 Hello everyone, welcome to the next lecture in the course. In the last lecture, we discussed and concluded the topic platforms used for remote sensing observations. We discussed about several kinds of platforms and we concentrated more on the satellite based platforms where we discussed the commonly used satellite orbit such as the geostationary orbit, near polar orbit, near polar sun synchronous orbit and all. Also, we have discussed how near satellites in the near polar orbit can achieve global coverage by staggering the orbital trace the satellite does on the earth every time by staggering it we are able to achieve a global coverage. So, today we are going to start a new topic called LIDAR. So, LIDAR is primarily a developed as like a surveying technique in order to measure the coordinates of points, but in recent years maybe like a decade or two, it has been used very widely for various applications including vegetation monitoring and so on. So, it is one of the important topic in remote sensing, but here we are not going to discuss it elaborately, rather we will talk in brief about LIDAR its principles and so on. And interested users or interested readers can refer to some other sources like some papers I have cited in the lecture slides during the course, I will provide like few other websites which you can look upon for additional information and so on. So, all these things you can like look up those who are really interested to do further research on the topic. Here in this course we will get just briefly introduced to the concepts of LIDAR. So, what LIDAR is? LIDAR is an acronym it stands for light deduction and ranging that is here the term is very similar to radar right. So, there in radar what we have seen we use microwave signals or originally it was like radio waves which was used for deduction and ranging of different objects. Here we employ similar concepts, but we use laser light for measuring the distances and detecting objects and that is why the name LIDAR light deduction and ranging it is again an active remote sensing method. In LIDAR 2 we will generate the wavelength we need, we will transmit it towards the target, get the backscatter signal back here also we are talking about backscattering whatever we send it has to come back to the same sensor in the same direction. So, backscattering only we are talking about. So, whatever is coming back we use that to range the distance between the objects and map them and also to infer some of its properties. And primarily LIDAR developed as a tool for engineering survey purposes like those students coming in from like a civil engineering background or like earth science background would have done some courses on engineering surveying where our primary aim will be to map the relative position of various points on the earth surface like with respect to your station where your instrument escape you will try to measure the coordinates of different objects surrounding you. Or with respect to like a boundary or with respect to some sort of like control points relative to that we will try to measure the x, y, z coordinates of other points. So, LIDAR primarily developed or came into use in the field of like geomatics for that particular purpose. We use it for like surveying applications and it first started its use in terrestrial instruments. So, like total stations, electronic distance magnetic measurement, they are like different ways in which we use electromagnetic signals for engineering surveying purposes. LIDAR also came into picture like that we use laser light for ranging the objects. Slowly people understood the advantages of LIDAR and it developed into different platforms like now LIDAR is available or LIDAR observations are available not only from terrestrial scanners but also from airborne platforms as well as satellite platforms. And now LIDAR has moved from just being like a survey technique from like a independent remote sensing technique where we not only measure the coordinates of points but we also try to get other useful information about the objects. We will see toward the end of this particular lecture, this particular topic. So, LIDAR is basically a ranging device. It measures the distance between the transmitter and the object that is in front of it or that is below it. If it is airborne platform, whatever the object is below it, it will measure the range. Essentially, it is a ranging device. So, it will measure the three-dimensional coordinates. Finally, that is what we will get. So, how we will get three-dimensional coordinates anyway just measures the distance which is like a one-dimensional distance like this. If the platform is aircraft is flying like this, it will just measure the distance in this particular axis. So, how we are going to translate that into a three-dimensional coordinate that we will see and also LIDAR came into use pretty long ago like into surveying into ground-based surveying LIDAR was beginning to be used few decades back. But in order for it to be used properly for airborne surveying or space-borne remote sensing surveying and remote sensing purposes, other technologies had to mature which actually prolonged the arrival of LIDAR technology in into airborne and space-borne platforms. So, essentially LIDAR cannot work independently. If you want a perfect three-dimensional coordinates of earth's surface features, we need to use LIDAR in combination with other technologies such as GNSS, a global navigation satellite system. And we need to have like an IMU like initial measurement unit and so on. So, that is the main reason why it took some time for LIDAR technology to mature and come into existence for remote sensing purposes through airborne and space-borne platforms. Also, LIDAR since it is like light-based ranging system, we use wavelengths in green band or NIR band. Traditionally, it is I told it is like a laser light. So, characteristically laser lights are monochromatic collimated beam of light. That is like if you take our normal white light what we get from sun or we get from our electric tube lights or whatever we use, it is not like properly monochromatic. They can come from like, they will have like a mix of wavelengths that we have already seen like white light composed of 7 different colors or 3 primary colors basically. But laser light are monochromatic. So, whenever we generate laser light, we will ensure that the light comes out in a single wavelength or a frequency. So, practically we may not get like an extremely single frequency, but we will have like a very narrow bandwidth. But for all practical purposes, we can assume it to be like a single wavelength. Plus it is collimated beam of light. Collimated means like all the laser pulses that are coming out will be like parallel to each other, will be perfectly coming in kind of like a straight line like the divergence. Say if you switch on a torch light, that light diverges very fast, whereas for a laser beam, if this is highly collimated, the divergence will be very narrow. Like as an example, say if you have like a torch light here, the light may suddenly spread like this. They are non-collimated, divergence may be large. But for a laser pulse or like a laser light, which is like highly collimated, the light may come out like this and the divergence will be very narrow. Divisions will be there, but it will be very narrow when we compare this with our normal light. So, here we use monochromatic collimated light source that is laser beams for as the medium of remote sensing. And conventionally, green wavelength roughly about 532 nanometers or NAR wavelength roughly 1064 nanometers, like 1.064 micrometers or 0.53 to micrometers. They are like the conventionally used wavelengths for laser remote sensing or LiDAR remote sensing. Basically, for land surfaces remote sensing, NAR may produce like a very high reflectance. So, that is why like NAR is often produced, like whatever be the feature, whether it is vegetation or bare soil, etc. NAR traditionally has like a higher reflectance, that is one. And also green wavelength, among vegetation green again can produce like higher reflectance compared to other visible wavelengths. Okay. So, basically green, if you look at like land surface, green or NAR typically has higher reflectance among its counterparts. If you look at visible, green will have slightly higher reflectance than others, like red and blue visible wavelengths. Similarly, NAR typically has high wavelengths, that is one thing. And in very rare cases, in some selected airborne platforms, 1550 nanometers, which is like a short wave infrared frequency that is also used. But more conventionally, green wavelength or NAR wavelength is widely used for LiDAR remote sensing purposes. So, how basically a LiDAR system works? We will first start with a terrestrial laser scanner, then we will move on to airborne systems and briefly have a look at space bond systems. So, first this picture on the left is a schematic representation of terrestrial laser scanner. Terrestrial means from ground, like the instrument that produces and transmits laser will be fixed in the ground. So, from this particular instrument, it will be sending in short pulses of laser, like while discussing about radar, we saw like radar, the antenna will send in short pulses of radar. Similarly, here also it will send in short pulses of laser light. There are some systems which transmits continuous waves but not very often used. Mostly we will discuss about this pulsed laser system but continuous laser transmission is also possible. But for our discussions, we will restrict ourselves to pulsed laser. So, it will send in or the instrument will transmit short pulses of laser and whatever object is present in front of it, this pulse will be reflected back. So, once this pulse is reflected back, the instrument can measure the distance between the instrument station and the target. So, this will produce the ranging in this particular line. Also, if the instrument is able to scan in the entire hemisphere surrounding it, say if I look from the top view, this is like the instrument location. So, if you look it from the top, instrument can potentially scan in all the 360 degree angles surrounding it. Normally like when we do ground based surveying using like a total station or something that instrument is able to rotate in the horizontal plane for the entire 360 degrees. Same thing is applicable with LIDA sensor too. It can rotate freely in 360 degrees. Similarly, it can rotate in the vertical plane from a horizontal to like vertical like this. So, it can rotate in both directions. When it does, it will be sending in pulses in all directions. Say when you like say there is like a building standing in front of you, you want you are like just keeping the laser instrument pointed upwards. So, all the laser beams will be targeting the top portion of the building. Then slowly you come back and target up to the bottom of the building. Then we move from left to right covering the entire portion. Maybe we can start like this. It is kind of like doing like a scanning and stitching images. You do multiple scans to collect like the entire feature present before you. So, essentially this radar system will store information about like the distance and in which direction and which angle the laser beam was transmitted. Like if we know the angle of elevation or angle of depression. Like angle of elevation means from horizontal by which angle the instrument was looking up, that is angle of elevation. Or from horizontal at which angle the instrument was looking down, that is angle of depression. If we know this angle and also if we know the azimuth angle, maybe with respect to like one particular direction. Let us say this is my north, with respect to my north in which direction my instrument is pointing at. Say here maybe like 30 degrees from north or here maybe like 320 degrees from north like that. If you know like these angles like to in both horizontal and vertical directions. And if you know the distance from the point of instrument to the target, then relative to the instrument station we will be able to locate this point. Say I have the coordinate here or I know this particular position. From this particular position using the angle in both horizontal and vertical axis like this is say here there is some target. This is like and then with respect to say this is my north or let us say sorry, let us say this is my north. So, with respect to this north what is like this angle. So, this is in horizontal plane. Say this is let us say this is beta. So, alpha is like the elevation angle, beta is a depression angle. I know this range, it can be calculated by laser pulse. So, I know two angles horizontal angle and vertical angle, I know the range. So, with respect to my position the instrument station, I will be able to locate the target in front of me. So, if I have like 100 measurements of the building like if I have taken range for 100 different points on the building, I will be able to locate all the 100 points with respect to my station. Imagine if I know the 3 dimensional coordinate of my station with respect to some reference datum. Say in surveying students who learn surveying will know we will always have some set of like reference system like an origin from which we will measure our horizontal coordinates and vertical coordinates. Like similarly earth has it is like there are like lot of datum available. Let us take for example, we are using this WGS 1984 reference system. Like you have some x y origin and some z origin above, z origin means above which you measure all the elevation, x y means like from which you measure all the horizontal coordinates. If we know that and if we know the coordinate of the instrument station using in that particular reference system, then with all the measurements we made, we will be in a position to calculate the 3 dimensional coordinates of all the points that we measured. So, that is why I said LIDAR cannot work independently if your aim is to get like 3 dimensional coordinates of all the points with respect to some reference system. First of all, you need to know the coordinate of your instrument station with respect to the reference system you are working on. Then only from that position you can calculate the relative position of all other features. So, this is like the basic working principle of a terrestrial LIDAR system. Coming to aircraft, exactly same principle. Aircraft will be moving like this. We will have like a LIDAR system attached with it, looking down vertically downwards. So, let us say the LIDAR system is just fixed like this, looking at the nadir. So, as the aircraft flies, it will measure the range between the instrument and the target along the line of its flight path. Say flight is aircraft is flying like this, this is like the terrain. When it flies, if it sends in laser pulses, like aircraft is moving continuously. So, each of this position or the range for each of this point will be measured by this laser system. So, if you know the coordinate that is the x, y, z coordinate of the instrument station from which you measure the range, we will be able to position or we will be able to estimate the x, y and z coordinate of each of this ground points. So, again we need to know the coordinates of the instrument station from which the laser beam is transmitted. So, this will produce what we just saw, it will produce like a profile or like a x, y coordinate information along the line of flight. Traditionally, this will give like elevation. The LIDAR will only give like, if you look at in terms of like vertical direction, it will measure elevation or if you talk in terms of like horizontal direction, you are measuring distance. Basically, it is a distance measuring device. So, using this distance you can and if you know the coordinate of your instrument station, you can calculate the coordinate of ground point. Imagine the aircraft system, now the LIDAR is also fitted with a scanner, a across truck scanning mechanism like a mirror or something. So, what it will do? The laser beam will be transmitted, but now since scanner is present, it will be able to scan across a swath width. So, it will scan like this while it is moving. So, now in take it in analogy with the ground based scanner, aircraft is moving like this, you are continuously recording the coordinates of your aircraft station, you are also measuring the angle at which the laser beam is transmitted and received. So, we know the coordinate, we know the angle and the direction in that particular angle. So, using this geometry, we will be able to calculate the coordinates of the ground points. So, essentially this is the working mechanism of LIDAR. But how do we get the coordinate of our platforms? In ground based terrestrial laser scanners, it is simple like, simple in the sense we know like the ground measurement techniques. We already have established ground control points from that we can take it or we can do our own surveying like you can do a total station based survey or you can extend the control net and establish new control points from which you can do the surveying all these things. So, that is like a different topic engineering surveying. That is possible, but to do it from an aircraft or a spacecraft, we need a independent source of data which provides the coordinates. That is where the role of GNSS comes in like global navigational satellite system or very commonly one of the most widely used GNSS system is GPS, global pushing system. We all know GPS as a common in, GPS is just one of the GNSS system it is deployed and maintained by United States which we commonly use. But apart from GPS there are other systems available that are implemented and maintained by different countries. India has its own, China has its own, Russia has its own and so on. So, collectively all these systems are called GNSS, Global Navigation Satellite System. So, if you have a receiver, it is like a satellite based coordinate measurement system. So, it uses observations from multiple satellites and it uses like basic surveying principles using which we can establish the coordinate of our instrument station. So, our aircraft should have this GNSS receiver which will collect measurements from all the satellites surrounding it whatever the system that is there and whatever the system the receiver is capable of receiving like certain GNSS receiver may receive only like GPS, certain may receive GPS, Galileo, GLONASS, some may receive like Indian IRNSS and so on. It depends on which receiver we use. So, based on our receivers capacity, we can receive signals from several satellites in space from which the coordinate of our own ground point or our own platform can be established. So, essentially your aircraft should have like a high response high quality GPS receiver which will give the coordinates of the aircraft continuously. So, that will give you the XYZ of the aircraft location. Let us know within the aircraft we know where the laser system like say the GPS antenna may be kept here, the laser scanner may be kept here. So, the GPS coordinate system will measure the XYZ with respect to this is entirely enclosed in an aircraft box. So, the GPS will give the coordinates of this particular point, the antenna position. With respect to this antenna position, we can measure at which location this is installed within the aircraft. So, basically we know the coordinate of this LiDAR system also. So, that coordinate can be established as the aircraft flies continuously. Then we also have what is known as an IMU, Inertial Measurement Unit, which will tell us in which direction or what is the attitude of the platform. Attitude means when an aircraft is like flying like this, we normally expect the aircraft to fly perfectly straight horizontally. But sometimes due to any disturbances, it can change its attitude like it can undergo a roll like this, it can undergo pitch like this, it can undergo yeah like this, 3 different axis XYZ. If it rotates around the X axis along the direction of flight, it is called roll. If it moves like this, like the nose goes up and the nose goes down, we call it as pitch or if it rotates like this above the vertical axis, we call it yeah, it can happen anytime. So, when the attitude changes, maybe the angle in which the instrument is pointed will be changing, like as a very simple example I tell you. We are, let us say we have planned the aircraft to fly straight like this and we have like a LiDAR scanner attached that looks only at the nadir, okay. Now, when the aircraft is flying like this, suddenly a gust of wind blew or the pilot has to do some urgent maneuvering to save the aircraft, some emergency came, something happened, the pilot has to do some sort of like quick maneuvering. So, let us say there was a roll that is given to the aircraft like this. Suddenly, the aircraft like with, instead of moving like this, it had a roll like this, one instant it went like this and came back, okay. When a roll is given what will happen, we will be thinking the laser beam is pointing downwards. So, the range is measured like this, but at the instant of this roll, the laser beam is now pointing not at the nadir, but with certain angle. We have to account for that angle of roll if we want to properly establish the ground point at which the distance is measured, right. We need to know the angle of roll from the vertical, then only we will be able to calculate the range. Some range it would have measured, but not the intend at point, but some other point that is also fine. But even in order to measure that particular point properly, we did not know this roll. So, this is not the only case, attitude can change in all three axes together, like a plane may completely look like this in three dimensions. There can be like a roll, there can be like a pitch, there can be like a yaw. Everything can happen simultaneously, which will totally change the way or change the point in which we are doing the measurement. That is the important, that is the need for having an IMU, initial measurement unit which will measure the attitude of this aircraft. So, once we know the attitude of the aircraft, we will be able to establish geometrically at which point our system is looking at. So, by knowing the coordinates, our positions x, y, z and by knowing the attitude of the aircraft at the time of data acquisition and by knowing the range, we will be able to establish the distance between each point at which it is being, that is being eliminated by the laser light in the ground. This is again the basic working principle of how airborne remote sensing is done, like LIDAR remote sensing from airborne platform is done. Similar concepts apply even to spaceborne systems, like from satellites. It measures the distance and the satellite also has its own GPS receivers and IMUs. It also has like some sort of stabilization mechanisms in order to keep the beams perfectly pointed downwards and all. So, it is like a combination of different technologies, like all the technologies had to mature, like measuring your coordinates using GPS from an aircraft is not an easy task. It needs a very high quality precision grade like the GPS receiver. So, everything like matured in the recent few decades. So, the laser technology was there in the last like four, five decades, but for other technologies to develop and mature it took some time and when everything came together, we are now able to move the LIDAR system to airborne platforms. It developed quite extensively in the last like two, two and a half decades, this airborne and spaceborne remote sensing of LIDAR technology. And also the LIDAR beam will have like now let us imagine we know the basics how it works. Now, the laser beam that is being transmitted, like I told you it will have like a collimated beam it will send, like perfectly straight that is what we will think, but there will be some divergence. So, when it starts it may start like this slowly it may diverge very slowly. So, this is like the laser beam divergence gamma. So, each laser pulse after it is like divergence it will produce like a small footprint on the ground instantaneous laser footprint. So, whatever be the objects there that will reflect this particular laser beam back towards the sensor. So, and whatever the objects there if we will be measuring a kind of average elevation not like average elevation, but it is like whatever is there and how many reflections we receive the laser system can store this and from which we will be able to calculate the elevation of points. Maybe we will discuss it in detail in the next lecture. So, as a summary in this particular lecture we just got introduced to a LIDAR system which is like laser which uses laser for surveying purposes stands for light deduction and ranging. And we also seen the basic working principle of LIDAR system. With this we in this particular lecture thank you very much.