GIS 1710 GPS Applications: Differential Positioning

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GIS 1710GPS ApplicationsDifferential
Positioning

• Dr. Walter Goedecke
• Spring 2007

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GPS ApplicationsIntroduction

• Since the inception of GPS as a tool for the
  military, many civilian applications have sprung
  forth, such as
• Ship and aviation navigation
• Surveying
• Rescue operations
• Scientific applications
• Consumer applications

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GPS ApplicationsIntroduction Cont.

• Several of such widespread applications require
  extra triangulation accuracy and precision.
• Methods developed to increase such accuracy and
  precision are
• Differential GPS, or DPGS
• Local area augmentation system (LAAS)
• Wide area augmentation system (WAAS)
• GPS networks

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GPS ApplicationsDifferential Positioning

• Single receiver positioning using the current GPS
  depends on pseudorange measurements, or code
  positioning
• Since every chip is unique, the receiver
  correlator can match a received chip with a
  reference
• Each chip is 1,540 L1 wavelengths
• Accuracy is based upon the chip length of 293
  meters.

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Differential PositioningDGPS

• DPGS (differential GPS) positioning, or relative
  positioning, overcomes the position location
  error of one receiver by using two receivers
• The differential method depends upon the theory
  that location errors are similar to both
  receivers, so differences will cancel the common
  errors out.

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Differential PositioningDGPS

• An accuracy on the order of a two meters is
  possible if the roving receiver is in motion or
  on the order of a centimeter (1cm) when the
  roving receiver is stationary
• This method is reliable when both the reference
  and roving receivers are not far apart

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Differential PositioningDGPS Example

• One receiver is a stationary reference at a known
  location
• The GPS reported position is compared to the
  actual position (x, y, z)ref, and a reference
  vector error ?(x, y, z), is determined
• ?(x, y, z) (x, y, z)true - (x, y, z)GPS
• The other receiver, the rover or remote receiver,
  is triangulated at an unknown location (x, y,
  z)gps
• The triangulation error at the reference is
  transmitted by radio to the roving receiver
• The reference error is summed with this GPS
  position, thus giving a fix that is more
  accurate
• (x, y, z)corrected (x, y, z)gps ?(x, y,
  z)

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Differential Positioning

• Illustration of the Differential or Relative
  Multipath MethodEl-Rabbany

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Differential PositioningDGPS

• Another method of DPGS positioning is for the
  reference receiver to receive several
  pseudoranges broadcast by GPS satellites, and
  report the range errors to the roving station
• These errors can then be compensated by the
  roving station to get an accurate fix.

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Differential PositioningKinematic DGPS

• Another form of DPGS is carrier positioning,
  differential phase, or kinematic DGPS
• Every carrier wavelength is very nearly the same
  as another.
• This creates range ambiguity by distances of a
  whole number of wavelengths.
• Errors by integral number of wavelengths is also
  known as cycle slips
• But since there are a fixed number of L1 carrier
  wavelengths of 19 cm in a 293.255 meter C/A-code
  chip, 1,540 of them, they can be counted from the
  start of a chip in differential mode, and
  furthermore, fractions of a carrier wavelength
  can be measured

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Differential PositioningKinematic DGPS

• Also, the P-code modulation on the L1 carrier
  produces 29.326 meter chip lengths, and can also
  be counted, in spite of the encrypted P-code,
  since only time of arrival of the start of chip
  is needed.
• The advantage is that the P-code never has
  selective availability (S/A) applied as can the
  C/A-code.
• Accuracies of 2 cm are possible when enough time
  is allowed for signal averaging
• Comparing two different path distances is a
  similar method known as interferometry

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Differential Positioning Additional Accuracy
Considerations

• Repeated measurements at the unknown roving site
  improve accuracy.
• If the baseline distance, the distance between
  the reference and the roving locations, is less
  than about 20 km, accuracy is good, since
  differences in errors are small, or the errors
  are mostly common
• Larger distances between reference and roving
  receivers may introduce uncommon errors, such as
  ionospheric signal path differences.

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Differential PositioningRTK

• Real Time Kinematic (RTK) differential GPS
• A carrier phase differential method used when the
  roving receiver is in quasi-motion
• The base station compares actual position with
  the GPS position, and transmits this difference
  via radio to the roving station
• Accuracy is improved if the roving station pauses
  a few seconds during a reading, to allow for
  position averaging

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Differential PositioningDGPS

• Real time differential GPS (DGPS) is a code-based
  relative positioning technique.
• Used when the roving receiver is constantly
  moving and accuracy need only be within a few
  meters.
• Base station compares actual position with GPS
  position, and radios the difference to the roving
  station via a format called RTCM (Radio Technical
  Commission for Maritime Service)
• Accuracy is from sub-meter to 5 m range.

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Differential PositioningWAAS

• Wide Area Augmentation System (WAAS), or Wide
  Area Differential GPS (WADGPS), a code based
  system, was developed by the FAA for aircraft
  navigation over North America
• Rather than have a pair of GPS receivers for
  differential measurements, there is an
  established reference station network of
  stations, 24 of them, with precisely surveyed
  locations that will broadcast the DGPS
  corrections
• The WAAS stations use both C/A and P-code to
  accurate differential measurements.

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Differential PositioningWAAS

• The 24 stations send the interpreted information
  to a central ground station that validates and
  integrates all the information
• This combined data is uplinked to a GEO satellite
• Corrected pseudorange downlink information is
  available to all ground WAAS users
• Suspect GPS information is also sent, thus
  allowing users to discriminate against inaccurate
  data, such as heavily distorted signals from any
  GPS satellites.

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Differential PositioningWAAS

• Although previously GPS receivers needed beacon
  receiver to receive the additional information,
  now WAAS supplemental data is broadcast on L1,
  and GPS receivers only need software to decode
  the GEO satellite information to distinguish is
  from standard GPS satellites
• WAAS GEO satellites also could serve as
  positioning spacecraft since they operate on the
  same frequency

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Differential PositioningWAAS

• The method of collective error determination and
  broadcasting could be adapted for other
  applications, such as small areas about the size
  of a city using carrier phase positioning, thus
  allowing sub-decimeter accuracy

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Differential PositioningLAAS

• Local area augmentation systems (LAAS) may be
  implemented at many airports to replace existing
  ILS approach systems
• DGPS real-time accuracies of 1 meter allow
  coupling to auto-pilot controls so that automatic
  landings in bad weather are possible
• DGPS autoland demonstrations were made on Boeing
  737 and 757 aircraft in the 90s

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Differential PositioningGPS Networks

• RTK Network
• If enough reference stations existed in a region
  that could compute DGPS corrections,
  interpolation could allow corrections for all
  areas within that region.
• The corrections could then be sent to any roving
  station in that region.
• A similar system is implemented by FM
  transmitting radio stations, where DGPS
  corrections are sent to subscribers having
  special RDS (radio data system) FM receivers.

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Differential PositioningGPS Networks

• Data Organizations
• Many organizations have GPS reference stations
  world-wide for geodetic purposes
• The IGS had 250 tracking stations as of April
  2001 for monitoring GPS signals.
• Data is archived and provided online.
• The data is comprised of
• Precise ephemeris
• Satellite and tracking station coordinates and
  clock information
• Earth rotation parameters
• This network could be used for transmitting DGPS
  corrections.

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References

• Ahmed El-Rabbany, Introductions to GPS The
  Global Positioning System, Publisher Artech
  House.
• Pratt, Timothy, Bostian, Charles, Allnutt,
  Jeremy, Satellite Communications, 2003, John
  Wiley Sons.
• http//www.trimble.com/gps/