Introduction to Remote Sensing WMO-RMTC-EUMETSAT Satellite Meteorology Course, Alanya, Turkey 22-25...

WMO-RMTC-EUMETSAT Satellite Meteorology Course, Alanya, Turkey 22-25 September 2003

Introduction to Remote Sensing

Yrd. Doç. Dr. Ali DENİZYrd. Doç. Dr. Ali DENİZ



Remote sensing is defined as the way to infer about the objects from distance i.e size, consentration, content etc.

What is remote sensing?



The interaction of electromagnatic waves with the objects modifies the incident wave;

The resulting sinature depands on the composition and structure of the medium;

The principle of measurements of the Atmospheric parameters i.e. temperature and humidity is the interpretation of measured radiation in the specific spectral intervals which are sensitive to the constituent;



In the infrared and microwave regions of the spectrum atmospheric constituents absorbs the radiation; then emitAccording to Kirckhoff’s law;

Since the emitted radiance is a function of distribution of objects, measurements of radiance gives information about them.



Absorption and Transmission of Monochromatic (spectral) Radiation:

The amount of energy, radiance, crossing a differencial area dA in a time integral dt and wavenumber v is given as:

L= dE / Cos dA dt d d

its unit is W/m2 sr cm-1

(...)



is a solid angle and is defined as:

d

d

(...)

2

1

2

1

sin dd



X

Absorption:

dx

Incident beam

Attenuated exit beam

Absorbing med

ium

Figure-.. Absorption through a Medium

L



When a monochromatic radition in Figure- whose radiance is L

peneterates into absorbing medium (non scatering) the fractional decrease is:

d L/ L = -k dx

Where is the density of medium, k is the spectral absoption

Coefficient.

(...)



When integrating equation ... between 0 and x, becomes:

(...)

Where L(0) is the radiance entering the medium at x=0,

is called optical depth. And

)exp()0()(0x

dxkLxL

x

dxk0

)exp(0x

dxk is called transmittance.



Black Body Radiation:

Blackbody radiation field is characterised as:

Isotropic and nonpolarized;Independent of shape of cavity;Depends on only temperature (T).

In a perfect blackbody emisivity is equal to unity due tothermodynamic equilibirium.



The ratio of emitted radiance by an object to the radiance emitted by a blackbody at the same temperature is calledemisivity ().

= 1 for blackbody.

< 1 for greybody.

Blackbody radiation

Greybody radiationE



Planck Law:

To explain the spectral distribution of radiance emitted by solid bodies, Planck found that the radiance per unit frequencyemitted by a blackbody at temperature (T) is given as:

1)exp(

2)(

3

Tchch

TB

Where h is Planck const,k is Boltzmann const.

(...)



Figure-... Spectral Radiance of Blackbody

Kelvin



Stefan Boltzmann Law:

It gives the total radiation of a cavity (blackbody) not spectral distribution of radiation. When Planck function (...) is integrated 0(zero) to infinity (), S-B is given as:

E(Exitance)=E4

Where is stefan Boltzmann constant.

(...)



Wien Law:

When Planck equation is differentiated w.r.t wavenumber() or wavelength () and equated to 0(zero), one can find max for a given temperature (T) called Wien Law.

max =0.2897/T (cm) E

What wavelengthggives maximum energy



The radiative temperature of the sun surface is about 5780 K. After applying Wien law, maximum Planck radiance is obtained at the wavelength (max ) of 0.50 m which is the

center of the visible region of te spectrum.

On the other hand eath’s atmospheric temperature is about 255 K. Maximun emitted energy takes place around 11 m which is infrared region (Figure...).



Brightness Temperature (or equivalent blackbody temperature) is the temperature estimated by inverting Planck function.



Gaseous Absorption:

In the atmosphere, the absorption of radiation ismostly due to gases. Major interest for the transfer of radiant energy is the value of absorption coefficient (spectroscopy).Total energy of a molecule consists of rotation, vibration,electronic, and translation.

E=Erot+Evib+Eelc+Etrans



Absorption or emmision occurs when molecule changes from energy level E1 to E2 with a frequency f = (E1- E2 ) / h.

Where h is a planck constant.

rotational energy in the microwave and far IR regions;vibrational energy in the near IR region;electronic as well as vibrational and rotational energy in the visible and UV regions.

Significance in the spectrum:



In order to posses rotational energy (interact withthe elecrtromagnatic field), molecules shall have dipolemoment.

Atmospheric such gases important for satellite meteorology as CO, N2O, H2O and O3 have dipole moments

while N2, O2 , CO2 and CH4 do not.

However, as CO2, and CH4 vibrate electrical dipole moment

is produced and so rotational interaction take place. Therefore,vibration-rotation interaction takes place with the incident wave.



O-C-O

O-C-O

O-C O

Symetric Stretching

Bending

Asymetric Stretching

Figure-... Vibration modes of CO2

=7.46 m

=14.98 m

=4.26 m

(Wavelength of vibration)

No static and dynamic electric dipoleSo no interection with the incident radiationSo no absoption (Figure-...).

Dipole occurs due to bending Vibration (vibration-rotation) soAbsoption takes place (Figure-...).



Temperature Sounding and measurements of some trace gases in the atmosphere are based on vibrational transition. For example;

The 15 m and 4.3m of CO2 bands are used for temperature

soundings.

The 6.3 m H2O band is used for water vopour.

The 9.6 m O3 band is used for the total ozone measurements.



Figure-... IR transmittance of several gases in the atmosphere (after Kidder,S.Q)



Scattering:

Radiation scattered from a particle depends on:

Size;Shape;Index of refraction;Wavelength of radiation;View geometry.



Fundemental two types of scattering are Mie and Rayleight.

For Rayleight scattering, >> For Mie scattering, ≈

Where is particle size.

Scattering properties of such aerosols as smoke, dust, haze in the visible part of the spectrum and of cloud dropletsin the IR region can be explanined by Mie scattering, while of air molecules in the visible part can be explained by RayleightScattering (Figure-...).



Figure-... Scattering Properites of atmospferic Contitiuens (after Kidder, S.Q).



Scattering phase function determines the direction in whichthe radiation is scattered. As the size parameter (x=2πr/) inreases, more scattering takes place in the forward direction (Figure-...).

Figure-...Scattering phase function of water droplets (after Kidder, S.Q).



Radiative Transfer Equation (RTE):

Consider a volume of gas (Figure-...) where absorption and emission takes place but no scattering, energy transferequation can be written as:

dxJdxkLdL

Where first therm on the right hand side is abrorption within dxand second term is emission within dx.

(...)



Applying Kirchhof law and some manpulation and integrationfinally we get:

aa a

dxdxkTBkdxkLaL00 0

)exp()()exp()0()(

The first term on the right hand side is the radiance at the Boundary multiplied by the transmitance from the boundary to a.Second term is the contribution due to emission from the medium in the direction of incident wave.

(...)



Similar equation can be computed for the emitted radiance in the atmospfere with zenith angle as:

dzzTBk

zLzLz

),()(cos

),(),0(),(0

(...)

)cos

exp( dzkb

a

where and

cos

k

dz

dW

z

is called weighting function.



Figure-... NOAA HIRS weighting functions.



So equation (...) can be written in a more compact form as:

dzz

TBzLzLz

)(),(),0(),(

0

(...)

surface

z

dz

L(0)=εs,ν B(T)

, B(T)

TOA

τ

Surface contribution Atm layer cont.Radianceat the TOA



Previous RTE (Figure-...) is driven in cartesian coordinate systemdrive it in presure coordinate system in p by using hydrostatic Equation.

pdp

pTBppLpL p

o

p

lnln

),()(),(),(),(

)secexp( dpg

kqb

a

Where q is mixing ratio and g is gravity.

Good luck...

Brain Storming!



Processes of Atmospheric Radiation:

LandOcean

emitted

transmitted

reflected

absorbed,emitted and scattered by aerosols and molecules

absorbed

emitted reflected

absorbedtransmitted

emitted

transmitted

reflected

transmitted

absorbed&scattered

Figure-... Process of Atmospheric Radiation



Sensors:Sensors are the devices for detecting the photons. The criticalpart of the sensors is the detectors which works based on photoelectric effect. That is, There will be an emmision of negative particles (electrons) when negatively charged plate is subject to a beam of photons.

The electrons then can be made to follow,collected and counted as signals.

The magnitude of electric current (number of photoelectrons per unit volume is directly proportional to light intensity.



Thus, the change of electric current can be used to measure the change in the photons (number, intensity) which strikes the plate during the given time interval.

NegativelyCharged Plate

Photon beam

R

C

C

Figure-... Shemetic view of a detector.



Remote Sensor Types:

Sensor types

passive

aktive

non-scaning

scanning

non-scaning

scanning

non-imaging

imaging

Microwave radiometerMagnatic ensorGravimeterFourier spectrum

CameraMonochromIR

imaging image plane scanning

object plane scanning

TV camera

Solid scanner

Optical mechanical scan.Microwave radiometer

Microwave radiometerMicrowave altimeter

non-imaging

imagingimage plane scanning

object plane scanning

Passive phased array radar

Real aperture radar

Synthetic aperture radar



Passive sensors: Radiation comes from the external sources.Active sensors: Radiation is generated within the sensor.Non-imaging: Measured radiation received from all points in the sensed target and integrated. Imaging: Radiaiton is received from a specific points (pixels) in the targetend result is an image(picture).Sensors which instantaneously measure radiation coming from entire scene called framing systems e.g eye, camera; if the scene is sensed point by point along successive lines over finite time called scanning systems.The size of scene which is determined by the aperture and optics called fieldof view (FOV).



Radiometer is the general term for any instrument which quantitatively measures the EM radiation in some interval of EM spectrum.When the radiation is light from the narrow visible band, the term photometer is used.If the sensor includes such components as prism or difractiongrating which can break incoming radiation into discrete wavelengths and despers them to detectors called spectrometer.Spectroradiometer implies that dispersed radiation is in bands (Δλ) rather than discrete wavelenght (); most space sensors are of this type.



Retrieval Method:

Physical;

Statistical and;

Hybrid.

Retrieval methods can be classified in three general categories:



To predict the obserable parameters from arbitrary model parameters called forward problem; on the other hand, invers problem is to infer the model from observed parameters,

Invers problems are “ill posed”; that is, the solution is not unique.



dzz

TBzLzLz

)(),(),0(),(

0

Example:

Estimation of L(z) from known temperature profıle, T,is a forward problem, while estimation of T profile fromsatellite measurement of L(z) is an invers problem.



Physical Retrieval:

It is based on itiration of RTE by using first guess NWPprofile until the desired solution is optained.

Statistical Retrievals:

The siplest is to make regression between radiosonde sounding called training data and measured radiance.

Temperature Sounding Retrieval:



Hybrid Retrieval:

RTE can be written in the dicrete formas:

1

1

0

j

j

ijijii WBLL After putting surface contribution intoSummation and replacing Li by Ri.

j

j

ijji WBR1

in maxrix notation

BWR .

Where W is a matrix containing discrete weighting function.



Assuming linearity and introducing basis function bB .

bAbWR ... by matrix inversion;

Which is an exact solution of RTE. However not asatisfactory solution because it is ill conditioned;smaller error in R results in larger errors in B.

Trying least square fit of gives the solution as:

b=A-1R

Σ(R-Σab)2

(...)

b=(AT.A)-1.AT.R (...)



Which is better solution than equation (...), yet it can be improved by applying other methods (e.g. minimum variance)



Wind Retrieval:Atmospheric wind retrieval from the images is based oncross correlation method of successive three images.

32x32 pixel

96x96 96x96H

Image 3 (H+1/2)Image 1 (H-1/2)Image 2 (target image)

First, cross-correlation matrix produced between target image andLast image (H+1/2) to find the correlation peak .

cc pick

cc pickcenter of search area

Wind wector VH+1/2

VH-1/2



The distance between the center of search and the cc peak is the cloud tracking wind vector.

In order to eleminate the spurious (false) peak cc is estimated between the target image and first image (H-1/2). If cc peaks are not symetric, it is rejected. If not, wind speed is claculated as:

V=1/2 (VH-1/2 + VH+1/2 )

Wind direction is the vectorial sum of two winds:VH-1/2

VH+1/2 V



Height assignment: After finding the most contributing cloud cluster to cc peak, its top temperature is used to assign height toV by means of NWP forecast temperature profile. CO2 slicingmethod can also be used for height assignment gives better solution.Sea surface wind speed and direction are estimated by meansof scatterometer which works like a radar. Tracking winds can also be retrieved from atmospheric soundings data.



CO2 Slicing: Radiance measurements around 15m CO2 band ( e.g HIRSsounder) allows to detect clouds at various level. In the center of band, upper level clouds and at the wings lower level cloudsare detected (Figure …).

Cloud amont and level (top pressure) can be estimated by means of RTE.

Effectively detects thin cirrus clouds which are missed by IR window and VIS channels.



Radiance from the partly cloudy regions are given by L=Lcd+(1-)Lcl where is a fractional cloud cover,

Lcd is radiance from clouds and Lcl is radiance from clear air.

The cloud radiance is given by Lcd=Lbcd+(1-)Lcl

where Lbcd is radiance from completely opaque cloud.

After expresing Lcl and Lbcd in the form of RTE and some

algebric manipulation, it becomes:

c

s

cl

P

pdBpLL )(, (…)



Where is called effective cloud amount. Lcl estimated from

known temp and moisture profiles and L is a satellite measurement (e.g HIRS). Right hand side is calculated from known temp profile and atmospheric transmitance profile.

Representing left hand side with, L, and right hand side, B, and taking the ratio of two spectral channels which see the same FOV (e.g 14.2m/14.0m or 14.2m/13.3m), it becomes:

2

1

2

1

B

B

L

L

(…)



Once the cloud height is determined, an effective cloud amount can be estimated by using IR window channel.

clc

clw

LpTB

LL

))((

(…)

Opaque cloud radiance.

The optimum cloud pressure is obtained when the absolute differences of right hand side and left hand side is minimum.

Calculated from temp profile

Sat measurement.



Clouds:

Objective cloud retrieval is carried out by using imagingradiometers rather than sounders due to their high resolution.Widely used cloud analysis techniques are:

Threshold:

Temperature thresh is set for the spectral channels such that if pixel temp is colder, it is assigned as a cloudy pixel.Level of the cloud is determined by comparing brightness temp of pixel with a known temp sounding.



Histogram:

It is based on the histogram analysis of pixels which represent cloud types and surface. Various dimentional histograms may be constructed.

Figure-… shows two Dimentional ( IR vs VIS) historam analysis.High clouds are located on the cold region while middle clouds are clustered at the center and sea pixels are clustered warm (dark) region.



10 30 50 90 110

130

220

150

180

200

230

VIS

IR

Cloud id VIS count IR Count

High clouds 64 219Cirrus thin 32 108Medium clouds 44 185Low clouds 56 166Warm land 28 145Sea 16 153

Figure_... Bi-dimentional histogram of IR vs VIS

Kernels of the classes are:



Pattern recognition:

For an area of pixels, standard deviations (SD) and mean values are Calculated. Uniform scenes (cloudy or clear) tend to have low standard deviation while partly cloudy scene tends to have high standard deviation.

Estimated statistical values (SD and mean etc) compared with the background information (e.g. Climatological) to classify the scene (cloudy, land, sea).



SST:For the scene classified as sea, after applying atmospheric correction, the radiance is converted to brightness temperatureby means of Planck function.

For NOAA, SST estimation is based on the reression Between Brightness temperatures of the channels (11 m, 12 m and 3.7 m) ans SST.

For sea scene, in the daytime SST is:

SST=1.9346T11+2.5779(T11-T12)-283.21 (…)

Where T in K and SST in Celsius.



At nightime SST is given by:

SST1 =1.0088T3.7+0.4930(T3.7-T11)-273.34

SST2 =1.0350T11+2.5789(T11-T12)-283.18

SST3 =1.0170T11+0.9694(T3.7-T12)-276.58

All three SST result must agree within 1K to be accepted.

(…)



Split Window Technique:

Making of two measurements rather then one in the atm window region (near 11 m ) called split window. The twochannel see the same absorbers but in different amount.

The aim of the split window is to correct atm attenuation (mostlydue to water vapour) to estimate better surface temperature.

The siplit window equation driven as follows: in the windowsregion transmitance can be expressed as = exp(-ku), using talor expention, = 1-ku and d=-kdu then RTE becomes



duTBkkusLLu

)()1)((0

Where B is mean atm radiance and u total atm absorbtion path length due to water vapour.

(…)

After linearizing the RTE and using for two channels (e.g.11 and 12 m ), we get

2

1

2

1

k

k

TT

TT

s

s

Where Ts is a surface skin temp corrected for water vapour,

(…)



k is absorption coefficient and T 1 and T 2 are brightness temp for the windows channels.

In a similar manner atm precipitable water can be retrieved.



Calibration:

Calibration is the process to obtain physical parameters(e.g. Tempareture) from the radiometric measurements (count)

On board Calibartion:

Hot and cold targets located in the spectrometer are sensed to get relative radiomatric responses to estimate calibration slope.

Vicarious calibration:

The method of vicarious calibration by means of calculated radiance allow absolute calibartion of radiometers.



It works by comparing radiometric count to be calibrated with corresponding alsolute radiance which is calculated from actual atmospheric parameters (e.g. Temperature and humidity profile).

Radiometric respons to the radiance can be linear or nonlinear.In the case of linear response (e.g. METEOSAT) calibration coefficient is calculated as:

space

calir

CC

L



Orbits:

The motion of the satellites around the earth is governed by the Newton’s law of motion.

satelliter

earthSatellite orbit

2r

mgMF

e

The attractive force is:



Where M is earth mass, m is satellite mass, and g is garvitational Constant.The centrifugal force of the spacecraft’s motion in orbit must balance the attraction force such that:

2

2

r

mgM

r

mv e

introducing period T=2r/v and substituting in the equation we get:

32

2 4r

gMT

e

Ex: NOAA satellites are app 850 km above the earth surface, inserting into equation T=102 min.



However, real satellite orbit is nerly elliptical due to the external foreces(e.g. gravitational potential of earth, solar pressure). Eliptical orbit is to be explained by Kepler’s law.

Six orbital elements are used to express the spacecraft positiongiven by:

•Semimajor axis,a (km);•Eccentricity,e;•Inclination,i (degree);•Right ascensing of ascending node, (degree);•Argument of perigee, (degree);•And true anomaly, , degree.



a

spacecraft

Ascending node

perigeeapogee earth

X

Y

Z

Equator plane

Orbit plane

i

Ascending node

For the meteorological satellites, mainly two orbits are used: and sunsynchronous and geostationary.



Sunsynchronous orbit have high inclination angle (e.g98.7 for NOAA sat), pass the equator at the same local time,and located in the lower orbit (e.g . 850 km).

Geostationary orbit consides with the earth’s equatorial plane,located nearly 36 km above the equator. Geostationary satellitesdrift from the desired orbit so that periodic orbit manoeuvres areneeded in the east-west and north-south directions and vise versa.

Introduction to Remote Sensing WMO-RMTC-EUMETSAT Satellite Meteorology Course, Alanya, Turkey 22-25...

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Transcript of Introduction to Remote Sensing WMO-RMTC-EUMETSAT Satellite Meteorology Course, Alanya, Turkey 22-25...