In the next one and a half hour, we will be having a discussion on basics of remote sensing. So, in this context, if we need to collect any kind of scientific data, we have two options with us. One is collection of the data in situ, that means go to the site itself, collect the samples, analyze it and interpret the results. This is one method of collection of data.
information which is scientific. This is also called field data collection. The another option with us is remote sensing. That means you are collecting the information by not going to the site itself but from a remote location.
So in this context we will be discussing the second option with us which is remote sensing and we will first try to answer the basic three questions of science. which is what why and how so coming to what is remote sensing it is both science as well as an art of obtaining information about three things that the three things can be an object let's say a building of interest an area let us say a city or a phenomena for example rainfall or cloudburst etc now To get the information about these three things, we require the analysis of the data acquired by device which is not in physical contact with the same. It is science because it the sensing is involving scientific principles and we build technology to do that.
It is art because the remote sensing interpretation depends upon the skill of the interpreter. So, collectively it is both science and art in fact it started as an art right so once we have understood what is remote sensing why should we go about it we have seen the first option with us was field measurements which is basically more accurate and can give you a real picture or the ground picture of the site whereas the second option is remote sensing which is having its own limitations then still we are going to discuss the subject in the next one hour or you will be discussing further topics in continuation that means there are some advantages of remote sensing over field based measurements the very first is systematic data collection so if i ask you to collect the samples from the ground let's say 10 or 20 people go to the ground take the samples analyze and interpret now this procedure might involve human errors it may also involve the differences in the sampling so the overall information can become subjective depending upon how much how the samples have been collected whereas remote sensing enables it gives you an option to have a systematic data collection which is let's say if it is a scanner so it's going in a particular fashion click click click and you collect the data in a systematic pattern the objectivity comes into the data collection the second very important advantage with remote sensing is it enables the three-dimension information of real objects all of us are using google earth these days and you can see the data coming from stereo satellites is giving us the spectra pictures of three dimensions of real objects. Then another advantage with remote sensing is repetability in both space as well as time. So we can revisit a particular area after a particular interval of time depending upon which type of satellites or platforms we are using. So same area can be covered in different points of time which is repeatability.
Then comes a very important advantage which is global coverage. For several applications we require the data at a global scale as a whole. So that is also supported by remote sensing. The only solution sometimes for the otherwise inaccessible areas for example let's say there is a disaster in some region.
Now in order to collect information we can't be risking lives to collect field measurements right so in that case remote sensing becomes the only possible solution There are a lot of other advantages also associated with remote sensing, but by and large, it gives you a multi-purpose information. That means the same data which is collected by your satellite or by aerial platforms or by UAVs, etc. That can be interpreted or analyzed in a differential manner by, let's say, a person of agriculture or from forestry or from photogrammetry.
or from marine science or from atmospheric science etc. So same information but it can have multiple purposes. However, it is not everything.
That means we cannot do away with the field measurements. In order to calibrate the instruments before sending them on board, it has to be first calibrated with respect to the ground targets. Or field measurements. So for calibration as well as once the data is achieved by a remote sensing instrument it has to be analyzed and it has to be assessed for its accuracy with respect to field measurements. The field measurements are also called ground truth because that's the best possible measurement of any phenomena that you can have.
So the measurements or the analysis by remote sensing has to be validated with respect to the field measurements so both for calibration as well as for validation we are still dependent on field measurements plus some topics of research are better addressed by using field measurements for example for subsurface analysis up to a certain extent remote sensing is helpful but beyond that we are still dependent on ground measurements. So, as long as remote sensing survives the field measurements have to survive. Now, when we understand that there are several advantages of remote sensing over field measurements, let us see how to have this process happen. So, to enable remote sensing process, we need to have certain components to realize it.
So the very first component is the source of electromagnetic radiation. So it can be natural source or it can be a source on the platform itself. So it has to be a source of electromagnetic energy.
Now this source illuminates your target, the target that whose properties you want to assess or analyze. So you need to have a target you need to have a source of light that illuminates the target. Now the target depending upon its properties will be reflecting or it will be absorbing and then re-emitting or if it is having some transparency it will be transmitting the radiation.
Now let's say one of these components reaches your sensor. The sensor here has to be a combination of your detector that is your camera or any sensor plus the platform that can carry that sensor so sensor alone won't be able to function it has to be supported by a proper platform this platform can be space based it can be airborne it can be ground based also so you need to have a sensor mounted on a platform after that once the data is collected by the sensor on board it has to be transmitted to the ground receiving station. So, for let us say this picture or this cartoon depicts the example of a space based measurement. In that case whenever your ground receiving station or the antennas here come in line of sight that is called locking. Whenever this comes in line of sight of the satellite that time the data will be.
transmitted to the ground receiving terminal. In India we have this at Shardnagar. Now once the data is collected here at ground receiving station it has to be pre-processed.
What does pre-processing mean here? Means we free the data from some known errors. This can be geometric correction, this can be radiometric correction or it can be just the conversion of the data into a usable format.
by the user. So some pre-processing some level of pre-processing is done here and you can get different levels of products the raw data usually comes as level 0 then some sort of corrections applied to it comes as level 1 followed by level 2 with some further processing applied to it let's say it can be a product and level 3 is generally gridded products right so different levels of products are available For each satellite you can browse through what are the levels defined by the satellite providers. It can vary from one satellite to another. Then once the data is converted to a usable format here it is given to different applications or the users.
Another point to notice here is for earth based observations there is also an atmosphere that covers the earth and the signals both while incoming from the source to the target as well as from target to the sensor encounters this interaction with the atmosphere. Okay, so this may be wanted, this may be unwanted. Unwanted when we are looking at the earth's surface targets.
Wanted when we are looking at specific properties of atmosphere. So for atmospheric based remote sensing we are concerned with these interactions and for earth surface based interactions we would like to remove this noise from the signal whatever alterations atmosphere has done to the signal that part we would like to correct for so that that's where atmospheric correction comes in so as a whole what we have is we have to have a source of light we have to have a target We need to have a sensor plus a platform. We need to have a ground receiving station. We also need to pre-process the data and then we can use it for our applications. This example showed here that light, a streak of light is falling on the target.
Part of it is getting reflected and is going to the sensor. So this type of sensing where The source of light is not on the platform itself. For example, sun here.
It's called passive remote sensing. Also, in this case, because we are observing the reflections from the target, this is also known as reflective remote sensing. Another way of remote sensing could be this dotted blue line.
You can see here, it is a dotted line but with double arrows. That means the platform is... carrying a source on board so the source on the platform illuminates the target that means you can control the illumination you can control the source of light both during day as well as night so night and daytime remote sensing is possible with this and you can receive the backscatter signal and record it so this type of remote sensing where the source you light or the source of illumination is used which is carried on board is called active remote sensing right there is another form of remote sensing you can see this where the light is coming from the target there is no incoming light arrow right so this is an example of emissive remote sensing see we all the objects on which are above zero kelvin are emitting some source of ready some amount of radiation for earth this amount this radiation falls maximum in the thermal regime of electromagnetic spectrum So that's why this Emissive remote sensing sometimes is also called thermal remote sensing here You can see that the target itself becomes the source right because we are also source of energy.
We are also emitting some form of energy. If the sensor is able to capture that form of energy then it is called emissive remote sensing. But it is also passive remote sensing because we are not using any source on board.
So this red lines are examples of passive remote sensing and this blue dotted line with double arrow is an illustration of active remote sensing. So with this we also get an a purview of what kinds of remote sensing are possible. In the following class maybe tomorrow or day after you will also come across in detail. how active remote sensing is different from passive remote sensing which one is costlier what are the pros and cons etc but in today's class you just try to understand that based on what we are sensing and how we are sensing we have different categories of remote sensing we have reflective remote sensing we have emissive remote sensing there is passive remote sensing and active remote sensing here right so once we have understood the three basic questions that is what, why and how.
Next let us try to see what will be the next focus of today's class. We will try to learn the properties of electromagnetic radiation because it is the electromagnetic radiation or energy that we are going to sense. So we need to understand its properties. We also need to understand how the electromagnetic radiation is going to be used. interacts with the target as well as the atmosphere.
The rest part of it like platforms and sensors and data reception on the ground or data pre-processing will be followed or covered in the subsequent classes. But for today let's try to understand how the electromagnetic radiation interacts with matter. Okay so to understand that first let's see some basic understanding of electromagnetic radiation. There are two theories in physics.
One is wave theory another is particle theory. Wave theory considers that electromagnetic energy is in the form of continuous harmonic sinusoidal waves. These waves are of oscillating electric fields and oscillating magnetic fields. So oscillating electric fields and magnetic fields both perpendicular to each other.
So if this is the electric field and this is the magnetic field so the energy propagation will be in the perpendicular direction to these two directions. So electric and magnetic oscillating fields at each point in space and time and energy is propagating in the perpendicular direction. This is the wave theory.
The another theory is particle theory which considers that electromagnetic energy consists of discrete. units which is called quantum. For light particularly it is called photon. Both the theories are just opposite to each other but light exists both as particle as well as wave.
Let us see some basics of wave theory. So electromagnetic wave as I just discussed this is a transverse wave. Transverse why? Because the propagation of energy is in perpendicular direction to the oscillations of electric and magnetic fields.
Then there is something called crest which is the topmost point on the wave. This wave can be of electric field or of magnetic field. One of them is shown here that means the other is also there at the background. So trough is the lowest point of the wave.
Wavelength is the distance between two identical points on the wave. Frequency is the number of cycles per second right so the number of wavelengths that pass a point in a set period of time so these are some basic terms and the frequency and wavelength are inversely related longer is the wavelength smaller is the frequency associated with that wave right so and this is connected using a constant which is the speed of light c this is the speed of light 3 into 10 to the power 8 meters per second c is equal to lambda that that means the wavelength times the frequency that is nu then the energy of a photon is given by h nu h is the planck's constant and nu is the frequency so these are some terms which are related to electromagnetic waves it's an interesting equation which is also called wave particle duality. So one theory says that light exists in the form of waves another theory says it exists in the form of particles but actual existence of light till date we could not separate both the behaviors and light exists both as particle as well as waves right.
It's just that we are tapping one nature to study some properties and the other for the other properties. So there is an equation which is very interesting. This is a hypothesis proposed by de Broglie. What it says is the momentum of photon is inversely proportional to its wavelength.
Here wavelength is also called de Broglie wavelength. You see that momentum is a property of a particle right. So momentum is mass into velocity.
So photon usually you are using have the photons having zero mass. So momentum should be zero. But there is a finite momentum which is explainable by this equation where momentum is inversely proportional to wavelength and equated using the Planck's constant.
So this this duality or this equation embodies in itself the wave nature of light in terms of wavelength and the particle nature of light in terms of momentum both connected at one place. So this is an electromagnetic spectrum illustration. You can see that if you put on there are different components of electromagnetic radiation. We have categorized them based on the wavelengths or you can say frequency or energy because these are all connected.
So if you see from here cosmic rays going to gamma rays then to x-rays then ultraviolet visible IR microwaves and radio waves. You can see that the wavelength is increasing from 10 to the power minus 18 meters and going up to 10 to the power 2 meters. And associated frequency if you see the highest frequencies are borne by cosmic rays and the lowest by radio waves. So corresponding highest energies by cosmic rays and lowest by radio waves.
For Earth based observation studies we are restricting ourselves to this range. Right. So ultraviolet, visible, infrared and microwaves. Ultraviolet is mainly used for atmospheric studies.
visible IR and microwaves both for atmosphere as well as for land surfaces, surfaces of earth. Then coming to visible range extends from 0.4 to 0.7 micrometers. Infrared range ranging from 0.7 to 3 micrometers and from 3 to 100 micrometers. 0.7 to 3 micrometers is the range of reflected IR. So, here the remote sensing that takes place uses the reflective principles.
So, it is sensing is similar to that of visible light. In fact, together with visible light and the reflected portion of IR that means 0.4 to 0.7 and from 0.7 to 3 micrometers this entire range is also called optical range of remote sensing. And from 3 to 100 micrometers it is the thermal IR.
We do not use the entire range. We use a portion of it depending on where is the earth emitting maximum. But its entire range from 3 to 100 micrometers is called thermal IR.
Coming to microwave range it is usually expressed in the form of frequency bands P band L band S band C band etc. This will be covered later in detail in the microwave classes. Here it is important to mention here that microwave attained its popularity because of its ability to penetrate below the clouds also and bring in the signals from the surface.
But now it's being used in almost all the applications. Now once we understand that we are going to measure the radiation. Now how do we measure this radiation? There is a measurement of radiation in some terms. So let's see those terms here.
Before we go further, let's see a solid angle concept. So as we have an angle in two dimensions, similarly we have an angle in three dimensions. So in two dimensions, the angle is defined as the length of the arc subtended on the circle divided by the radius.
This is how you define an angle. on the surface of a circle from the center. But when in three dimensions this circle becomes a sphere and this arc will become a circle on the sphere. So the solid angle is also defined as the surface area of that circle on the surface of the sphere divided by the square of the radius. So this is solid angle and its units are in ste radian.
Ste means solid. 3D and radian is the unit of angle. So, in three dimensions the angle which is solid angle is defined in steradian. These are some terms associated with radiometry.
The basic unit of measurement of radiation is energy. If you add the information of time to it that means energy per unit time you get radiant flux. If you further add the information of area to it that means rate of change of radiant flux with respect to surface area. Actually these are all the differentials. So differential of energy with respect to time or differential of flux with respect to surface area like that.
So that becomes flux density. Now depending upon whether the light is incoming or it is outgoing we have two terminologies. For incoming light flux density is called irradiance. For outgoing light this is called excitance. And for a point source we are not concerned about the surface area rather we talk in terms of intensity.
So energy the radiant flux per unit solid angle is called radiant intensity. This is mostly used for point sources. And then comes the radiance which is the flux density per unit solid angle per unit projected surface area right. So this sorry the flux density per unit solid angle or you can move from this direction also that is intensity per unit projected surface area.
So this becomes radiance. This is the quantity that we generally measure. using the sensors. So, the energy which is falling on the ground from coming from the sun is usually measured in terms of irradiance and energy which is measured at the sensor level is in terms of radiance.
All these terms can be measured spectrally. So, all these terms can also called spectrally like spectral radiance, spectral flux density, spectral power. Why spectral?
Because your sensor will be limited to certain range of wavelengths. According to that range, you will measure the energy. Right.
So, then coming to black body. Black bodies are some idealized objects which absorb and re-emit the radiation in a characteristic continuous spectrum. okay this spectrum is dependent on the temperature okay so this radiation is called black body radiation this is not uh radiate the emitted radiations are not from a real object this is an hypothetical idealized object right but some real objects may behave close to that of black body in certain situations we will see them how so This is how a black body spectrum looks like. On the x-axis you have wavelength and on the y-axis you have spectral radiant exitance. That means the exitance the quantity which we just saw for flux density light coming out from the source.
So it is spectral because it is measured in certain spectral range. So you see that if you focus on one of the curves let's say this curve here you see that as the wavelength increases The spectral radiant excitance increases, reaches a maximum here and then starts falling down. I am sorry.
And so this wavelength at which there is a peak emission from the source. This wavelength is called peak emission wavelength or wavelength of peak emissivity. Okay. And below this wavelength if you see here the behavior is as you increase the wavelength the excitance is increasing.
But after that. the excitance falls down but it never touches the x-axis whatsoever. That means there is a finite amount of energy at all the wavelengths from a black body and this is this behavior is shown at a particular temperature at a fixed temperature and if you see that while you increase the temperature this the curves keep on rising these are not crossing each other anywhere. That means at all the wavelengths the Exitance keeps on increasing building up.
So if you keep on heating a blackbody the overall exitance from the blackbody keeps on increasing irrespective of the wavelengths. For all wavelengths it's increasing. But you see that the peak is shifting towards a lower wavelength. That means the wavelength of peak emission it starts decreasing as you increase the temperature.
It is for example let's say if you have an iron rod if you heat it first it will become red hot then further you heat it it will become yellowish so the wavelength keeps on shifting. Though iron is not iron rod is not a black body but sometimes certain objects behave close to that of black body. Right so from Planck's law of radiation there are two corollaries if you integrate the Planck's law that means you will get the overall area under the curve that means total existence in the entire spectrum you will get if you integrate the Planck's law and this gives you the information that total existence is directly proportional to fourth power of temperature.
This law is known as Stephen Boltzmann law. While if you differentiate the Planck's law you will see that the peak wavelength and the wavelength of peak emission shifts towards lower side as you increase the temperature. So this wavelength of peak emissivity is inversely proportional to temperature.
This behavior is called Wien's displacement law. These are nothing but coming out from the Planck's law itself. Then Why do we study black body at all?
It's a hypothesized ideal object but real objects in certain situations behave close to that of black body. In those cases you can model the real object as close to the black body as its behavior is. So how close a real object behaves with respect to a black body is given by a figure of merit.
This figure of merit is called spectral emissivity. So spectral emissivity is the efficiency with which the real materials emit the thermal radiation at different wavelengths. As I said it is a figure of merit that means it is a ratio.
So the ratio or the spectral emissivity is given by the ratio of spectral existence of a material real object which is exists in nature. Divided by the spectral emissivity of a black body at the same temperature as that of real body. Okay. So spectral existence of a real object divided by spectral existence of a black body at a constant temperature.
This quantity is known as spectral emissivity. Now because it is normalized with respect to spectral emissivity of black body. Its maximum value will be 1 and because it is the spectral existence that means the energy coming out from a source or from a target whatever the minimum value can be 0. So the spectral emissivity varies between 0 and 1. For black body the real object is the black body itself. The figure of merit is 1 at all the wavelengths. There is something called a grey body also we define it.
In such a manner that its behavior is same as that of blackbody but emissivity is less than 1. Okay it is that means the spectral emissivity is independent of wavelength. We call it spectral emissivity but it is independent of wavelength. For all wavelengths the value is same less than 1. For perfect reflector, perfect reflector means those objects which reflect every energy that falls on them. That means nothing is absorbed so nothing is re-emitted so spectral emissivity is 0. All other real objects this spectral emissivity will be varying somewhere between 0 and 1 but it's a function of wavelength.
Now coming to as I said this was all about the electromagnetic radiation, the source of electromagnetic radiation. The laws of electromagnetic radiation now coming to how the electromagnetic radiation interacts with matter. At this point we will take a 2 minutes break and we will be back. Alright so welcome back. Now let us continue with the electromagnetic interaction with matter.
So there are 2 types of interactions possible. One interaction at the boundary of two surfaces so at the boundary of surf two media so there is medium one there is medium two and there is a boundary creating the separation between the two mediums so here when the light comes it will either have a reflection or it will have refraction but within a particular medium same single medium light comes and interacts with the particle it can part of it It can be absorbed and part of it can be scattered. Scattered means it can be redirected in different directions.
So we have these two scenarios with us. Either the electromagnetic radiation is going to interact at the boundary of two media or within a single medium. So coming to the interaction of electromagnetic radiation with the earth's surface.
Here the light which is incoming is. composed of two signals one it gets reflected another it gets transmitted and part of it which is traveling inside can be absorbed by the medium. Now this absorbed energy can be re-emitted by the target depending upon its emissivity and that means temperature and wavelength. So here in case of optical remote sensing that means we are talking of reflections from the object we are concerned with the reflected energy. In case of thermal remote sensing we are concerned with the absorbed energy because it is a function of absorbed energy which will be re-emitted. No object is a perfect source or sink of energy.
So whatever energy we are absorbing we will be re-emitting also depending on our spectral emissivity. So for having the thermal remote sensing you should be concerned about the absorbed energy. So for optical remote sensing we are concerned with the reflected energy. So let us take the case of reflected energy thermal remote sensing you can have a separate discussion with a scheduled syllabus. Now for the reflected energy we are concerned with the reflective properties of the target right.
So there can be two types of scenarios. One where the surface is smooth that means mirror like. So you will have a specular reflection.
Angle of reflection will be equal to the angle of incidence. Whereas if the surface is rough like this your incident light can be in one direction But the reflected light can be in multiple directions. It can be evenly spread in all the directions or it may be having more directionality in some particular direction depending upon what kind of surface you have. As an example for optical remote sensing if you have a smooth road the surface here may behave as specular reflector.
Whereas if you look at a tree canopy. Because the leaves are in oriented in different directions. So the light if it is coming in one direction may be reflected in different directions.
Just because the orientation of leaves is different. So your targets can be smooth targets or it can be rough target. Your surface can be smooth surface or can be a rough surface. But how to tell whether the surface is rough or smooth.
So here is a criteria which is called Rayleigh criteria. That tells you if H is greater than lambda by 8 cos theta where H is the average undulation of the surface. See every surface will have some form of undulation.
If it is having some friction there is some undulation in the surface. Now it can be questionable whether the undulations are in microscopic level or at nanoscale. or it is in centimeter level or it is in meters level depending on which type of surface we have. So if you have an average undulation of the surface and you can measure it that is roughly taken as h actually it is the root mean square height variation above a reference plane. But for simplicity just take it as average undulation of the surface and we have one on the other side lambda by 8 cos theta is a factor.
where lambda is the wavelength in which you are observing the surface and theta is the angle of incidence. So, if h becomes greater than this factor the surface appears rough on a remote sensing image whereas if the reverse happens the surface appears smooth. Okay. So, there is a question you can try on your own if the surface is the same h is the average undulation let us say its undulation is one micrometer and if If I have two types of wavelengths one is lambda A which is 80 micrometer another is lambda B which is 80 nanometer. If you try out this checking this criteria you will find maybe the surface changes its behavior from rough to smooth or from smooth to rough in going from one wavelength to another wavelength.
So whether to say a surface is rough or smooth also depends upon which wavelength we are looking at. And what is the angle of incidence. The next important subject is spectral signatures. I guess there is a complete one or one and a half hour dedicated in the following classes to you. But here for the time being let's understand the concept itself.
Spectral signatures are the unique identification of each feature. So signatures mean. something which is your identification and it has to be unique. So the spectral signature of let's say here vegetation is different from that of soil. The spectral signature of dry soil will be different from that of wet soil.
The spectral signature of dry soil or dry sand and dry silt will be different. Right. So you understand the concept that each feature in different stages also have a different spectral signature.
You can identify them based on their spectral behavior. In optical remote sensing. These signatures are defined in terms of spectral reflectance. So that means on the x-axis you have wavelength and on the y-axis you have spectral reflectance.
And the different features will exhibit different characteristics here. As I said the details will be covered in the forthcoming class. So I will refrain from discussing much on this. But here you just try to understand that. signatures of different material is different the signature of forest will be different from agriculture the signature of rice will be different from that of wheat the signature of basmati will be different from non basmati the signature of basmati a will be different from basmati B the signature of diseased basmati a will be different from healthy basmati a you understand the there are variations to the health of the vegetation that also is covered in their spectral signatures.
So knowing the spectral signature means you know the material. Next coming to the interaction of electromagnetic radiation with atmosphere. For earth observation we can't do away with this part whether or not we like it or not. For if we are looking at as I discussed in the very beginning if we are looking at the atmosphere itself You will be decoding this interaction.
You will want this interaction so that you can get the information about the atmosphere. For earth surface remote sensing that means let's say you want to take a picture of earth surface. That times atmosphere behaves as a and the atmosphere alters the signal both while incoming as well as outgoing.
So it behaves as noise. This noise can manifest itself in terms of absorption and scattering. With absorption by atmosphere, we are limited to atmospheric windows. What are atmospheric windows?
These are those spectral regions where the electromagnetic radiation is passed through without much attenuation. That means the atmosphere becomes transparent to these electromagnetic radiation. If you look at this x-axis, you have wavelength and on the y-axis, you have percentage of transmission. You can see that this transmission is high at certain range of wavelengths.
Then there are some dips here, here, then here. Then again there is a rise, peak and then there is a dip. Wherever there is a peak that means the atmosphere is allowing the passage of these electromagnetic radiation. So these regions of electromagnetic radiation are atmospheric windows. So if...
my interest is to look at the earth surface i'll be creating the op the sensors in these range wherever you have a peak but whereas wherever you have a dip this is a broad dip let's say if i want if my purpose is to study the earth targets the earth surface i cannot be creating a sensor here in this region right so i am left only with the atmospheric windows whereas you If the purpose of my study is to let's say understand the water vapor or carbon dioxide then I will be interested to have a sensor in this range because the greater dip in this channel means the greater amount of that particular gas or constituent of the atmosphere right. So depending upon what is my purpose or what is the application I will be looking at different channels and also. Absorption by the atmosphere limits the signal both while incoming as well as while outgoing. So it diminishes or it lowers down the signal overall. So the effect of absorption on remote sensing is we are left with atmospheric windows to observe the earth's surface and the signal is lowered down.
It is diminished. The other process which is scattering that means the redirection of light depends upon the wavelength which is interacting. and the size of the scatterer there are other properties like chemical structure of the scatterer its physical properties etc but by and large these two quantities that means the wavelength of interaction and the size of the scatterer decides what kind of scattering it is i am restricting myself here to Elastic scattering that means the wavelength of outgoing radiation is same as that of incoming radiation.
So in elastic scattering there are three forms Rayleigh, Mie and non-selective. If you go to inelastic scattering that means the wavelength of scattered light is different may be different from that of the incoming light that becomes inelastic scattering. Raman scattering is one such form.
So here I am restricting my discussion to elastic scattering that means Rayleigh, MIE and non-selective scattering. So for Rayleigh scattering what happens is the amount of forward scattering is equal to the amount of backward scattering. For MIE scattering the amount of forward scattering is more compared to the backward scattering.
These two types of scattering Rayleigh and MIE are also called selective scattering. Why selective because Their behavior depends upon the wavelength of light which is interacting and larger is the wavelength, smaller is this type of selective scattering. For Rayleigh, this dependency is lambda to the power minus 4 and for MIE, it is roughly lambda to the power minus 2 but it can vary between lambda to the power 0 to lambda to the power minus 4. These two are dependent on wavelengths. So, the scattering cross section is dependent on the wavelength of light which is interacting. If you have blue and red wavelengths, blue will be scattered more in Rayleigh as well as in mile.
But in Rayleigh, blue will be much scattered compared to whatever it is scattered in mile. In non-selective, whatever wavelength is coming, it is equally scattered. So, here if it is blue or it is red, both are equally scattered.
So, this type of scattering, Rayleigh scattering is shown by very very small particles. When the size of the scatterer is much smaller compared to the wavelength of light which is interacting, it is called Rayleigh scattering. When the size of the scatterer is comparable to that of wavelength of light which is interacting, it becomes in the range of Mie scattering.
And when the Size of the scatterer is much much larger compared to the wavelength of interaction. Then it is falling in the range of non-selective scattering. Rayleigh scattering is exhibited by gaseous molecules in the atmosphere. Mice scattering is experienced by smoke, is shown by smoke and haze particles that basically aerosols and non-selective scattering is exhibited by bigger particles like cloud droplets. But the effect of scattering on remote sensing is it can modify the spatial as well as spectral distribution of incoming and outgoing radiation.
More is the amount of scattering at a particular place more will be the haziness in the image. So contrast will reduce right. So scattering will reduce the overall image quality right.
So this is the effect of atmosphere on remote sensing. In this last one hour, we tried to learn the overview of remote sensing. I'll just wind it up.
We have learned about what, why and how to carry out the remote sensing process. Then we learned about what we are measuring. We are measuring the electromagnetic radiation. So we need to understand its properties.
Some terms, some definitions, wave particle duality. laws of radiation, spectral emissivity, etc. Then we also discussed the interaction between electromagnetic radiation and matter. This matter can be target or atmosphere.
When the matter is target, we have restricted ourselves to the reflective properties in the optical remote sensing range. So we learnt about specular and diffused reflectors. We also learnt about Rayleigh criteria for rough surfaces and while discussing the interaction with the atmosphere we've discussed the manifestation of absorption and scattering by the atmosphere on the remote sensing image.
Now with this I'll stop here and now I open this class for discussion. We'll have a five minutes break and I'll collect your questions and we'll discuss. Thank you.