Global Navigation Satellite Systems

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Global Navigation Satellite Systems

• Introduction to the
• Global Positioning System

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Overview

• Basics of GPS
• Applications and uses
• Error sources
• Use of GPS in Surveying
• Advantages and disadvantages of traditional
  methods
• Other satellite navigation systems

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What is GPS?

• Navigation Satellite Timing and Ranging Global
  Positioning System (NAVSTAR GPS)
• All weather global navigation, positioning and
  time-transfer system
• Created and maintained by the US Department of
  Defense

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Capabilities of GPS
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GPS Segments
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Space Segment

• Constellation currently consists of 27 satellites
  on six orbital planes
• Orbit the Earth at a altitude of approx. 20,200km
  with a period of 11h 58m
• Each SV has an atomic clock

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Space Segment

• SV transmit two carrier signals, L1 and L2, which
  are modulated with pseudo-random noise (PRN)
  codes
• The C/A (Civilian Access) PRN code is modulated
  on L1
• The P (Protected) code is modulated on both L1
  and L2

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Carrier Waver (L1, L2)
Pseudo-random Noise (C/A, P codes)
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Control Segment
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Control Segment

• Tasks of the control segment
• Monitor and control the constellation
• Establish GPS time and maintain synchronisation
• Predict satellite ephemerides and satellite clock
  corrections
• Update navigation message for each SV

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Control Segment

• Consists of
• 5 Monitor stations, 3 of which act as Ground
  Control Stations
• 1 Master Control Station
• GPS time, which is based on Universal Coordinated
  Time (UTC) is maintained by the US Naval
  Observatory

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User Segment

• GPS used for
• personal, vehicle, marine and aircraft navigation
• Surveying, deformation monitoring, geodesy,
  geodynamics
• Precision farming, soil mapping, ionospheric
  modeling, asset management
• Time transfer

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User Segment

• Standard Positioning Service
• Based on C/A code
• Calculates the time the signal takes to travel
  from the satellite to the receiver by measuring
  the shift between the incoming C/A code and an
  internally generated copy
• Distances (or ranges) between each satellite and
  the receiver are calculated using the speed of
  light

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Time Measurement Using PRN Code
Receiver Generated Code
Signal Received from Satellite
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User Segment

• Standard Positioning Service
• Because of errors the distances are called
  pseudoranges
• Because quartz clocks are used must solve for
  receiver clock offset from GPS time
• ? need 4 satellites to solve for 3D position, 3
  satellites for a 2D position

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User Segment

• Standard Positioning Service (cont)
• Until July 2000 US-DoD deliberately degraded the
  civilian service by imposing Selective
  Availability (SA)
• Under SA errors were introduced into the
  satellite clock and position to degrade accuracy
  to approx. 100m horizontally and 300m vertically
• US military has the ability to deny civilian
  access to GPS signal for any particular region of
  the Earth

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User Segment

• Precise Positioning Service
• Based on P-code
• Access restricted to US military and its allies
  by encryption of the P-code to prevent hostile
  use and impersonation of the GPS signals
• Operates in the same way as the SPS
• More precise because it has a lower effective
  wavelength and is dual-frequency

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Error Budget
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DGPS

• Requires 2 or more receivers
• One or more receivers have known coordinates and
  are used to calculate corrections to the others
• Determines a 3D vector (called a baseline)
  between the reference and roving receivers
• Spatially correlated errors cancel
• As distance between receiver increases less error
  is eliminated

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DGPS

• Block Shift Method
• Coordinates are calculated for the reference
  station and rover station using SPS
• Difference between the calculated and known
  coordinates are used to correct the position of
  the roving receiver
• Simple and effective
• Effectiveness reduced when reference and roving
  receivers do not use all the same satellite

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DGPS

• Pseudorange Correction Method
• Using the known coordinates of the reference
  station true pseudoranges are calculated
• Corrections are applied to the pseudoranges at
  the rover
• Effective and relatively simple
• Forces use of common satellites

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Carrier Phase Positioning

• Based on used of sinusoidal L1 and L2 carrier
  phases
• Carrier phases can be measured more precisely
  than the PRNs
• Observable is the fractional part of the carrier
  phase and the number of full cycles since lock on
  satellite

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Carrier Phase Positioning

• Unknown number of full cycles of the occur
  between the satellite and receiver
• Determination of the integer ambiguities is
  very complicated
• Requires 2 or more receivers and measures a
  baseline not absolute coordinates

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GPS vs Traditional Surveying

• For GPS
• does not require line of sight between survey
  points
• easy to measure baselines of many kilometres very
  precisely
• highly productive once ambiguities are solved
• can provide almost continuous positioning
• suitable for many non-traditional applications

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GPS vs Traditional Surveying ...

• Against GPS
• requires 4 (preferably 5 or more) satellites to
  be visible
• cannot be used underground, under trees, in urban
  canyons
• high equipment costs
• time lost recovering ambiguities
• gives ellipsoidal height

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Surveying with GPS

• Mission planning
• Data collection methods
• Processing algorithms

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Mission Planning

• Recce survey to measure the height (in degrees)
  and distribution of any obstructions
• Use of mission planning software with a current
  almanac to determine satellite visibility

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Mission Planning ...

• Number of satellites and the Dilution of
  Precision (DOP) used to chose an appropriate date
  and time for the survey
• More satellites means higher redundancy and
  ability to detect poor measurements
• Lower DOP translates into higher precision

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Mission Planning ...
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Mission Planning ...
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Data Collection and Processing

• Static surveys
• Fast (rapid) surveys
• Stop and go
• Kinematic
• Real Time Kinematic (RTK)

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Other GNSS Systems

• GPS Modernisation
• 3rd frequency, L5
• C/A code on L2
• Higher frequency of updates to broadcast
  ephemeris
• Result in faster ambiguity resolution, better
  ionosphere modelling

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Other GNSS Systems ...

• GLONASS (Global Navigation Satellite System)
• Russian system similar to GPS
• currently only 8 operational satellites

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Other GNSS Systems ...

• GALILEO
• European system currently under development
• High integrity -gt user will be aware of any error
  in the system within 6 seconds of it occurring
• Interoperability with GPS

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Questions?
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Practical