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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 and Inertial Nav 1940’s: Loran-A

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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 and Inertial Nav
  • 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
  • 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)
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
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 in 5 easy steps
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
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

the gps constellation
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.
gps visibility
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!!)
gps time
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
the almanac
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
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.
available receivers
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+ ??