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Global Positioning SystemsThe Basics

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Applying Global Positioning Applying Global Positioning SystemsSystems

GPS provides navigational aidesGPS provides navigational aides– Locating a single pointLocating a single point– Navigating between pointsNavigating between points

GPS provides the basis for mappingGPS provides the basis for mapping– Tracking changing locational informationTracking changing locational information– Collecting coordinates of features for use in GISCollecting coordinates of features for use in GIS– Collecting information about features for use in GISCollecting information about features for use in GIS

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How GPS Works GPS works by triangulating GPS works by triangulating

your position on the earth, your position on the earth, based on satellite signalsbased on satellite signals

Satellites broadcast radio Satellites broadcast radio signalssignals

Receivers pick up the signalsReceivers pick up the signals Receivers calculate Receivers calculate

geographic coordinates from geographic coordinates from the satellite signalsthe satellite signals

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Satellites GPS satellites are controlled and GPS satellites are controlled and

operated by the Dept. of Defense, but operated by the Dept. of Defense, but it is an open system it is an open system

24 satellites in orbit dedicated to GPS24 satellites in orbit dedicated to GPS 6 satellites are within view of any 6 satellites are within view of any

location at one time, provided that location at one time, provided that physical terrain, or structures do not physical terrain, or structures do not block themblock them

Satellites constantly transmit their Satellites constantly transmit their locational information, and time datalocational information, and time data

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Receivers Receiver picks up signals Receiver picks up signals

broadcast from satellites in known broadcast from satellites in known orbitsorbits

Radio signals travel near the Radio signals travel near the speed of lightspeed of light

Receiver calculates how long the Receiver calculates how long the signal took to reach the earthsignal took to reach the earth

Using velocity of the signal and Using velocity of the signal and time, receivers calculate distance time, receivers calculate distance to satelliteto satellite

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Calculating Distance with Speed and Time

Speed x time = distanceSpeed x time = distance

Satellite radio transmission consists of a Satellite radio transmission consists of a series of dots and dashes in a “pseudo-series of dots and dashes in a “pseudo-random” coderandom” code

All satellites transmit a unique code with a All satellites transmit a unique code with a time stamp, synchronized by atomic clockstime stamp, synchronized by atomic clocks

Receivers decode each signal to determine Receivers decode each signal to determine which satellite the signal is originating fromwhich satellite the signal is originating from

Receiver compares time stamps with code to Receiver compares time stamps with code to determine the time difference between satellite determine the time difference between satellite and ground positionand ground position

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The Mathematics

Once the first satellite distance is calculated, the Once the first satellite distance is calculated, the receiver has narrowed its location down to a sphere receiver has narrowed its location down to a sphere with the radius of that distance.with the radius of that distance.

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The Mathematics

From the second From the second satellite, the satellite, the receiver can narrow receiver can narrow its position to the its position to the intersection of the intersection of the two possible two possible spheres.spheres.

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The Mathematics

Adding a third satellite Adding a third satellite narrows the receiver narrows the receiver position down to two position down to two possible locations.possible locations.

The fourth satellite will The fourth satellite will provide more provide more accuracy, narrowing accuracy, narrowing to a single location.to a single location.

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Position Calculations Adding a fourth satellite into the calculations helps Adding a fourth satellite into the calculations helps

calibrate timing of the atomic clockscalibrate timing of the atomic clocks The fourth satellite also greatly improves the level of The fourth satellite also greatly improves the level of

accuracy on your positional dataaccuracy on your positional data

• Four satellites = Four satellites = 3-D data 3-D data collection collection Accuracy +/- 1 meterAccuracy +/- 1 meter

•Three satellites = Three satellites = 2-D data 2-D data collection collection Accuracy +/- 200 Accuracy +/- 200 meters: meters: NOT RECOMMENDEDNOT RECOMMENDED

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Sources of Error Atmosphere slows down the satellite signalsAtmosphere slows down the satellite signals Multi-pathing -- signals bounce off metal fences, large Multi-pathing -- signals bounce off metal fences, large

trees, buildingstrees, buildings

Static and interferenceStatic and interference Atomic clocks are not Atomic clocks are not perfectperfect Selective availabilitySelective availability

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Selective Availability Inaccuracy introduced by the US Department of Inaccuracy introduced by the US Department of

Defense for national securityDefense for national security Signals from the satellites are deliberately Signals from the satellites are deliberately

mistimedmistimed Results in average error of 30 meters, but can Results in average error of 30 meters, but can

be as high as 200 metersbe as high as 200 meters Planned phase out over next 10 yearsPlanned phase out over next 10 years

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Average Horizontal Error

Satellite clocksSatellite clocks 1.5 meters1.5 metersOrbit errorsOrbit errors 2.5 meters2.5 metersIonosphereIonosphere 5 meters5 metersTroposphereTroposphere 0.5 meters0.5 metersReceiver noise/staticReceiver noise/static 0.3 meters0.3 metersMultipathingMultipathing 0.6 meters0.6 metersSelective AvailabilitySelective Availability 30 meters30 meters

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Differential Correction Most sources of error can be Most sources of error can be

eliminated through eliminated through Differential GPS Differential GPS (differential correction)(differential correction)

Uses two GPS units to correct errorsUses two GPS units to correct errors Works on the principle that two Works on the principle that two

receivers placed relatively close to receivers placed relatively close to each other will have the same each other will have the same conditions, and the same errorsconditions, and the same errors

Cannot correct local error sources, Cannot correct local error sources, such as static and multi-pathingsuch as static and multi-pathing

Only way to get +/- 1 meter accuracyOnly way to get +/- 1 meter accuracy

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Differential Correction Uses two GPS receivers: rover and baseUses two GPS receivers: rover and base Base unit set up on a known locationBase unit set up on a known location Base measures and records errors by calculating the Base measures and records errors by calculating the

correct timing based on its know locationcorrect timing based on its know location Base and rover files compared using time tagsBase and rover files compared using time tags Correction factor applied to rover filesCorrection factor applied to rover files

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Differential Correction Usually done in the office, after field work is Usually done in the office, after field work is

completedcompleted New technology is making real-time differential New technology is making real-time differential

correction possiblecorrection possible Base station beacon broadcasts correction data to Base station beacon broadcasts correction data to

the receiversthe receivers Receivers correct positions immediatelyReceivers correct positions immediately Important for navigationImportant for navigation

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Vocabulary SVSV : Space Vehicle, a satellite: Space Vehicle, a satellite Ephemeris: Information on position and orbit of Ephemeris: Information on position and orbit of

SV’s which is broadcast to the satelliteSV’s which is broadcast to the satellite 3-D: GPS data collection using 4 SV’s3-D: GPS data collection using 4 SV’s 2-D: GPS data collection using 3 SV’s2-D: GPS data collection using 3 SV’s SA: Selective Availability; deliberate mistiming of SA: Selective Availability; deliberate mistiming of

satellite signals by DoDsatellite signals by DoD

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Vocabulary Position: Set of x,y,z coordinates collected by the Position: Set of x,y,z coordinates collected by the

GPS unitGPS unit Feature: Specific object or place on the ground to Feature: Specific object or place on the ground to

be mapped; a collection of positions. May be a be mapped; a collection of positions. May be a point, line, or areapoint, line, or area

File: format in which positions and descriptions File: format in which positions and descriptions are stored in the GPS unit and transferred to the are stored in the GPS unit and transferred to the PCPC

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Vocabulary Data Dictionary: Selected list of features to be Data Dictionary: Selected list of features to be

mappedmapped Attribute: Descriptive information collected for Attribute: Descriptive information collected for

features i.e. feature=road, attribute=name of features i.e. feature=road, attribute=name of roadroad

Attribute Value: List of possible values to answer Attribute Value: List of possible values to answer the attribute. e.g., attribute = road surface, att. the attribute. e.g., attribute = road surface, att. values=paved,unpavedvalues=paved,unpaved

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Vocabulary: PDOP Positional Dilution of PrecisionPositional Dilution of Precision Measure of the quality of the GPS calculationsMeasure of the quality of the GPS calculations Based on the geometry of the visible satellitesBased on the geometry of the visible satellites Best geometry is with SV’s spread evenly across Best geometry is with SV’s spread evenly across

the skythe sky Low PDOP = high accuracyLow PDOP = high accuracy

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Our EquipmentGPS receiver, with GPS receiver, with dome antenna and dome antenna and battery packbattery pack

Data logger, or hand-Data logger, or hand-held computer for held computer for collecting attribute collecting attribute datadata

People to conduct People to conduct fieldwork and fieldwork and collect locational collect locational datadata

People and computers to People and computers to correct and edit datacorrect and edit data

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Basic Steps in Collecting and Using GPS Data

Create a data dictionaryCreate a data dictionary Conduct fieldwork to gather Conduct fieldwork to gather

locational and attribute datalocational and attribute data Differentially correct and edit Differentially correct and edit

locational datalocational data Bring your edited data into a Bring your edited data into a

GIS for analysis and use with GIS for analysis and use with other data setsother data sets