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GPS Overview TEC7132 April 2004 What is GPS? Radio-based navigation system developed by DoD Initial operation in 1993 Fully operational in 1995 System is called NAVSTAR NAV igation with S atellite T iming A nd R anging Referred to as GPS

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GPS Overview


April 2004

what is gps
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
global navigation satellite systems gnss
Global Navigation Satellite Systems (GNSS)
    • USA
    • Russians
  • Galileo
    • Europeans
gps involves 5 basic steps
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
how gps works
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
determining range
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

signal structure
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)
three sv ranges known
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

accurate timing is the key
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)
satellite positioning
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
sources of errors


(Charged Particles)


Sources of Errors
  • Largest source is due to the atmosphere
    • Atmospheric refraction
      • Charged particles
      • Water vapor
other sources of errors
Other Sources of Errors
  • Geometry of satellite positions
  • Satellite clock errors
  • SV position or “ephemeris” errors
  • Quality of GPS receiver
  • Multi-path errors
dilution of precision dop
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
selective availability
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”
differential correction
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 receiver’s (known as the “rover”) calculated position
dgps methods
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
wide area augmentation system waas
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
waas satellites
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)
  • On west coast, WAAS satellite sits over Pacific ocean at 178.0° East (#47 on Garmin)
  • 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)
gps accuracy comparison
GPS Accuracy Comparison

Some common GPS devices used by FWS:

Accuracy given in meters

gps accuracy issues
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!
some issues to consider when purchasing gps devices
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 network
  • How well does unit interface with GIS?

Latest Technology

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

Bring GIS data into the field!

Integrate GPS with GIS

Custom forms for data collection

ArcPad Software

arcpad training
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