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

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? 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 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 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 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 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 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 Largest 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 errors SV position or “ephemeris” errors Quality of GPS receiver Multi-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 Real-time 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 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 Trimble - GeoXT ~ 10 1-3 Sub-meter 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?

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

ArcPad Software Bring GIS data into the field! Integrate GPS with GIS Custom forms for data collection

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