Download What is remote sensing?

What is remote sensing?
Remote sensing is defined
as the way to infer about
the objects from distance i.e
size, consentration, content
etc.
The interaction of electromagnatic waves
with the objects
modifies the incident wave;
The resulting signature depends 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:
2 2

  sin dd
1
1
d
d
Absorption:
Attenuated exit beam
L
X
dx
Incident beam
Figure-.. Absorption through a Medium
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:
x
L ( x )  L (0) exp(   kdx )
0
Where L(0) is the radiance entering the medium at x=0,
x
  kdx is called optical depth. And
0
x
  exp(   kdx)
0
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 to
thermodynamic equilibirium.
The ratio of emitted radiance by an object to the radiance
emitted by a blackbody at the same temperature is called
emisivity ().
 = 1 for blackbody.
Blackbody radiation
E
Greybody radiation
 < 1 for greybody.

Planck Law:
To explain the spectral distribution of radiance emitted
by solid bodies, Planck found that the radiance per unit frequency
emitted by a blackbody at temperature (T) is given as:
2h c
B (T ) 
hc
exp(
) 1
T
3
Where h is Planck const,
k is Boltzmann const.
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)
What wavelengthg
gives maximum energy
E

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 is
mostly 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.
Significance in the spectrum:
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.
In order to posses rotational energy (interact with
the elecrtromagnatic field), molecules shall have dipole
moment.
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.
=7.46 m
O-C-O
Symetric Stretching
(Wavelength
of vibration)
=14.98 m
O-C-O
Bending
O-C
O
=4.26 m
Asymetric Stretching
Figure-... Vibration modes of CO2
No static and dynamic electric dipole
So no interection with the incident radiation
So no absoption (Figure-...).
Dipole occurs due to bending
Vibration (vibration-rotation) so
Absoption 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 4.3m 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 droplets
in the IR region can be explanined by Mie scattering, while
of air molecules in the visible part can be explained by Rayleight
Scattering (Figure-...).
Figure-... Scattering Properites of atmospferic Contitiuens (after Kidder, S.Q).
Scattering phase function determines the direction in which
the 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 transfer
equation can be written as:
dL   Lkdx  Jdx
Where first therm on the right hand side is abrorption within dx
and second term is emission within dx.
Applying Kirchhof law and some manpulation and integration
finally we get:
a
a
a
0
0
0
L (a )  L (0) exp(   kdx)   kB (T ) exp(   kdx)dx (...)
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:
k
L( z, )  L(0, ) ( z, )  
B(T ) ( z, )dz
cos
0
z
k
where   exp( 
a cos dz)
b

z
and W 
is called weighting function.
d
k


dz
cos 
Figure-... NOAA HIRS weighting functions.
So equation (...) can be written in a more compact form as:
z
L( z, )  L(0, ) ( z, )   B(T )
Radiance
at the TOA
z
0

dz
z
Surface contribution
Atm layer cont.
TOA
dz , B(T)
τ
surface
L(0)=εs,ν B(T)
Brain Storming!
Previous RTE (Figure-...) is driven in cartesian coordinate system
drive it in presure coordinate system in p by using hydrostatic
Equation.
o
 ( p, )
L( p, )  L( p, ) ( p, )   B(Tp )
d ln p
 ln p
p
b
kq
  exp(   sec dp)
g
a
Where q is mixing ratio and g is gravity.
Processes of Atmospheric Radiation:
transmitted
absorbed,
emitted and
scattered by
aerosols and
molecules
transmitted
absorbed
&scattered
emitted
reflected
transmitted
reflected
emitted
Land
emitted
reflected
transmitted absorbed
absorbed
Figure-... Process of Atmospheric Radiation
Ocean
Sensors:
Sensors are the devices for detecting the photons. The critical
part 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.
Photon beam
C
Negatively
Charged
Plate
R
C
Figure-... Shemetic view of a detector.
Remote Sensor Types:
non-imaging
non-scaning
Microwave radiometer
Magnatic ensor
Gravimeter
Fourier spectrum
passive
imaging
Monochrom
IR
Camera
TV camera
imaging
Sensor
types
scanning
image plane scanning
object plane scanning
Solid scanner
Optical mechanical scan.
Microwave radiometer
non-scaning non-imaging Microwave radiometer
Microwave altimeter
aktive
scanning imaging
image 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 target
end 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 field
of 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 difraction
grating 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:
Retrieval methods can be classified in three general categories:
Physical;
Statistical and;
Hybrid.
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.
Example:
z
L( z, )  L(0, ) ( z, )  
0

B(T )
dz
z
Estimation of L(z) from known temperature profıle, T,
is a forward problem, while estimation of T profile from
satellite measurement of L(z) is an invers problem.
Temperature Sounding Retrieval:
Physical Retrieval:
It is based on itiration of RTE by using first guess NWP
profile until the desired solution is optained.
Statistical Retrievals:
The siplest is to make regression between radiosonde
sounding called training data and measured radiance.
Hybrid Retrieval:
RTE can be written in the dicrete formas:
j 1
Li  L0i   BijWij
j 1
After putting surface contribution into
Summation and replacing Li by Ri.
j
Ri   BjWij
in maxrix notation
R  W .B
j 1
Where W is a matrix containing discrete weighting function.
Assuming linearity and introducing basis function B   .b
R  W . .b  A.b
by matrix inversion;
b=A-1R
(...)
Which is an exact solution of RTE. However not a
satisfactory solution because it is ill conditioned;
smaller error in R results in larger errors in B.
Trying least square fit of Σ(R-Σab)2 gives the solution as:
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 on
cross correlation method of successive three images.
center of search area
cc pick
Wind wector VH+1/2
VH-1/2
96x96
Image 1 (H-1/2)
32x32 pixel
H
Image 2
(target image)
cc pick
96x96
Image 3 (H+1/2)
First, cross-correlation matrix produced between target image and
Last image (H+1/2) to find the correlation peak .
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
V
VH+1/2
Height assignment: After finding the most contributing cloud
cluster to cc peak, its top temperature is used to assign height to
V by means of NWP forecast temperature profile. CO2 slicing
method can also be used for height assignment gives better
solution.
Sea surface wind speed and direction are estimated by means
of scatterometer which works like a radar.
Tracking winds can also be retrieved from atmospheric
soundings data.
CO2 Slicing:
Radiance measurements around 15m CO2 band ( e.g HIRS
sounder) allows to detect clouds at various level. In the center
of band, upper level clouds and at the wings lower level clouds
are 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=Lcd+(1-)Lcl where  is a fractional cloud cover,
Lcd is radiance from clouds and Lcl is radiance from clear air.
The cloud radiance is given by Lcd=Lbcd+(1-)Lcl
where Lbcd is radiance from completely opaque cloud.
After expresing Lcl and Lbcd in the form of RTE and some
algebric manipulation, it becomes:
Pc
L , cl  L     ( p )dB
ps
(…)
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 14.2m/14.0m or 14.2m/13.3m), it becomes:
L 1
B 1

L 2
B 2
(…)
The optimum cloud pressure is obtained when the absolute
differences of right hand side and left hand side is minimum.
Once the cloud height is determined, an effective cloud amount
can be estimated by using IR window channel.
Sat measurement.
L  Lcl
w 
B(T ( pc))  Lcl
Calculated from temp profile
Opaque cloud radiance.
(…)
Clouds:
Objective cloud retrieval is carried out by using imaging
radiometers 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.
230
Kernels of the classes are:
220
Cloud id
VIS count
High clouds
Cirrus thin
Medium clouds
Low clouds
Warm land
Sea
200
IR
180
150
64
32
44
56
28
16
130
10
30
50 90
VIS
110
Figure_... Bi-dimentional histogram of IR vs VIS
IR Count
219
108
185
166
145
153
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 temperature
by 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 two
channel see the same absorbers but in different amount.
The aim of the split window is to correct atm attenuation (mostly
due to water vapour) to estimate better surface temperature.
The siplit window equation driven as follows: in the windows
region transmitance can be expressed as  = exp(-ku), using
talor expention, = 1-ku and d=-kdu then RTE becomes
u
L  L( s)(1  ku)  k  B(T )du
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
Ts  T 1
k1

Ts  T 2
k 2
(…)
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.
Orbits:
The motion of the satellites around the earth is governed
by the Newton’s law of motion.
The attractive force is:
r
satellite
earth
Satellite orbit
gMem
F 
r2
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:
mv2 gMem

r
r2
2
4

T2 
r3
gMe
introducing period T=2r/v and
substituting in the equation we get:
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 position
given by:
•Semimajor axis,a (km);
•Eccentricity,e;
•Inclination,i (degree);
•Right ascensing of ascending node,  (degree);
•Argument of perigee,  (degree);
•And true anomaly, , degree.
Z
spacecraft
apogee
earth
a
Orbit plane

perigee
Y

Ascending node

i
Equator plane
X
Ascending node
For the meteorological satellites, mainly two orbits are used:
sunsynchronous and geostationary.
Sunsynchronous orbit have high inclination angle (e.g
98.7 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 satellites
drift from the desired orbit so that periodic orbit manoeuvres are
needed in the east-west and north-south directions and vise versa.