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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 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)
NAVSTAR USA
GLONASS Russians
Galileo Europeans
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
GPS involves 5 Basic Steps
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
Receiver and satellite use same code
Synchronized code generationCompare incoming code with
receiver generated code
From satellite
Measure time difference between the same part of code
From receiver
Series of ones and zeroes repeatingevery 1023 bits. So Complicated alternation of bits that pattern looks random thus called“pseudorandom code”.
Signal StructureEach satellite transmits its own
unique codeTwo 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
20,000 Km radius22,000 Km radius
21,000 Km radiusLocated at one of these 2 points. However, one point can easily be eliminated because it is either not on earth or moving at impossiblerate of speed.
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 PositioningAlso 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 ErrorsLargest source is due to the
atmosphere Atmospheric refraction
• Charged particles• Water vapor
Ionosphere(Charged Particles)
Troposphere
Other Sources of Errors
Geometry of satellite positions Satellite clock errorsSV position or “ephemeris” errorsQuality of GPS receiverMulti-path errors
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
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 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
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
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
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)
GPS Accuracy Comparison
GPS Device Autonomous
WAASDGPS
Real-time DGPS
Post-process DGPS
Garmin GPSMap 76s ~ 10 - 15 ~3 3 1 - 3
Rockwell – PLGRFederal Users Only ~ 8 - 15 NA 3 NA
Trimble - GeoXT ~ 10 ~3 1-3 Sub-meter
Some common GPS devices used by FWS:
Accuracy given in meters
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
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?
Mobile mapping software for WindowsCE devices
TerraSync (Trimble)
ArcPad (ESRI)
Multi-path rejection technology
Trimble GeoXT
Bluetooth
Allows for cable free operation
Latest Technology
Bring GIS data into the field!
Custom forms for data collection
ArcPad Software
Integrate GPS with GIS
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