GPS Technology

1
GPS Overview
TEC7132 April 2004
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What is GPS?

• Radio-based navigation system developed by DoD
• Initial operation in 1993
• Fully operational in 1995
• System is called NAVSTAR
• NAVigation with Satellite Timing And Ranging
• Referred to as GPS
• Series of 24 satellites, 6 orbital planes, 4
  satellite vehicles (SV) on each plane
• Works anywhere in the world, 24 hours a day, in
  all weather conditions and provides
• Location or positional fix
• Velocity
• Direction of travel
• Accurate time

3
Global Navigation Satellite Systems
(GNSS)

• NAVSTAR
• USA
• GLONASS
• Russians
• Galileo
• Europeans

4
GPS involves 5 Basic Steps

• Trilateration
• Intersection of spheres
• SV Ranging
• Determining distance from SV
• Timing
• Why consistent, accurate clocks are required
• Positioning
• Knowing where SV is in space
• Correction of errors
• Correcting for ionospheric and tropospheric delays

5
How GPS works?

• Range from each satellite calculated
• range time delay X speed of light
• Technique called trilateration is used to
  determine you position or fix
• Intersection of spheres
• At least 3 satellites required for 2D fix
• However, 4 satellites should always be used
• The 4th satellite used to compensate for
  inaccurate clock in GPS receivers
• Yields much better accuracy and provides 3D fix

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Determining Range

• Receiver and satellite use same code
• Synchronized code generation
• Compare incoming code with receiver generated code

Measure time difference between the same part of
code
Series of ones and zeroes repeating every 1023
bits. So Complicated alternation of bits that
pattern looks random thus called pseudorandom
code.
From satellite
From receiver
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Signal Structure

• Each satellite transmits its own unique code
• Two frequencies used
• L1 Carrier 1575.42 MHz
• L2 Carrier 1227.60 MHz
• Codes
• CA Code use L1 (civilian code)
• P (Y) Code use L1 L2 (military code)

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Three SV ranges known
22,000 Km radius
20,000 Km radius
Located at one of these 2 points. However, one
point can easily be eliminated because it is
either not on earth or moving at impossible rate
of speed.
21,000 Km radius
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Accurate Timing is the Key

• SVs have highly accurate atomic clocks
• Receivers have less accurate clocks
• Measurements made using nanoseconds
• 1 nanosecond 1 billionth of a second
• 1/100th of a second error could introduce error
  of 1,860 miles
• Discrepancy between satellite and receiver clocks
  must be resolved
• Fourth satellite is required to solve the 4
  unknowns (X, Y, Z and receiver clock error)

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Satellite Positioning

• Also required in the equation to solve the 4
  unknowns is the actual location of the satellite.
• SV are in relatively stable orbits and constantly
  monitored on the ground
• SV position is broadcast in the ephemeris data
  streamed down to receiver

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Sources of Errors

• Largest source is due to the atmosphere
• Atmospheric refraction
• Charged particles
• Water vapor

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Other Sources of Errors

• Geometry of satellite positions
• Satellite clock errors
• SV position or ephemeris errors
• Quality of GPS receiver
• Multi-path errors

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Dilution of Precision (DOP)

• Geometric location of the satellites as seen by
  the receiver
• The more spread out the satellites are in the
  sky, the better the satellite geometry
• PDOP (position dilution of precision) is a
  combination of VDOP and HDOP
• The lower the PDOP value, the better the
  geometric strength
• PDOP value less than 6 is recommended

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Selective Availability

• The intentional introduction of errors for
  civilian users is called Selective Availability
• SA was terminated on May 2, 2000
• When SA was on, civilian users accuracy was 100
  meters
• Military has capability to degrade signal in
  certain theaters of operation this is called
  spoofing

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Differential Correction

• Technique used to correct some of these errors
• Referred to as differential GPS or DGPS
• In DGPS, two GPS receivers are used
• One receiver is located at an accurately surveyed
  point referred to as the base station
• A correction is calculated by comparing the known
  location to the location determined by the GPS
  satellites
• The correction is then applied to the other
  receivers (known as the rover) calculated
  position

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DGPS Methods

• Post-processing
• Corrections performed after the data is collected
• Special software required
• Real-time
• Corrections are performed while the data is being
  collected
• Need special equipment to receive the DGPS signal

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Wide Area Augmentation System - WAAS

• New real-time DGPS
• Satellite based
• FAA initiative.now fully operational
• Series of 25 ground reference stations relay
  info to master control station
• Master control station sends correction info to
  WAAS satellite
• http//gps.faa.gov/programs/waas/howitworks.htm

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WAAS Satellites

• WAAS satellites are geo-stationary
• On east coast, WAAS satellite sits off coast of
  Brazil over equator at 53.96 West (35 on
  Garmin)
• http//www.lyngsat.com/tracker/inmar3f4.shtm
• On west coast, WAAS satellite sits over Pacific
  ocean at 178.0 East (47 on Garmin)
• http//www.lyngsat.com/tracker/inmar3f3.shtml
• Ability to get signal deteriorates in northern
  latitudes (satellite is lower on the horizon)
• If you can get WAAS satellite signal..3 meter
  accuracy
• However, cannot always get signal due to
  obstructions
• More WAAS satellites becoming available in future
• Europeans (EGNOS)
• Japanese (MSAS)

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GPS Accuracy Comparison
Some common GPS devices used by FWS
GPS Device Autonomous WAAS DGPS Real-time DGPS Post-process DGPS
Garmin GPSMap 76s 10 - 15 3 3 1 - 3
Rockwell PLGR Federal Users Only 8 - 15 NA 3 NA
Trimble - GeoXT 10 3 1-3 Sub-meter
Accuracy given in meters
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GPS Accuracy Issues

• Ways to improve the accuracy of your GPS
  collected data
• Standardize data collection methods
• Establish protocols for your applications
• Employ averaging techniques
• Perform mission planning
• Utilize DGPS
• Understand how the selection of datums and
  coordinate systems affect accuracy
• GPS data collected in wrong datum can introduce
  200 meters of error into your GIS!

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Some issues to consider when purchasing GPS
devices

• What is the accuracy level required for your
  application?
• (10 meters or sub-meter)
• How is unit going to be used in field?
• External antenna required, in heavy canopy, ease
  of use, durability, data dictionary capability,
  waterproof
• Cost from 100 to 12K
• Staff expertise..training..support network
• How well does unit interface with GIS?

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Latest Technology

• Mobile mapping software for WindowsCE devices
• TerraSync (Trimble)
• ArcPad (ESRI)
• Multi-path rejection technology
• Trimble GeoXT
• Bluetooth
• Allows for cable free operation

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ArcPad Software
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ArcPad Training

• NCTC 3 day course (TEC7133)
• Utilize ArcPad Tools for ArcGIS
• Geodatabase - check out check in
• Design custom forms for data collection
• Applets ArcPad Studio
• Utilize GPSCorrect extension
• Customize the ArcPad interface

Next offering September 1-3, 2004