gps workshop 7-12-05

8/6/2019 GPS Workshop 7-12-05

http://slidepdf.com/reader/full/gps-workshop-7-12-05 1/84

GPS in Land SurveyingGPS in Land Surveying

Evergreen Valley CollegeEvergreen Valley CollegeEngineering and EngineeringEngineering and Engineering

TechnologyTechnology

H. Johnston, T. Redd, A. TabriziH. Johnston, T. Redd, A. TabriziJuly 12, 2005July 12, 2005

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Today¶s TopicsToday¶s TopicsPart IPart I ± ± Background InformationBackground Information ± ± Accuracy and Precision Accuracy and Precision ± ± What is GPS?What is GPS?

± ± Why and who uses it?Why and who uses it? ± ± How does it work?How does it work? ± ± GPS SurveyingGPS Surveying ± ± The BasicsThe Basics

Part IIPart II

± ± GPS Surveying TechniquesGPS Surveying Techniques ± ± Mission Planning and DesignMission Planning and Design

Part IIIPart III ± ± Field ExercisesField Exercises ± ± Post Processing Field DataPost Processing Field Data ± ± Advanced Topics Advanced Topics

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GPS Course InformationGPS Course Information

Lecture component:Lecture component: ± ± Accuracy, precision, & error Accuracy, precision, & error

± ± Oral & written communicationOral & written communication ± ± Introduction to GPSIntroduction to GPS

Laboratory component:Laboratory component:

± ± Field activitiesField activities ± ± Data processingData processing

Expected learning competenciesExpected learning competencies

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HHistory of Measurement &DDevices

MechanicalOpto-MechanicalElectronic

Electronic Distance Measuring

Total StationSatellite Assisted Systems

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Mechanical & OpticalMechanical & Optical

DevicesDevicesSimple to useSimple to useUsually cheapUsually cheap

Poor accuracyPoor accuracySimple applicationsSimple applicationsPoor productivityPoor productivityPoor practicalityPoor practicality

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Some OldiesSome Oldies

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Wireless Communication Technology

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Electronics Devices

Electronic Distance Meter Electronic Distance Meter

d

Two-way ranging by EDMone clock used to measure ( t

Two way travel time: ( t=2d /c.Distance: d=c ( t/2

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Total StationTotal Station

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Accuracy and Precision Accuracy and Precision

Accuracy: Degree of perfection obtained in Accuracy: Degree of perfection obtained inany measurement, i.e. closeness to theany measurement, i.e. closeness to the

actual valueactual valuePrecision: Degree of refinement of Precision: Degree of refinement of measurement, i.e. degree of repeatability or measurement, i.e. degree of repeatability or consistency of a group of observationsconsistency of a group of observationsBoth are important in SurveyingBoth are important in Surveying

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Accuracy & Precision Accuracy & PrecisionGood percisionGood accuracy

Poor percisionGood accuracy

Good percisionPoor accuracy

Good Precision Poor Precision Good PrecisionGood Accuracy Good Accuracy Poor Accuracy

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Can HiCan Hi--Tech EquipmentTech Equipment

be Trusted?be Trusted? Accuracy and precision may be improved: Accuracy and precision may be improved: ± ± If we follow directionsIf we follow directions ± ± If we stay within the operating limits of the equipmentIf we stay within the operating limits of the equipment

± ± If we use the equipment properlyIf we use the equipment properly ± ± If we use the right equipment for the jobIf we use the right equipment for the job ± ± If we use care and preplanningIf we use care and preplanning ± ± If we build redundancies into the measurementIf we build redundancies into the measurement

± ± If we can trust the people who are using the equipment!If we can trust the people who are using the equipment!

So nothing is new here! HiSo nothing is new here! Hi- -tech or not, we still need to usetech or not, we still need to usecaution.caution.

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What is a GPS?What is a GPS?

DefinitionsDefinitions

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What is GPS?What is GPS?

A system capable of providing position A system capable of providing positioninformation anywhere on earthinformation anywhere on earth ± ±

GGloballobal PPositioningositioning SSystemystem A constellation of orbiting satellites A constellation of orbiting satellitesVarious orbits around the earthVarious orbits around the earthNAVSTAR GPSNAVSTAR GPS

User receivers acquire signal and determines itsUser receivers acquire signal and determines itspositionposition

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GPSGPS

Global Positioning SystemGlobal Positioning System

Developed by DODDeveloped by DODCost $10 billionCost $10 billionTriangulationTriangulation--basedbased

technologytechnology

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Why use it?Why use it?

AAA AAA (who can resist it(who can resist it ! ! )) ± ± A All weather operationll weather operation ± ± A Always available (24 /7 operation)lways available (24 /7 operation) ± ± A Anywhere availablenywhere availableEconomicalEconomicalIncreased ProductivityIncreased ProductivityImproved Customer serviceImproved Customer service

Accuracy (3 Accuracy (3- -D data, Velocity and timing)D data, Velocity and timing)

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Some ApplicationsSome Applications

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Could be used to track mail

if properly used!

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Mapping

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The GPS System ComponentsThe GPS System Components

ser se ment Control se ment

Sp ce se ment

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The User SegmentThe User Segment

GPS user equipmentGPS user equipment ± ± portable and fixedportable and fixed ± ± MilitaryMilitary

± ± CivilianCivilianNavigationNavigationSurveyingSurveyingGISGIS

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The Control SegmentThe Control Segment

Ground facilities responsible for Ground facilities responsible for ± ± satellite trackingsatellite tracking ± ± telemetrytelemetry ± ± orbit & ephemeris computationsorbit & ephemeris computations ± ± uplinking of the computed datauplinking of the computed data ± ± supervising the daily management of the spacesupervising the daily management of the space

segmentsegment ± ± Five ground control stations (Monitor Stations)Five ground control stations (Monitor Stations) ± ± One Master Control StationOne Master Control Station

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Master Control StationMaster Control Station

Receive tracking data from the monitor Receive tracking data from the monitor stationsstations

Calculates satellites ephemerisCalculates satellites ephemeris Adjusts satellite clocks Adjusts satellite clocksManeuvers satellites, if neededManeuvers satellites, if needed

Encrypts signalsEncrypts signalsMaintains GPS reference system (WGS84)Maintains GPS reference system (WGS84)

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The Space SegmentThe Space Segment

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The Space SegmentThe Space Segment

Constellation of 24 SatellitesConstellation of 24 SatellitesIn six orbital planes around the equator In six orbital planes around the equator

(60 degrees apart)(60 degrees apart)Four satellite per orbitFour satellite per orbitOrbital planes inclined 55 degrees from theOrbital planes inclined 55 degrees from the

equator equator

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Satellite ConstellationSatellite Constellation

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GPS SatelliteGPS Satellite

Seven satellites are typically visible 10Seven satellites are typically visible 10degrees or more above the horizondegrees or more above the horizon

Each satellite is about 2 to 3K lbsEach satellite is about 2 to 3K lbsSatellites orbit the earth every 12 hoursSatellites orbit the earth every 12 hoursTime can be figured to within 100 nanosecsTime can be figured to within 100 nanosecs

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BLOCK IIA SATELLITE CHARACTERISTICSBLOCK IIA SATELLITE CHARACTERISTICS ± ± Weight (in orbit): 2,1 7 5 poundsWeight (in orbit): 2,1 7 5 pounds ± ± Orbit altitude: 10,988 nautical milesOrbit altitude: 10,988 nautical miles ± ± Power source: solar panels generating 7 00Power source: solar panels generating 7 00

wattswatts ± ± Dimensions: 5 feet wide, 1 7 .5 feet longDimensions: 5 feet wide, 1 7 .5 feet long

(including wing span)(including wing span) ± ± Design life: 7 .5 yearsDesign life: 7 .5 years

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BLOCK IIR SATELLITE CHARACTERISTICSBLOCK IIR SATELLITE CHARACTERISTICS ± ± Weight (in orbit): 23 7 0 poundsWeight (in orbit): 23 7 0 pounds ± ± Orbit altitude: 10,988 nautical milesOrbit altitude: 10,988 nautical miles ± ± Power source: solar panels generating 1136Power source: solar panels generating 1136

wattswatts ± ± Dimensions: 5 feet wide, 6.33 feet in diameter,Dimensions: 5 feet wide, 6.33 feet in diameter,

6.25 feet high (38.025 feet wide including wing6.25 feet high (38.025 feet wide including wingspan)span)

± ± Design life: 10 yearsDesign life: 10 years

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BLOCK IIF SATELLITE CHARACTERISTICSBLOCK IIF SATELLITE CHARACTERISTICS ± ± Weight (in orbit): 3 7 58 poundsWeight (in orbit): 3 7 58 pounds ± ± Orbit altitude: 10,988 nautical milesOrbit altitude: 10,988 nautical miles ± ± Power source: solar panels generating up toPower source: solar panels generating up to

2900 watts2900 watts ± ± Dimensions: 8 ft x 6.4 7 ft (stowed) 7 0.42 ftDimensions: 8 ft x 6.4 7 ft (stowed) 7 0.42 ft

(deployed 4 panel solar arrays) x 12 ft(deployed 4 panel solar arrays) x 12 ft ± ± Design life: 15 yearsDesign life: 15 years

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What is so special about anWhat is so special about an

11,000 mile orbit?11,000 mile orbit?

MathematicallyMathematicallyperfect orbitperfect orbitµOrbits¶ twice per dayµOrbits¶ twice per dayLarge µviewable¶ areaLarge µviewable¶ area

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Basic ConceptBasic Concept

Satellites are reference points to locationsSatellites are reference points to locationson earth (their location are known)on earth (their location are known)

A location of a point on earth is identified by A location of a point on earth is identified by³triangulation´³triangulation´Signals from three satellites are usedSignals from three satellites are usedTravel time of each signal is determinedTravel time of each signal is determinedSignals travel at Speed of lightSignals travel at Speed of lightDistance = Travel Time * Speed of LightDistance = Travel Time * Speed of Light

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Triangulation (2Triangulation (2- -D)D)

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1 satellite1 satellite

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2 satellites2 satellites

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3 satellites3 satellites

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Where are the satellites?Where are the satellites?

From orbital mechanics, the location of From orbital mechanics, the location of satellites are determinedsatellites are determined

An almanac of orbital information for all An almanac of orbital information for allsatellites are stored in each satellitesatellites are stored in each satelliteGround controlGround control- -stations continuously updatestations continuously updatelocation information of each satellite andlocation information of each satellite andtransmit it to them (i.e. ephemeris)transmit it to them (i.e. ephemeris)

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Satellite MechanicsSatellite Mechanics

ist r in f rces

s lar ra it

al e

l nar ra it

air ra

s lar ra iati nearth's ra it

ti esEA TH

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Satellite SignalsSatellite Signals

A GPS satellite transmits continuously at A GPS satellite transmits continuously attwo frequencies in L band:two frequencies in L band:

± ± 15 7 5.42 MHz (L1, civilian & military use)15 7 5.42 MHz (L1, civilian & military use) ± ± 122 7 .6 MHz (L2, military use only)122 7 .6 MHz (L2, military use only)

These signals are modulated by aThese signals are modulated by apseudorandom noise (PRN)pseudorandom noise (PRN)

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GPS Codes & CarriersGPS Codes & Carriers

S

P(Y) Code. Encrypted. US Military Use

P(Y) Code. Encrypted. US Military Use

C/ A Code. Degraded (Until 2000). Civil Use

L21227 .6 Mhz

L1157 5.42 Mhz

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Makeup of a SignalMakeup of a Signal

Each signal contains:Each signal contains: A carrier (L1 or L2) A carrier (L1 or L2)

A unique PRN code A unique PRN code ± ± C / A code (Coarse / Acquisition for L1)C / A code (Coarse / Acquisition for L1) ± ± P code (Precise or Private)P code (Precise or Private)

A binary data message A binary data message

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Elements of a GPS SignalElements of a GPS Signal

P-code

L1 carrier

navigation message

C/ A-code

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L1 Carrier SignalL1 Carrier SignalHas a unique 1023Has a unique 1023- -bitbit--long C / A codelong C / A codeC/ A code repeats every 1C/ A code repeats every 1- -millisecondmillisecondHas 50 bits /s navigation message containing:Has 50 bits /s navigation message containing: ± ± Data on satellite orbitData on satellite orbit

± ± ClockClock ± ± HealthHealth ± ± Etc.Etc.The chipping rate is 1.023 MHzThe chipping rate is 1.023 MHzLength of each chip (wavelength) is 293 mLength of each chip (wavelength) is 293 m

Each satellite transmits a different set of C / A codeEach satellite transmits a different set of C / A codeThis is the basis for the Standard Positioning ServiceThis is the basis for the Standard Positioning Service(SPS)(SPS)

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Signals for Military UseSignals for Military Use

L1 & L2 signals with PRN codes encryptedL1 & L2 signals with PRN codes encryptedThe chipping rate for these is 10.23 MHzThe chipping rate for these is 10.23 MHzThe length of each chip is 29.3 mThe length of each chip is 29.3 mThe nonThe non--encrypted version called P codeencrypted version called P codeThe encrypted version called Y codeThe encrypted version called Y codeP code is longP code is long ± ± repeats every 3 7 weeksrepeats every 3 7 weeks

Each satellite transmits a portion of P codeEach satellite transmits a portion of P codeThese signals are basis for Precise PositioningThese signals are basis for Precise PositioningService (PPS)Service (PPS)

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Comparison of Signals on L1Comparison of Signals on L1& Their PRN& Their PRN

C/ A co e PN

C/ A co e

P co e

(10 P-co e chips for e er C / A co e chip)

P co e PN

C/ A co e signal

P co e signal

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Satellite RangingSatellite Ranging

Calculation of distanceCalculation of distanceOneOne--way rangingway ranging

Needs two clocksNeeds two clocks ± ± OnOn--board satellite (accurate)board satellite (accurate) ± ± OnOn--board user receiver (not as accurate)board user receiver (not as accurate)

Called pseudoranging due errors presentCalled pseudoranging due errors present

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PseudorangingPseudoranging

d

ne- a ranging in P* attellite clock generates a signal* ece i er c lock detec ts t e s ignal a rri a l* T e t o clocks ust kee t e sa e ti e

Tra el ti e: ( t d / c (one a )is tance : d c ( t

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Signal Travel TimeSignal Travel Time

GPS satellite and GPS receiver generateGPS satellite and GPS receiver generatethe same signal at the same timethe same signal at the same time

Satellite transmits the generated signalSatellite transmits the generated signalReceiver acquires the satellite signalReceiver acquires the satellite signalThe receiver generated signal and theThe receiver generated signal and the

acquired satellite signals are comparedacquired satellite signals are comparedThe difference between these is the travelThe difference between these is the traveltime of the satellite signal (about 0.0 7 sec)time of the satellite signal (about 0.0 7 sec)

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Time Lag of SignalsTime Lag of Signals

Satellitegeneratesignal

ecie er generatesignal

a ro imatel 7 0 msec

( t

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What is the Distance?What is the Distance?

Range (distance) = Time * Speed of LightRange (distance) = Time * Speed of LightThree satellites will provideThree satellites will provide

± ± LatitudeLatitude ± ± LongitudeLongitude ± ± HeightHeight

Fourth satellite is needed to account for Fourth satellite is needed to account for clock time differenceclock time difference ± ± solve for timesolve for time

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Pseudoranging to four SatellitesPseudoranging to four Satellites

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Accuracy of Pseudoranging Accuracy of Pseudoranging

P codeP code ± ± 10 meters10 metersC/ A codeC/ A code ± ± 20 to 30 meters20 to 30 metersWith Selective Availability (SA)With Selective Availability (SA) ± ± 100 meters100 metersSA was turned off since 2000SA was turned off since 2000Other techniques are needed to improveOther techniques are needed to improveaccuracy:accuracy: ± ± Carrier Phase measurement (Surveying)Carrier Phase measurement (Surveying) ± ± Differential GPSDifferential GPS

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Sources of Error Sources of Error

Atmospheric scattering Atmospheric scatteringClock errorsClock errors

Receiver errorsReceiver errorsMultiMulti--Path InterferencePath InterferenceEphemerisEphemeris

GDOPGDOPSASA

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MultiMulti--path Error path Error

Satellite

Building

M u l t i p a t h

s i g n a l

D i r e c t s i g n a l

Receiver

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Carrier Phase MethodCarrier Phase Method

Measure the end segment of the carrier Measure the end segment of the carrier signal that is not a complete cyclesignal that is not a complete cycle

Determine the number of whole cyclesDetermine the number of whole cyclesNote that the difference with the codeNote that the difference with the codecomparison technique (binary comparison)comparison technique (binary comparison)

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DGPSDGPS

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Distance AdjustmentDistance Adjustment

A djusted Range = ( S- P ) - ( d

S - C S

CS

E X A C T

d

S

S - P

P

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Errors CompensatedErrors Compensated

The adjustments made by DGPS techniqueThe adjustments made by DGPS techniquerepresents a net sum of various errorsrepresents a net sum of various errorspresent in the process.present in the process.This correction doesn¶t address problemsThis correction doesn¶t address problemswith the receiver clockswith the receiver clocksThis correction may not be sufficient whenThis correction may not be sufficient whenthe receiver and the base station are too far the receiver and the base station are too far from each other from each other

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GPS SurveyingGPS Surveying ± ± The BasicsThe BasicsCarrier phase measuring employedReference system: World Geodetic System (WGS84)WGS84 ± a geocentric 3-D Cartesian coordinate system ± Primary parameters: define the shape of an ellipsoid for the earth,

angular velocity and mass of earth ± Secondary Parameters: define detailed gravity model of the earth

which are used to define the orbits of satellites ± Defined and maintained by the U.S. Defense Mapping AgencyRelative positioning method is used for increased accuracy

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Selection of Ranging MethodSelection of Ranging Method

According the clients expected accuracy, select a According the clients expected accuracy, select aranging methodranging method ± ± StaticStatic

± ± Rapid StaticRapid Static ± ± PseudoPseudo- -KinematicKinematic ± ± KinematicKinematic ± ± Real Time KinematicReal Time Kinematic

Some methods require dual frequency systems or Some methods require dual frequency systems or multiple receiversmultiple receiversSize of the crew depends on the method usedSize of the crew depends on the method used

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Survey PointsSurvey PointsIdentify points to be surveyed, i.e. stationsIdentify points to be surveyed, i.e. stationsOrganize stations into groupsOrganize stations into groupsFirst group should contain a control stationFirst group should contain a control stationEach group should include at least one station fromEach group should include at least one station fromanother groupanother group ± ± pivoting stationpivoting station

All stations in a group should be observed during the same All stations in a group should be observed during the samesessionsessionPivoting stations are observed twicePivoting stations are observed twice

Collect GPS data at each stationCollect GPS data at each stationProcess the data in the office using corrections at theProcess the data in the office using corrections at thecontrol stationcontrol station

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Grouping of Survey PointsGrouping of Survey Points

X=Survey Stations

Control Station

PivotingStations

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