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The Global Positioning System (GPS)

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 and Inertial Nav 1940’s: Loran-A

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The Global Positioning System (GPS)

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  1. The Global Positioning System(GPS)

  2. 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 and Inertial Nav • 1940’s: Loran-A • 1960’s: Omega and Navy Transit (SatNav) • 1970’s: Loran-C • 1980’s: GPS

  3. 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 • Timation satellites, 1967-69 • Fullscale GPS development begun in 1973 • Renamed Navstar, but name never caught on • First 4 SV’s launched in 1978 • GPS IOC in December 1993 (FOC in April 1995)

  4. 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

  5. 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

  6. How It Works (In 5 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 ranging signals from several satellites • Each transmission is time-tagged • Each transmission contains the satellite’s position • The time-of-arrival is compared to time-of-transmission • The delta-T is multiplied by the speed of light to obtain the range • Each range puts the user on a sphere about the satellite • Intersecting several of these yields a user position

  7. 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

  8. The GPS Constellation • 24 operational space vehicles (“SV’s”) • 6 orbit planes, 4 SV’s/Plane • Plus at least 3 in-orbit spares • Orbit characteristics: • Altitude: 20,180 km (SMA = 26558 km) • SV’s perform full revs 2/day.

  9. Simulation: GPS and GLONASS Simulation

  10. GPS Visibility • GPS constellation is such that between 5 and 8 SV’s are visible from any point on earth • Each SV tracked by a receiver is assigned a channel • Good receivers are > 4-channel (track more than 4 SV’s) • Often as many as 12-channels in good receivers • Extra SV’s enable smooth handoffs & better solutions • Which SV’s are used for a solution is a function of geometry (math is useful!!)

  11. GPS Time • GPS time is referenced to 6 January 1980, 00:00:00 • GPS uses a week/time-into-week format • Jan 6 = First Sunday in 1980 • GPS satellite clocks are essentially synched to International Atomic Time (TAI) (and therefore to UTC) • Ensemble of atomic clocks which provide international timing standards. • TAI is the basis for Coordinated Universal Time (UTC), used for most civil timekeeping • GPS time = TAI + 19s • Since 19 leapseconds existed on 1/6/1980 • GPS time drifts ahead of UTC as the latter is “held” (leapseconds) to accommodate earth’s slowing

  12. The Almanac • In addition to its own nav data, each SV also broadcasts info about ALL the other SV’s • In a reduced-accuracy format • Known as the Almanac • Permits receiver to predict, from a cold start, “where to look” for SV’s when powered up • GPS orbits are so predictable, an almanac may be valid for months • Almanac data is large • Takes 25 subcommutations of subframes 4,5 • 12.5 minutes to tranfer in entirety

  13. 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.

  14. Available Receivers • Garmin, Magellan, Lowrance, DeLorme, Trimble, etc. • Basic 6-12 channel receivers ~$100 • Usually includes track & waypoint entry • With built-in maps ~$150 • Combination GPS receiver/cell phone ~$350 • Survey-quality: $1000 and up • Carrier tracking • FM receiver for differential corrections • RS232 port to PC for realtime or post-processing • Military Standard: $10000+ ??

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