Hello everyone and welcome to the 17th lecture of this introduction to remote sensing course
and especially we will be focusing on radar remote sensing and more on towards the SAR
interferometry or in short we call in SAR technique So far we have been discussing mainly
the passive remote sensing technique where or more optical where the illumination sources
generally shun but in radar remote sensing which is active remote sensing the sources
will be itself on the sensor or satellite
And as we know that microwave remote sensing it is having 1 centimetre to 1 metre in wavelength
and it senses in compasses both active and passive form of remote sensing So if we divide
this microwave remote sensing microwave remote sensing can also be divided as also earlier
mentioned in the beginning of this course as active microwave and passive microwave
remote sensing
Even for some time we had on passive microwave sensors on an passive micro region but because
of very coarse resolution those were never become very popular However active microwave
is very very popular remote sensing and especially because of SAR interferometry it has become
further popular The main reason is the longer wavelength that and it allows the of microwave
radiation which allows the penetration
So it can penetrate through cloud cover it can penetrate haze dust and except in where
rare weather phenomena like a heaviest rainfall when we are having otherwise because being
in a longer wavelength it can penetrate though all this metrological or atmospheric distortions
which we consider in other optical remote sensing and this detection of microwave energy
is almost in all weather and a day and night kind of technology
This penetration of microwave remote sensing allowed us in India to discover the old sources
of Saraswati river in Rajasthan because if you are having dry soil even microwave signals
can penetrate up to few metres and if there were the moisture was found that has been
detected So old course of Saraswati River which still had some water supply through
its old channels and today on the surface though you see just sand or sand dunes but
on the top in the in beneath that sand are below few metres there old courses were present
and those were detected using remote sensing microwave remote sensing or radar remote sensing
So this is the advantage of having longer wavelength in penetration capabilities of
this one
As a we have seen this one earlier also that in active remote sensing in micro like radar
remote sensing we instruments and the pulse in like here in aircraft is shown but we can
assume also in a satellite sense the and whatever the back is catered after interacting with
different objects of the surface of the earth returned and these are recorded and based
on this the distance are measured but still basically a ranging technique and then we
then we ultimately create though complex numbers we create an image which is we call as radar
image so likewise with the time we that travel time it gives us the distance of different
objects Whereas in passive microwave the illumination source is generally sun or natural emissions
and can give a problem Here it is shown that if there are clouds then you are having problem
in passive microwave
But in a normal remote sensing even so now in just slightly about the passive microwave
rather than active microwave that passive microwave as I have said the sensor detects
the naturally emitted microwave energy related to temperature and moisture properties of
the emitting objects of surface within the field of view Passive microwave sensors are
typically radiometer scanners and antenna is used to detect and record microwave energy
but this energy is so small this energy is so small that high resolution passive microwave
images cannot be generated so easily and therefore in order to get sufficient energy which can
reach up to the satellite large area was involved and that means compromising on special resolution
and that is why I said like SSMI SSMR sensors were there on board of different missions
which were in the passive microwave region but they had relatively very poor special
resolution as I mentioned about 30 kilometre
So because of the less energy available to the sensor passive microwave never became
that popular so the energy available is very small as I have already mentioned compared
to optical wavelengths like in case of simple passive remote sensing thus field of view
must be large to detect the enough energy to record as a signal and the most passive
microwave sensors are therefore characterised by low special resolution and that became
a problem with them
The popular one is the active microwave where it provides their own source of microwave
radiation to illuminate the target and this source is coming basically from the sensor
itself so there are 2 types of sensors systems are exist one is imaging another one none
imaging the most common form of imaging active microwave sensor is radar which is what is
being used radar is a abbreviation which stands for Radio Detection And Ranging so basically
radar remote sensing or microwave remote sensing in short we say is a ranging technique and
based on that everything then later on are tried
So the sensor transmits a microwave signal radio signal towards the target if it is on
the on board of a satellite towards the earth which is having different objects present
and detects the back scattered portion of the signal and this strength of that back
scattered signal is measured to discriminate between different targets and time between
transmitted and reflected signals determines the distance or the range to the target So
the strength the changes in its strength and time it has taken these 2 parameters will
help us to make a make an image through a complex number
So here this is shown through an aircraft that airborne radar system also exists that
a pulse is sent towards the earth and then back scattered which is shown in this view
graph as a or in short we say eco also and that is recorded So this is transmitted this
is back scattered and likewise the aircraft or satellite keeps forward moving having movement
So radar is essentially ranging or distance measuring device it consists fundamentally
of a transmitter
It has to have its own illumination source a receiver and antenna and electronic system
to process and record the data and this transmitter generates successive short pulse or pulses
of microwave as soon at a regular intervals which are focused by antenna and into the
beam and this radar beam illuminates the surface Illuminate means the pulse is there and then
oblique and this is not Nadir viewing so far the passive remote sensing is always Nadir
viewing
In most of the cases except in generating stereo pairs but this is it is oblique because
if satellite moving like forward then it sense a pulse and when the back scatter comes back
by that time the satellite is there and to collect that back scatter otherwise a if it
is Nadir then it will move off and it will not be able to collect it unless we are having
a very huge antenna which is not possible in a space bond platforms
So the antenna receives a portion of transmitted energy reflected or back scattered from various
objects within the illuminated beam and they say then by measuring the timely between the
transmission of the pulse and the reception of the back scattered are also called eco
from different target their distance is from the radar and thus their location can be determined
And as sensor platform moves forward recoding and processing of back scattered signals build
up a 2 dimensional image of a complex numbers of that part of the area or that part of
As a there are different radar frequency bands which are which have been used like L band
which is located here then you are having S band then C band and X band so C band and
X band have become very popular but there are different satellites are having different
bands which we will see which are still operational then you are having KUKK and W bands are there
the most popular one are the C and X bands
So microwave region of spectrum is quite large as you can see from 1 centimetre to you know
1 metre and in that way in quite large to visible and infrared there are several wavelengths
range or bands commonly used in this and this the the basic advantage the major advantage
is the penetration so penetration is the key factor for selecting the wavelength which
band C band or X band depending on our requirements accordingly the sensors are designed the longer
the wavelength
If we go in this example right way the smaller the frequency is going to be the stronger
the penetration into vegetation and soil especially dry vegetation Soil and sand and I gave the
example of discovering the lost course of Saraswati River over dry sand If it is having
water then dry electric constant are different than energy is observed and then detection
the penetration does not become possible
So different satellite like airborne AESR or this Alos Pulsar which is still operational
Alos Pulsar too is working it is n L band so it is on relatively in a lesser wavelength
and then like a sea band as I mentioned is very popular and the first two radar remote
sensing satellite was radar sat then ERS Envisat these were the European satellite which has
the ASAR sensor then Risat is our own Indian remote sensing satellite radar remote sensing
satellite and Sentinal these are still working in sea band these are still working
Alos 2 is also working and then X band in the TERRA which is also being used and then
some some bands are purely reserved for army or military purposes like KKU bands and other
things so mainly L band because of Alos Palsar or C band or X bands these are the very common
bands of radar remote sensing which are being used here
Now these values are important because when we go for SAT interferometry we see that what
is the the the wavelength in a particular band because this wavelength we divide by
2 in order to measure the deformation which has taken place so one has to remember that
what wavelength is Indian bands are having against a satellite So like sentinel if I
am using sentinel data in interferogram if I have to estimate the ground deformation
then I will divide this value Basically it is not exactly 5 in case of this Envisat or
in Sentinel it was 5.6 centimetre so where it is located that value exact value has to
be used and then divided by 2 and count a 1 width of a fringe
So microwave remote sensing in this respective of wavelength radar signals can transmit horizontal
or vertical electric field vectors and they receive either horizontal or vertical return
view so there can be different combinations in it There are certain advantages uh and
the basic physical processes responsible for like polarise HH or VV return are quasi-specular
surface reflection and for example like calm water that is without any wave or disturbances
will appear black in such situations and we can also as a cross polarise also possible
return is usually but this kind of back scatter is very poor so in certain applications one
can think in that but generally cross polarisations are not used
4 combinations one is horizontal HH another one is VV or we can have in combination that
we can have HV or VH for vertical that like is when we say HS means the transmission and
receiving both are in the same and like in horizontal transmit horizontal receive vertical
transmit vertical receive horizontal transmit vertical receive or in reverse is also possible
So depending on the requirements and design of sensors these are decided and once they
are decided for a particular sensor it is fixed throughout the life of that sensor
So radar as a but how that image is created that as I have mentioned that each pixel give
a complex number it is not a simple image like we have seen in passive remote sensing
that you are having a pixel and it is having a digital number which might be reflection
value or might be emitted value It is not like that here radar image each pixel is a
complex number and which (comple) that complex number contains 2 major information one is
amplitude another one is phase information and about the microwave field scattered by
the scatters like caterers like rocks vegetation building etc
So different objects will scatter differently so doing the corresponding resolutions and
projected on the ground so this this is very important to note that in normal passive remote
sensing the pixel value is indicate a just having one value but here is a complex number
having a amplitude information as well as phase information So different rows of image
are associated with different azimuth locations because for every role the azimuth location
is hanging because satellite is also moving and whereas different columns indicate different
slant range location we will see through a diagram
That this is what the slant range is called if flight path is like this this is the slant
range oblique view is there this is the height that is altitude and this is the range projected
on a horizontal plain So this is the band or the main width or the swath we call so
only this much strip of the part of the earth will be covered and there the furthest part
is called far swath the nearest is called the near range or near swath or middle swath
and then this is the incident angle because it is a complex number so all these things
are very very important here
Now interferometry interferometry requires that the same area should have been looked
at least twice maybe from different angle or maybe from the same angle but there might
be in between changes on the ground so if this happens then we can measure or estimate
the ground deformation which has taken or changes on the ground has taken place within
those 2 dates when the 2 data sats have been acquired
So in this one that is the synthetic aperture radar the satellites SAR can observe the same
area from slightly different look angles it is very difficult in a space bond remote sensing
that you can visit or revisit the same area with same look angle There might be some changes
does not matter these things can be processed and still deformation estimation or change
estimations can be done very accurately
So this can either be done simultaneously this is also possible this was done in case
of this SRTM Settle Radar Topographic Mission instead of revisiting the same site a mass
long pool was put on the on the spacecraft itself and it was taking the it was looking
the same area or making a radar image from 2 different angles and that data was used
to create a digital elevation model
So that is also possible but that was an mission only it lasted for about 20 days it covered
almost entire globe and purpose was solved but having such a long mass is not possible
all the time on the spacecraft so different (ti) or different times rather than simultaneously
at one time with 2 different angles or at different times by exploiting repetitive orbits
of the same that is for the change detection
So interferometry SAR or in short as I have mentioned we also is known now as a in SAR
allows accurate measurement of radiation travel path because of its coherence Coherence is
very much required for this this kind of change detection using radar remote sensing especially
in SAR The measurements of travel path variations as a function of satellite position and time
of acquisition allow generation of digital elevation model like in Settle Radar Topographic
mission but otherwise also digital elevation model at very high resolution can also be
generated and measurement of centimetric surface deformations of the earth This is very very
important of a millimetre accuracy deformations of the earth can be measured we will see some
examples as well
How it is done basically suppose there is orbit one the satellite next time it is orbiting
may not be from exactly from same location in both these overpass it has covered an area
this much area and this is the strip Now using this data and if they this is the perpendicular
base line and this is very important if this perpendicular base line is very less that
means the both during the both visits the satellite was almost in the same location
then we can achieve better coherence and whatever the deformations which has taken place on
ground maybe induced by an earthquake or maybe flooding landslide subsidence any other such
regions can be measured or very accurately So this is that is why this perpendicular
base line is very very important and this is how in 2 visits of the same satellite can
allow us to create interferograms
The distance between two satellites as mentioned here or orbits in the plain perpendicular
the orbit is called the interferometric base line and its cross backs in perpendicular
to this is interferometric base line in black and red one is the perpendicular base line
Perpendicular base line becomes very important for any deformation studies
And this SAR interferograms is generated by cross multiplying pixel by pixel the first
SAR image with the complex conjugate of the 2nd and the again here the phase difference
because a this complex number or if this if I say very in a normal term the pixel of a
radar image there is getting 2 values 1 is amplitude another one is your phase so I can
I can use either one so if I use the phase of image 1 and image 2 then the phase difference
can give me the deformation information so this is how it is achieved Thus the interferogram
amplitude is the amplitude of the first image multiply by that of the second one whereas
it phase the interferometric phase is the phase difference between the images
The example here in which this is Envisat data and of your sea band and the wavelength
was 5.6 metre and here sorry 5.6 centimetre and here 2 images having baseline difference
of only point 6 metre very ideal were acquired and of 3rd December 2003 and 11th February
2004 and in between an earthquake on 26th December 2003 of magnitude 6.6 which is known
as Bam earthquake of Iran occurred
Now using this 2 SAR images interferograms were generated and all these fringes are telling
that how much deformation and where deformation is taking place Now this color pattern these
have to be interpreted but there are automatic wave also so like if I get from if I from
the centre I have to see the colors if I see that cyan magenta cyan yellow magenta this
kind of scheme I have found then I say that this is probably the subsidence which has
occurred here so like in this example if I start from centre to away where I am having
less concentration of in fringes then cyan yellow magenta so that means this area must
have gone subsided
Whereas (megen) like if I start here magenta yellow cyan magenta yellow cyan that means
this area must have visit and half the wavelength so 5.6 centimetre was the wavelength of Envisat
SAR sensor so half the wavelength is counted so if I count number of these fringes multiply
by 2.6 or 2.8 half the wavelength that is 5.6 divided by 2 2.8 then I get exact deformation
and this can also be done using some software’s so deformation maps was also generated in
that case
Now wherever wherever the (fing) if no changes has occurred then it will show no fringes
will be observed but if the ground has got deformations then fringes will have a far
distances and wherever the more concentration or steep changes have occurred on the ground
then we will see very close fringes like in 3 examples no changes no deformation some
deformation and very sharp deformation and they are also demonstrated through these models
So this this interferogram was later as I mentioned was converted through the (int)
you know computer based interpretation in a form of deformation map and as you can see
that this area subsided by 20 centimetre and this area got uplifted because of that earthquake
by 30 centimetre So this is these are the plus values in terms of before earthquake
and after earthquake so after earthquake this area got uplifted by 30 centimetre and this
green area got subsided by 20 centimetre
So this is very unique powerful technique that is the in SAR technique to exploit or
to measure a such deformations which otherwise are impossible to measure and as you know
I have been saying in this course of remote sensing that remote sensing not only provides
the synoptic view not only provides the digital data but is a completely unbiased recording
and these satellites are orbiting continuously acquiring the data if an such even occurs
I get just we have to get the pre earthquake data as close as possible and post-earthquake
data
And once these data is there coherence is available then things can be why the less
coherence was available in these white patches because because of the these areas have the
buildings or built up areas and that got completely wiped out because of that earthquake So that
is why there were less coherence otherwise no problem the still this can be measured
and all of a sudden these fringes stopped here all of a sudden these fringes because
there is a fault line was going something like this only roughly north south in this
and the epicentre was on the other side
So the major deformations have taken place like this some area has got subsided and there
some area has got uplifted and this is remember this is in against the range or slant range
so that means what we can conclude here that this area the distance because after all radar
remote sensing is a ranging so from the between these 2 scenes and the baseline difference
in this particular case was only point 6 metre so that means thus the this area has gone
away the green area has gone away by 20 centimetre from the satellite and this area has come
close to 30 centimetre of the satellite
Similarly people have used the these things SAR interferometric technique for land subsidence
due to ground water and this is the example of Kolkata city where in between 92 to 98
in roughly 6 yeas time about 5 to or about 6 millimetre per year subsidence have been
observed in this part of the country So subsidence or ground deformations might be earthquake
landslide ground water mining all these now is possible to measure
Another very important thing is earlier like Envisat data Radarsat data even Alos Palsar
these data becomes were very expensive but the Sentinel 1 data is free and it is providing
interferometric data Risat they say radar remote sensing but it does not have the capability
of providing interferogram interferometric data for interferogram but the Sentinel data
is free provides the data one can wherever one find problem being related to ground deformation
induced by some factor then one can imply and estimate this thing
And the last one here is example of recently occurred major earthquake in Nepal there were
2 successive earthquake and using Alos Palsar data this this is how the fringes were generated
and the subsidence the upliftment of about 1 metre in Kathmandu valley was induced by
that that particular earthquake of 25th April 2015 of magnitude 7.8 so this has become very
very popular technique of remote sensing where very accurately deformations can be measured
So this brings to the end of this presentation thank you very much
Hello everyone and welcome to the 17th lecture of this introduction to remote sensing course
and especially we will be focusing on radar remote sensing and more on towards the SAR
interferometry or in short we call in SAR technique So far we have been discussing mainly
the passive remote sensing technique where or more optical where the illumination sources
generally shun but in radar remote sensing which is active remote sensing the sources
will be itself on the sensor or satellite
And as we know that microwave remote sensing it is having 1 centimetre to 1 metre in wavelength
and it senses in compasses both active and passive form of remote sensing So if we divide
this microwave remote sensing microwave remote sensing can also be divided as also earlier
mentioned in the beginning of this course as active microwave and passive microwave
remote sensing
Even for some time we had on passive microwave sensors on an passive micro region but because
of very coarse resolution those were never become very popular However active microwave
is very very popular remote sensing and especially because of SAR interferometry it has become
further popular The main reason is the longer wavelength that and it allows the of microwave
radiation which allows the penetration
So it can penetrate through cloud cover it can penetrate haze dust and except in where
rare weather phenomena like a heaviest rainfall when we are having otherwise because being
in a longer wavelength it can penetrate though all this metrological or atmospheric distortions
which we consider in other optical remote sensing and this detection of microwave energy
is almost in all weather and a day and night kind of technology
This penetration of microwave remote sensing allowed us in India to discover the old sources
of Saraswati river in Rajasthan because if you are having dry soil even microwave signals
can penetrate up to few metres and if there were the moisture was found that has been
detected So old course of Saraswati River which still had some water supply through
its old channels and today on the surface though you see just sand or sand dunes but
on the top in the in beneath that sand are below few metres there old courses were present
and those were detected using remote sensing microwave remote sensing or radar remote sensing
So this is the advantage of having longer wavelength in penetration capabilities of
this one
As a we have seen this one earlier also that in active remote sensing in micro like radar
remote sensing we instruments and the pulse in like here in aircraft is shown but we can
assume also in a satellite sense the and whatever the back is catered after interacting with
different objects of the surface of the earth returned and these are recorded and based
on this the distance are measured but still basically a ranging technique and then we
then we ultimately create though complex numbers we create an image which is we call as radar
image so likewise with the time we that travel time it gives us the distance of different
objects Whereas in passive microwave the illumination source is generally sun or natural emissions
and can give a problem Here it is shown that if there are clouds then you are having problem
in passive microwave
But in a normal remote sensing even so now in just slightly about the passive microwave
rather than active microwave that passive microwave as I have said the sensor detects
the naturally emitted microwave energy related to temperature and moisture properties of
the emitting objects of surface within the field of view Passive microwave sensors are
typically radiometer scanners and antenna is used to detect and record microwave energy
but this energy is so small this energy is so small that high resolution passive microwave
images cannot be generated so easily and therefore in order to get sufficient energy which can
reach up to the satellite large area was involved and that means compromising on special resolution
and that is why I said like SSMI SSMR sensors were there on board of different missions
which were in the passive microwave region but they had relatively very poor special
resolution as I mentioned about 30 kilometre
So because of the less energy available to the sensor passive microwave never became
that popular so the energy available is very small as I have already mentioned compared
to optical wavelengths like in case of simple passive remote sensing thus field of view
must be large to detect the enough energy to record as a signal and the most passive
microwave sensors are therefore characterised by low special resolution and that became
a problem with them
The popular one is the active microwave where it provides their own source of microwave
radiation to illuminate the target and this source is coming basically from the sensor
itself so there are 2 types of sensors systems are exist one is imaging another one none
imaging the most common form of imaging active microwave sensor is radar which is what is
being used radar is a abbreviation which stands for Radio Detection And Ranging so basically
radar remote sensing or microwave remote sensing in short we say is a ranging technique and
based on that everything then later on are tried
So the sensor transmits a microwave signal radio signal towards the target if it is on
the on board of a satellite towards the earth which is having different objects present
and detects the back scattered portion of the signal and this strength of that back
scattered signal is measured to discriminate between different targets and time between
transmitted and reflected signals determines the distance or the range to the target So
the strength the changes in its strength and time it has taken these 2 parameters will
help us to make a make an image through a complex number
So here this is shown through an aircraft that airborne radar system also exists that
a pulse is sent towards the earth and then back scattered which is shown in this view
graph as a or in short we say eco also and that is recorded So this is transmitted this
is back scattered and likewise the aircraft or satellite keeps forward moving having movement
So radar is essentially ranging or distance measuring device it consists fundamentally
of a transmitter
It has to have its own illumination source a receiver and antenna and electronic system
to process and record the data and this transmitter generates successive short pulse or pulses
of microwave as soon at a regular intervals which are focused by antenna and into the
beam and this radar beam illuminates the surface Illuminate means the pulse is there and then
oblique and this is not Nadir viewing so far the passive remote sensing is always Nadir
viewing
In most of the cases except in generating stereo pairs but this is it is oblique because
if satellite moving like forward then it sense a pulse and when the back scatter comes back
by that time the satellite is there and to collect that back scatter otherwise a if it
is Nadir then it will move off and it will not be able to collect it unless we are having
a very huge antenna which is not possible in a space bond platforms
So the antenna receives a portion of transmitted energy reflected or back scattered from various
objects within the illuminated beam and they say then by measuring the timely between the
transmission of the pulse and the reception of the back scattered are also called eco
from different target their distance is from the radar and thus their location can be determined
And as sensor platform moves forward recoding and processing of back scattered signals build
up a 2 dimensional image of a complex numbers of that part of the area or that part of
As a there are different radar frequency bands which are which have been used like L band
which is located here then you are having S band then C band and X band so C band and
X band have become very popular but there are different satellites are having different
bands which we will see which are still operational then you are having KUKK and W bands are there
the most popular one are the C and X bands
So microwave region of spectrum is quite large as you can see from 1 centimetre to you know
1 metre and in that way in quite large to visible and infrared there are several wavelengths
range or bands commonly used in this and this the the basic advantage the major advantage
is the penetration so penetration is the key factor for selecting the wavelength which
band C band or X band depending on our requirements accordingly the sensors are designed the longer
the wavelength
If we go in this example right way the smaller the frequency is going to be the stronger
the penetration into vegetation and soil especially dry vegetation Soil and sand and I gave the
example of discovering the lost course of Saraswati River over dry sand If it is having
water then dry electric constant are different than energy is observed and then detection
the penetration does not become possible
So different satellite like airborne AESR or this Alos Pulsar which is still operational
Alos Pulsar too is working it is n L band so it is on relatively in a lesser wavelength
and then like a sea band as I mentioned is very popular and the first two radar remote
sensing satellite was radar sat then ERS Envisat these were the European satellite which has
the ASAR sensor then Risat is our own Indian remote sensing satellite radar remote sensing
satellite and Sentinal these are still working in sea band these are still working
Alos 2 is also working and then X band in the TERRA which is also being used and then
some some bands are purely reserved for army or military purposes like KKU bands and other
things so mainly L band because of Alos Palsar or C band or X bands these are the very common
bands of radar remote sensing which are being used here
Now these values are important because when we go for SAT interferometry we see that what
is the the the wavelength in a particular band because this wavelength we divide by
2 in order to measure the deformation which has taken place so one has to remember that
what wavelength is Indian bands are having against a satellite So like sentinel if I
am using sentinel data in interferogram if I have to estimate the ground deformation
then I will divide this value Basically it is not exactly 5 in case of this Envisat or
in Sentinel it was 5.6 centimetre so where it is located that value exact value has to
be used and then divided by 2 and count a 1 width of a fringe
So microwave remote sensing in this respective of wavelength radar signals can transmit horizontal
or vertical electric field vectors and they receive either horizontal or vertical return
view so there can be different combinations in it There are certain advantages uh and
the basic physical processes responsible for like polarise HH or VV return are quasi-specular
surface reflection and for example like calm water that is without any wave or disturbances
will appear black in such situations and we can also as a cross polarise also possible
return is usually but this kind of back scatter is very poor so in certain applications one
can think in that but generally cross polarisations are not used
4 combinations one is horizontal HH another one is VV or we can have in combination that
we can have HV or VH for vertical that like is when we say HS means the transmission and
receiving both are in the same and like in horizontal transmit horizontal receive vertical
transmit vertical receive horizontal transmit vertical receive or in reverse is also possible
So depending on the requirements and design of sensors these are decided and once they
are decided for a particular sensor it is fixed throughout the life of that sensor
So radar as a but how that image is created that as I have mentioned that each pixel give
a complex number it is not a simple image like we have seen in passive remote sensing
that you are having a pixel and it is having a digital number which might be reflection
value or might be emitted value It is not like that here radar image each pixel is a
complex number and which (comple) that complex number contains 2 major information one is
amplitude another one is phase information and about the microwave field scattered by
the scatters like caterers like rocks vegetation building etc
So different objects will scatter differently so doing the corresponding resolutions and
projected on the ground so this this is very important to note that in normal passive remote
sensing the pixel value is indicate a just having one value but here is a complex number
having a amplitude information as well as phase information So different rows of image
are associated with different azimuth locations because for every role the azimuth location
is hanging because satellite is also moving and whereas different columns indicate different
slant range location we will see through a diagram
That this is what the slant range is called if flight path is like this this is the slant
range oblique view is there this is the height that is altitude and this is the range projected
on a horizontal plain So this is the band or the main width or the swath we call so
only this much strip of the part of the earth will be covered and there the furthest part
is called far swath the nearest is called the near range or near swath or middle swath
and then this is the incident angle because it is a complex number so all these things
are very very important here
Now interferometry interferometry requires that the same area should have been looked
at least twice maybe from different angle or maybe from the same angle but there might
be in between changes on the ground so if this happens then we can measure or estimate
the ground deformation which has taken or changes on the ground has taken place within
those 2 dates when the 2 data sats have been acquired
So in this one that is the synthetic aperture radar the satellites SAR can observe the same
area from slightly different look angles it is very difficult in a space bond remote sensing
that you can visit or revisit the same area with same look angle There might be some changes
does not matter these things can be processed and still deformation estimation or change
estimations can be done very accurately
So this can either be done simultaneously this is also possible this was done in case
of this SRTM Settle Radar Topographic Mission instead of revisiting the same site a mass
long pool was put on the on the spacecraft itself and it was taking the it was looking
the same area or making a radar image from 2 different angles and that data was used
to create a digital elevation model
So that is also possible but that was an mission only it lasted for about 20 days it covered
almost entire globe and purpose was solved but having such a long mass is not possible
all the time on the spacecraft so different (ti) or different times rather than simultaneously
at one time with 2 different angles or at different times by exploiting repetitive orbits
of the same that is for the change detection
So interferometry SAR or in short as I have mentioned we also is known now as a in SAR
allows accurate measurement of radiation travel path because of its coherence Coherence is
very much required for this this kind of change detection using radar remote sensing especially
in SAR The measurements of travel path variations as a function of satellite position and time
of acquisition allow generation of digital elevation model like in Settle Radar Topographic
mission but otherwise also digital elevation model at very high resolution can also be
generated and measurement of centimetric surface deformations of the earth This is very very
important of a millimetre accuracy deformations of the earth can be measured we will see some
examples as well
How it is done basically suppose there is orbit one the satellite next time it is orbiting
may not be from exactly from same location in both these overpass it has covered an area
this much area and this is the strip Now using this data and if they this is the perpendicular
base line and this is very important if this perpendicular base line is very less that
means the both during the both visits the satellite was almost in the same location
then we can achieve better coherence and whatever the deformations which has taken place on
ground maybe induced by an earthquake or maybe flooding landslide subsidence any other such
regions can be measured or very accurately So this is that is why this perpendicular
base line is very very important and this is how in 2 visits of the same satellite can
allow us to create interferograms
The distance between two satellites as mentioned here or orbits in the plain perpendicular
the orbit is called the interferometric base line and its cross backs in perpendicular
to this is interferometric base line in black and red one is the perpendicular base line
Perpendicular base line becomes very important for any deformation studies
And this SAR interferograms is generated by cross multiplying pixel by pixel the first
SAR image with the complex conjugate of the 2nd and the again here the phase difference
because a this complex number or if this if I say very in a normal term the pixel of a
radar image there is getting 2 values 1 is amplitude another one is your phase so I can
I can use either one so if I use the phase of image 1 and image 2 then the phase difference
can give me the deformation information so this is how it is achieved Thus the interferogram
amplitude is the amplitude of the first image multiply by that of the second one whereas
it phase the interferometric phase is the phase difference between the images
The example here in which this is Envisat data and of your sea band and the wavelength
was 5.6 metre and here sorry 5.6 centimetre and here 2 images having baseline difference
of only point 6 metre very ideal were acquired and of 3rd December 2003 and 11th February
2004 and in between an earthquake on 26th December 2003 of magnitude 6.6 which is known
as Bam earthquake of Iran occurred
Now using this 2 SAR images interferograms were generated and all these fringes are telling
that how much deformation and where deformation is taking place Now this color pattern these
have to be interpreted but there are automatic wave also so like if I get from if I from
the centre I have to see the colors if I see that cyan magenta cyan yellow magenta this
kind of scheme I have found then I say that this is probably the subsidence which has
occurred here so like in this example if I start from centre to away where I am having
less concentration of in fringes then cyan yellow magenta so that means this area must
have gone subsided
Whereas (megen) like if I start here magenta yellow cyan magenta yellow cyan that means
this area must have visit and half the wavelength so 5.6 centimetre was the wavelength of Envisat
SAR sensor so half the wavelength is counted so if I count number of these fringes multiply
by 2.6 or 2.8 half the wavelength that is 5.6 divided by 2 2.8 then I get exact deformation
and this can also be done using some software’s so deformation maps was also generated in
that case
Now wherever wherever the (fing) if no changes has occurred then it will show no fringes
will be observed but if the ground has got deformations then fringes will have a far
distances and wherever the more concentration or steep changes have occurred on the ground
then we will see very close fringes like in 3 examples no changes no deformation some
deformation and very sharp deformation and they are also demonstrated through these models
So this this interferogram was later as I mentioned was converted through the (int)
you know computer based interpretation in a form of deformation map and as you can see
that this area subsided by 20 centimetre and this area got uplifted because of that earthquake
by 30 centimetre So this is these are the plus values in terms of before earthquake
and after earthquake so after earthquake this area got uplifted by 30 centimetre and this
green area got subsided by 20 centimetre
So this is very unique powerful technique that is the in SAR technique to exploit or
to measure a such deformations which otherwise are impossible to measure and as you know
I have been saying in this course of remote sensing that remote sensing not only provides
the synoptic view not only provides the digital data but is a completely unbiased recording
and these satellites are orbiting continuously acquiring the data if an such even occurs
I get just we have to get the pre earthquake data as close as possible and post-earthquake
data
And once these data is there coherence is available then things can be why the less
coherence was available in these white patches because because of the these areas have the
buildings or built up areas and that got completely wiped out because of that earthquake So that
is why there were less coherence otherwise no problem the still this can be measured
and all of a sudden these fringes stopped here all of a sudden these fringes because
there is a fault line was going something like this only roughly north south in this
and the epicentre was on the other side
So the major deformations have taken place like this some area has got subsided and there
some area has got uplifted and this is remember this is in against the range or slant range
so that means what we can conclude here that this area the distance because after all radar
remote sensing is a ranging so from the between these 2 scenes and the baseline difference
in this particular case was only point 6 metre so that means thus the this area has gone
away the green area has gone away by 20 centimetre from the satellite and this area has come
close to 30 centimetre of the satellite
Similarly people have used the these things SAR interferometric technique for land subsidence
due to ground water and this is the example of Kolkata city where in between 92 to 98
in roughly 6 yeas time about 5 to or about 6 millimetre per year subsidence have been
observed in this part of the country So subsidence or ground deformations might be earthquake
landslide ground water mining all these now is possible to measure
Another very important thing is earlier like Envisat data Radarsat data even Alos Palsar
these data becomes were very expensive but the Sentinel 1 data is free and it is providing
interferometric data Risat they say radar remote sensing but it does not have the capability
of providing interferogram interferometric data for interferogram but the Sentinel data
is free provides the data one can wherever one find problem being related to ground deformation
induced by some factor then one can imply and estimate this thing
And the last one here is example of recently occurred major earthquake in Nepal there were
2 successive earthquake and using Alos Palsar data this this is how the fringes were generated
and the subsidence the upliftment of about 1 metre in Kathmandu valley was induced by
that that particular earthquake of 25th April 2015 of magnitude 7.8 so this has become very
very popular technique of remote sensing where very accurately deformations can be measured
So this brings to the end of this presentation thank you very much