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GPS Triangulation Procedure

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Triangulation Requirements

To triangulate, a GPS receiver measures distanceusing the travel time of radio signals.

To measure travel time, GPS receiver needs very

accurate timing.

Along with distance, receiver need accurate dataon where satellites are in space.

System will also need to correct for any delaysthe signal experiences as it travels throughatmosphere.

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Components of GPS System

Control Segment: five ground stations located onearth.

Space Segment: satellite constellation (24 activesatellites in space).

User Segment: GPS receiver units that receivesatellite signals and determine receiver locationfrom them.

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Ground Monitor Stations

Falcon AFB

Colorado Springs, CO

Master Control Monitor Station

Hawaii

Monitor StationAscension Island

Monitor Station

Diego Garcia

Monitor Station

Kwajalein

Monitor Station

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Basic Functions of Monitor Stations

These stations are the eyes and ears of GPS,monitoring satellites as they pass overhead bymeasuring distances to them every 1.5 seconds

This data is then smoothed using ionosphericand meteorological information and sent toMaster Control Station at Colorado Springs.

The ionospheric and meteorological data isneeded to get more accurate delaymeasurements, which in turn improve locationestimation.

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Functions of Monitor Stations (Contd)

Master control station estimates parametersdescribing satellites' orbit and clockperformance,. It also assesses health status of

the satellites and determines if any re-positioningmay be required.

This information is then returned to three uplink

stations (collocated at the Ascension Island,Diego Garcia and Kwajalein monitor stations)which transmit the information to satellites.

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Space Segment

Space segment is the satellite constellation.

24 satellites with a minimum of 21 operating

98% of the time 6 Orbital planes

Circular orbits

20-200 km above the Earth's surface

11 hours 58 minute orbital period Visible for approximately 5 hours above the

horizon

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GPS Satellite Orbits

We can obtain updates of GPS satellites at

http://www.ngs.noaa.gov/GPS/GPS.html

http://www.ngs.noaa.gov/GPS/GPS.htmlhttp://www.ngs.noaa.gov/GPS/GPS.html

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GPS Satellite Orbits (Contd)

Orbits of GPS satellites need to be updatedevery once in a while because orbit does notstay circular without adjustments.

Adjustments needed because: Other objects exert gravitational force on each

satellite (e.g. sun, moon) Effect of gravity is non-uniform during orbit.

Radiation pressure (due to solar radiation). Atmospheric drag Other effects

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Interesting Aside on GPS Orbits

When GPS satellites are decommissioned,

they are placed on a disposal orbit (outside

the operating GPS orbit).

Some studies show that satellites in disposal

orbits can eventually, perhaps over 20-40

years, encroach into operating constellation.

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Aside (Contd)

This is because disposal orbits, while circularinitially, become increasingly elliptical, mostlyas result of sun-moon gravitational

perturbations.

Besides intersecting GPS constellation, these

satellites eventually could pose a threat tooperational satellites in low Earth andgeosynchronous orbits

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Aside (Contd)

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Third Component of GPS: User

Segment

User segment comprises receivers that have beendesigned to decode signals transmitted fromsatellites for purposes of determining position,

velocity or time.

Receiver must perform the following tasks:

select one or more satellites in view

acquire GPS signals

measure and track signal

recover navigational data

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Synchronizing Clocks

In order to make this measurement, the

receiver and satellite both need clocks that

can be synchronized down to the nanosecond.

Accurate time measurements are required. If

we are off by a thousandth of a second, at the

speed of light, that translates into almost 200

miles of error.

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Synchronizing Clocks (Contd)

To make a satellite positioning system using onlysynchronized clocks, you would need to haveatomic clocks not only on all the satellites, butalso in the receiver itself.

But atomic clocks cost somewhere between$50,000 and $100,000, which makes them a just abit too expensive for everyday consumer use.

The Global Positioning System has a clever,

solution to this problem. Every satellite containsan expensive atomic clock, but the receiver itselfuses an ordinary quartz clock, which it constantlyresets.

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Synchronizing Clocks (Contd)

The receiver can easily calculate the necessary

adjustment that will cause the four spheres to

intersect at one point.

Based on this, it resets its clock to be in sync

with the satellite's atomic clock.

The receiver does this constantly whenever

it's on, which means it is nearly as accurate as

the ex ensive atomic clocks in the satellites.

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Knowing Satellite Locations

In order to properly synchronize clocks and

figure out which PRC signal to listen to, the

receiver has to know where the satellites

actually are.

This isn't particularly difficult because the

satellites travel in very high and predictableorbits.

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2 Types of Errors

Errors can be categorized as intentional andunintentional.

Intentional errors: government can and doesdegrade accuracy of GPS measurements. Thisis done to prevent hostile forces from usingGPS to full accuracy.

Policy of inserting inaccuracies in GPS signalsis called Selective Ability (SA). SA was singlebiggest source of inaccuracy in GPS. SA was

deactivated in 2000.

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Typical Errors

Source of Error Typical Error in Meters

(per satellite)

Satellite Clocks 1.5

Orbit Errors 2.5

Ionosphere 5.0

Troposphere 0.5

Receiver Noise 0.3Multipath 0.6

SA 30

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Using GPS Data

A GPS receiver essentially determines the receiver'sposition on Earth.

Once the receiver makes this calculation, it can tell youthe latitude, longitude and altitude of its current

position. To make thenavigation more user-

friendly, most receivers

plug this raw data into

map files stored inmemory.

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Using GPS Data (Contd)

You can use maps stored in the receiver's memory,

connect the receiver to a computer that can holdmore detailed maps in its memory, or

simply buy a detailed map of your area and find yourway using the receiver's latitude and longitudereadouts.

Some receivers let you download detailed mapsinto memory or supply detailed maps with plug-in map cartridges.

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Using GPS Data (Contd)

A standard GPS receiver will not only place youon a map at any particular location, but will alsotrace your path across a map as you move.

If you leave your receiver on, it can stay inconstant communication with GPS satellites to

see how your location is changing.

This is what happens in cars equipped with GPS.

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