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The Global Positioning System (GPS)

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The Global Positioning System (GPS). Brief History of Navigation. PreHistory - Present: Celestial Navigation Ok for latitude, poor for longitude until accurate clock invented ~1760 13 th Century: Magnetic Compass 1930’s: Radar 1940’s: Loran-A 1960’s: Omega and Navy Transit (SatNav)

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brief history of navigation
Brief History of Navigation
  • PreHistory - Present: Celestial Navigation
    • Ok for latitude, poor for longitude until accurate clock invented ~1760
  • 13th Century: Magnetic Compass
  • 1930’s: Radar
  • 1940’s: Loran-A
  • 1960’s: Omega and Navy Transit (SatNav)
  • 1970’s: Loran-C
  • 1980’s: GPS
brief history of gps
Brief History of GPS
  • Original concept developed around 1960
    • In the wake of Sputnik & Explorer
  • Preliminary system, Transit, operational in 1964
    • Developed for nuclear submarines
    • 5 polar-orbiting satellites
  • Fullscale GPS development begun in 1973
    • Renamed Navstar, but name never caught on
  • First 4 satellites launched in 1978
  • GPS approved for commercial use in 1993
gps tidbits
GPS Tidbits
  • Development costs estimate ~$12 billion
  • Annual operating cost ~$400 million
  • 3 Segments:
    • Space: Satellites
    • User: Receivers
    • Control: Monitor & Control stations
  • Prime Space Segment contractor: Rockwell International
  • Operated by US Air Force Space Command (AFSC)
    • Mission control center operations at Schriever (formerly Falcon) AFB, Colorado Springs
gps visibility
GPS Visibility
  • GPS constellation is such that between 5 and 8 satellites are visible from any point on earth
  • Each satellite is tracked by a receiver (the user)
  • Trees and buildings will block the radio signals from the satellites.
who uses it
Who Uses It?
  • Everyone!
  • Merchant, Navy, Coast Guard vessels
    • Forget about the sextant, Loran, etc.
  • Commercial Airliners, Civil Pilots
  • Surveyors
    • Has completely revolutionized surveying
  • Commercial Truckers
  • Hikers, Mountain Climbers, Backpackers
  • Cars! Cell phones!!
  • Communications and Imaging Satellites
    • Space-to-Space Navigation
  • Any system requiring accurate timing
how it works easy steps
How It Works (Easy Steps)
  • GPS is a ranging system (triangulation)
    • The “reference stations” are satellites moving at 4 km/s
  • A GPS receiver (“the user”) detects 1-way signals from several satellites
    • Each transmission contains information when it was sent
    • Each transmission contains the satellite’s position
  • The time-of-arrival is compared to time-of-transmission
    • the radio waves travel at 300,000km/s
    • The GPS can measure how long the signal took to travel from the satellite to the GPS and using the formula: distance = speed x time to calculate how far away the GPS is from the satellite.
  • The receiver then picks up a signal from another satellite. Each range puts the user on a sphere about the satellite. The GPS knows it is somewhere on that sphere.
  • Intersecting several of these yields a user position
multi satellite ranging
Multi-Satellite Ranging

A 3rd range constrains user to 1 of the 2 points.

1 range puts user on the spherical face of the cone.

Intersecting with a 2nd range restricts user to the circular arcs.

Pictures courtesy http://giswww.pok.ibm.com/gps

how accurate is it
How Accurate Is It?
  • The expected accuracy of a standard GPS (they do a vary!) is around 7 meters vertically and horizontally.
  • Testing with the Magellan GPS receivers will typically experience accuracy improvements to around 3 meters.
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