Global Positioning System (GPS) 2

8/6/2019 Global Positioning System (GPS) 2

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By

Ms. Sangeeta Saini


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GPS i.e. Global Positioning System, is the only

system today which is able to show you your

exact position on the Earth anytime, in any

weather, anywhere.

The three parts of GPS are:

`Satellites

`Receivers/ Software

`Ground Control


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` Satellite-based radio positioning systems thatprovide 24 hour 3D position, velocity and timeinformation to suitably equipped users anywhereon or near the surface of the Earth

`A system of satellites and receiving devicesused to compute positions on the Earth

` GPS satellites transmit signal at frequencies L1=1576.42MHz and L2= 1227.6 MHz modulatedwith two types of code ( P- Code and C/A code)with navigation message.


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` NAVSTAR (US)

` GLONASS (Russia)

` Galileo (EU)


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` NAVigation System for Timing and Ranging: Global PositioningSystem

` Operated by US DoD,

` Fully operational from 8th Dec 1993

` Consists of 24 32 Satellites orbiting earth (+ 4 spare)

In 6 orbital planes

At an altitude of 20,180 km


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` GLObal Navigation Satellite System

` Operated by Russian Federation Ministry of Defense

` Consists of

24 Satellite

Orbiting in 3 planes (8 in each) Altitude 19,130 km


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` Operated by ESA (European Space Agency)

` Operational from 2011

` Will consists of

27 active and 3 spare satellites In 3 orbital planes (10 satellites in each plane)

Altitude 23,616 km


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` The basic component of a communicationssatellite is a receiver-transmitter combinationcalled a transponder.

`A satellite stays in orbit because thegravitational pull of the earth is balanced by thecentripetal force of the revolving satellite.

` Satellite orbits about the earth are either circularor elliptical.


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` Satellites orbit the earth from heights of 100 to 22,300miles and travel at speeds of 6800 to 17,500 mi/h.

` A satellite that orbits directly over the equator 22,300

miles from earth is said to be in a geostationary orbit.

` Geostationary (GEO) satellite: revolves in synchronismwith the earths rotation, so it appears to be stationarywhen seen from points on the earth.


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` Only some of the satellites have circular orbits.

` Others have elliptical orbits. These orbits have furtherclassifiers:

Apogee

Perigee


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` Perigee: point on orbit when satellite is closest toearth.

` Apogee: point on orbit when satellite is farthestfrom earth.


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`A satellite is stabilized in orbit by spinning it onits axis or building in spinning flywheels for eachmajor axis (roll, pitch, yaw).

` Altitude adjustments on a satellite are made byfiring small jet thrusters to change the satellitesposition or speed.

` Satellites are launched into orbit by rockets that

give them vertical as well as forward motion.


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` All satellites have a metal or composite frame and body,usually known as the bus. The bus holds everythingtogether in space and provides enough strength tosurvive the launch.

` They have a source ofpower(usually solar cells) andbatteries for storage.

` They have an onboard computerto control and monitorthe different systems.

` They have a radio system and antenna.

` All satellites have an altitude control system. The ACSkeeps the satellite pointed in the right direction.


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` GEO (Geostationary Earth Orbit) satellites orbitabout 36,000 km above Earths surface.

` LEO (Low Earth Orbit) satellites are about 500-

1500 km above earths surface.

` MEO (Medium EO) satellites are about 6000-20,000 km above earths surface.

` There are also HEO (Highly Elliptical Orbit)satellites.


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Earth

1000 km

35,768 km

10,000 km

LEO GEO

HEO

MEO

N t ra t al


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` The majority of communications satellites are GEOs.These support voice, data, and video services, mostoften providing fixed services to a particular region.

` GEO systems are less complicated to maintain because

fixed location requires relatively little tracking capabilityat ground.

` High orbital altitude allows GEOs to remain in orbitlonger than systems operating closer to earth.


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` These characteristics, along with their highbandwidth capacity, may provide a cost advantageover other system types.

` However, their more distant orbit also requiresrelatively large terrestrial antennae and high-powered equipment and are subject to delays.


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` Typical LEO satellite takes less than 2 hours to orbit theEarth, which means that a single satellite is "in view" ofground equipment for a only a few minutes.

` If transmission takes more than few minutes that any

one satellite is in view, a LEO system must "hand off"between satellites to complete the transmission.

` Handoffs can be accomplished by relaying signalsbetween satellite and various ground stations, or bycommunicating between satellites themselves using

"inter-satellite links."` LEO systems designed to have more than 1 satellite in

view from any spot on earth at any given time.


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` LEO systems must incorporate sophisticatedtracking and switching equipment to maintainconsistent service coverage.

`Advantages: very little delay, operate usingsmaller equipment (because signals travelshorter distance), etc.

` Disadvantages: highly complex andsophisticated control and switching systems,

shorter life span (subject to greater gravitationalpull and higher transmission rates lead toshorter battery life).


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` MEOs are in between a GEO and a LEO.

` Advantages/Disadvantages are also in between:

PRO: MEO systems will require far fewer satellites

than LEOs, reducing overall system complexity andcost, while still requiring fewer technological fixes toeliminate signal delay than GEOs.

CON: MEO satellites, like LEOs, have a much shorter

life expectancy than GEOs, requiring more frequentlaunches to maintain system over time.


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` Elliptical orbit causes satellite to move around earth faster when it istraveling close to earth and slower the farther away it gets.

` Satellites beam covers more of earth from farther away.

` Orbits are designed to maximize amount of time each satellitespends in view of populated areas.

` Delay characteristics depend on where the satellite is in its orbit.

` Several of proposed global communications satellite systems

actually are hybrids of the four varieties reviewed above.


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` Weather satellites help meteorologists predict theweather or see what's happening at the moment. Thesatellites generally contain cameras that can returnphotos of Earth's weather.

` Communications satellites allow telephone and data

conversations to be relayed through the satellite.` Broadcast satellites broadcast television signals from

one point to another (similar to communicationssatellites).

` Scientific satellites perform a variety of scientific

missions. The Hubble Space Telescope is the mostfamous scientific satellite, but there are many otherslooking at everything from sun spots to gamma rays.

` Navigational satellites help ships and planes navigate,e.g., GPS.


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` Rescue satellites respond to radio distresssignals.

` Earth observation satellites observe the planetfor changes in everything from temperature toforestation to ice-sheet coverage.

` Military satellites are up there, but much of theactual application information remains secret


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` Satellite launches don't always go well; there is a great deal atstake. The cost of satellites and launches to name one.

` For example, a recent hurricane-watch satellite mission cost $290million. A missile-warning satellite cost $682 million.

` A satellite launch can cost anywhere between $50 million and $400million. Russian launches are generally the cheapest and theFrench launches are the most expensive.

` A shuttle mission pushes toward half a billion dollars (a shuttlemission could easily carry several satellites into orbit).


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Monitor stations Diego Garcia

Ascension Island

Kwajalein Hawaii

USER SEGMENT

SPACE SEGMENT

GPS ControlColorado Springs

GROUND CONTROL SEGMENT


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` 24+ satellites 6 planes with 55 inclination Each plane has 4-5 satellites Broadcasting position and

time info on 2 frequencies Constellation has spares

` Very high orbit 20,180 km 1 revolution in approximately

12 hrs Travel approx. 7,000mph

` Considerations

Accuracy Survivability Coverage


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` Located at Falcon AFB in Colorado Springs, Colorado

` It is responsible for overall management of the remote monitoring andtransmission sites.

` It calculates any position or clock errors for each individual satellite, basedon information received from the monitor stations,

` Orders the ground antennas to relay requisite corrective information back tosatellites

` Each of the monitor stations checks the exact altitude, position, speed, andoverall health of the orbiting satellites.

` The control segment uses measurements collected by the monitor stationsto predict the behavior of each satellite's orbit and clock.

` The prediction data is up-linked, or transmitted, to the satellites fortransmission back to the users.


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` The control segment also ensures that the GPS satelliteorbits and clocks remain within acceptable limits.

` A station can track up to 11 satellites at a time.

` This "check-up" is performed twice a day, by eachstation, as the satellites complete their journeys aroundthe earth.

` Noted variations, such as those caused by the gravity of

the moon, sun and the pressure of solar radiation, arepassed along to the master control station.


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` Over $19 Billion invested by DoD

` Dual Use System Since 1985

(civil & military)

` Civilian community was quick to

take advantage of the system Hundreds of receivers on the market

3 billion in sales, double in 2 years

95% of current users

` DoD/DoT Executive Board sets GPS policy


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` All satellites have clocks set to exactly the same time.

` All satellites know their exact position in space from datasent to them from the systems controllers.

` Each satellite transmits its position and a time signal.

` The signals travel to the receiver delayed only bydistance traveled.

` The receiver calculates the distance to each satellite andtrilaterates its own position.


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Each GPS satellitetransmits data thatindicates its location andthe current time. All GPSsatellites synchronizeoperations so that theserepeating signals aretransmitted at the same

instant.

Physically the signal is just acomplicated digital code, or in

other words, a complicatedsequence of on and offpulses.


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Measuringthe distance fromasatellite

` Done by measuring travel time of radio signals


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The GPS receiver

compares the time a signal

was transmitted by a

satellite with the time itwas received. The time

difference tells the GPS

receiver how far away the

satellite is.


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Velocity x Time = Distance

Radio waves travel at the speed of light, roughly 186,000

miles per second (mps)

If it took 0.06 seconds to receive a signal transmitted by asatellite floating directly overhead, use this formula to find yourdistance from the satellite.

186,000 mps x 0.06 seconds = 11,160 miles


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GeometricPrinciple:

You can find one

location if youknow its distancefrom other,already-known

locations.


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Latitude and Longitudeare sphericalcoordinates on thesurface of the earth.Latitude is measuredNorth or South of theEquator. Longitude is

measured East orWestof Greenwich. GPS usesLatitudes andLongitudes to reference

locations.


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Waypoints are locations or landmarks that can

be stored in your GPS. Waypoints may be

defined and stored in the unit manually by

inputting latitude and longitude from a map orother reference.

Latitude and

Longitude

Your

location

Direction of

waypoint

Date

and

Time

Waypoint

Or more usually,waypoints may be

entered directly bytaking a reading with theunit at the location itself,giving it a name, andthen saving the point.


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GPS units collect datain:

Points

L

ines Areas

Thesearecalledfeatures.

A data dictionary is ameans by which wecollect specificinformation about adata feature.


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A roving file is like a

drawer of a filing

cabinet containing

many feature files.

Collect all the features collected in one interval in one rovingfile.


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` Land, Sea and AirNavigation andTracking

` Surveying/ Mapping

` Military Applications

` Recreational Uses


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` Police and

Emergency

Medical

Services

` Firefighters

` Map makers

` Science


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` 100 meters: Accuracy of the original GPS systemunder the government-imposed SelectiveAvailability (SA) program. (turned off 5/1/2000)

`

15 meters: Typical GPS position accuracy withoutSA.

` < 3 meters: Typical WAAS position accuracy.


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` Multipath Use a ground plane on antenna Move away from multipath source

` Receiver Channel noise

Use a different GPS receiver` 2-D fix with a bad altitude

Use Manual 3D when collecting data

` High PDOP (Position Dilution of Precision)

Wait for the geometry to change


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GPS satellites use Atomic Clocks for

accuracy, but because of the expense,

most GPS receivers do not.


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Line of sight is the ability to

draw a straight line between

two objects without any

other objects getting in theway. GPS transmission are

line-of-sight transmissions.

Obstructions such as trees, buildings, or natural formations may preventclear line of sight.


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Sometimes the GPSsignal from the satellitedoesnt follow astraight line.

Refraction is thebending of light as ittravels through one

media to another.


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Signals from satellites can be like light. When they

hit some interference (air patterns in the atmosphere,

uneven geography, etc.) they sometimes bend a little.


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Sometimes the

signals bounce off

things before they hit

the receivers.


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When the satellites are all in the same part of the

sky, readings will be less accurate.


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All of this combines

to make the signal

less accurate, and

gives it what we call a

high PDOP.

PDOP = Positional Dilution of Precision

11,000miles

11,000miles

11,000 miles

11,000 miles

A P Pof 8ispoor


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Differential correction isa technique that greatlyincreases the accuracyof the collected GPSdata. It involves using areceiver at a knownlocation - the "basestation- and comparing

that data with GPSpositions collected fromunknown locations with"roving receivers."

ISU Base Station - http://134.50.65.125/


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Before

After


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Apply estimatedcorrections

` The signals aredelayed by theionosphere andtroposphere

` Receiver makes

estimated correctionsfor these delays

Ionosphere

Troposphere


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U.SGovernment can (and does!) degrade the accuracy

` Largest source of error in GPS

` To prevent hostile forces from using GPS to full

accuracy` By introducing intentional errors in timing signals

and/or satellite ephemeris

` S A was removed on 02.05.2000


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` Ionospheric effects 5 meters

` Tropospheric effects 0.5 meter

` Ephemeris errors 2.5 meters

` Satellite clock errors 2 meters` Multipath distortion 1 meter


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Global Positioning System (GPS) 2

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