Department of Geomatics

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451 - 200 Geomatics Science 2
Lecture 9 Introduction to The Global Positioning
System (GPS)
The Navstar Global Positioning System (GPS) is
an all-weather, space based navigation system
developed by the Department of Defense to satisfy
the requirements for the military forces to
accurately determine their position, velocity
and time in a common reference system anywhere on
or near the Earth on a continuous basis
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GPS

• has revolutionised traditional surveying
  techniques
• intervisibility between stations not required
• independent of weather conditions
• can operate day or night
• highly accurate
• quick and easy to use
• baselines of 100s of km in length possible
• economic advantage

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Applications of GPS

• geodetic surveying (position and height)
• GIS data capture
• in-car navigation systems
• ship and aircraft navigation
• geophysical surveys
• recreational uses

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System Availability

• GPS is free
• Anyone with a receiver can pick up GPS signals
  anywhere in the world
• Military system - downgraded by the Americans

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GPS Downgrading

• Selective Availability(SA)
• Anti-spoofing(AS)

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The Space Segment

• 24 satellites
• 12 hour, geosynchronous orbits
• 20,000km
• Block I, Block II, Block IIR
• atomic clocks - key to the systems accuracy
• spread spectrum signal - less subject to
  intentional jamming
• transmit within the L band of the frequency
  spectrum, microwaves
• L1 - 1575.42MHz
• L2 - 1227.60MHz

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The Space Segment

• Dual frequencies eliminate the effect of the
  atmosphere - major source of error
• modulated with 2 pseudo random noise codes
• coarse acquisition code
• precise code
• data message
• satellite identification number PRN codes
• 4-8 satellites can be observed anywhere on the
  Earth above 15o

L1
L2
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The Space Segment

• C/A code allows access to the standard
  positioning service (SPS)
• positioning accuracy of around 100m,
• P code allows access to the precise positioning
  service (PPS)
• positioning accuracy of around 15m
• C/A code gives approximate position, it assists
  with acquisition of the P-code for more precise
  positioning

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Denial of Accuracy and Access

• Selective Availability (SA)
• dithering of the satellite clock frequency
• affects one receiver operation - eliminated by
  differencing between receivers
• expected to be removed within the next 10 years
• Truncation of the transmitted navigation message
  so that the coordinates of the satellites cannot
  be accurately computed
• Anti Spoofing(AS) - turn off the P code of invoke
  an encrypted Y code. Denies the P code to all
  but authorised users

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The Control Segment

• Operational control System (OCS) - master control
  station, worldwide monitor stations and ground
  control stations
• stations track the GPS satellites
• predict satellite orbits
• satellite clock corrections computed
• updated in the data message every hour
• master control station - Colorado
• calculates satellite orbit and clock parameters
  from the monitor stations
• results passed to ground control stations for
  upload to the satellites

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The Control Segment

• 5 monitor stations globally
• orbit determination using precise cesium clocks
  and P code receivers
• private monitoring networks also exist5 monitor
  stations globally
• 3 ground control stations collocated with the
  monitor stations
• mainly consist of ground antennas
• communication links to the satellites

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The User Segment

• Military - Department of Defense
• Civilian
• high precision - geodetic surveying
• low precision - leisure activities, hiking,
  yachting

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Calculation of Position

• GPS positioning based on geometry
• Resection
• 4 satellites

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Calculation of Position

• Using a single receiver accuracy affected by
• accuracy of satellite position
• accuracy of the pseudorange measurement
• satellite geometry

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GPS coordinates are computed in a Cartesian (XYZ)
coordinate system on the WGS84 reference
system. To mean anything to us these
coordinates must be converted (1) to geodetic
coordinates in WGS84 (2) from WGS84 to AGD
geodetic coordinates (on ANS66) (3) from AGD
coordinates to AMG
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GDA

• ANS66 change to GRS80 (WGS84)
• GDA will be fully compatible with GPS
• Transformations from WGS84 to AGD will become
  unecessary

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GPS Time

• Time very important in GPS
• Uses a global timeframe - universal time (UT)
• Melbourne is 10 hours ahead of UT
• in reality GPS uses its own atomic time system

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Drawbacks of GPS

• no good in tunnels, underwater or in buildings
• need clear sky to see satellites
• gives out coordinates in WGS84, must be
  transformed to local datum
• produces ellipsoidal not orthometric heights

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GPS hardware and mission planning

• GPS receivers
• signal tracked (C/A and/or P code) L1 and/or L2
  carrier phase.
• number of channels
• realtime or post processed
• static/kinematic
• data transmission link
• size/portability
• power source
• price
• time to first fix
• update rate

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GPS hardware and mission planning

• GPS antennas
• used to receive signals from GPS satellites
• designed to give high SNR
• designed to reduce multipath
• size depends on application
• antenna phase centre must be stable

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GPS Survey

• Field Reconnaissance
• reconnaissance saves time, effort and money in
  the long term
• similar to standard surveying reconnaissance
  except sky visibility is important
• need maps, plans, site descriptions

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Site Evaluation

• find the point
• check for obstructions - elevation mask
• multipath
• construct visibility diagram

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GPS Survey Planning

• What accuracy do you require
• how many receivers
• single or dual frequency
• what type of receivers (preferably same types of
  antenna and receivers)

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GPS Survey Planning

• how long will you observe
• 1-10km (30-60 minutes)
• 10-20km (60-120 minutes)
• when should you observe
• satellite avaiability plot
• DOP Plot
• data sampling rate
• logistics

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GPS Field Practice

• Code range vs carrier phase
• code ranges accurate to 1m, carrier phase to 1mm
• code unambiguous, carrier phase ambiguous
• real time vs post processing
• static vs kinematic - kinematic less precise
• point positioning vs relative positioning
• point positioning - coordinates of a single point
  determined using a single receiver (almost always
  code)
• relative positioning - combining simultaneous
  observations from two receivers to give the
  position of one receiver relative to the other
  (code or carrier phase)
• code relative positioning is usually called
  differential GPS

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Relative GPS

• solves for the vector between two receivers
• for short baselines (lt30km) most errors cancel
• A GPS baseline is a 3D vector between two points
• the coordinates of the unknown station are solved
  relative to a control station
• require good control

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GPS Accuracies

• kinematic positioning (code) 100m
• static positioning (code) 5 - 20m
• kinematic relative positioning (code -
  DGPS) 3- 5m
• kinematic relative positioning (phase) lt10cm
• static relative positioning (code) 0.5 - 1m
• static relative positioning (phase) 0.01 -
  1ppm

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GPS Field Procedure

• Make sure you have all your equipment and
  scheduling sheets and field teams are
  synchronised
• on arrival at the point
• set up precisely over the point
• ensure everything is level
• check orientation of antenna
• check receiver configuration

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Logging the Data

• check satellites available
• check and maintain power supply
• check satellite visibility
• measure antenna, vertical height

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GPS Data Processing

• computes the coordinates
• user must be alert to what is going on
• look for QA
• software inputs
• satellite ephemerides
• measurements
• antenna heights
• coordinates of control points
• software output
• dX, dY, dZ

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GPS Error Sources

• Code and phase pseudoranges affected by by
  systematic errors, biases and random noise

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Biases and Noise

• Systematic errors can be modelled and solved in
  the observation equations
• differencing between receivers eliminates
  satellite specific errors
• differencing between satellites eliminates
  receiver specific errors
• random noise contains actual observation noise
  plus multipath effects

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Other Space Positioning Systems

• GLONASS
• GALILEO
• EGNOS
• GNSS