8/8/2019 GE1 GPS Lecture 2[1]

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GE 1 Earth TrekLecturer: Ariel C. Blanco, Dr.Eng.

GPS Lecture 2

Anatomy of a GPS :Satellites, control stations,

and receivers

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Review

What can you rememberfrom last week’s lecture?

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Users of GPSGovernment – Military – Agencies (e.g. survey, mapping)

Industry – Fishing – Agriculture – Transportation – ConstructionGeneral Public – Recreation – Security reasons

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GPS Navigation

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Application of GPS

TechnologyLocation - determining a basic positionNavigation - getting from one location toanotherTracking - monitoring the movement ofpeople and things

Mapping - creating maps of the worldTiming - bringing precise timing to theworld

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What you will learn today:

Components of GPSHow GPS works

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NAVSTAR GPSNavigationNavigation SatelliteSatelliteTiming and RangingTiming and RangingGlobal PositioningGlobal Positioning

SystemSystemFundedFunded and controlledand controlledby the US Departmentby the US Departmentofof DefenseDefense

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Components of GPS

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The Space SegmentSpace segment

24 satellite vehicles(nominal)Six orbital planes – Inclined 55 o with respect to

equator – Orbits separated by 60 o

20,200 km elevationabove Earth

Orbital period of 11 hr 55minFive to eight satellitesvisible from any point onEarth

Block I Satellite Vehicle

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The GPS Constellation

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GPS SatelliteVehicle

Four atomic clocksThree nickel-cadmium batteriesTwo solar panels – Battery charging – Power generation – 1136 wattsS band antenna—satellitecontrol12 element L band antenna—user communication

Block IIF satellite vehicle(fourth generation)

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The User SegmentGPS antennas &receiver/processorsPositionVelocityPrecise timingUsed by – Aircraft

– Ground vehicles – Ships – Individuals

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Handheld GPS ReceiversExamples:

Garmin eTrex

Garmin-12

Casio GPS

wristwatch

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The Control Segment

Master control station – Schreiver AFB, ColoradoFive monitor stationsThree ground antennasBackup control system

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GPS Ground Control Stations

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How does GPS work?

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How does GPS work?

Satellite ranging – Satellite locations – Satellite to user distance – Need four satellites to determine positionDistance measurement

– Radio signal traveling at speed of light – Measure time from satellite to userMeasurements collected from 4 satellites areprocessed to solve for the 3D position,velocity and time.

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How does GPS work?Pseudo-Random Code

Complex signalUnique to eachsatelliteAll satellites usesame frequency

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SIGNALSSIGNALSSatellites send out signals at different frequenciesSatellites send out signals at different frequencies

(L1/L2)(L1/L2)Codes are used to transmit timeCodes are used to transmit timeThe receiver uses this information to calculate aThe receiver uses this information to calculate a

position through the following steps:position through the following steps:

1.1. Determine distanceDetermine distance2.2. Position of satellites (orbit/ephemeris)Position of satellites (orbit/ephemeris)3.3. TriangulateTriangulate

Calculating a Position

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How does GPS work?

Distance to a satellite is determined bymeasuring how long a radio signal takes to reachus from that satellite.To make the measurement we assume that boththe satellite and our receiver are generating thesame pseudo-random codes at exactly the sametime.By comparing how late the satellite's pseudo-random code appears compared to our receiver'scode, we determine how long it took to reach us.Multiply that travel time by the speed of light andyou've got distance.

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1. Determine Distance1. Determine DistanceDISTANCEDISTANCE == VELOCITYVELOCITY XX TIME DELAYTIME DELAY

TheThe signalssignals travel through radio waves, therefore, theytravel through radio waves, therefore, theytravel the speed of light!travel the speed of light!

Velocity = Speed of lightVelocity = Speed of light (3 x 10(3 x 10 88 m/s)m/s)

The measured delay between the transmitted code andThe measured delay between the transmitted code and

the received code is thethe received code is the TIME DELAY.TIME DELAY.

Now the receiver can calculate the distance to the satellite.Now the receiver can calculate the distance to the satellite.BUT…WHERE are the satellites?BUT…WHERE are the satellites?

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1. Determine Distance1. Determine Distance

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How does GPS work?

Accurate timing is the key to measuringdistance to satellites.

Satellites are accurate because they havefour atomic clocks ($100,000 each) onboard.Receiver clocks don't have to be tooaccurate because an extra satellite rangemeasurement can remove errors.

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2. Position of Satellites2. Position of SatellitesWhere are the satellites in space?Where are the satellites in space?

TheThe satellites send information about its:satellites send information about its:

1.1. Orbital Path (ephemeris)Orbital Path (ephemeris) – – indicates its position in itsindicates its position in itselliptical path around the earth.elliptical path around the earth.

2.2. This is used to determine the position of the satellitesThis is used to determine the position of the satellites

exact position relative to the earth (x,y,z)exact position relative to the earth (x,y,z)

Now the receiver has distance to satellites and position. ItNow the receiver has distance to satellites and position. Itneeds to triangulate!needs to triangulate!

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How does GPS work?

To use the satellites as references for rangemeasurements we need to know exactly wherethey are.GPS satellites are so high up their orbits are verypredictable.All GPS receivers have an almanac programmedinto their computers that tells them where in thesky each satellite is, moment by moment.Minor variations in their orbits are measured bythe Department of Defense.The error information is sent to the satellites, tobe transmitted along with the timing signals.

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3. Triangulation3. TriangulationA receiver will need the distance and position of atA receiver will need the distance and position of at

least three satellites to getleast three satellites to get the position.the position.

D 2

D 1

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3. Triangulation3. TriangulationA receiver will need the distance and position of atA receiver will need the distance and position of at

least three satellites to get an accurate position.least three satellites to get an accurate position.

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3. Triangulation3. TriangulationA receiver will need the distance and position of atA receiver will need the distance and position of at

least three satellites to get an accurate positionleast three satellites to get an accurate position. .

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3. Triangulation3. TriangulationA receiver will need the distance and position of atA receiver will need the distance and position of at

least three satellites to get an accurate positionleast three satellites to get an accurate position. .

EARTH

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GPS Position Determination

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System PerformanceStandard PositioningSystem – 100 meters horizontal accuracy

– 156 meters vertical accuracy – Designed for civilian use – No user fee or restrictions

Precise PositioningSystem – 22 meters horizontal accuracy – 27.7 meters vertical accuracy – Designed for military use

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System PerformanceSelective availability

Intentional degradation of signalControls availability of system’s fullcapabilitiesSet to zero May 2000Reasons

– Enhanced emergency response – Car navigation – Adoption of GPS time standard – Recreation

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System Performance

The earth's ionosphere and atmospherecause delays in the GPS signal thattranslate into position errors.Some errors can be factored out usingmathematics and modeling.The configuration of the satellites in thesky can magnify other errors.Differential GPS can reduce errors.

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The Russian Global Navigation Satellite System (GLONASS) is managed for theRussian Federation Government by theRussian Space ForcesGLONASS is very similar to the NAVSTARGPSThis is an all weather global navigationsystemThere are 21 operational satellites with 3spares

– The satellite mass is about 1,300 kg – The size is 2.35 m diameter

Other Navigation Systems

Russian GLONASS

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GLONASS Satellite, Russian Federation Ministry of

Defense

Russian GLONASS

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Integration of GLONASS may be possiblewith GPS

– Several applications use both now

This would allow extra coverage during poorvisibilityHowever, there are problems:

– Two different coordinate frames usedGPS uses WGS 84 systemGLONASS uses Earth Parameter System 1990 (PZ-90)

– Because of different reference times, this differencedrifts

– This problem could be solved by considering thetime error as an additional variable for solution

Russian GLONASS

Integration of GLONASS with GPS

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The European Global Satellite Navigation System (GNSS), or Galileo , will be aEuropean civil controlled satellite systemEurope’s reason for development is for anavigation system not dependant on theU.S. NAVSTAR system.The venture is a joint public and private

partnershipThere will be two types of service: – A free, basic service – A premium chargeable service with

additional features

Other Navigation Systems

European Galileo

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The Galileo development plan has threephases:

– The definition phase that ended in 2000 – The development and validation phase, that

began in 2001, to include:Ground control infrastructurePrototype satellitesCommunication allocation

– Constellation deployment began in 2006

European Galileo

Development

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Galileo should be available by 2013/2014,with additional ground control and satelliteimprovementsThe European Geostationary NavigationOverlay System (EGNOS) will augment thesystem

European Galileo

Development

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China has launched two navigation satellitesThese geostationary orbits are at 36,000 kmaltitudeThese are intended for land and marine

transportationThe second generation system is beingplanned

Other Navigation Systems

Chiness Regional Satellite NavigationSystem (Beidou)

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That’s all for today!Thanks!

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