GPS Overview

TEC7132April 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 RangeReceiver and satellite use same

codeSynchronized 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 ErrorsGeometry 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 devicesTerraSync (Trimble)ArcPad (ESRI)

Multi-path rejection technology Trimble GeoXT

BluetoothAllows 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