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Introduction
               to
         NAVSTAR GPS
Charlie Leonard, 1999
(revised 2001, 2002)
The History of GPS
   Feasibility studies begun in 1960’s.
   Pentagon appropriates funding in 1973.
   First satellite launched in 1978.
   System declared fully operational in April, 1995.
How GPS Works
Three Segments of the GPS

   Space Segment




                                           User Segment

            Control Segment

                                              Ground
                                              Antennas
Master Station          Monitor Stations
Control Segment

      US Space Command



                 Cape Canaveral
  Hawaii
                                                             Kwajalein Atoll

                                              Diego Garcia
                                  Ascension
                                     Is.




Master Control Station            Monitor Station      Ground Antenna
Space Segment
User Segment
   Military.
   Search and rescue.
   Disaster relief.
   Surveying.
   Marine, aeronautical and terrestrial navigation.
   Remote controlled vehicle and robot guidance.
   Satellite positioning and tracking.
   Shipping.
   Geographic Information Systems (GIS).
   Recreation.
Four Basic Functions of GPS
   Position and coordinates.

   The distance and direction between any two waypoints, or a
    position and a waypoint.

   Travel progress reports.

   Accurate time measurement.
Position is Based on Time
    Signal leaves satellite
    at time “T”

                              T



                Signal is picked up by the
       T+3      receiver at time “T + 3”
         Distance between satellite
         and receiver = “3 times the
         speed of light”
Pseudo Random Noise Code

               Time
               Difference



                            Satellite PRN




Receiver PRN
What Time is It?
                 Universal Coordinated Time



Greenwich Mean Time          GPS Time + 13*        Zulu Time


         Local Time: AM and PM (adjusted for local
                        time zone)


                            Military Time
                  (local time on a 24 hour clock)


* GPS Time is ahead of UTC by approximately 13 seconds
Signal From One Satellite

                     The receiver is
                     somewhere on
                     this sphere.
Signals From Two Satellites
Three Satellites (2D Positioning)
Triangulating Correct Position
Three Dimensional (3D) Positioning
Selective Availability (S/A)
   The Defense Department dithered the satellite time
    message, reducing position accuracy to some GPS users.
   S/A was designed to prevent America’s enemies from using
    GPS against us and our allies.
   In May 2000 the Pentagon reduced S/A to zero meters error.
   S/A could be reactivated at any time by the Pentagon.
Sources of GPS Error
Standard Positioning Service (SPS ): Civilian Users
      Source                  Amount of Error
    Satellite clocks:        1.5 to 3.6 meters
    Orbital errors:          < 1 meter
    Ionosphere:              5.0 to 7.0 meters
    Troposphere:             0.5 to 0.7 meters
    Receiver noise:          0.3 to 1.5 meters
    Multipath:               0.6 to 1.2 meters
    Selective Availability   (see notes)
    User error:              Up to a kilometer or more
Errors are cumulative and increased by PDOP.
Receiver Errors are Cumulative!


    System and other flaws = < 9 meters




   User error = +- 1 km
Sources of Signal Interference

                         Earth’s Atmosphere




    Solid Structures



                 Metal   Electro-magnetic Fields
Using GPS Receivers for
Positioning and Navigation
GPS Navigation Terminology
                                                                         Active GOTO
                                                                         Waypoint




                                                                t              ypoin
    N (0000)




                                               g
                                           Le




                                                                Distance to Wa
                                           e
                                        tiv
               Desired Track                                         )




                                                                           G)
                                      Ac
                                                                 0




                                                                     nd ver
                                                                (X




                                                                       (SO
               (DTK) (xº)                                   g




                                                                Gr e d O
                                                          in
                                                        ar
       (CMG) (xº)                        N (00)    Be




                                                                    e
                                                                  ou
                                                                 Sp
                               XT
                                  E
                                 (CD
                                    I)


                                                                 Tracking (TRK) (xº)
Active Course Made Good (CMG)
                                       Present
From
                                       Location
Waypoint
GPS Navigation: On the Ground

                                                         Active GOTO




                                   L eg
                                                         Waypoint
   N




                                  tive
                Bearing = 780




                                Ac
                COG = 3500
Bearing = 650   XTE = 1/3 mi.
COG = 50 XTE
= 1/2 mi.
                                         Bearing = 400
                                         COG = 1040
                                         XTE = 1/4 mi.

                            Course Over Ground (COG) =
Location Where GOTO         Bearing =
Was Executed                Cross Track Error (XTE) =
Position Fix
   A position is based on real-time satellite tracking.
   It’s defined by a set of coordinates.
   It has no name.
   A position represents only an approximation of the
    receiver’s true location.
   A position is not static. It changes constantly as the GPS
    receiver moves (or wanders due to random errors).
   A receiver must be in 2D or 3D mode (at least 3 or 4
    satellites acquired) in order to provide a position fix.
   3D mode dramatically improves position accuracy.
Waypoint
   A waypoint is based on coordinates entered into a GPS
    receiver’s memory.
   It can be either a saved position fix, or user entered
    coordinates.
   It can be created for any remote point on earth.
   It must have a receiver designated code or number, or a user
    supplied name.
   Once entered and saved, a waypoint remains unchanged in
    the receiver’s memory until edited or deleted.
Planning a Navigation Route




                     = Waypoint
Start
How A Receiver Sees Your Route
GPS Waypoint Circle of Error




             X
GPS Dilution of Precision and Its
  Affects On GPS Accuracy
GPS Satellite Geometry
   Satellite geometry can affect the quality of GPS signals and accuracy
    of receiver trilateration.
   Dilution of Precision (DOP) reflects each satellite’s position relative
    to the other satellites being accessed by a receiver.
   There are five distinct kinds of DOP.
   Position Dilution of Precision (PDOP) is the DOP value used most
    commonly in GPS to determine the quality of a receiver’s position.
   It’s usually up to the GPS receiver to pick satellites which provide the
    best position triangulation.
   Some GPS receivers allow DOP to be manipulated by the user.
Ideal Satellite Geometry
               N




W                              E




               S
Good Satellite Geometry
Good Satellite Geometry
Poor Satellite Geometry
               N




W                             E




               S
Poor Satellite Geometry
Poor Satellite Geometry
Differential GPS
Real Time Differential GPS


                                                x+5, y-3
        x+30, y+60

                                x-5, y+3


   Receiver          DGPS Receiver
                                                  DGPS Site
DGPS correction = x+(30-5) and             True coordinates =
y+(60+3)                                   x+0, y+0

True coordinates = x+25, y+63              Correction = x-5, y+3
NDGPS Ground Stations
            National Differential Global Positioning System




Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage.
Topography may also limit some areas of coverage depicted here.
NDGPS Ground Stations
            National Differential Global Positioning System




Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage.
Topography may also limit some areas of coverage depicted here.
Wide Area Augmentation System
  Geostationary                               GPS Constellation
  WAAS satellites




                                                         WAAS Control
                                                         Station (East
WAAS Control           Local Area System (LAAS)          Coast)
Station (West Coast)
How good is WAAS?
With Selective Availability set
to zero, and under ideal
conditions, a GPS receiver
without WAAS can achieve                            +-15 meters
fifteen meter accuracy most
of the time.*
                                                        +-
                                                     3 meters


Under ideal conditions a
WAAS equipped GPS
receiver can achieve three
meter accuracy 95% of the
time.*



* Precision depends on good satellite geometry, open sky view, and no user
induced errors.

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How gps works

  • 1. Introduction to NAVSTAR GPS Charlie Leonard, 1999 (revised 2001, 2002)
  • 2. The History of GPS  Feasibility studies begun in 1960’s.  Pentagon appropriates funding in 1973.  First satellite launched in 1978.  System declared fully operational in April, 1995.
  • 4. Three Segments of the GPS Space Segment User Segment Control Segment Ground Antennas Master Station Monitor Stations
  • 5. Control Segment US Space Command Cape Canaveral Hawaii Kwajalein Atoll Diego Garcia Ascension Is. Master Control Station Monitor Station Ground Antenna
  • 7. User Segment  Military.  Search and rescue.  Disaster relief.  Surveying.  Marine, aeronautical and terrestrial navigation.  Remote controlled vehicle and robot guidance.  Satellite positioning and tracking.  Shipping.  Geographic Information Systems (GIS).  Recreation.
  • 8. Four Basic Functions of GPS  Position and coordinates.  The distance and direction between any two waypoints, or a position and a waypoint.  Travel progress reports.  Accurate time measurement.
  • 9. Position is Based on Time Signal leaves satellite at time “T” T Signal is picked up by the T+3 receiver at time “T + 3” Distance between satellite and receiver = “3 times the speed of light”
  • 10. Pseudo Random Noise Code Time Difference Satellite PRN Receiver PRN
  • 11. What Time is It? Universal Coordinated Time Greenwich Mean Time GPS Time + 13* Zulu Time Local Time: AM and PM (adjusted for local time zone) Military Time (local time on a 24 hour clock) * GPS Time is ahead of UTC by approximately 13 seconds
  • 12. Signal From One Satellite The receiver is somewhere on this sphere.
  • 13. Signals From Two Satellites
  • 14. Three Satellites (2D Positioning)
  • 16. Three Dimensional (3D) Positioning
  • 17. Selective Availability (S/A)  The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users.  S/A was designed to prevent America’s enemies from using GPS against us and our allies.  In May 2000 the Pentagon reduced S/A to zero meters error.  S/A could be reactivated at any time by the Pentagon.
  • 18. Sources of GPS Error Standard Positioning Service (SPS ): Civilian Users Source Amount of Error  Satellite clocks: 1.5 to 3.6 meters  Orbital errors: < 1 meter  Ionosphere: 5.0 to 7.0 meters  Troposphere: 0.5 to 0.7 meters  Receiver noise: 0.3 to 1.5 meters  Multipath: 0.6 to 1.2 meters  Selective Availability (see notes)  User error: Up to a kilometer or more Errors are cumulative and increased by PDOP.
  • 19. Receiver Errors are Cumulative! System and other flaws = < 9 meters User error = +- 1 km
  • 20. Sources of Signal Interference Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields
  • 21. Using GPS Receivers for Positioning and Navigation
  • 22. GPS Navigation Terminology Active GOTO Waypoint t ypoin N (0000) g Le Distance to Wa e tiv Desired Track ) G) Ac 0 nd ver (X (SO (DTK) (xº) g Gr e d O in ar (CMG) (xº) N (00) Be e ou Sp XT E (CD I) Tracking (TRK) (xº) Active Course Made Good (CMG) Present From Location Waypoint
  • 23. GPS Navigation: On the Ground Active GOTO L eg Waypoint N tive Bearing = 780 Ac COG = 3500 Bearing = 650 XTE = 1/3 mi. COG = 50 XTE = 1/2 mi. Bearing = 400 COG = 1040 XTE = 1/4 mi. Course Over Ground (COG) = Location Where GOTO Bearing = Was Executed Cross Track Error (XTE) =
  • 24. Position Fix  A position is based on real-time satellite tracking.  It’s defined by a set of coordinates.  It has no name.  A position represents only an approximation of the receiver’s true location.  A position is not static. It changes constantly as the GPS receiver moves (or wanders due to random errors).  A receiver must be in 2D or 3D mode (at least 3 or 4 satellites acquired) in order to provide a position fix.  3D mode dramatically improves position accuracy.
  • 25. Waypoint  A waypoint is based on coordinates entered into a GPS receiver’s memory.  It can be either a saved position fix, or user entered coordinates.  It can be created for any remote point on earth.  It must have a receiver designated code or number, or a user supplied name.  Once entered and saved, a waypoint remains unchanged in the receiver’s memory until edited or deleted.
  • 26. Planning a Navigation Route = Waypoint Start
  • 27. How A Receiver Sees Your Route
  • 28. GPS Waypoint Circle of Error X
  • 29. GPS Dilution of Precision and Its Affects On GPS Accuracy
  • 30. GPS Satellite Geometry  Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.  Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver.  There are five distinct kinds of DOP.  Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position.  It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.  Some GPS receivers allow DOP to be manipulated by the user.
  • 38. Real Time Differential GPS x+5, y-3 x+30, y+60 x-5, y+3 Receiver DGPS Receiver DGPS Site DGPS correction = x+(30-5) and True coordinates = y+(60+3) x+0, y+0 True coordinates = x+25, y+63 Correction = x-5, y+3
  • 39. NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.
  • 40. NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.
  • 41. Wide Area Augmentation System Geostationary GPS Constellation WAAS satellites WAAS Control Station (East WAAS Control Local Area System (LAAS) Coast) Station (West Coast)
  • 42. How good is WAAS? With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve +-15 meters fifteen meter accuracy most of the time.* +- 3 meters Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.* * Precision depends on good satellite geometry, open sky view, and no user induced errors.