L4b-GPS - Introduction Notes

7/24/2019 L4b-GPS - Introduction Notes

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Global P

ositioning System

GLOBAL

POSTTTONTNG

SYSTEM

(GPS)

1.0 lntroduction

The Global

Positioning System

(GPS)

is a

burgeoning

technology,

which

provides

unequalled

accuracy

and

flexibility of

positioning

for

navigation, mapping,

surveying

and GIS

data capture.

1.1

Why GPS?

.

Trying

to

figure out where

you

are

and

where

you're

going

is

probably

one of

man's

oldest

pastimes.

Figure

1.1:

Helping

you get

from

point

A

to

point

B

Navigation

and positioning are

crucial

to so many activities and yet

the

process

has

always been

quite

cumbersome,

Over

the

years

all

kinds

of technologies

have

tried to

simplify the

task

but

every

one

has had some disadvantage.

Finally,

the U.S.

Department

of

Defense decided

that

they

have

a

super

precise

form

of worldwide

positioning.

r

The

result

is

the

Global Positioning System,

a system

that's

changed

navigation

forever.

1.2

What is GPS?

The

NAVSTAR

(Alavigational

Satellite

7'iming

And

Ranging)

Global

Positioning

System

(GPS)

is a

satellite-based radio

positioning

and

time-transfer

system

designed,

financed,

deployed,

and operated

by

the U.S.

Department

of

Defense.

GPS has also

demonstrated

a

significant benefit

to the civilian community

who are

applying

GPS

to a

rapidly

expanding

number

of applications.

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Global

P

ositioning

System

What attracts

us

to GPS

is:

.

The

relatively

high

positioning

accuracies,

from

tens of meters

down

to the

millimeter

level.

.

The capability of determining

velocity and

time,

to

an accuracy

commensurate

with

position.

o

The signals are available

to

users

anywhere on the

globe:

in

the air,

on

the

ground,

or at

sea.

o

lt a

positioning

system

with no user

charges,

that

simply

requires

the

use

of

relatively

low

cost

hardware.

.

lt

is an

all-weather

system, available

24 hours

a day.

o

The

position

information

is

in

three

dimensions,

that is, vertical

as well as

horizontal

information is

provided.

The number

of

civilian users is already significantly

greater

than that of the military users.

However,

for the time being the U.S. military still operates

several levers

with which

they

control

the

performance

of GPS.

Nevertheless, despite

the

handicap

of GPS

being

a

military

system

there

continues

to

be

tremendous

product

innovation

within

the

clvilian

sector, and

it

is ironic

that this

innovative

drive

is

partly

directed

to developing

technology

and

procedures

to overcome some of

the

constraints

to GPS

performance

which

have

been applied

by

the

system's military

operators.

{.3

GPS

System

configuration

GPS

is

a

space

based radio

positioning

system

that

provides

24

hour

three

dimensional

position,

velocity

and

time

information

to

suitably

equipped users

anywhere

on

the

surface of

the

earth. GPS

involve

three

major

components,

the satellites,

the

ground

based

control

of

the

satellites

and

the

user. These

are

often

referred

to

as the

Space, Control

and

User

Segment.

Figure

1.22 A

constellation

of 24

satellites

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Glob al P ositioning System

ds

Figure

1.3:

GPS System Elements

1.3.1

Space Segment

The

Space Segment of the system consists

of the

GPS satellites

comprising the

satellites

and the transmitted signals. These space

vehicles

(SVs)

send radio

signals

from

space.

Figure 1.5:

Navstar

GPS

Constellation

Name: NAVSTAR

Manufacturer: Rockwell lnternational

Altitude:

10,900 nautical

miles

(20,200

km),

Weight:

1900

lbs

(in

orbit)

Size:

17

ft with

solar

panels extended,

Orbital

Period:

12

hours

Orbital Plane: 55 degrees

to equatorial

plane,

Planned

Lifespan: 7.5

years

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GPS Noninsl

Constellation

2*

Satcllites in 6

Orbital

Plancs

4

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2{1p00

krn .dltitudss,

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Degrrc

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Global

P

ositioning

System

Current constellation:

24

Block

ll

production

satellites

Future satellites:

21 Block

lll

developed by

Martin Marietta.

The nominal

GPS

Operational

Constellation consists of 24

satellites

that

orbit the earth

in

12

hours.

1.3.2

ControlSegment

The Control Segment

consists

of a

system of tracking stations

located around

the

world.

These

ground

facilities carrying out the

task of

satellite tracking,

orbit computations,

telemetry and

supervision

necessary for the daily control of

the

space

segment.

n.?-r,

M

ooitti 's'

igtf

Monitor

Station

t'

a

'wajaleirr

Z*rq.rr,station

L---.{

I

.

Yf

Glabal

Positioning

System

iGFSi

Master

Cantrol

and

Manitor

Station

Network

ipi*eo

Garcia

lVtonitor Station

Figure

1.6:

GPS

Master

Control

and Monitor

Station

Network

Figure

1.7: GPS Control

The Master Control facility is located

at

Falcon Air

Force Base

in

Colorado.

These

monitor

stations measure signals from the SVs which are incorporated

into orbital models

for

each

satellites.

The

models

compute

precise

orbital data

(ephemeris)

and

SV

clock

corrections

for

each satellite.

The Master Control station

uploads

ephemeris

and

clock data to

the

SVs.

The

SVs

then

send subsets

of

the

orbital ephemeris

data

to GPS

receivers

over

radio signals.

1.3.3 User Segment

The

GPS

User Segment

consists

of the GPS receivers and the user

community.

GPS

receivers convert

SV

signals

into

position, velocity, and

time

estimates. Four satellites

are

required

to compute

the four dimensions

of

X, Y, Z

(position)

and Time.

GPS.receivers

are

4

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Global

P

ositioning

System

used

for navigation,

positioning,

time

dissemination,

and other research.

Navigation in three

dimensions is

the

primary

function

of GPS.

Navigation receivers

are

made

for

aircraft,

ships,

ground

vehicles,

and

for

hand

carrying

by individuals.

Figure 1.8:

Users, GPS Navigation

Precise

positioning

is

possible

using GPS

receivers

at

reference

locations

providing

corrections

and

relative

positioning

data for remote

receivers. Surveying,

geodetic

control,

and

plate

tectonic

studies are examples. Time and frequency

dissemination,

based on

the

precise

clocks on

board the SVs and

controlled

by

the monitor

stations, is

another use for

GPS.

Astronomical

observatories, telecommunications

facilities, and

laboratory

standards

can be set

to

precise

time

signals

or

controlled

to

accurate frequencies by special

purpose

GPS

receivers.

1.4

How

GPS

Works?

Here's how GPS

works

in

five

logical steps:

.

The

basis

of

GPS

is triangulation from satellites.

.

To

triangulate,

a

GPS

receiver measures

distance

using the

travel time

of radio

signals.

.

To measure

travel time, GPS

needs very accurate

timing

which

it

achieves with

some

tricks.

o

Along

with

distance,

you

need

to

know

exactly

where

the

satellites are

in

space.

High

orbits and

careful

monitoring

are the secret.

.

Finally

you

must correct for

any delays the

signal

experiences

as

it travels through

the

atmosphere.

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Global

P

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1.5

GPS

Applications

GPS

technology

has matured into a

resource

that

goes

far

beyond its

original design

goals.

These

days

scientists, sportsmen, farmers, soldiers,

pilots,

surveyors, hikers,

delivery

drivers,

sailors,

dispatchers,

lumberjacks,

fire-fighters, and

people

from many

other

walks

of

life

are using

GPS

in ways

that

make their work more

productive,

safer,

and sometimes even

easier.

These applications

fall

into

five

broad categories.

o

Location

-

determining

a basic

position

.

Navigation

-

getting

from

one

location

to

another

o

Tracking

-

monitoring

the

movement of

people

and

things

.

Mapping

-

creating

maps

of

the

world

. Timing

- bringing

precise timing to the world

1.5.1

Location

-

determining a basic

position

(q,I,h

)

or

(X,Y,Z)

'Where

am l? The first and most obvious

application

of GPS

is

the

simple determination

of

a

position

or location. GPS is the

first

positioning

system to

offer highly

precise

location

data

for

any

point

on

the

planet,

in

any

weather.

GPS is also being applied

in

countries

to

create exact location

points

for their

nationwide

geodetic

network,

which will be

used

for surveying

projects.

Once

in

place

it

will

support

the

first

implementation

of a nationally created

location

survey

linked

to the WGS-84

global

grid.

For example,

getting

to

the

height

of

a mountain was

tricky,

but

GPS made

it

possible.

1.5.2 Navigation

-

getting

from

one location to

another.

'Where

am I

going?

GPS

helps

you

determine exactly

where

you

are, but

sometimes

important

to

know how to

get

somewhere else. GPS

was

originally

designed

to

provide

navigation information for

ships and

planes.

So

it's

no surprise

that while

this technology is

appropriate

for

navigating on

water,

in

the

air and on

the

land.

6

Figure

1.9: Satellites

are

reference

points

for

location

on earth.

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Global P os itioning System

1.5.3

Tracking

-

monitoring

the movement.

lf

navigation

is

the

process

of

getting

something

from

one

location

to another,

then

tracking

is the

process

of

monitoring

it as

it

moves along.

GPS used

in

conjunction

with

communication

links and

computers

can

provide

the

backbone

for

systems tailored

to

applications in

agriculture,

mass

transit,

urban

delivery,

public

safety,

and

vessel

and vehicle

tracking.

So it's no

surprise

that

police,

ambulance,

and fire

departments

are adopting

systems

like

GPS-based

AVL

(Automatic

Vehicle Location).

Manage

to

pinpoint

both

the

location

of the emergency and the

location

of the

nearest

response

vehicle

on

a computer

map.

With

this

kind

of

clear

visual

picture

of

the situation,

dispatchers

can

react

immediately

and confidently.

W

Figure

1.11: Tracking

1.5.4 Mapping

-

creating

maps of the world.

Using GPS

to survey and

map

it

precisely

saves

time

and

money

in

this

most stringent

of

all

applications. Today, GPS makes it

possible

for a single

surveyor

to

accomplish

in

a day

what

used

to take weeks

with

an entire

team.

And

they can

do

their work

with a

higher level

of

accuracy

than

ever

before. Mapping is the art and science

of

using GPS

to

locate

items,

and

then create maps and

models of

everything

in

the

world.

And we

do mean

everything,

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Global P ositioning

System

such as:

mountains, rivers,

forests,

landforms,

roads,

routes,

city

streets,

endangered

animals,

precious

minerals,

disasters,

trash, archeological

treasures

and

all sorts

of

resources.

GPS is mapping

the

world.

&dffiS'r*m

F.,*l**fu**

Figure

1.12: Mapping

1.5.5 Bringing

precise

timing

to the world.

GPS

is

also used

to

disseminate

precise

time,

time

intervals,

and

frequency.

Time

is

a

powerful

commodity,

and exact time

is more

powerful

still.

Knowing

that

a

group

of

timed

events

is

perfectly

synchronized is

often very important.

GPS

makes

the

job

of

synchronizing

our

watches

easy and reliable.

There are three fundamental ways

we use time. As

a

universal

marker,

time tells us

when

things happened

or

when

they

will.

As

a

way

to

synchronize

people,

events,

even

other

types of signals,

time

helps keep

the world

on schedule.

And as a way

to tell

how long

things

last,

time

provides an

accurate,

unambiguous sense of duration.

Astronomers,

power

companies, computer networks,

communications

systems, banks,

and

radio

and television stations can

benefit

from

this

precise

timing.

One

investment

banking

firm

uses

GPS

to

guarantee

their

transactions are recorded

simultaneously

at all otfices

around

the

world.

1.6 Methods

of

observations

The

ditferent methods of observations with GPS

include

absolute

positioning,

relative

positioning

in

translocation

mode, relative positioning using differential GPS technique,

and

kinematic GPS surveying technique.

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Figure

1.13:

Timing


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Global

P ositioning System

1.

Absolute Positioning:

The

pseudo

ranges

(the

satellite antenna

range,

contaminated

by

the

receiver block

bias)

from

minimum

four

satellites are observed

at the

given

epoch, from which

the

four

unknown

parameters

-

the

3D

position

of

the

antenna

(X,

Y,

Z) and the

receiver

clock error

can

be determined. The accuracy of

the

position

obtained from

this

method

depends

upon

the

accuracy of the time and

position

messages

received from

the

satellites.

With the

selective availability operational,

the absolute

positioning

in

realtime

is limited

to about

100 meters,

which

can

be

improved

to

a

few2

meters

level by using

post-processed

satellite

orbit information

in

the

post-processing

mode.

The accuracy

of

absolute

positioning

with GPS is limited

mainly due

to

the

high

orbit

of

the

satellite.

2.

Relative

Positioning:

ln the

translocation

mode with

tow

or more GPS receivers

observing the same

satellites

simultaneously

many common errors, including

the

major effect

of SA

(selective

availability)

get

cancelled out,

yielding

the

relative

positions

of the

two

or

more stations

with

a

very

high

accuracy.

The

length

of

the

base line between

two

stations, and also

the

absolute

position

of

one of

the stations,

if

accurate

position

of

the

other

station

is known, can be obtained

to

cm-level

accuracy,

using carrier phase

observations.

ln

differencing

mode

of

observation,

using

single difference

(difference

of carrier

phase

observations

from

two

receivers

to

the

same satellite),

double

difference

(between

observations from two receivers to

two satellites)

and triple

difference

(difference

of double differences

over

two

time

epochs),

effect

of

many

errors such as

receiver

and satellite clock errors

etc.,

can be minimized.

Use of dual frequency observations

(both

L1 and L2

frequencies)

eliminates

the

major

part

of

ionosphere

effect

on

the

signal,

thus

improving

the

accuracy

of

positioning.

With accurate satellite orbit and

use

of

such refined

procedures

cm-level

accuracy

is

possible

even in regional and

global

scale

surveys.

3.

Differential

GPS:

A

modification

of

the

relative

positioning

method is

the differential

GPS technique,

where

one of

the two

receivers

can

receive

the

messages

given

by this transmitter.

The

transmitting

receiver

is kept

fixed

on a

point

whose

location

is known

to

high

degree of

accuracy.

Based

upon

this

position.

The

receiver

computes

observations

to

the

range/phase observations

from

GPS observations.

Such as

system

is

suited

for

applications

such

as vehicle

guidance

system,

location-fishing

boats

close to

the

seashore, etc.

The

limited range

of

the transmitter restricts

the

use of

such system to

few km.

4.

Kinematic GPS:

ln

the

kinematic GPS

technique,

one

of

the receivers is in

relative

motion with

respect to

the other

receiver having been mounted

either

on a

vehicle or ship or

aircraft. This

technique

has

a

number of important

applications,

including

ship and

aircraft

navigation,

photogrammetric

survey

control

etc.

I

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Global P

ositioning System

1.7 GPS Reference Systems and Transformation

of

Coordinate.

Normally, regional surveys are conducted with respect

to a

local

datum.

In

Malaysia, the

Modified Everest Ellipsoid

is in

used and

the

satellite

positioning

system

provides

ground

coordinates

of any

point

in

an earth-centered

coordinate

system.

As for

NAVSTAR GPS,

the

worldwide

datum

of

WGS 84

is being used.

ln

order to relate

the

coordinates

determined by

GPS

to the local

geodetic

datum,

the

coordinate transformations

need

to be

done.

The

transformations

which are

often to

be

performed

in

geodetic

computations

are:

o

The

transformation of the ellipsoidal coordinates

(


11/14

Global P ositioning System

Ge*detic

Height,rf

Point P

?oint P

Ellipsoid

Surface

Geodetic

Longitnde

af

Point

P

Figure

1.15:

(9,1,,

h) Coordinate

System

The

geodetic

latitude

(


12/14

Global P ositioning

System

1.7.4

Coordinate Conversion.

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SeHSJSS*

1.8

Conducting GPS Survey

1.8.1

GPS

Survey Equipments

Selection of

the

right

GPS

receiver

for

a

particular project

is critical

to

its success.

1.8.2 Receiverapplications

Land

applications

include

surveying,

geodesy,

resource

mapping,

navigation,

survey

control, boundary

determination, deformation monitoring,

and

transportation.

Marine

applications include navigation

and

positioning

of

hydrographic

surveys.

Airborne

applications include

navigation

and

positioning

of

photogrammetric

based mapping.

1.8.3

Accuracy

requirements

Afirm definition

of the accuracy requirements

(e.9.,

point

accuracy

to

100m,

50m, 25m,

5m,

1m,

cm or mm)

helps

to

further

define

procedure

requirements

(static

or kinematic),

signal

reception requirements

(whether

use

of

C/A- or L1lL2 P-codes

is

appropriate),

and type of

measurement

required

(pseudo-range

or carrier beat

phase

measurements).

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Global P ositioning System

1.8.4 Power

requirements.

The

receiver

power

requirements

are an

important

factor

in

the

determination

of

receiver

type.

Receivers currently run on a variety of

power

sources from fuC

to 12-volt car

batteries

or small camcorder

batteries.

A high

end

GPS receiver

can

operate

3

to

4

hr on

a

set of batteries,

whereas a

low

end

may operate

1 to

2

days on

the

same

set.

1.8.5 Cost.

Cost

is

a

major factor

in

determining

the type

of

receiver

the user can

purchase.

Receiver

hardware and software

costs arc a function of

development

costs, High

end

receivers

are

upwards

of

RM

60,000

down

to

a low end receiver

of

RM 1,500.

1.8.6

Processing requirements.

Operational

procedures

required before, during, and after

an observation

session are

very

manufacturer-dependent

and should

be

thought-fully

considered

before

purchase

of a

receiver.

1.9

GPS

Method

of

Observation and

Precision.

1.9.1 Static mode.

Minimum

# of

Sv's: 4

Min.Observation Time:

t

hour

Precision:

Single-freq:

20 mm

+l-2ppm

Dual-freq: 5 mm

+/-

1

ppm

Other

Characteristics:

Best baseline

Documents

L4b-GPS - Introduction Notes