The Global Positioning System GPS

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The Global Positioning System(GPS)
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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
• 1930s Radar and Inertial Nav
• 1940s Loran-A
• 1960s Omega and Navy Transit (SatNav)
• 1970s Loran-C
• 1980s 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
• Timation satellites, 1967-69
• Fullscale GPS development begun in 1973
• Renamed Navstar, but name never caught on
• First 4 SVs launched in 1978
• GPS IOC in December 1993 (FOC in April 1995)

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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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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 (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 satellites
  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

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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 http//giswww.pok.ibm.com/gps
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The GPS Constellation

• 24 operational space vehicles (SVs)
• 6 orbit planes, 4 SVs/Plane
• Plus at least 3 in-orbit spares
• Orbit characteristics
• Altitude 20,180 km (SMA 26558 km)
• SVs perform full revs 2/day.

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Simulation GPS and GLONASS Simulation
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GPS Visibility

• GPS constellation is such that between 5 and 8
  SVs 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
  SVs)
• Often as many as 12-channels in good receivers
• Extra SVs enable smooth handoffs better
  solutions
• Which SVs are used for a solution is a function
  of geometry (math is useful!!)

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GPS Time

• GPS time is referenced to 6 January 1980,
  000000
• 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 earths
  slowing

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The Almanac

• In addition to its own nav data, each SV also
  broadcasts info about ALL the other SVs
• In a reduced-accuracy format
• Known as the Almanac
• Permits receiver to predict, from a cold start,
  where to look for SVs 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

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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.

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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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