Saturday 31 August 2013

GPS TECHNOLOGY

                                                          GPS TECHNOLOGY

What is GPS

·         Radio-based navigation system developed by DoD
ü  Initial operation in 1993
ü  Fully operational in 1995
·         System is called NAVSTAR
ü  NAVigation with Satellite Timing And Ranging
ü  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:
ü  Location or positional fix
ü  Velocity
ü  Direction of travel
ü  Accurate time


Global Navigation Satellite Systems  (GNSS)

q  NAVSTAR
ü  USA
q  GLONASS
ü  Russians
q  Galileo
ü  Europeans
GPS involves 5 Basic Steps
q  Trilateration
ü  Intersection of spheres
q  SV Ranging
ü  Determining distance from SV
q  Timing
ü  Why consistent, accurate clocks are required
q  Positioning
ü  Knowing where SV is in space
q  Correction of errors
ü  Correcting for ionospheric and tropospheric delays


How GPS works

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


Determining Range

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

 Signal Structure

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


Accurate Timing is the Key

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


Satellite Positioning

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


Sources of Errors

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

Other Sources of Errors

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


Dilution of Precision (DOP)

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


Selective Availability

q  The intentional introduction of errors for civilian users is called Selective Availability
q  SA was terminated on May 2, 2000
q  When SA was on, civilian users accuracy was ~100 meters
q  Military has capability to degrade signal in certain “theaters of operation” – this is called “spoofing”
Differential Correction
q  Technique used to correct some of these errors
q  Referred to as “differential GPS” or DGPS
q  In DGPS, two GPS receivers are used
q  One receiver is located at an accurately surveyed point referred to as the “base station”
q  A correction is calculated by comparing the known location to the location determined by the GPS satellites
q  The correction is then applied to the other receiver’s (known as the “rover”) calculated position
DGPS Methods
q  Post-processing
ü  Corrections performed after the data is collected
ü  Special software required 
q  Real-time
ü  Corrections are performed while the data is being collected
ü  Need special equipment to receive the DGPS signal
Wide Area Augmentation System – WAAS
q  New “real-time” DGPS
q  Satellite based 
q  FAA initiative….now fully operational
q  Series of ~25 ground reference stations relay info to master control station
q  Master control station sends correction info to WAAS  satellite
GPS Accuracy Issues
q  Ways to improve the accuracy of your GPS collected data
ü  Standardize data collection methods
ü  Establish protocols for your applications
ü  Employ averaging techniques
ü  Perform mission planning
ü  Utilize DGPS
ü  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
q  What is the accuracy level required for your application?
     (10 meters or sub-meter)
q  How is unit going to be used in field?
ü  External antenna required, in heavy canopy, ease of use, durability, data dictionary capability,  waterproof…
q  Cost…… from $100 to $12K
q  Staff expertise..training..support network
q  How well does unit interface with GIS?

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