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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

  • 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

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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

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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

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Simulation: GPS and GLONASS Simulation

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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.

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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

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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

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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

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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.