gps_lect

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GPS

Global PositioningSystem

Dr.G.Bhaskaran

Senior Lecturer

Department of Geography

University of Madras, Chennai-5

[email protected]

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Contents

Radio Navigation

GPS History

Parts of GPS

GPS Signals

How Does GPS Work?

GPS Error Sources Differential GPS

Application of GPS

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Radio Navigation Systems

Radio navigation consists of findingposition and heading by usingelectromagnetic wave propogation.

Examples:

Radar

VHF Omnidirectional Range (VOR,

VOR/DME) Long Range Navigation (LOR AN-C)

Global Positioning System (GPS)

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What is GPS ?

GPS is a satellite based navigation

system.

It is developed and financed by the U.S.Department of Defense.

It provides position velocity and timing

information anywhere in the world under any weather condition.

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1969²Defense Navigation Satellite System (DNSS) formed

1973²NAVSTAR Global Positioning System developed

1978²first 4 satellites launched

Delta rocket launch

1993²24th satellite launched; initial operational capability

1995²full operational capability

In 1980s it became available for civilian use.

May 2000²Military accuracy available to all users

Today it is being used in land, air and marine applications bymillions of people.

Development of GPS ?

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USA ± NAVSTAR

(Navigation System with Time and Ranging)

Department of Defense (DoD)

Russia ±GLONASS Global Navigation Satellite System

EEC ± GALILEO (Under development)

Who maintains GPS?

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Government Military

Agencies - Conservation, Weather, Rescue

Industry Transportation

Fishing

Agriculture

General Public Recreation

Emergency / Security Reasons

Health Related Reasons

Who uses GPS?

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Current Configuration

20,200 km elevation above Earth

Currently there are 29 operational BlockII/IIA/IIR/IIR-M satellites

Nominally there are 24 satellites (4 on 6 orbital planes)

Inclined 55o with respect to equator

Orbits separated by 60o

Orbital period of 11

hr 55 min

Five to eight satellites visible from any pointon Earth

Block I Satellite Vehicle

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ACTUAL SHAPE OF EARTHACTUAL SHAPE OF EARTH

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10 / 55GPS CONSTELLATIONGPS CONSTELLATION

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GPS Segments

There are 3

segments

Space Control

User

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Control Segment

Space Segment

User Segment

Three Segments of the GPS3Three Segments of the GPS3

Monitor Stations

Ground

Antennas

Master Station

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GPS Space Segment

Consitsts of the spacevehicles (satellites) andthe radio signals sent bythese satellites.

GPS satellites Height ~20200 km

6 orbits with at least 4satellites on each orbit

Period ~ 1 revolution / 12 hour

Weight ~950 kg

Size 1,6 x 6 m

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GPS Space Segment

At least 5 satellites are visible from

anywhere on the earth

There are solar panels and 12 navigationantennas on each satellite.

Block II/IIA

Block IIR ve IIR-M

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GPS Control Segment

Monitors and Controls the

GPS satellites.

One Master ControlStation (MCS),

Five Monitor Stations

(MS)

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GPS Control Segment

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GPS Control Segment

Functions of the Control Segment

Detection and determination of Satellite orbits

Correction of satellite clocks Updating the satellite messages

Monitoring the status of each satellite and

performing the maintanence tasks

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GPS User Segment

Consists of receivers

that can decode the

satellite signals

GPS receivers

transform the satellite

signals into position,

velocity and time

information.

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GPS Services

GPS has two levels of information

Precise Positioning Service - PPS

Standard Positioning Service - SPS

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GPS Services ± PPS

Pr ecise Positioning Ser vice (PPS)

Can be used by authorized users only

Planned for military purposes

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GPS Services ± PPS

Access to PPS is controlled by two

methods

SA (Selective Availability), GPS accuracy isdegraded intentionally by adding pseudo-

random errors on the signals.

A-S (Anti-Spoofing), Encrypted code

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GPS Services ± SPS

Standard Positioning Service (SPS) Open to all users but less accurate

With Selective Availability 100 m SEP (3D - %50) position accuracy

337 ns (1 ) time accuracy

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GPS Services ± SPS

SA has been removed on May 2000

SPS users have accuracies close to PPS

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GPS Signals

GPS satellites send very weak

radio signals on two L ± band

frequencies (L1

and L2

) L1 and L2 are carrier

frequencies.

These are sinusoidal signals

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GPS Signals

All GPS satellites use the same frequency

carriers (L1 and L2)

But each satellite has its own identificationcode

These are two types of codes modulating

the L1

and L2

carriers. C/ A ± Code

P ± Code

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L2

P(Y)P(Y)

L1

C /AC /A

P(Y)P(Y)

GPS Signals

L1: 1575.42 MHz Modulated byC/ A-code & P-code

Signal Power: -160 dBW

L2 : 1227.6 MHz Modulated by P-code only

Signal Power : -166 dBW

1227 MHz

1575 MHz

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GPS Signals

Carr iers (L1 /L2)

Bipolar Phase Shif t Keying

(BPSK) Modulation

C /A - Code (L1)

P - Code (L1 /L2)

Phase Quadr atur e O

SA Degr edation

Nav Data (L1 /L2)

A-S Encr yption P ±

P(Y)

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GPS Signals

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GPS Signals

GPS receivers generate the equivalent of these codes

internally and compares with the ones coming from the

satellites.

GPS receiver shifts the internally generated code until it

matches with the received one (cross-correlation)

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GPS Signals

Another Message on the L1 ve L2 carrier frequency is the ³Navigation Message´

Navigation Message

50 Hz Clock rate Has information specific for each satellite

Has the satellite position and time delay information

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How Does GPS Work?

Based on a geometric principle

³Position of a point can be calculated if the distances between this point andthree objects with known positions canbe measured ´

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2kms

5kms

11kms

Tr iangulation

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How Does GPS Work?

If the distance to one object is known:

Then I am on a sphere with the object at the center

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How Does GPS Work?

If I know the distance to a second object:

Then I am on a circle which is the intersection of two

spheres

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How Does GPS Work?

If I know the distance to a thrid object:

Then I am on one of the two points which are at the

intersection of three spheres

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How Does GPS Work?

To find the distance to a satellite ³SignalTime of Transmission´ is used

How is Signal Time of Transmissioncalculated?

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How Does GPS Work?

Satellite

Receiver

GPS receiver generates the same signalthat is coming from the satellite (C/ A -Code) starting at the same time.

But the code coming from the satellite isdelayed because it travels the distancebetween the satellite and the receiver.

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How Does GPS Work?

GPS receiver shifts the internallygenerated code until it matches with thereceived one and finds T, Signal Time of

Transmission

Code generatede by thereceiver

T

Code generatede bythe sattelite

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How Does GPS Work?

Signal Time of Transmission is actually an

indication of the distance between the

receiver and the satellite Signal travels with the speed of light and in

t time travels a distance of

Pr =

C.T

(C = Speed of light)

Pr - Pseudo-Range

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How Does GPS Work?

Pr is the ³Pseudo-Range´

It is called Pseudo-Range because it is not

the real range between the receiver andthe satellite due to uncertainties such as:

Synchronisation error between the receiver

and satellite clocks

Change in the medium in which the signal

travels

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How Does GPS Work?

The dominant source of error in Pseudo-

Range calculation is the synchronisation

between the receiver and the satellite Satellites have very accurate and very

expensive atomic clocks

It is not practical to use atomic clocks in

the receivers. Standard crystal oscillators

are used instead

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How Does GPS Work?

This syncrhronisation error is called Clock

Bias

To eliminate clock bias a forth satellite isused

4 unknowns (3 dimensional position + Clock

Bias)

4 equations

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How Does GPS Work?

b Z Z Y Y X X P

b Z Z Y Y X X P

b Z Z Y Y X X P

b Z Z Y Y X X P

!

!

!

!

2

4

2

4

2

44

2

3

2

3

2

33

2

2

2

2

2

22

2

1

2

1

2

11

)()()(

)()()(

)()()(

)()()(

Pi = Pseudo-Range to satellites

X i , Yi , Zi = 3 Dimensional satellite cartesian coordinates

X , Y , Z = 3 Dimensional satellite cartesian coordinates

b = Receiver clock bias (in terms of distance)

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How Does GPS Work?

These 4 non-linear equations are solved

and receiver coordinates and clock bisa

are obtained These equations are in ECEF (Cartesian)

Coordinates

Latitiude, Longitude and hight values can

be obtained by a transformation

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How Does GPS Work?

ECEF and Latitude / Longitude

X

Y

Px

Py

Pz

R

h

Z

GREENWICHMeridian

Px: ECEF Pos x (M)

Py: ECEF Pos y (M)

Pz: ECEF Pos z (M)

O

Equator

LOCAL

Meridian User position

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GPS Error Sources

SA (Selective Availability)

Satellite clock errors

Satellite orbit errors

Atmospheric effects

Receiver noise

Multipath

Number of satellites in range

Satellite geometric

configuration

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GPS Error Sources

DOP (Dilution of Precision)

GDOP - Geometric DOP

It is a metric to define the effect of the satellitegeometry on the accuracy of the solution:

PDOP ± Position DOP (3 D Position)

HDOP ± Horizontal DOP (Horizontal position) TDOP ± Time DOP (Time)

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Ideal Satellite GeometryIdeal Satellite Geometry

N

S

W E

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Good Satellite GeometryGood Satellite Geometry

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Good Satellite GeometryGood Satellite Geometry

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Poor Satellite GeometryPoor Satellite Geometry

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GPS Error Sources

Satellites close to each other have larger uncertainty

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GPS Error Sources

Satellites far away from each other have less

uncertainty

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GPS Error Sources

1 signifies the ideal situation

Satellites grouped on the sameside cause larger DOP ± Bad

accuracy

Well distributed, smaller DOP ±

better accuracy

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Common Problems - Errors

GPS

Antenna

Hard Sur face

Satellite

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GPS Error Sources

GPS Pseudo-Range Error Budget

Segment Error Sour ce

Error contr ibution (m,

%95)

P-Code C /A-Code

SpaceFrequency stability 6

.5

6.5

D-Band Delay 1.0 1.0

Satellite acceleration uncertainty 2.0 2.0

Other 1.0 1.0

ControlEphemeris Estimation 8.2 8.2

Other 1.8 1.8

User

Ionospheric Delay compensation 4.5 9.8 ± 19.6

Troposphere Delay compensation 3.9 3.9

Receiver noise 2.9 2.9

Multipath 2.4 2.4

Other 1.0 1.0

Total System Error (m, %95) 13.0 15.7 - 23.1

Reference: Navstar GPS User Equipment Introduction

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Differential GPS - DGPS

Used for applications where GPS accuracy is

not enough

In a typical DGPS application

There is a reference receiver (base receiver) at an

exactly known location

And there are other receivers (rover receivers) thatcan receive the correction signals sent by the base

receiver.

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DG PS Correction Signals

GPS

Refer ance

Station

DGPS Tr ansmitter GPS &

DGPS Receiver

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Since the exact location of the reference stationis known it can calculate the distances tosatellites accurately

It compares these distances with its ownsolutions as a GPS

Calculates corrections from thesemeasurements

Sends these corrections to the rover receiversfrom a different frequency than the GPSfrequencies.

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Transmission is usually over a FM channel

The rover receivers are able to receive

these corrections and they use them tocorrect their solutions

Corrections are valid within a certain range

Referance and rover receivers must havethe same satellites in view

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Application of GPS Technology

Private and recreation

Traveling by car

Hiking, climbing, biking

Vehicle control

Mapping, survey, geology English Channel Tunnel

Agriculture

Aviation

General and commercial

Spacecraft

Maritime

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Thanks for your interest in theGlobal Positioning System

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Differential leveling for height measurements

(Mean Sea Level is the starting point for the height measurements)

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Relationships between the earth's surface, the geoid and a

reference ellipsoid

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Geoid

Sur face of the

Earth

CG

P

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geoid

Globally Fitting Ellipsoid

(Geocentr ic) e.g., WGS - 84

CG

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Locally

Fitting

Ellipsoid

Ar ea of

Best Fit

Locally Best Fitting

Ellipsoid

e.g., EVEREST

SP

HEROID

geoid

CG

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T he geoid and two best fitting local

N.AMERICA

S.AMERICA

INDIA

NN

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