satellite orbits and sensors

7/24/2019 Satellite Orbits and Sensors

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Platformsand

Satellite

Orbits

OSTM/Jason2BoostProfile


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TypesofPlatform

Thevehicleorcarrierforremotesensorsiscalledtheplatform.Typicalplatformsaresatelliteand

aircraft,buttheycanalsoincluderadiocontrolledairplanes,balloons,kitesforlowaltituderemote

sensing,aswellasladdertrucksor"cherrypickers"forgroundinvestigations.

Platformswith

the

highest

altitude

are

geo

synchronous

satellites

such

as

the

Geosynchronous

MeteorologicalSatellite(GMS),whichhasanaltitudeof36,000kmattheEquator.

Mostoftheearthobservationsatellites,suchasLandsat,SPOT,MOSetc.areatabout900km

altitudewithasunsynchronousorbit.

Fromlower

orbit,

there

are

space

shuttle

(240

280

km),

radio

sonde

(

100km),

high

altitude

jet

plane(10km),lowormiddlealtitudeplane(0.58km),radiocontrolledplane(~0.5km)andsoon.

Thekeyfactorfortheselectionofaplatformisthealtitudewhichdeterminesthegroundresolution

IFOV(instantaneousfieldofview)ofthesensor.

Theselectionofplatformalsodependsonthepurposewhichissometimerequestedforexamplea

constantaltitudeisrequiredforaerialsurveys,whilevariousaltitudesareneededtosurveyvertical

atmosphericdistribution,forexample.


3/16

Variousplatforms,altitudesand

objectsbeingsensed


4/16

Atmosphericcondition

is

different

depending

on

the

altitude.

This

factor

must

be

considered

intheselectionofplatformsorsensors.Inthissection,airpressure,airdensityand

temperatureareconsidered.

AtmosphericConditionandAltitude

Theverticalstructureoftheatmosphereiscomposedofthefollowinglayers.

Troposphere:fromthegroundsurface(1017km)to11km,

Stratosphere:from1017kmtoabout50km

Mesosphere

:

from

about

50

km

to

about

90

km

Thermosphere:fromabout80kmto500km


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Exosphere

Aboveaheightofabout500kmistheexosphere,alayerwherethe

atmospheremerges

into

space.

Satellites

are

stationed

in

this

area,

500kmto1000kmfromEarth.

Thermosphere

Thethermosphereisthefourthlayerintheatmosphere,between

80kmand110kmabovetheEarth.Spaceshuttlesflyinthisareaandit

isalsowheretheauroralightsarefound.Aurorasarewispycurtainsof

lightcaused

when

the

sun

strikes

gases

in

the

atmosphere

above

the

Poles.

Mesosphere

Beyondthestratospheretheairisverythinandcold.Thisareais

knownasthemesosphere,andisfoundbetween50kmand80km

abovetheEarthssurface.

Stratosphere

Thestratosphere isthesecondlayerofairabovetheEarthssurface

andextendstoaheightof50km.Itisherethatwefindtheozone

layer.Theozonelayerabsorbsmuchofthesunsharmfulradiation

thatwouldotherwisebedangeroustoplantandanimallife.

Troposphere

ThetroposphereisthelayerclosesttotheEarth,approximately

11kmhigh.Weatheroccursonlyinthetropospherebecauseitisthis

layerthatcontainsmostofthewatervapour.Weatheristheway

waterchangesintheair,andsowithoutwatertherewouldbeno

clouds,rain,

snow

or

other

weather

features.


6/16

Thegeometricdistortiondependsonnotonlythegeometryofthesensorbutalsotheattitudeofthe

platform.Therefore

it

is

very

important

to

measure

the

attitude

of

the

platform

for

the

consequent

geometriccorrection.

Theattitudeoftheplatformisclassifiedbythefollowingtwocomponents.

Rotationanglesaroundthethreeaxes;roll,pitchandyaw;

Random

and

unsystematic

vibration

which

cannot

be

measured.

Therotationangles;roll(),pitch()andyaw()aredefinedastherotationanglesaroundtheflightdirection,themainwingandtheverticallinerespectively,asshowninthenextslide.Thebottom

panel(nextslide)showsthesatelliteattitudeparameters.

Inthe

case

of

satellites,

the

variation

of

the

positionand

the

attitude

will

be

continuous,

Incaseofaircraft,thevariationwillnotalwaysbesmooth,whichmakesthegeometriccorrection

moredifficult.

Thetypicalattitudesensorsforaircraftareasfollows.

Speedometer

Altimeter

Gyrocompass(forattitudemeasurement)

Dopplerradar(formeasurementofaltitude)

GPS(forpositioning)

Gyrohorizon

TV

camera

Flightrecorder

AttitudeofPlatform


7/16

Theaircraftandsatelliteattitudeparameters.


8/16

OrbitalElementsofSatellite

Asetofnumericalvaluestodefineanorbitofasatelliteorplanetarecalledorbitalelements.The

independentorbitalelementsoftheearthobservationsatellitearesixelementsoftheKeplerianorbit(satellitesorbitinanellipseofconstantshapeandorientation).

Asatellitecanbeconsideredtorotatearoundtheearthinaplane,calledtheorbitalplane,because

theinfluenceofgravityofthemoonandthesuncanbeneglectedascomparedwiththegravityof

theearth.

Apointinspacecanbeexpressedintheequatorialcoordinatesystemasfollows.Theoriginof

equatorialcoordinatesystemisthecenteroftheearth.

Thereferencegreatcircle:theequatorialplane

Theorigin

of

astronomical

longitude

(right

ascension)

:the

vernal

equinox

Theastronomicallongitude(rightascension):024hourstotheeastfromthevernalequinox

Theastronomicallatitude(declination):anglefromtheequatorialplane(+90degreeinthenorth

pole;90degreeinthesouthpole)

The

six

elements

of

Keplerian

orbit

are

;(1)Thesemimajoraxis(A):

(2)Eccentricityoforbit(e):

(3)Inclinationangle(i):

(4)Rightascensionofascendingnode(h)

(5)Argumentofperigee(g)

(6)Time

of

passage

of

the

perigee

(v)


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Figure(below)showstheaboveelements.Theshapeandsizeofanorbitcanbedefinedby

Aande,whiletheorbitplanecanbedefinedbyiandh.Thelongeraxisoftheorbitellipse

canbe

determined

by

g.

The

positionof

asatellite

can

be

located

by

T.

OrbitalelementofKeplerArgumentofPerigee(g): Theanglebetweenthe

ascendingnodeandperigee,measuredcounter

clockwisealongtheplaneoftheorbit.

Apogee(A):Pointinanorbitwhenthesatelliteis

farthestfromtheEarth

Perigee(P):

The

point

in

an

orbit

when

the

satellite

isclosesttotheEarth.

Eccentricity(e):Halfofthedistancebetweenthe

fociofanellipsedividedbythesemimajoraxis.

Inclination(i):Theanglebetweentheplaneofthe

orbitandtheplaneofthecelestialequatorforEarth

orbitingsatellites(ortheplaneoftheeclipticforsun

orbitingsatellites).

RightAscension

of

the

Ascending

Node

(h):

AnothertermforLongitudeoftheAscendingNode,

ItistheangleoftheascendingnodemeasuredEAST

ofthevernalequinoxalongthecelestialequator.

SemiMajorAxis(A):Thehalfofthelongerofthe

twoaxesoftheorbitalellipse

VernalEquinox:Oneoftwopointswherethe

ellipticcrosses

the

celestial

equator,

the

other

being

theAutumnalEquinox.

CelestialEquator:TheplaneoftheEarth'sequator

projectedontothecelestialsphere.Thecelestial

equatoristilted23.5degreesinrelationtothe

planeoftheEarth'sorbit(theelliptic).

Ecliptic:TheplaneoftheEarth'sorbitaroundthe

sun.


10/16


11/16

Satelliteorbits

Thepath

followed

by

asatellite

is

referred

to

as

its

orbit.

Orbitselectioncanvaryintermsofaltitude(theirheight

abovetheEarth'ssurface)andtheirorientationand

rotationrelativetotheEarth.

Threeprincipally

employed

types

of

Earth

observation

orbitsare(1)geosynchronous, (2)sunsynchronousand

(3)nearpolarorbits.

(1)Satellitesatveryhighaltitudes,whichviewthesame

portion

of

the

Earth's

surface

at

all

times

have

geostationaryorbits(ataltitudesofapproximately36,000

km).

Thesesatellitesrevolveatspeedswhichmatchthe

rotationoftheEarth,sotheyseemstationary,relativeto

theEarth's

surface.

This

allows

the

satellites

to

observe

andcollectinformationcontinuouslyoverspecificareas.

Examplesareweatherandcommunicationssatellitesthat

commonlyhavethesetypesoforbits.


12/16

(2)Manyremotesensingplatformsaredesignedtofollowan

orbit(basicallynorthsouth)which,inconjunctionwiththe

Earth'srotation

(west

east),

allows

them

to

cover

most

of

the

Earth'ssurfaceoveracertainperiodoftime.Thesearenear

polarorbits.

(3)Manyofthesesatelliteorbitsarealsosunsynchronous such

thattheycovereachareaoftheworldataconstantlocaltimeof

daycalled

local

sun

time.

If

asatellite

is

in

polar

orbit

at

an

altitudeof800km,itwillbetravellingataspeedof

approximately7.5kmpersecond.

Atanygivenlatitude,thepositionofthesunintheskyasthe

satellite

passes

overhead

will

be

the

same

within

the

same

season.Thisensuresconsistentilluminationconditionswhen

acquiringimagesinaspecificseasonoversuccessiveyears,or

overaparticularareaoveraseriesofdays.Thisisanimportant

factorformonitoringchangesonthesurface.

Mostof

the

remote

sensing

satellite

platforms

today

are

in

near

polarorbits,whichmeansthatthesatellitetravelsnorthwards

ononesideoftheEarthandthentowardthesouthernpoleon

thesecondhalfofitsorbit.Thesearecalledascendingand

descendingpasses,respectively.


13/16

AsasatelliterevolvesaroundtheEarth,thesensor

"sees"acertain

portion

of

the

Earth's

surface.

The

areaimagedonthesurface,isreferredtoasthe

swath.

Imagingswathsforspacebornesensorsgenerally

vary

between

tens

and

hundreds

of

kilometres

wide.

AsthesatelliteorbitstheEarthfrompoletopole,its

eastwestpositionwouldn'tchangeiftheEarth

didn'trotate.However,asseenfromtheEarth,it

seemsthatthesatelliteisshiftingwestwardbecause

theEarth

is

rotating

(from

west

to

east)

beneath

it.

Thisapparentmovementallowsthesatelliteswath

tocoveranewareawitheachconsecutivepass.

Thesatellite'sorbitandtherotationoftheEarth

worktogethertoallowcompletecoverageofthe

Earth'ssurface,afterithascompletedonecomplete

cycleoforbits.

Mercatormap

of

the

satellite

ground

track

for

the

orbit

for(a)nonrotatingEarth,and(b)rotatingEarth.

swath


14/16


15/16

Orbitalvelocitiesandperiods

Tocalculate

the

velocity

and

orbital

period

of

the

sensor,

we

equate

the

gravitational

andcentripetalforcesonthesatelliteattheorbitalaltitudeofinterest,

Where,

G

=gravitational

constant

=6.671011 Nm2 kg2

=massofthesatellite

=massoftheearth=5.9761024 kgr =meanradiusoftheearth=6.371106mH =orbitalaltitudeinmeters(h=a(1e2)/1+ecos)

=

velocity

of

the

satellite

Wederivethe asfollows,

Thesubsatellite velocityattheground,vg,isgivenby

Hr

vm

Hr

mGm sses

2

2)(

sm

em

sv

1

2/1

ms

Hr

Gmv es

Hr

rvv sg

Therefore,

Usingthisequation,wecancalculatevg forvarious

satellites.Forexample,vg andvs forLandsatTM

having

an

the

Earth

orbit

altitude

of

705103

m

is

6.76and7.51kms1 respectively.

2/3

2/1

)(

)(

Hr

Gmrv eg

1

2/36

11

)10371.6(10272.1

kms

Hvg

sv


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satellite orbits and sensors

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