GPS A Guide For Users

8/9/2019 GPS A Guide For Users

1/24

GPSguidea fo r users


2/24

Fo rew o rd 1

Section 1

What is GPS? 2

Section 2

GPSapplications 4

Section 3

Choosing appropriate 6GPSequipment

Section 4

Introductory GPSconcepts 9and definitions

Section 5

GPS and accuracy 12

Section 6

Quality standards 16

Furt her info rma t io n 17

Glo ssa ry 18

Contents


3/24

Fo rew o rd 1

As a mo dern society

w e a ll t a ke

a d va n t a g e o f

info rmat ion t echnolog ies

undreamt o f a f ew

deca de s a g o. The Globa l

Positioning System (GPS)

is one such advanced

te chno log y. GPS receiverscan b e boug ht for a

relat ively sma ll

investme nt . When placed

in t he ha nd s of a user,

they can insta nt ly a nsw er

one of huma nkind's most

funda menta l q uest ions -

w here on Ea rth am I?

Wheth er w e arebushw alking in remot e

locat ion s, an g ling in a

new f ishing hole, or fo ur

w heel driving in rugg ed

locations it is important

to b e a ble to a ccura tely

a nd q uickly det ermine

our locat ion .

Modern mapping systems

a r e no w g e a r ed t o w a r d s

the use of a dvanced

satellite navigation

syste ms. It is th eref ore

importa nt for new GPS

users to understa nd the

ba sics of th is techno log y

and how best to use it .

I com men d th is boo klet

to you a nd ho pe it helps

you keep pace w ith t he

rapid chang es in

technolog y tha t surround

us all.

John Landy, AC, MBEGovernor of Victoria

Foreword


4/24

2 Wha t is GPS?

GPS (Global Posit ion ing

System) is a mea ns of

det ermining locat ion a nd

navigat ion.

Just a s the Inte rnet

revolut ionised the w a y

w e collect a nd d istribut e

info rmat ion, GPS has

t ransfo rmed the w ay w e

determine locations on

Earth.

Expla ined simp ly, G PS

uses a netw ork of

sa tellites (of ten ca lled the

GPS constella tion ) in

conjunct ion w ith g round-

ba sed technology to

det ermine a precise

loca t ion anyw here o n

Earth.

GPS is a vaila ble t o

everyone , is simple to

lea rn, a nd f or most

purposes it’s free.

GPS is divide d int o t hree

key segm ent s:

The spa ce seg ment is th e

con ste lla t ion o f 24

sa te llites tha t o rbit Ea rth

tw ice a d a y emit t ing

rad io sign a ls. The y a ct a

bit like a rtificial sta rs

be ca use you ca n use

th ese sign a ls a s a

reference point f orfinding locations on

Earth.

The user seg ment consists

o f a handhe ld GP S

receiver a nd th e person

operat ing it .

The t ype o f G PS receiver

a nd t he techniq ues you

use determine the

a ccuracy and na ture of

th e po sit ion it comput es.

Most ha ndhe ld GPS

receivers ha ve a relia ble

a ccuracy of a bo ut 10 to

20 metres depending on

operat ing cond it ions.

Adva nced GPS receivers

a nd t echniques ca n

provide rea l-time

a ccura cy in cent ime tres.

The control segment

mo nito rs GPS sa te llites

from g round sta t ions on

Ea rth, uploa ding

correction s a s ea ch

satellite drifts from its

original orbit due to solar

w inds and o ther sma ll

variables.

How GPS calculates aposition on Earth

GPS is ba sed on the

principle of trilateration ,

w hich a llow s you to

ca lcula te a posit ion o n

Ea rth by know ing its

dista nce from t hree oth er

known locations.

It ’s a simple ma the ma tica l

equa t ion: Velocity x

Tra vel

Time

Distance .

A GPS receiver kno w s the

velocity of a satellite

1 Wha t is GPS


5/24

sig na l is th e speed o f

lig ht (299,792.458

kilomet res per second ).

The GPS receiver t he n

ca lcula te s ho w lon g it

to ok for the sig na l to

reach the Ea rth (travel

t ime ). By multiplying t he

time w ith th e velocity,

the receiver can calculate

the d ist a nce to tha t

satellite.

The crucia l pa rt o f th is

eq uat ion relies on a GPS

receiver ob ta ining

accurate t ime

measurements from

precise atomic clocks on

bo a rd t he sa te llites. The

sa te llite s’ orb its a re

arrang ed so a GPS

receiver can lock on t o a tlea st fo ur sa te llite s

(a l though it w ould b e

po ssible t o u se o nly

three) no ma t ter w here

you a re located on Earth.

Using the info rmat ion

sent from fo ur sa tellite s,

th e G PS receiver

de te rmines its position inth ree d imen sion s w hich

a llow s it t o calcula te its

posit ion on Ea rth.

A brief history of satellitepositioning

Many countries use satellites

to determine positions on

Ea rth. The ge neric te rm fo r

all of these satellite systems

is Globa l Na vig a tion Sat ellite

Systems or GNSS.

Altho ug h t he American

Globa l Positioning Syste m is

the most popular, the

Russia n Fede rat ion ope rat es

a similar one called GLONASS

an d t he European Union is

developing a third ca lled

Galileo .

The United Sta te s of America

(USA) started developing GPS

for military purposes in the

1970s a nd first a llow ed

pub lic access to t he

technology in the 1980s.

How ever, it w a sn’t un til 1994

that all 24 GPS satellites

w ere in place to provide t he

accuracy available today.

There a re tw o levels of GPS

services currently available –

on e fo r public use a nd

an o t h er

encrypted service

for military use.

The sat ellite s

used in the G PS

prog ram a re

kn ow n as

Navstar

(Na vigat ion

System by

Timing And

Ranging) space

vehicles.

GPS—not ‘bigbrother’

Ma ny peo ple b elieve GPS ca n

be used to moni tor thew hereab outs of people or

o b jects. This is inco rrect .

Sa tel lites have no w a y of

telling w hen a GPS receiver is

locked on to it . A GPS

receiver simp ly receives

info rmation from GPS

sa te llites, similar t o tu ning

a nd receiving a sig na l from a

radio sta t ion.

Tracking te chno log y is a n

a ddi t iona l capa bi lity tha t can

be u sed in conjunction w ith

GPS. For exa mple, th e lat est

transport security systems

use tra nsmitt ers fi t te d t o

vehicles. Ma pping soft w a re is

then used to locate t he

vehicle’s po sitio n w hile

direction a nd speed

information is transmitted toa moni tor ing centre.

Wha t is GPS? 3


6/24

4 GPS a pplica t io ns

GPS is rapidly being

applied to a broad rang e

of a ctivities.

GPS receivers are ea sy t o

use a nd t he funct ions

the y offe r are improving

a ll th e t ime. They a re a lso

be coming less expensive.

Great er memo ry capa city

an d t he a bility to conne ct

to the Internet and

person a l com puters w ith

mapping so f tw are are

no ta bly increasing

bu siness app licat ions of

GPS. The spat ia l

informat ion GPStechnolog y can n ow

provide is also improving

the safety and efficiency

of vita l com munity

services like f iref igh t ing

a nd sea rch a nd rescue.

Around 85 per cent of a ll

info rmat ion used b y

g overnment ha s a spa t ia l

compo ne nt . The

Victorian Government

uses spa tial inf orma tion

fo r environmenta l an d

na tura l resource

ma na g ement , risk a nd

asset ma nag ement , land

a dministra t ion a nd land

use pla nning ,infra structure pla nning

a nd provision, a nd service

pla nning a nd de livery.

Industry uses GPS for

a g ricultu re, pub lic sa fe ty,

telecommunications and

asset ma nag ement .

The scient ific co mm un itya pplies it t o a rcha eo log y,

oceano graphy, w ea t her

resea rch, g eo log y, w ildlife

conservation and

environme nt a l research.

Recrea tiona l a nd sporting

users em ploy GPS fo r

activities like

bushwalking, four wheel

driving , fishing a nd

vehicula r na vig a tion.

A new g lob a l a dventure

sport , ge ocaching , ha s

even de velope d using GPS

coo rdinat es listed on t he

In terne t a t w w w.

geocaching.com.

2 GPS a pplica t io ns


7/24

GPS a pplica t io ns 5

Precision farming

One industry that surprises

ma ny people w ith i ts

advanced use of GPS is

farming.

New fa rm machinery f it ted

w ith hig h a ccuracy

equipment al lows ‘hands

free ’ driving. Fa rmers ca n

no w precisely na vig a te

around a pa ddock w ith

‘centimetre accuracy’ when

ploug hing , seeding a nd

ha rvesting , w hich reduces

overla ps a nd g a ps. Fa rmers

can a lso d ete rmine exa ctlyw h a t par t s o f a paddock

need more fertiliser, apply

the r igh t amo unt , and

mea sure ha rvests from t ho se

areas.

Techn o log y-savvy fa rmers

can sig nificant ly save o n

seed , f ertiliser, pe sticide a nd

fu el costs. Impro vemen ts in

crop yields and profi ta bility

ha ve a lso b een reported.

Precision farming also helps

the environment because

fa rmers do n’ t need to

blan ket-spread pesticide a nd

fertilisers over entire

paddocks.

Mapping Victoria’sforests

Finding your w ay t hrough

Victo ria ’s fo rests a nd pa rks

has become much easier

tha nks to GPS.

The Depa rtment of

Susta ina bility a nd

Environment (DSE) is

a ccurat ely recording every

tw ist a nd turn an d piece of

infrastructure along the

sta te’s ma ny w a lking tra cks.

The se ma ps a re a llow ing

be t t e r m an ag em ent o f our

fo rests and pa rks, are usedby bushw a lkers a nd a re also

vital for Victoria’s

f i ref ight ing a nd prevent ion

strategies.

Image abo ve cou rt esy Victo ria

Pol ice Search and Rescue

Image below court esy of Victo r ian Inst i t ut e of Dryland

Agricul tu re Walpeup


8/24

6 Cho o sing a ppro pria t e GPS eq uipment

When selecting a GPS

receiver, your main

con side rat ion sho uld b e

the a ctivity you int end to

use it f o r. This w ill he lp

det ermine t he complexity

of receiver you ne ed , its

a ccuracy, size, w eight a nd

relevant accessories. Forexam ple, you ma y need a

waterproof GPS receiver

tha t f its in a pocket o r

one suited fo r mapping.

The re a re hun dred s of

GPS receivers and

a ccessories to cho ose

from . Tw o ma in g roups

exist:

Basic , low cost units fo r

recreat ion a l use.

Complex , mo re expen sive

units w ith g reater

a ccura cy, w hich a lso a llow

GPS receivers to be linked

to personal computers

an d ma chinery for

comme rcia l use.

It is ad visa ble t o req uest a

demo nstrat ion from a

reta iler b efo re purcha sing

a GPS receiver. It w ou ld

a lso b e preferable to test

them in the f ield b efo re

buying.

Don’t e xpect to rely on

reta ilers fo r GPS ad vice

a nd tra ining a s th is is a

specia lised a rea . Ta lking

to experien ced G PS users

w ill provide first-ha nd

informat ion abo ut the

best equipment fo r your

a ctivity. Recrea tion a l club s

(bushw a lking , four w heel

driving or bo a t ing) areusua lly ha ppy to provide

informat ion a nd

introductory training to

ne w GPS users.

For more a dva nced GPS

activit ies and equipment,

ma nufa cturers of ten ha ve

specia lised distribut ors

w ho hire out GPS

receivers a nd provide

t ra ining and ong oing

support.

Key things to consider

Accuracy

The a ccura cy you n ee d

fro m your G PS receiver isa key facto r in t he choice

you m a ke. For exa mple, if

you a re using GPS fo r

recreat ion a l purposes,

such a s f inding a g oo d

fishing spot or

bushw a lking , an

inexpensive b a sic unit w ill

proba bly be sufficient .

Manuf a cturers a nd

reta ilers of low cost

recrea tion a l GPS receivers

often generalise about

th e a ccuracy of th eir

eq uipment – many sta te

th e ‘b est ca se scena rio’

a ccura cy level, w hich is

rarely achievab le in t he

field. Ma ny G PS receivers

3 Cho o sing a ppropria t e G PSequipment


9/24

Cho o sing a ppro pria t e GPS eq uipment 7

a re a lso ma rketed a s

‘ma pping ’ receivers yet

only ha ve enoug h

memo ry to sto re

calculated positions.Accura te ma pping a nd

navigation needs satellite

rang e da ta w hich uses a

lot of receiver memory.

Extra memory ad ds to th e

cost o f t he receiver but

provide s a b et ter result .

Maps

You ma y think you w ill

no t n eed a GPS receiver

w ith built-in m a ps fo r

your a ctivity, bu t o nce

you b eg in using GPS,

cha nces are you’ll w a nt to

explore it mo re. Con sider

purcha sing a GPS receiver

th a t d ispla ys GPS

posit ions, to g ether w ithma ps a nd ot her spat ia l

da ta such a s ima g ery an d

la yered info rmat ion, or

one t ha t ca n be linked to

ot her ha ndheld d evices

tha t provide t hese

functions. A simple GPS

do t o n a blank screen or

a set of coo rdina tes

do esn’t provide a g reatdea l of useful

informat ion f or

inexperienced users.

It is po ssible to ha ve

specific ma pping da ta

geo-referenced fo r use on

yo ur GPS receiver. The

technolog y for geo-

referencing da ta is

evolving a ll th e time. For

more informat ion a bout

w here to get q ua lity

Victorian data for your

GPS, visitw w w.la nd.vic.go v.au.

Display

Mo st GPS receivers displa y

betw een four and six

na vig at ion screen pa g es,

an d o fte n sub-menu

pag es, w hich show things

like sa te llite visib ility a nd

sta tus, current locat ion ,

w a ypoints (or ‘ in

bet w een’ points) a nd

landmarks.

It ’s a g oo d idea t o ha ve

most o f t he informat ion

you need fo r your

pa rticular a ctivity

displa yed o n a sing le

screen – it elimina te s the

need to cont inua lly

sw itch men us. While t his

ma y seem unimportant

no w , it w ill qu ickly

become anno ying w hen

using your GPS receiver if

you can’t see a ll th e

informat ion you need at

a g la nce.

The q ua lity o f t he LCDs

(liquid crystal displays) o n

GPS receivers also va ries.

Test t he LCD in d irect

sunlig ht be fo re purcha se

to be sure you can read

it. Some GPS receivers

a lso ha ve screens tha t

rota te f rom vertica l (fo rholding in the ha nd) to

horizontal (for mounting

on a car d a sh).

Information Storage

GPS receivers can usua lly

sa ve fro m 100 to 1000

locations. Most GPS

receivers a llow you t o

na me locat ions for ea sy

ide ntifica tion la ter (eg

‘loo kout ’). This a llo w s

you to ea sily navig at e to

a po int o f interest (eg

‘ca mping g round ’) or in a

seq uence (eg ‘ca mping

g round 1’, ‘ca mping

g round 2’ etc).

Image left cou rt esy Victo ria



10/24

8 Cho o sing a ppro pria t e GPS eq uipment

Accessories and Software

GPS receivers ca n no w be

linked to , or a re d irectly

integ ra t ed w ith , a w ide

rang e o f o ther

eq uipmen t. This a llow s

posit ional information tobe a pplied to a w ide

variety o f a ctivities.

The m ost po pular a nd

widely available

accessories are:

- Au t o pilo t s

- Laser ranger f inders

- Dig it a l compass

- P ersona l Dig i t a l

Assistants (PDAs)

- GPS enab led mobile

phones

- Bo a tin g

instrumentation

- Chartp lo t t ing devices

- Echosounders

Basic GPS receiverchecklistIs the receiver ea sy to use?

Wha t sort o f a ccuracy w ill

you need?

Will your receiver nee d to be

wa t er pr oo f?

How sma ll doe s your receiver

n eed t o be?

How many waypo in ts w ill

you r GPS receiver sto re?

Wha t sor t of ba t te ry life w ill

you need?

Ca n th e receiver be run of f a

vehicle cig a rette lig hte r?Will you need to use yo ur

GPS receiver hands free?

Will the receiver sound a

sig na l w hen you a re moving

closer or furthe r from yo ur

t a r ge t ?

Do you need a d ig i t a l

compass?

Can your receiver be

connected to other handheldequipment that increases i ts

abilities?

How ea sy is th e LCD screen

t o r ead?

If you a re buying GPS

equipment for business

purpo ses, th e Victo ria n

Go vernment ha s produced a

more detailed GPS technical

standards publication, The

Glob al Posit ioni ng Syst em

Handbook , a vaila ble o nline

at www.land.vic.gov.au


11/24

Int ro d uct o ry GPS co ncept s a nd d ef init io ns 9

The po pularity o f G PS

receivers has highlighted

the need fo r users to

understa nd the

limita tions of t heir

receivers.

The re a re comm on

miscon ceptions a bo utw ha t your GPS ca n

a ctua lly do . By

understa nding some key

GPS con cepts, you w ill

minimise the risk of

errors, in b ot h G PS

posit ioning a nd in the use

of a ssocia ted ma pping

products.

Som e of th e ba sic GPS

concept s you shou ld

understand are:

Datum & Coordinates

Ma ps creat ed using GPS

technolog y a re ba sed on

a reference frame o r

d a t u m. A da tum loo ks a

litt le like a local stree t

directory except w ith GPS

it a pplies to a coun try or

the e nt ire surface of t he

Earth.

There a re man y da tums

used a round the w orld .

The mo st re cog nised

g lob al da tum is WGS84(th e World Geodet ic

System), established in

1984, w hich is w ide ly

used f or ma rine a nd

aeronautical activities.

How ever, mo st coun tries

have their ow n da tum,

w hich is spe cifica lly

designed to bet ter f i t the

shape o f t he ea rth inthe ir locat ion , and is

updated t o a ccount for

cont inenta l drif t an d

ot her varia t ions.

In Austra lia , mo st m a ps

create d by go vernment

agencies use GDA (t he

Geocent r ic Datum of

Austral ia ).

Most Austra lia n m a ps

de veloped be fo re 2000

use AGD (the Aust ra l ian

Geodet ic Datum ) . This

da tum w a s superseded b y

GDA. The re is, roug hly, a

200 metre difference

be tw een these tw o

datums.

To a void a po ssible 200

metre discrepancy, you must

ensure t ha t your GPS receiver

a nd t he ma ps you a re using

(w het her in t he receiver

itself, o r pape r map s you

refer to) ha ve the sa me

datum. For consistency, and

to minimise t he po tent ia l for

errors, you sho uld use G DA.

4 Int ro d uct o ry GPSco ncept s &def init io ns

M aps using

GDA carry th is

logo


12/24

10 Int ro duct o ry GPS co ncept s a nd def in it io ns

Ba sic GPS rece ivers w ill

d ef a ult to WGS84. The

posit iona l differences

calculated using WGS84

and GDA is minimal (in2004 this w a s ab out 0.7

met res a nd increasing by

0.07 me tres per yea r) a nd

is not not icea ble w hen

using b a sic GPS receivers.

How ever, you m ust use

the GDA reference frame

if pla nning t o sha re

business-critical

information (like land

surveys) w ith t he

Victorian Government

an d o ther orga nisa t ions.

More info rmat ion on

GDA a nd da tums is

availa ble a t

w w w.icsm.go v.au

Positional Uncertaintyand Local Uncertainty

These a re new te rms

ad opte d in Austral ia a nd

a re expressed in m et res

to de scrib e a circle of

uncerta inty around a

g iven po int. The y a re

used t o q uant i fy the

reliability of GPS andot her po sit ions, w hich

can b e a f fec ted by

satellite signals bouncing

of f b uilding s or

veget a tion (ca lled

multipath ), va rying

a tm ospheric con ditions

a nd sa te llites mo ving

slig ht ly out o f o rbit .

Posit iona l Uncerta int y is

the uncerta inty of GPS

posit ions, rela t ing to th e

horizonta l and height

com pon ent s, in met res a tthe 95 per cent

con fidence level w i th

respect to the d at um you

a re using (eg GDA).

Loca l Uncerta int y is th e

a verag e mea sure in

met res a t a 95 per cent

con fidence level w ith

respect to the ad ja centpoints in a da tum (eg

GDA). It is calculated

betw een tw o points in

q uest ion or from the

point in q uest ion to

adjacent points in the

same area .

Dilution of Precision


(DOP) is a mea sure of th e

q ua l ity o f the geo metryof the satellite

con stella t ion. A g reat er

ang le be tw een the

sa te llite s low ers the DOP,

a nd provides bet te r

posit iona l a ccuracy tha n a

hig he r DOP, w hich

indicates poor satellite

geometry .

Precision versusAccuracy

Precision a n d accuracy a re

of ten assumed to mea n

the same thing but a n

importa nt technica l

difference exists for GPS

users. It is po ssible to

ha ve a GPS reading t ha t isprecise b ut ina ccurat e, o r

is a ccura te bu t imprecise.

Precision refers to how

clo sely severa l GPS

readings taken from the

sa me loca t ion ma tch each

ot her. If a rea ding is

precise, it can be ea sily

repea te d. A precisereading w ill ha ve the

positions clustered close

to g ether (see f ig ure on

page 11).

Accuracy rela tes to the

correctness o f t h e

inf orma tion collected

using GPS. The mo re t ime

you spend


13/24

Int ro d uct o ry GPS co ncept s a nd d ef init io ns 11

collect ing d at a w ith your

GPS receiver at one

locat ion, the more

a ccurat e th e posit ion

mea surements you ta ke

be com e. In th e figure

a bo ve, an a ccurate

reading ha s the points

clustered in o r nea r the

correct result cent ret a rge t .

Metho ds fo r ob ta ining

highe r a ccuracy are

ou tlined in Section 5 GPS

a nd a ccuracy.

Maps and heights

People of ten w onder why

the height det ermined by

the ir GPS is diffe rent t o t he

same height show n on a

to pog raphic ma p. Theexplan a t ion is in the w ay

GPS computes heights and

the w ay ma ps are def ined .

A vertical datum is a point

from w hich heig hts are

measured. Australia’s

vertica l da tum is the

Austra lia n Heig ht Dat um

(AHD) w hich a pproxima te s

mea n sea level (or geoid)height s a nd is the ba sis of

most pub lished ma ps.

GPS receivers generate

(ellipsoid ) height s using the

WGS84 dat um rat her tha n

mea n sea level. These GPS

heights need to be corrected

to compute loca l AHD

heights.

The diffe rence bet w een th etw o d a tums is ca lled t he

geoi d-ellipsoid separat ion . In

Australia the separation may

rang e betw een -40 and + 75

met res. Globa lly it can be in

the ra ng e o f -100 to + 100

metres.

AUSgeoid is a free he ig ht

transla t ion program

ava ila ble from theGe o science Australia w eb site

w w w .ga .go v.a u/nmd/g eod esy

/a usg eo id/.

What does 95 percent confidencemean?

GPS a ccura cy levels are

sta ted a s hav ing a 95 per cent conf idence fact or . This

mea ns, for example, tha t

w hen using a ba sic GPS

receiver, 95 per cent o f t he

time it ca lcula tes your

position w ithin a 13 metre

rad ius of you r true position.

The o the r 5 per cent o f t he

time your G PS receiver could

sho w your position o utside a

13 met re rad ius from yourtrue position. Experiments

ha ve show n t hese po sit ions

ca n be hundreds of metres

from their true location.

Accuracy

P r e c i s i o n

Low

Precision

High

Precision

Low Accuracy High Accuracy

Precision vs Accuracy


14/24

12 GPS a nd a ccura cy

GPS is a very eff icien t a nd

effective means of

locat ing posit ions on

Ea rth. How ever, even

und er ide a l con ditions it

is un likely to b e ‘100 per

cent ’ e rror f ree . This is

beca use th e posit ion your

GPS receiver calculatescont a ins some deg ree of

uncertainty (or bias ).

Sometimes the effect of

this uncertainty ca n mea n

posit ions a re te ns or even

hundreds of m etres from

your true po sit ion o n

Earth.

These posit ional inaccuracies probably

w on’ t bo ther recrea t iona l

users. How ever, if you a re

using GPS fo r surveying ,

mapping or business-

rela ted a ctivit ies or

intend to share da ta w ith

oth er org a nisa t ions, you

need to understa nd w hy

th ese ina ccura cies occuran d ho w to m inimise

them.

Errors

Errors exist in GPS

posit ions fo r a variety o f

rea sons. The y can b e

categorised into two

main groups:

Environment al ly indu ced

errors , w hich includ e

a tmo spheric errors a nd

mult ipath. The se e rro rs

are generally inconsistent

and can occur a t a ny

time.

Techn ical erro rs , includ ing

sa te llite errors a nd

receiver no ise .

Environme nt a l a nd

technica l errors ca n b ot h

be minimised using the

rig ht techniq ues and

equipment .

Rem emb er, your GPS

receiver know s w here

ea ch sat ellite in t he G PS

netw ork is a t a ny givenmo men t. It ca lcula te s its

distance from t hese

sa te llite s by m ea suring

the t ime it t a kes for

sig na ls to t ravel do w n to

it.

Using th is dista nce

inf orma tion, the receiver

w orks out your posit ion

on Ea rth.

Your GPS receiver

a ssume s sa te llite sign a ls

a re a lw a ys t ravelling a t

the speed of l ight in a

direct pa th w hen ma king

these calculations.

If t he sig na l is slow ed

do w n or do esn’t t ravel in

a com plet ely stra ig htpa th to your receiver, the

t ime a nd rang ing

mea surements can be

th row n o ut . Timing

calculations can also be

a f f ected b y the a to mic

clocks in the GPS satellites

w hen compared to the

time kept b y your GPS

receiver. Errors can also

occur w hen de la ys in



15/24

GPS a nd a ccura cy 13

processing signals occur

insid e you r receiver.

If yo u ma ke a mista ke in

set t ing the configurat ionof your G PS receiver, th e

result m a y also b e

ina ccura te. Accura cy ca n

a lso b e a f fected i f a user

neglects significant

det ai ls or fa ils to g at her

enough informat ion to

ad eq uate ly def ine a

ground fea ture .

Don’t forget tocollect enoughinformation

GPS receivers are often used

to collect location

information for mapping,

ho w ever no t collecting

enough in format ion can

result in a n ina ccurat epicture.

For example, if t he ed g e o f a

carpark pavement w as

curved b ut you only collected

three points of the a rc

shape, you w ould no t ha ve

a n a ccurate shape in your

ma p file. Curves req uire

more positions to define the

sha pe of t he a rc.

Environmental errors

Atm ospheric errors a re

one o f t he ma in causes of

GPS errors an d occurbeca use cha rged pa rticles

an d moisture in t he

a tm osphere ca n slow

sa tellite sig na ls do w n.

Yo u ca n minimise

a tm ospheric errors in a

sta nd a rd G PS receiver by

predict ing w ha t a typica l

delay might be on atypical day, but

a tm ospheric con ditions

a re ra rely co nsist en t. The

best w a y to minimise

a tm ospheric errors is

using Diff erent ial GPS

(DGPS) (explained lat er in

t hi s sect ion).

Multipath occurs w hensatellite signals bounce

of f ob jects such a s cars,

bu ilding s, fe nces,

veget a t ion, w a t er or even

th ick smo ke, as the y

tra vel to your GPS

receiver. Multipath is

difficult to detect and is

somet imes hard t o a void.

It can be corrected tosom e e xte nt using

sophisticated receivers,

specia l ant enn a s, ca reful

p lanning a nd g ood f ie ld

procedures (like

a vera g ing posit ions over

t ime or repeat ing

ob serva t ions w ith a

dif ferent set o f GPS

satellites).

Don ’t e xpect inexpensive

GPS receivers to

eff ectively dea l w ith

multipa th errors.

Technical errors

Satellite orbit (or

‘ephemeris’) a nd t im ing

errors occur w hen t he

satellites in the GPS

con ste lla t ion d rift from

th eir pred icte d orbits,

causing errors in the

distance calculations yourGPS receiver makes.

Ephemeris errors are

usually sma ll bu t m ust be

ta ken into a ccount if

you’re seeking greater

a ccura cy. Altho ug h

sa te llite sig na ls a re

synchronised to your

receiver w ith high-

accuracy atomic clocks,your GPS receiver

maintains less accurate

time. As a result yo ur GPS

needs to receive data

from fo ur or more

sa tellites to eliminat e t his

effect .

Receiver noise refers to

ho w w ell a GPS receivermea sures a nd ca lcula tes

info rma tion received

fro m sa te llites. The level

of receiver no ise d epend s

on t hings like the q ua lity

of a ntenn a , electronics

a nd sig na l processing .

Som et imes a receiver w ill

round off numbers or

electromagnet icinterf eren ce w ill disto rt


16/24


the sa tellite sig na l. Low er

cost receivers usually have

a g reat er receiver no ise

tha n higher cost

alternatives.


Dilut ion of Precision

(DOP) is a simple w a y to

th eo retically a ssess th e

reliability of your GPS

position and its accuracy

according to w here the

GPS sa te llite s arecurrently located in space.

The value o f DOP o n yo ur

receiver screen relat es to

the rela tive a ccuracy of

your calcula ted posit ion.

If your receiver records a

posit ion w ith a low DOP

value, it me a ns th e

sa te llites fro m w hich it is

calculating its position are

mo re w ide ly dispersed.

The streng th of the

position signal is stronger

so t he ma rgin of e rror is

sma ller (mo re a ccura te ).

A high DOP value mea ns

tha t the sat ellites are

clustered closer together.The relia b ility of th e

po sition sig na l w ill be

w eaker so t he ma rgin o f

erro r is hig he r (less

accurate).

Low DOP 8 = less

accurate positionA q ua lity G PS po sitio n

w ill be calcula ted from

sat ellites tha t a re w ell

distr ibute d a round a nd

ab ove the horizon o f the

GPS receiver. DOP w ill

cha ng e over t ime

however, because the

sa te llites a re consta nt lymoving.

While DOP is a good wayto quickly assess theaccuracy of your position,it is only one of manyerror assessments youneed to consider,particularly when

applying data to businesspurposes.

Differential GPS

Differen tia l GPS (DGPS)

reduces errors and

improves the relia bility o fpo sition s a GPS receiver

calculates.

The DGPS met ho d is

ba sed o n the concept

tha t a t mospheric a nd

satellite errors affecting

th e a ccuracy of your GPS

receiver are very similar

to errors ca lcula ted byother receivers located in

the surrounding a rea .

This is be cau se t he sig na ls

ob ta ined b y a ll of t hese

receivers ha ve t ravelled

th roug h virtua lly the

sa me slice o f a tm osphere.

DGPS uses a fixed receiver

(called a ba se sta t ion o r

reference sta t ion) w hich

ha s ha d its posit ion

on Ea rth a ccurat ely

surveye d . Vict o ria ’s

Department o f

Susta ina bility a nd

Environment

operat es a b a se

sta t ion netw orkcalled GPSnet (more

informat ion a bout

GPSnet is available

a t w w w. land .vic.

g o v.a u/g psnet ).

By tracking a t lea st

fo ur commo n

satellites

simultaneously,


17/24

GPS a nd a ccura cy 15

DGPS uses the kno w n

posit ion of the ba se

sta t ion to ca lculat e a nd

correct the GPS-derived

position.

The b a se sta t ion t hen

tra nsmits t hese

corrections to GPS

receivers in the

surrounding a rea w ith

DGPS capability.

DGPS sig nifica nt ly

improves GPS accura cy byproviding correctional

in format ion f or

a tm ospheric and sa te llite

errors. How ever, th e

dista nce betw een the

ba se st a t ion a nd your

receiver sho uld no t

exceed 300km.

DGPS does not correcterrors caused bymultipath or receivernoise.

The know n coordina te s a t

the ba se sta t ion a lso

provide a con venient

mea ns to a llow the user’s

ca lcula te d po sit ion to be

on t he correct da tum

(GDA) - it also complies

w ith Victo ria ’s dig ita l

mapp ing da t a , Vicmap

Corrections from ba se

sta t ions can be o bt a ined

a ft er fieldw ork is finished

(called post-processing ),

or in rea l-time.

Real-time positioning has

the advan t a ge o f

providing improved

a ccuracy w hile you a re

using your GPS receiver inthe f ield. How ever, things

like sig na l coverag e, t he

length of t ime corrections

ta ke to get t o your

receiver, a nd your

dista nce f rom b a se

sta t ions ca n a f fect

accuracy. Post-processing

allow s you to concentrate

on collect ing da ta a nd

orga nising info rmat ion

int o files. GPSnet

provide s fo r bot h real-

time a nd po st-processing

operat ions.


18/24

16 Qua lit y st a nda rds

GPS is used t o collect a nd

process spa tia l

informat ion for a w ide

rang e o f go vernment ,

business a nd recreat ion a l

applications.

The Victo ria n

Go vernme nt uses GPS

techno log y to collect

informat ion f or

g eologica l ma pping ,

surveying, resource

mapping a nd

mana gement , emergency

respon se a nd

environmental

conservation.

This inf o rma t ion is

colla ted into a set o f

spat ia lly rela ted da ta

products called Vicmap.

Vicma p pro vide s spa tia l

da ta on t hing s like our

road an d rai l netw orks,

city an d to w n layout s,

fo rests a nd pa rks.

If yo u inte nd collecting

info rmat ion tha t is

accura t e enoug h to be

integrated into Vicmap

da ta or you intend t o use

GPS fo r bu siness

purpo ses, you m ust

a dhere to minimum

q uality sta nda rds a ndprocedures.

For e xa mple, you must

use DGPS if you w ish t o

integrate mapping

info rmat ion into t he

Victorian Government’s

ma pping da ta ba se. You

must a lso e nsure t ha t t he

da tum is GDA94.

The Victo ria n

Government ha s

produced a more det a iled

technical standards

publication, The Global

Posit ioning Syst em

Handbook, which outlines

th ese minimum

requirements.

You should refer to this

w hen collect ing spat ia l

info rmat ion you w ish to

sha re w ith ot hers.

6 Qua lity sta nd a rds


19/24

Furt her in fo rma t io n 17

For more informat ion a bo ut G PS, the

fo llow ing w ebsites are recommen ded .

Genera l GPS information

GPSnet, w w w .la nd .vic.g ov.a u/g psnet

GPS, www.navcen.uscg.gov/

NAVSTAR, ht t ps://g ps.losa ng eles.a f .mil/

To f ind m ore informa tion a bo ut GPStechnology and equipment suppliers,

type GPS into your prefe rred Int ernet

search engine.

Datums a nd Coordinates

Geo cent ric Datum of Austra lia ,

w w w .icsm.g o v.a u/icsm/g d a

Ge oscience Austra lia , w w w . g a . g o v . a u

Geocaching

www.geocaching .com

Professional

GPS

Techn ica l

Standards

The Global Posit ioning Syst em

Handbook , www.land.vic .gov.au

7 Furthe r inf o rma t io n

Image abo ve cou rt esy Vict oria



20/24

18 Glo ssa ry

95 per cent confidenceUsed to de scribe th e GPS

a ccuracy of a posit ion.

accuracy Refe rs to theclosene ss of GPS position s

to their t rue loca t ion on

Earth.

AGD Au str al ian Geodet ic Datum . Used to de velop

ma ps in Austra lia fro m

the 1960s up to 2000. It

ha s been superseded by

GDA94 since 1 January

2000.

AGD66 The 1966ad justment o f the

Austra lia n G eod et ic

Datum ad opted in

Victo ria th a t ha s be en

supersede d by GDA. GDA

a nd AGD coo rdina tes va ry

by a round 200m in the

no rth e a ste rly direction.

AMG66 UniversalTra nsverse Merca to r g rid

coo rdina tes (ea st , no rth

and zone) genera t ed

from AGD66 la titud es and

longitudes.

atmospheric errors Errorsintroduced into GPS

posit ions w hen cha rged

particles and moisture inthe a tmo sphere slow

sa te llite sig na ls tra velling

do w n t o a GPS receiver.

base station A fixedsta t ion on Ea rth tha t is

a ccurat ely surveyed a nd

send s sign a ls to he lp

redu ce errors in GPS

calculat ion s. Essen tia l fo rDGPS. Also know n a s a

ref erence st at ion .

control segment Groundsta t ions on Ea rth tha t up-

loa d corrections to G PS

satellites.

corrected The t ermg enera lly used in

con junction w ith DGPS

a nd refers to t he

ca ncella t ion o f errors tha t

affect GPS positions.

datumA ma thema tica lsurfa ce th at best f i ts the

shape o f the ea r th

allowing you to describeg eo g raphic posit ions.

Also kno w n a s a

reference frame .

DGPSDiff erent ial GPS .Increa ses the a ccura cy of

GPS position s by red ucing

a tmo spheric and sa te llite

errors.

DOP Di lu t ion of Precision . A measure of

the q ua l ity o f the a ngle

of a GPS satellite in

relat ion t o a GPS receiver.

The g reate r the a ng le,

the low er the DOP and

th e high er the a ccuracy.

Glossary8


21/24

Glo ssa ry 19

DSEDepar tment of Sust ainab ilit y and

Environment .

ellipsoid Mathematical lyth e ellipsoid is a rota tedellipse, a n o val tha t

revolves about its shortest

dimension. It is a

mathemat ica l

a pproxima t ion of the

geoid. It is used for exact

mea surement s over lon g

dista nces, a cross

con tinent s or ocea ns.

ephemeris errorsErrorsintroduced into GPS

positions caused by GPS

sa te llite s moving slig ht ly

out o f th eir orig ina l

orbits by solar w ind s a nd

ot her variab les.

GalileoThe GPS netw ork

being built b y the

Europea n Union.

GDA Geocent r ic Datum of Austral ia . Fo r mo st

practical purposes, GDA is

compatible with posit ions

genera ted b y

auto nomous GPS

handheld receivers.

GDA94 Geodet iccoordinates (lat itude and

lon g itud e) com puted in

terms of the G DA a t 1

Ja nua ry 1994.

geo-referenceAssigncoo rdina tes from a real-

w orld ref erence syste m,

such a s la t itude /

long itude , or ma p grid to

the pa ge coordina t es o f a

raster (ima g e) ma p. Geo -

referencing raster da ta

al low s it to be view ed,q ueried a nd a na lysed

w ith o ther geog raphic

da t a and used by a GP S

receiver.

geocaching An adventuresport involving finding

hiding places using GPS.

geoid A simplification ofth e Earth ’s surfa ce usingmea n sea level of the

ocean w ith a ll cont inent s

removed. Due t o

varia t ions in th e ea rth’s

mass distribution (oceans

and land ), the geo id ha s

an irreg ular shape t ha t is

describe d a s

“ undula t ing” .

GISGeographic Inf ormati on Syst em . A

computer-based system

tha t collects , ma na g es

an d a na lyses g eog raphic

spatial information.

GLONASSGlobal’nava Navigatsionnaya

Spu t ni ko vanva Sist ema .

The GPS netw ork

opera ted by th e Russia n

Federation.

GNSSGlobal Navigatio n Sat ell it e Syst ems, which

encom pa sses GPS,

GLONASS, Galileo and

ot her sa te llite b a sed

na vig a tion system s.

GPSGlob al Posit ion ing System .

GPS constellation GPS

sa tellites orbit the ea rthevery 12 ho urs, em itting

continuous navigation

signals. With the proper

eq uipment , users ca n

receive th ese sign a ls to

ca lcula te t ime, loca tion

a nd velocity.

GPSnet The net w ork of

permanent GPS ba sestations and supporting

infrastructure operated

by DSE th a t record s,

distributes and archives

GPS sa te llite correction

da ta fo r DGPS purposes.

w w w.land.vic.go v.au/

gpsnet

local uncertainty Thea verag e mea sure inmet res a t a 95 per cent

con fidence level w ith

respect to the ad ja cent

points in a da tum.

MGA94 UniversalTra nsverse Merca t o r g rid

coo rdina tes (ea st , no rth

and zone) genera ted

from GDA94 la titud es and

longitudes.

multipath When sa te llitesig na ls bo unce off

veg eta t ion or buildings as

the y travel from sat ellite s

to a GPS receiver. It

introduces errors into GPS

positions.


22/24

20 Glo ssa ry

positional uncertaintyExpressed in m et res, it

d escrib es a circle o f

uncerta inty around a GPS

position.

post-processingCorrecting errors in G PS

posit ions by using da ta

from b a se o r reference

sta t ions some t ime af ter

the mo ment o f posit ion

determinat ion.

precision Refe rs to ho w clo sely severa l GPSposit ions taken f rom th e

sa me loca t ion ma tch each

o th er. Precision is a

mea sure of repea ta bility.

receiver noise Refers toho w w ell a GPS receiver

mea sures a nd ca lcula te s

inf orma tion received

fro m G PS sa t ellite s. The

hig her the noise, the

g reat er th e risk of error.

reference frame Ama them at ical surface tha t

best f its the shape o f t he

Ea rth a llow ing you to

describe geographic

po sitions. Also kno w n a s

a datum .

space segment Thecon stella t ion o f 24

sa te llites used by G PS

receivers to calculat e

positions on Earth.

spatial informationSpa tial inf orma tion is a ny

info rmat ion recording or

describing the physical

locat ion of an ob ject .

trilateration The prin ciple

w hich a llow s you toca lcula te a posit ion o n

Earth using distance

measurements.

user segment A GPSreceiver a nd th e person

using it .

Vicmap A set of spa tia llyrela t ed d a t a products

prod uced b y DSE, w hichunderpin Victoria’s

mapping a nd geo graphic

info rma tion syste ms.

waypoints Points inbe tw een the st a r t and

end points of a part icula r

seq uence o f po ints of

interest.

WGS84 Wo rld Geodet ic Syst em 1984 . The g lob a lly

recog nised reference

frame or da tum used by

the GPS.


23/24

Published by

Spatial Information Infrastructure Division

Department of Susta ina bi lity a nd

Environment

PO Box 500

Ea st Me lbou rne Victo ria 3002

The De pa rtment of Susta ina bility a nd

Environmen t is responsible f or m a na g ing

Victoria 's GPS ba se sta tion net w ork,

GPSnet , a nd spat ia l info rmation

infrastructure.

Find more information about theDepartment of Sustainability and

Environment at www.dse.vic.gov.au orcall 136 186.

ISBN 1 74152 013 4

© The Sta te o f Victoria, Department of

Sustainability and Environment, 2004

This publica tion m a y be of a ssista nce to yo u but

the Sta te o f Victo ria a nd its employees do no t

gua rantee t hat the publicat ion is w ithout f law

of a ny kind o r is w holly appropriate for your

particular purposes and therefore disclaims all

liability for any error, loss or other consequence

w hich ma y arise from you relying on a ny

informat ion in this publica tion.

Printed by Chillipress

Print ed o n Han no Art Silk

- Mana ge d Planta tion Fibre

- To ta lly Chlorin e Free

- Acid Free

- Environmentally Accredited


24/24

Department

o f

Sustainability

a nd

Environment

Spat ia l Info rmat ion Infrastructure

Strategic Policy and Projects

PO Box 500

Ea st Melbourne VIC 3002

Tel (03) 8636 2333

Fa x (03) 8636 2813

www.land.vic .gov.au

Date post:	01-Jun-2018
Category:	Documents
Upload:	roger-groppi
View:	220 times
Download:	0 times

GPS A Guide For Users

Documents