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UNIT 4

NAVIGATION

GPS

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The Global Positioning System(GPS) is worldwide satellitenavigation and positioning

system.

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GPS provides multiple userswith highly accurate position,velocity and time data.

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The system is based on aconstellation of 24 operationalGPS satellites orbiting the Earth

twice a day in six orbital planes

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GPS RECEIVER

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The orbital planes have an inclinationrelative to the equator of 55° and anorbital height of 20,200 km.

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The GPS is controlled by the

United States Department of Defense.The cost of maintaining the

system is approximately $400million per year.

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The basis of GPS istriangulation from satellites.GPS satellites transmit signalinformation to earth.GPS receivers take thisinformation and use triangulationto calculate the user's exactlocation.

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. The GPS receiver compares thetime that a signal was transmitted bya satellite with the time it wasreceived. The time difference tellsthe GPS receiver how far away thesatellite is. The receiver can

determine the user's position anddisplay it on the electronic map.

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Each satellite transmits twotypes of orbit data;almanacephemeris

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Almanac data contains the healthand approximate location of every satellite in the system.

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Ephemeris data contains theprecise orbital parameters of each satellite.

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The satellites also transmit twocodes.

The encrypted code is onlyfor military use.

The unprotected code isintended for public access.

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GPS operations depend on a veryaccurate time reference which is

provided by atomic clocks.Each GPS satellite has atomicclocks on board.

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GPS uses a high frequency radio

signal where there is littleinterfering radiation.GPS uses spread spectrum

technology to protect itsnavigation signals.

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The GPS signal is extremely resistantto conditions that disturb otherelectronic navigation systems.In general, weather conditions, on-board electronics, passing ships, on-shore electronic installations, and

portable radio receivers do not affectthe GPS signal.

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ANSWER THE QUESTIONS

1- How many satellites form theGPS system?

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2- Who maintains the system?

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3- What is the system based on?

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4- What types of data does thesystem transmit ?

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5- Does the system easily disturbedand affected?

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ETYMOLOGY

Almanac < Latin, almanachus < Greekalmenichiaka, calendar Ephemeris < Greek, ephemeris , diary,

calender Global < Latin, globus, a ball Map <Latin, mappa, a cloth (on which

maps were painted) Military <Latin, militis, soldier Orbit < Latin, orbis, track, path, circle

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SOME WORDS

Constellation (n)- A group of starswhich appear close together.

Global (adj) - Universal, of the globe. Triangulation (n)- A technique for

precise determination of distancesand angles for a ship’s position.

Position (n)- A point defined bystated coordinates.

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Try to write the verb forms.

calculation calculate ignition ............................

information ............................installation ............................location ............................

navigation ............................observation ............................operation ............................

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Write the adverb forms of thefollowing words

1- accurate accurately 2- approximate ............................ 3- board ............................ 4- day ............................ 5- exact ............................ 6- extreme ............................ 7- general ............................ 8- high ............................ 9- possible ............................ 10- precise ............................

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Write the acronyms fully.

ARPA :…………………………… CW :…………………………… FM :…………………………… GPS :…………………………… IALA :…………………………… LORAN :…………………………… RADAR :…………………………… RDF :……………………………

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Complete the sentences

1- GPS is based on ................….from satellites

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2- The system’s maintenance cost is................ million per year.

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3- The system is based on aconstellation of ............... satellitesorbiting the Earth in ………….. orbital

planes

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4- Each satellite transmits twotypes of orbit data;................…. and ephemeris

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5- GPS satellites have ................on board.

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8- The receiver can determinethe user’s ……………… and displayit on the electronic map.

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How many satellites are there inGPS system?

a. 16

b. 18 c. 24

d. 36 e. 48