 Hello and welcome to the course on Microwave Remote Sensing and Hydrology. So we are going to start our first module and first lecture before we begin a particular course we should know why we are learning it in the first place, is not it? So as in what is the importance or relevance of getting to know more about microwave remote sensing. So for you to appreciate the subject let me try to begin with the history of microwaves, a little bit of history. So the history of microwaves goes back way back to at least 200 years ago when electricity, magnetism and a light were three different phenomena, they were considered unconnected at least in the minds of scientists and in the 18th and 19th century these three phenomenon that is electricity, magnetism and light were being investigated by a wide range of philosophers and scientists. So there were scientific investigations which were occurring on optics and light. For example you may be aware that Thomas Young who is an English physician and physicist was reintroducing light as a wave and also describing that there existed different types of cones in the retina, retina of the eyes. And these different cones in the retina were particularly sensitive to one of the three colours of red, green and blue RGB and that the other colours besides these are nothing but the cone cells stimulated in different combinations and that white was a result of combining three primary colours. So this theory was later developed by Helmholtz and is known as Young Helmholtz theory. So it were these ideas that laid the foundation for development of colour photography and colour displays. So remember we are trying to understand the importance or relevance of microwave remote sensing from the beginning. So in the 18th century Young demonstrated the wave theory of light which we now know as double slit experiment. The light from a single source with a colour was allowed to pass through a narrow slit and then the same light was directed to pass through two more slits which were placed centimetres apart and the resulting light from these two slits were directed to fall on a screen and what was observed was bright and dark bands. This was known as the first demonstration of the interference of light meant reducing a new term here interference of light. Of course we shall be learning in detail about interference through the module on radar interferometry which is coming as part of this course but for now remember our discussions are still in the 18th century where it was proposed that light comprised of transverse waves vibrating at right angles to direction of travel. So this allowed for something known as polarization which explains the direction of vibration of wave. Now in the 19th century the 19th century was being entertained with magnetism electricity and applications of light in different forms and at that time there was also a realization thanks to the Danish physicist Hans that electricity and magnetism were related. Now Christian Orsted had caused a compass needle to move by making electricity flow down nearby wire back then this was considered very interesting as both the wire through which the electricity was passing and the compass needle were not in physical contact. Just to give you a few glimpses of how we have arrived from 200 years ago to what we have in terms of knowledge now. So then you may have heard about Michael Faraday the great experimental physicist who conducted a similar experiment by switching on and off the magnetic force which generated electricity. So now there was a realization that somehow if electricity was made to vary change with time it resulted in generating magnetism and time varying magnetism generated electricity. Now we know this as induction but back then it was very much intriguing. Now again it was in the year 1845 that Faraday observed electricity, magnetism and light were related. So by now there was an understanding that a strong magnetic field could in turn affect the nature of light through a medium any medium. Now but then the most remarkable turning point was in fact considered as you know the beginning of modern physics was when Maxwell combined all the works on electricity and magnetism and how they behaved in matter in the form of four equations that you see in the screen. They are now known as Maxwell's equations. I may not go into detail into deriving Maxwell's equation and what each term means for now. Just to make you understand that back then Maxwell wondered as to how the equations would look like in vacuum. In a place where there were no electrical charges and absolutely no magnets. The answer obtained was that electric and magnetic fields propagated moved through empty space as if they were waves. Now these oscillating fields were like self-perpetuating, self-perpetuating meaning one created the other. So Maxwell then called them as electromagnetic waves and then he further went on to estimate the theoretical speed with which the electromagnetic waves propagated and interestingly it was found to be same as that of speed of light. So in microwave remote sensing, Maxwell's electromagnetic theory can be considered as a turning point even though it took like about 20 years for it to be proved. Now whatever be your motivation behind attending this course, whether it be to understand the concepts or to perform data analysis for applications of hydrology. So whatever be it, a sound understanding of the underlying physical properties of electromagnetic radiation are essential, crucial. So let us try to understand everything from the scratch starting with waves. To begin with, let us try to understand the measurable parameters of a wave. So right now we have not started discussing about electromagnetic radiation, we have just begun understanding about a wave, beginning with amplitude of a wave. Now amplitude refers to the maximum disturbance of a wave. That is the distance from the axis to the maximum height of a wave. Remember this is not a distance from maximum to minimum, it refers to the distance from the axis to the maximum height of a wave. So we can have waves with differing amplitudes, which means I can have a wave with higher amplitude which is shown towards your left side and a wave with lower amplitude that is shown towards your right side, amplitude of a wave. Moving further, we have frequency of a wave denoted usually by the Greek letter nu, frequency means the number of waves or cycles or oscillations per unit time. So shown here is a low frequency wave. What is frequency? The number of waves or cycles or oscillations per unit of time. So the unit of frequency is hertz, here T that is the period, it refers to the number of units of time per wave cycle. So shown here is an example of a wave with a smaller frequency and a higher frequency just so that you understand the meaning of frequency. So now let us go to wavelength of a wave. It is nothing but the distance between two successive crest or a trough of a wave. So distance between two successive crest or two successive trough of a wave, shorter wavelength and what you see here is a longer wavelength. So with this background let us try to understand what is polarization of a wave. Now assume I am holding a thick, very thick and a heavy rope and say I am holding one end of the rope while my partner on the other side is holding another end. Now imagine I am trying to send a wave down this rope. I am holding a very thick heavy rope, one end of it is with me and the other end is with my partner who is standing across the room and I am trying to send a heavy wave down this rope. So the wave start moving in the rope. So and you as an observer I am asking you to describe its motion which means you need to specify in which way the rope is moving. Is it up down or is it left right? So oscillation of an electric field vector is termed as polarization. Now why am I terming it as electric field vector and not a magnetic field vector that we will discuss shortly. But for now it denotes the direction of travel, oscillation of electric field vector. So assume what you see in your screen is an electric field vector, assume it is in the horizontal direction. So we call it as horizontal polarization. Similarly we can also have vertically polarized wave wherein the electric field vector is oscillating in the vertical direction, vertical polarization. Similarly we can also have circular polarization wherein the oscillation it can be right circular or it can be left circular clockwise or anti-clockwise, polarization, oscillation of electric field vector polarization. So now we have understood what is amplitude, wavelength, frequency, polarization of a wave. So now let us try to understand about an electromagnetic radiation, electromagnetic radiation. So to describe an electromagnetic radiation there are two conceptual ideas which have been used sort of interchangeably over the years. One is describing electromagnetic radiation as a wave and the second is to describe electromagnetic radiation as a flow of small packets of energy called as photons. So we are discussing about electromagnetic radiation and that there are two probable ways, two conceptual ideas which have been used interchangeably over the years. One is describing the electromagnetic radiation as a wave and second is to describe it as a flow of small packets of energy called as photons. Now with respect to microwave remote sensing, most of the important phenomena are better described using the wave theory, using frequency, using wavelength, using interference and so on. Hence throughout this course I shall be mostly using the wave description. So with that background let me try to define an electromagnetic wave which consists of time varying electric and magnetic fields. So in the figure shown for simplicity we are referring to the wave profile on a sine or a cosine curve so that it is easy for us to mathematically define it. So what is the polarization of the electromagnetic wave shown is it horizontal or vertical? As I mentioned earlier for electromagnetic radiation polarization is considered as the direction of electric field vector and not the magnetic field vector. This is because the most common means in which electromagnetic radiation manifests itself are mainly due to the electric field vector and not the magnetic field vector. We will shortly see how to mathematically define a wave and for that we may need to understand few concepts as well. So for that now before trying to understand the remaining concepts of a wave till now we have seen what is frequency of a wave, what is amplitude, what is polarization and we also saw an electromagnetic radiation through a wave you know we are describing it in terms of a wave. Now coming on to the phase of a wave, now before that we know that degree is a measurement of a plane angle and that one full rotation is 360 degree. We have that understanding with us, again we know that radian is another angular unit of measurement, another unit of angular measurement. And if you compare it to a pizza the central angle that subtends an arc length of one radius is one radian, is not it? Let me try to repeat the central angle that subtends an arc length of one radius is one radian which means one radian is nothing but 180 degree by pi. Several concepts which will be useful when we try to mathematically define a wave. Now with this background let us try to understand what is phase of a wave. Now waves can be at different initial stages of their oscillation cycle. They can have different initial phase which is denoted by pi. So if we have a wave which has a phase of 0, it is going to look something like this. For you to understand better let me try to show waves with phase of 90 degree, focus here phase of 90 degree let me play it again. And here we have waves which has a phase of minus 90 degree, I want you to focus here minus 90 degree. So we have seen what is phase of a wave with this background. Let me try to introduce a new terminology that is superposition. Now one very important question that we need to ask is till now we have seen few properties of waves. Now what will happen if we try to combine two or more waves together? So here we are going to assume that both the waves say we want to combine two waves which are of identical frequency and amplitude, identical frequency and amplitude I will let that sink in. Say you want to combine wave 1 and wave 2 and say the output is wave 3. So the amplitude of wave 3 will be affected or determined by the phase difference between both the waves. So again here we are assuming both the waves to be of identical frequency and amplitude. Say the phase of the first wave, wave 1 is same as phase of wave 2. This means phase difference is 0 and that the amplitude of the resulting wave 3 is going to be summation of amplitude of wave 1 and amplitude of wave 2 which means that waves are interfering constructively, waves are interfering constructively. When phase difference is 0, let me give you the reverse case wherein the phase of wave 1 is not equal to phase of wave 2 which means there is a phase difference that is not equal to 0. Then what happens? Then the waves tend to interfere destructively, I am giving you few terminologies. One is interference that is nothing but superposition of waves. We are trying to understand how we can combine two waves together and if we do what will be the amplitude of the resulting wave and then I gave you two scenarios. The first scenario is when the phase of wave 1 and wave 2 are equal which means there is no phase difference and then I told you that the waves are going to interfere constructively and then I gave you the second scenario wherein the phase difference is not equal to 0 which means the waves are going to interfere destructively. I will write this one more quantity used to describe wave motion, I cannot leave that out. So I am going to describe that as well which is known as angular frequency, angular frequency it is denoted by omega. So angular frequency is nothing but the rate of change of phase angle 2 pi by T radian equivalent for one full cycle. The units is radians per second, angular frequency why we use it to describe the wave motion. So by the beginning of 20th century it was clear that visible light, radiant heat and radio waves all these were part of the same forms of electromagnetic radiation wherein the key variable was frequency or wavelength and this range of possible electromagnetic waves was called as electromagnetic radiation. Now the conventional understanding that we have today about the electromagnetic spectrum is because of Maxwell, okay. Now for this course we shall be focusing on microwaves which are electromagnetic radiation with a wavelength between 1 millimeter to 30 centimeter. But then you know we have electromagnetic radiations in modern radars that operate at wavelength of 1 meter also, hence the terminology at the boundary is a little ambiguous, okay. So what you see in front of you is the electromagnetic spectrum and I want you to focus on the microwave region here. So this course will focus mainly on the satellite images captured in the microwave region of the electromagnetic spectrum. So the main technical progress of radar operating in microwave region happened during world war times, hence I need to introduce a little bit about that as well. So world war II was certainly a turning point for microwave remote sensing because the need and potential of tracking the enemy aircraft day or night was a valuable tool. So enormous amount of effort was being put forth to develop the radar technologies which was used extensively to locate aircrafts as well as storms. Now there is still a rather frustrating legacy that world war II has left in microwave remote sensing because during world war II several bands or windows or region of frequencies were used to relate to equipments to generate and detect them as they were being used during world war II they were given random names to confuse. So it has no logical naming convention and to this day we follow this same naming convention to different bands of microwaves which are being displayed here, okay. So to this day we use the frequencies of KA band, K band, KU band, X band, C band, S band, L band and P band. So what are they? They are the different names given to different range of wavelengths or frequencies which are all microwaves, okay. And as we move along in this course it will be more clear about the suitability of each of these bands as in where a C band microwaves more useful in hydrology or say where is KU band frequency more relevant in hydrology and so on, okay. Just for understanding that the nomenclature has no logical meaning from world war II it was named thus and it is being used in a similar manner even to this day. So even after the war radar had found many new applications as an early warning device in radar astronomy in radio astronomy and radars were also being used to observe planets or moon and solar system as a consequence of end of war. And there was an increasing realization that airborne radars, radars onboard aircraft could be used for navigation as well as for reconnaissance survey. Moreover microwaves offer the advantage of penetration through clouds. So we can see this in detail. For now in 19th century long antennas with high frequencies were used with projected beams sideways from an aircraft. Now such instruments we may now know as side-looking airborne radars. They were mainly developed for military reconnaissance in the 1950s. Again you know getting an image radar into space took a little bit longer which happened by 1970s. So moving further let me show you a small caricature. Again the images shown are not to scale. This is just a caricature to represent that if we have a microwave operating say nearly in 6 centimeter range you can say conveniently that it belongs to C band. And if you have a microwave of nearly 23 centimeter wavelength we can conveniently say that it belongs to L band because we have seen previously that the different windows ranges of frequencies in microwaves are further subdivided and each are denoted by an alphabet X, C, P band, L band and so on. So with this background you know when we try to understand about a course first of all we should know what the title of the course means. So right now we know what is a wave what is a microwave. Now with this background let us quickly try to understand what is remote sensing because we are going to learn about microwave remote sensing. So by remote sensing what we mean is we are trying to analyze and interpret the measurements of electromagnetic radiation that are either getting reflected from a target or which are getting naturally emitted by a target and then this reflected or emitted radiation is getting recorded by an observer or an instrument which is not in direct contact with the target. So let me try to repeat again here by target I mean it can be an urban area, it can be vegetation, it can be water body to state an example. So this target can either naturally emit electromagnetic radiation or it can reflect the electromagnetic radiation and this radiation is getting recorded by either an instrument or an observer who is not in direct contact with the target. So in remote sensing there are two broad categories known as passive remote sensing as well as active remote sensing we shall discuss about this in detail as part of this course. So just to reiterate as part of this course we shall be focusing on discussions on microwave remote sensing which means we will try to understand how microwave data are being collected and then we shall learn in detail about their applications in hydrology. So just to summarize just to introduce the terminology to you now microwave remote sensing what you see towards your right side here the active sensor illuminates the target itself and the passive sensor it measures the naturally emitted radiation. So as far as active sensors are considered we tend to compare it to a camera with flash wherein the camera itself is illuminating the target which is you and then collecting the information later on and when it comes to a passive sensor we usually compare it with a camera without flash because it is not illuminating the target but it is using the reflected radiation coming from the target to capture information. So we shall be discussing in detail about these. So this was just to introduce the terminologies of active remote sensing as well as passive remote sensing okay. Now it was somewhere in the 1960s that meteorological targets were launched based on the technology for detecting passive microwaves and microwave emissions at different wavelengths they carry highly useful information about the atmospheric chemistry and temperature and atmospheric sounding is the name given to making passive microwave measurements of the atmosphere we will see that in detail. Now with recent advances in technology there are more advanced founders which are being used a small video here because you know the microwave remote sensing is being relied for a variety of applications. So it has its own unique place in earth observation studies and through this module that is module one we shall be learning about the fundamentals of microwave remote sensing, its various applications in hydrology, the underlying laws and slowly try to understand the key terminologies which will be used time and again as part of this course which are part of microwave remote sensing all right. So with this background just let us try to solve two small problems just to evaluate yourself whether you have understood the basic terminologies that have been introduced till now. Assume I give you two waves at the top you have a wave with phase 0 and amplitude A and at the bottom you have the second wave that is wave 2 with phase 0 and amplitude A and I am asking you what is the output if you combine wave 1 and wave 2 what will be the amplitude of the resulting wave, wave 3, wave 1 phase 0 amplitude A, wave 2 phase 0 amplitude A. So I want you to tell me what is the amplitude of the resulting wave when you combine wave 1 and wave 2. So you can see there is no phase difference isn't it which means that the resulting wave is going to have an amplitude of 2A the amplitudes are going to constructively combine isn't it okay. Now what if I give you a reverse scenario as in we have wave 1 whose phase is 0 and amplitude is A say wave 2 there is a phase of 180 degree but the amplitude is A so now what is the amplitude of the resulting wave? There is a phase difference which means the waves are going to destructively interfere which means there is not going to be an amplitude isn't it 0 okay. Now so as part of this lecture of the first module we tried to understand what is the history of microwaves a little bit and then we were understanding about the properties of waves the terminology is that will be used and then we understood what is an electromagnetic radiation and what is an electromagnetic spectrum where does the microwaves lie and what are the different parts or frequencies within microwaves that are known by different alphabets like P band and you know X band C band and so on and then we tried to understand about what is remote sensing as in what is active remote sensing and what is passive remote sensing. So overall you will have a view about what is coming in the next lecture that is fundamental properties of microwave will be discussed as part of the upcoming lecture. So let me hope that you found this lecture informative and I will see you in the next class. Thank you.