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The End

• Thanks for listening
• For more information, view our website which will
  be available from the CompSci website at
  www.snc.edu/compsci/cs225F04/ProjectsF04.html
• it is currently available at
• G\PANKDC\cs225\projects\gps\Webpage\index.html
• Our webpage includes links to popular GPS
  websites throughout the explanations