 module and I have kept the tagline as radars and hydrology, how do they contribute? So as part of this module we shall be learning more about what is active microwave remote sensing and in particular radar remote sensing and how radars that are onboard satellites as well as in ground help to learn more about hydrology as in how do they give us information about precipitation that is rainfall intensity in millimeter per hours, how do they help us understand about soil moisture up to what depth of soil moisture can radars measure. We shall also try to cover topics wherein radar remote sensing is used to study more about the ground water levels as well as about radar altimetry. So that is the whole summary of this particular module 4. So let us begin. Welcome to first lecture of the fourth module and we shall be learning about active microwave remote sensing. So here in active microwave remote sensing microwaves are used by the instrument themselves to illuminate the earth surface and then the scattered energy that is getting scattered from these targets they are used to infer more about the physical properties of the earth surface. So remember the example when I told you that active microwave remote sensing can be compared with a camera with flash you know the instrument themselves trying to illuminate the target and the scattered energy from the target a part of it a fraction of it travels all the way back through the atmosphere to register a signal onboard the satellite. So the concept of active microwave remote sensing is shown here and the different frequency bands that we use in microwave remote sensing is also displayed that is we have k a band radar and k u band x band c band s band and so on. So each of these frequency bands have their own specific purpose application we will cover all this as part of the upcoming lectures. So moving forward numerous ways of using a radar you know either as an imaging device or a non-imaging device radar as an imaging device was covered as part of the second module wherein we were learning about synthetic aperture radar images you know SLC images all that we have covered. Now whether it is a imaging device or non-imaging device it is the radar equation which determines the proportion of transmitted energy that is returned from a target. So given here just to reiterate is the received power as a function of transmitted power square of antenna gain square of wavelength proportional to radar cross section sigma inversely proportional to fourth power of range that is r. Now we have discussed and derived in detail about the radar equation and also if you remember we solved a few numericals on this topic. Just to refresh your memory the radar equation is displayed here because irrespective of whether a radar is used as an imaging device or non-imaging device the radar equation is equally applicable because it gives us the power received as a function of transmitted power. So once again when I say radar I am referring to radio detection and ranging and shown here is the microwave transmitter and receiver but then I have shown just a single antenna is not it conceptually the pulse that is transmitted from the transmitter travels at the speed of light hits the target the hit pulse is getting reflected from the target and it travels back all the way to the antenna where it is detected and then the time taken between the transmission of pulse and when the reflected signal reaches the receiver is measured by a radar which is given the name as range range distance. The final objective here is to detect as well as quantify the radiation that is arriving at the detector. Now though we mentioned here that you know our interest primarily lies in measuring the power received at the detector please note that there are other properties like the polarization and phase that are of immense interest you know when we start discussing about interferometry they will be clear other properties of radar like the polarization and phase they are also of immense interest. Now please note here that a transmitter and receiver is shown the same antenna can be used as a transmitter as well as a receiver which means the same antenna can also transmit as well as a receiver which is why they will be used synonymously in this module. Moving on so whenever we think of radio telescopes like the one shown in the screen the image that comes automatically to our mind is of a large parabolic reflector is not it just like the one you see on the screen. So the ratio of wavelength to the size of aperture is important when discussing about focusing mechanisms any kind of focusing mechanism the ratio of wavelength to the size of aperture is important. Just a quick comparison with the optical remote sensing wherein the visible range of electromagnetic spectrum is used. So in optical remote sensing the wavelengths used are from 0.4 to 0.1 new meter while microwaves have wavelength which are nearly 6 times greater than that of visible region because they are somewhere here which means that the techniques that we use for focusing light in optical systems like radio telescopes they do not work in focusing microwaves because size is always an issue. And when sensors are operating in the microwave region when they are on board platforms like aircrafts and satellites the size as well as the weight are important. And we need to think about different mechanisms for focusing microwaves. See with microwave systems they collect energy from many directions okay they collect energy from many directions and then they are more sensitive to signal from a small range of angles. As I mentioned in one of the earlier lectures whenever there is a discussion about optical remote sensing the eyes are introduced as a remote sensing instrument. Isn't it? In all standard textbooks of remote sensing and image processing you will find that eyes human eyes are introduced as a simple example of a remote sensing instrument because it collects information about objects without coming into physical contact with the object okay. I mentioned this in the earlier lecture but then and re-iterating so that it gets registered that whenever we talk about microwave remote sensing microwave systems can be compared to the ear okay to the ear. And in microwave remote sensing the range is measured as a function of time or frequency. Please note that microwaves and sound waves both are different types of waves exhibiting wave properties and it is very important to remember that sound waves are longitudinal pressure waves. They don't exhibit polarization okay sound waves longitudinal pressure waves and they do not exhibit polarization whereas in the case of microwaves they do exhibit polarization and this comparison between ear and the antenna of a radar this comparison is solely not intended for a direct comparison between sound waves and microwaves but this is just comparing the analogous mechanisms of measurement okay. So in the previous lectures we were discussing more about imaging radars synthetic aperture radar and in the tutorials also we were covering about how to do processing with the complex numbers of synthetic aperture radar images. When it comes to this module we shall be discussing about non-imaging radars as well. Now we will shortly see how the images look like and what are the underlying principles of collecting data from a non-imaging radar okay. Now whenever there are two coherent sources of light that are placed at a small distance apart say a few orders of magnitude greater than the wavelength of light it helps us in understanding the wave theory which is explained with the help of a diagram here. I am talking about Young's double slit experiment. So constructive as well as destructive interference occurs and Thomas Young postulated that light being a wave is subjected to superposition principle you know superposition principle. So let us try to understand using sound waves please have a look at the video here. I am showing you two sources of sound waves which are placed at a distance apart. Do you see the interference pattern when I am changing the frequency. So now I am increasing the frequency slowly it affects the interference pattern. Constructive when destructive interference are occurring. Now what I will do is I will increase the frequency to the maximum okay. You can see how the pattern is changing. Two coherent sources that are placed at a distance apart which are emitting waves how do they interfere constructively and destructively is shown here. Again we are trying to indirectly understand using sound waves but as I mentioned earlier I am not trying to make a direct comparison because sound waves are longitudinal pressure waves and they do not exhibit polarization whereas microwaves they do exhibit polarization. But conceptually you will get a feel that microwaves they also tend to interfere constructively and destructively if they are being sent from two sources that are at a distance apart that are in the same horizontal plane and which send out coherent waves that are in phase coherent in phase alright. Now for radar imaging the wavelengths that we use are from 1 millimeter to 1 meter 1 millimeter to 1 meter. Now this is a real challenge when it comes to microwaves because we need resolving power as good as an optical system. At the same time we should use the technologies that are suitable for microwaves. So in this context let us try to learn about antennas or a aperture they collect the waves and as an antenna can work as both a receiver and a transmitter they are used interchangeably when talking about transmitting microwaves or receiving the return echoes okay. So as shown in the screen in front of you there are different types of antennas we can have dipole antenna, array antenna and parabolic antenna. When it comes to dipole antenna it is most commonly used as a transmitting and receiving device and when used as a transmitter this dipole antenna transmits the electromagnetic waves when an oscillating current is passed through it okay. So it starts transmitting the waves whenever an oscillating current is passed through it and when it is used as a receiver in the presence of electromagnetic waves it generates an oscillating current just the reverse. Now please note that with dipole antennas transmission and receiving it happens in multiple directions you can get the cue from the interference pattern that was discussed just now. And the dipole antenna as it has a very useful property as in it can transmit and receive electromagnetic waves that are polarized in the axis of dipole okay polarized in the axis of dipole all right. When we talk about array antennas as the name suggests they consist of an array an array of what an array of microwave transmitters and or receivers that are arranged across a panel array antennas. Now phased array antennas consist of each transmitter capable of transmitting microwaves of a specified frequency with a phase okay and they have the ability to specify the phase and amplitude of each transmitter which means I can steer the antenna pattern I can steer the antenna pattern. It is a very important point to note and the phased array antennas are commonly used in the case of imaging radars that require a relatively large antenna okay imaging radars all right. We also have parabolic antennas as the name suggests they focus the incident energy from a particular direction onto the detector very much like the mirror system on a reflecting telescope you know these are different types of apertures or antennas. They usually act as a link between the external environment and the inside of the instrument okay apertures or antennas all right. So since we are discussing about the antenna we have to touch upon gain of an antenna isn't it. The total amount of power that enters a microwave system is determined by the antenna sensitivity to direction to which direction is it more sensitive to total amount of power that enters a microwave system is determined by the antenna sensitivity to direction directional sensitivity of an antenna it is defined by the antenna gain pattern. So over here the boresight direction is shown wherever the maximum peak power is obtained the direction boresight direction and you see the main lobe and the side lobes okay. Now the gain pattern of an antenna it shall comprise of a well defined main peak that is a main beam with side lobes of much lower gain that is away from the main beam. Antenna gain pattern you can find the diagrams for every antenna and please note that we typically tend to use decibels for signifying the antenna gain decibels as we discussed earlier it is 10 let me write it down decibels as such we have discussed it it is nothing but 10 log 10 value of a quantity I am going to write it as value q by value of reference okay value of reference quantity it can be ratio of powers ratio voltages and the direction at which the peak of antenna pattern occurs as we discussed it was known as a boresight direction and antenna gain is given as a quantity expressed as shown here antenna efficiency multiplied by physical area of the antenna into 4 pi by lambda square. So we have discussed about this earlier but just to re-hydrate please remember that angular resolution for microwave antennas they are defined by half power beam width okay. Now we spoke about antenna we spoke about receiver now let us try to understand about monostatic and bistatic antenna systems see remember a detector it should be able to amplify the signal and measure the signal while minimizing the effects of noise and for coherent systems it is also important to measure the phase of a wave and not just intensity alone. So keeping all that in mind what you see here is a monostatic antenna system which can transmit and receive the signal and what you see here is a bistatic system where the transmitter and the receiver it can be in different locations okay and please do not forget that a Doppler effect occurs whenever the target is moving towards an antenna or it is moving away from the antenna whenever target is moving towards the instrument or away from the instrument. And also the antenna can tend to collect noise from the background because of the side loads and hence the usefulness of the instrument can very well be defined using a term known as signal to noise ratio okay it is nothing but received a signal divided by the system noise signal to noise ratio abbreviated popularly as SNR and whenever we have SNR that is less than 1 it implies that the system is generating more noise than the signal you are receiving remember the numerical that we solved wherein we were getting SNR in decibels that is negative the system generating more noise than the signal you are receiving. So higher the SNR better is the performance of the measurement system right. Now at this point let me also mention about calibration okay calibration. Now calibration is the quantitative characterization of performance of an instrument for any instrument that is used for measurement we need to determine the relationship between the recorded signal and the physical property of interest the recorded voltage and the physical property of interest and you know determining this relationship and its noise related uncertainty is known as calibration. And we need to carry out calibration on a regular basis please note that when I mention calibration it does not mean checking if the instrument is working or not no calibration refers to quantifying the manner in which the instrument is working. Say an instrument has to be launched in space and before launch the instrument has been tested under say different environmental condition different gravitational acceleration. Now once this instrument is in orbit there may be different factors which can affect the performance of an instrument it is there may be variations in thermal conditions variations in acceleration different vibrations and with time the machinery of the instrument can also degrade is not it. And by calibration the performance of the instrument is compared with respect to a reference signal performance of the instrument compared with respect to a reference signal and we can have internal calibration as well as external calibration in radar remote sensing we use microwave corner reflectors with well defined cross sections for calibration. So let me play a small video which shows the corner reflectors located in Shadnagar Hyderabad microwave corner reflectors. They have well defined cross sections in the region being observed so that they appear as a bright spot in the synthetic aperture radar imagery because they are placed at a certain angle so that maximum backscattering occurs in the direction in which collection of information is happening from the satellite. Corner reflectors as I mentioned earlier an antenna or an aperture is a link it is connecting the outside environment with the inside of the instrument and in a microwave system external calibration is required for an antenna and this can pose a problem for space bond systems as how do you calibrate a space bond antenna in far field condition. So till now we have been discussing about radar remote sensing and specially about the antenna sensitivity and calibration so what we will do is we will slowly try to understand the applications of radar remote sensing and hydrology from the next lecture onwards. So let me hope that you found these contents informative and I shall meet you in the next class. Thank you.