 Hello everyone, welcome to the next lecture in the topic platforms for remote sensing observations. In the last lecture we discussed about geostationary orbits and as a special case geostationary orbits and the main purpose of why satellites are launched in geostationary orbits and we also discussed about Lagrange points. So, if a satellite is placed in geostationary orbit it will be observing the same point on earth again and again. So, it will cover like a certain region of the globe like as given in the slide it will say this particular red circle here is kind of like an example for the coverage of geostationary satellite. It may cover like a much larger region, but for most of the useful applications will be contained within this particular zone. So, if a satellite is placed in geostationary orbit it will not provide global coverage it will be seeing only like a certain part of the globe, but it will provide continuous coverage over that particular part of the globe. Also we got introduced to concept of Lagrange points which are like special points which are which will come into effect because of the combined gravity of two objects which are in motion relative to each other like it is kind of like a two body problem it is moving in tandem. So, between sun and earth we can fix five Lagrange points L1 to L5 where L1 L2 L3 are like in the line joining sun and earth and L4 and L5 will be found as one of the corners or the vertex of equilateral triangle joining sun earth and that particular point sun earth and L4 sun earth and L5 something of that sort. So, for remote sensing application the point L1 is used there is a satellite called discovered with a camera called Epic in the L1 point roughly 1.5 million kilometers away like here there is again a small video by NASA I just play this. So, this is like how that particular epic camera sees the earth. So, this is like when moon crossed over the earth the epic camera captured it. So, this is three cyclones same time in Pacific Ocean captured by this camera the north pole here this north pole this is south pole this is smoke traveling moon shadow. So, these are like another moon crossing. So, basically this is kind of like example to tell the application of observations from L1 point actually L1 point the satellite is placed there is for mainly for like climate applications in order to observe the earth as a whole. So, there the time period is like it may appear like very close to the concept of geostationary orbit or geosynchronous orbit, but there are like differences huge differences between them. So, if some orbit if some satellite is placed in like L1 Lagrange point the time period and all is not synchronized earth will be this satellite will be here only earth is rotating around itself. So, earth will be rotating at a different speed and satellite will be moving along with the sun and earth as a whole like sun earth system the L1 point will be fixed because of the combined motion between them. So, the speed of revolution of satellite will not be synchronized with earth and all earth will be rotating at a different speed the satellite will be moving around in a different speed due to the combined effect of sun and earth. Okay. So, that is one thing. So, it will if a satellite is placed in L1 point it can observe the entire globe based on earth's rotation as the earth rotates the camera as it is like it appears stationary with related to earth. But at the same time due to earth's rotation it will be imaging different different portions of the earth it can see north pole and south pole and all because let us say during summer what will happen north will be earth is tilted at 23 and half degrees axis right earth's axis is tilted. So, the camera will be here let us say sun is here camera will be between earth and the sun. So, when earth is tilted like this during northern hemisphere summer the camera will be observing the north pole. During southern hemisphere summer earth will be tilted like this north will be pointing away from the sun. So, the camera will be seeing the south pole. So, normally if a camera is placed in L1 orbit it can observe different parts of the globe as the speeds are non synchronized this is not like a synchronized orbit on earth. Earth is rotating in a different speed satellite is moving around with a different speed whereas a geostationary orbit everything is synchronized earth speed and this thing and also since earth is tilted when you see from outer space when you move away from the earth and observe it we can see like the earth's tilt so similarly satellite also will be placed along the equator and it will also have the 23 and half degree tilt if you see this from outer space normally that will happen. So, these are like some small differences between L1 orbit and the geostationary orbit do not confuse them they are like completely different class of orbits with completely different purposes. Lagrange points if a satellite is placed there it will produce extremely coarse resolution images because the distance between earth and sun is as I mentioned it is 1.5 million kilometers 15 lakh kilometers huge zoom huge distance. So, the spatial resolution and all will be extremely coarse useful for climate monitoring climate applications ok. But I just wanted to let you know there is satellite from such long distances in a particular point L1 which is used for earth observation. So, the next class of orbit which we are going to see is polar or near polar orbit. So, this is one of the most widely used orbit for remote sensing purposes. So, what is polar or near polar orbit I already told you the satellite will be appearing as if it is moving from north to south like this. So, this is polar orbit earth will be like this satellite will be moving like this. So, in geostationary or geostationary orbit it will be moving along the equator like this in polar orbit it will be moving like this from north to south that is depicted here in this particular slide you can see like earth is rotating underneath it satellite is moving in its orbital plane and it will appear as if moving from north to south. So, this is how basically it will rotate. Let us say the satellite is starting from here it is kind of like start moving. So, what will happen is as a satellite completes one rotation what will happen it will earth would have moved underneath it like I draw here see this is earth let us say a satellite is starting from this particular point in this orbit. So, this is start moving from here by the time it completes one rotation and comes here earth would have moved and now the satellite will be landing in at a new location this is starting point by the time it comes and finishes it may be coming at a point to a west of it this is second orbit like this satellite will be moving in an orbit earth will be rotating underneath it if the satellite moves exactly over pole 90 degree north and 90 degrees south we call it as polar orbit. But normally for remote sensing applications satellite will not be placed exactly over poles not 90 degrees it will be placed with slightly higher inclination 98 degrees 99 degrees and so on the reason for it we will see later. If a satellite is placed in a near polar orbit what will happen is the satellite orbit itself will not be fixed in space that is just look at this animation in the previous slide here you can see the orbit as if it is appearing fixed in space and earth is moving normally we will think it will happen say geostationary orbits and all are fixed orbits it will be keep it will be fixed earth will be moving around with that is all. But if a satellite is placed in any angle other than like 0 degree inclination what that is away from the equator what will happen is the orbit will undergo precession what exactly precession is similar to earth rotating around itself the orbit also will move around itself. So, that is you can see from this particular video this is like highly exaggerated animation what is being shown here but this is just to tell you like the effect of orbital precession that is as the earth rotates around itself the orbit also will be keep on rotating say here you can observe orbit itself will be rotating around itself this is known as precision the orbit also will rotate in space say earth is fixed orbit is like this there will be kind of like an axis passing through earth center and the orbit center orbit center also will be aligned with earth center no issues earth is here orbit is here but what will happen is the orbit also will spin independently if the inclination is if it like lies above 0 degree. So, normally satellite will be placed with inclination greater than 90 degrees near polar orbits 98 degrees 99 degrees and so on. Such orbits will undergo this precision that is the orbital plane itself will rotate this is because of earth's asymmetry earth's asymmetry is like earth is not like a perfect sphere it is slightly flattened at poles bulging at the equator due to this thing and if a satellite is moving in an inclined orbit it will feel that asymmetrical nature of earth's mass distribution and it the orbit will undergo precision it will rotate around itself. So, this is actually we may think it is bad the orbit is not fixed the orbit itself is rotating around itself in a plane it is like kind of top you can imagine it will have an axis it will rotate around its axis the orbit also will rotate but this orbital precision is useful to us for remote sensing purposes we will see it how but before seeing that just one thing I would like to tell you if the inclination is like anything other than like 0 degrees or 90 degrees orbit will undergo precision if an orbit is placed exactly over poles 90 degree north and 90 degrees south the orbit will not undergo precision orbit will remain fixed in initial space if you go outside of earth the orbit will appear as if it is fixed around the earth ok it will not appear to undergo any sort of rotation but if any other inclination is there say 92 degrees 93 degrees 88 degrees whatever orbit will undergo precision that is orbit itself revolver on itself ok the orbit is processing and I told you it is useful in remote sensing how it is useful in remote sensing in order to achieve what is known as a near polar sun synchronous orbit we already seen geosynchronous orbit like the orbital rotation is synchronized with earth's rotation similarly for remote sensing applications we also have orbits which are classified as sun synchronous orbits what exactly sun synchronous orbits are why it is needed we will see let us say there is some orbit this is the orbital plane we are like seeing from the top ok so this is the orbital plane this is earth this is sun during different different time period in a year the earth will be keep on rotating around the sun right if the orbit is not undergoing any precision if it is fixed what will happen is the angle between the sun or the line joining the sun center earth center and the orbit will be keep on varying say here actually the earth is not seen it is like a mistake in the diagram earth will be like this ok so this is like angles so this is the orbital plane and the angle between the orbital plane and the line joining the sun and the earth will be keep on moving if the orbit is not undergoing any precision say let us say the orbit is permanently fixed earth is rotating like this orbit is fixed means earth will be rotating independently orbit will be there nothing would have happened if the orbit is not undergoing any precision due to earth's rotation around the sun the angle between the line joining the sun earth and the orbital plane will be keep on varying during different times of year this will this will change the elimination conditions like what will happen is during one time sun will be over at a place during morning at one time sun will be over at a place during evening solar elimination conditions will change drastically but for in optical remote sensing we learnt that the reflectance or same object will look completely different if the elimination changes right which is the BRDF effect this will be like a huge problem if the sun is continuous if the imaging happens during different different day let us say like I am going to acquire some images over Mumbai city today I am acquiring image at say 10 a.m tomorrow I am acquiring an image at say afternoon 2 p.m after that I am acquiring an image at evening 6 p.m and so on so what exactly will happen I am changing the elimination conditions as the time of observation changes the solar elimination will change the strength of elimination the angle at which the sun shines everything will change this will severely affect our remote sensing observations. So, if we were we will normally try to achieve uniform elimination conditions for that particular thing sun synchronous orbits will help. So, in sunscreen if a satellite is placed in sun synchronous orbit then the angle between the line joining the sun and the earth and the orbital plane will always be equal whatever be the season. So, the sun will be eliminating the earth surface from a constant angle there will be seasonal variation like the amount of solar radiation coming in will vary but at least the elimination conditions will be more or less fixed because the orbit is processing we are using that particular orbital precision to our advantage it is a natural phenomenon the orbit orbital plane rotating around itself is a natural phenomenon we are using it for our own benefit that is one thing and also if a satellite is placed in sun synchronous orbit then the satellite will be moving or observing the same spot on the earth at the same given means mean local solar time or mean solar time in short. What does mean solar time that means let us say if you look at like catalogs or brochures of different different satellites say landsat modus and dol they will mention what is known as a equatorial overpass time. What that signifies is it will mention at what time the satellite will cross the equator they will also give whether it is ascending mode or descending mode let us say a satellite has been mentioned 10 am equatorial overpass time in ascending mode what does it represent it will say at the equator whatever like earth is like a huge sphere we can imagine so along the equator 0 degree axis there will be lot of places right whatever be the place like whenever the satellite crosses the equator from north to south in ascending mode I told you from north to south the time will be 10 am at that particular location at which the satellite is overhead let us say if the satellite is overhead in Mumbai say Mumbai is not at 0 degrees but let us assume let us say Ethiopia somewhere closer to equator okay so the time will always be 10 am whenever the satellite crosses that particular location same thing will happening even to other places. So here what we are doing we are making sure the sun is having like a almost constant elimination conditions and the satellite will image those locations at a constant time in order to ensure almost uniform elimination conditions and to achieve this taking images at different different times of a day. So this is known as sun synchronous orbit okay so a sun synchronous orbit is one in which the angle between the orbital plane and the line joining the earth and the sun it will be kept constant that is we will make sure we will put a satellite in such an orbit that is the orbit undergoes precision the orbit itself will rotate and the rotation speed will be adjusted so that it will match this seasonal effect to some extent. This is sun synchronous orbit and also the time of observation over a given location on the earth will be equal. This is the advantage of launching satellites in sun synchronous orbits so most of the remote sensing satellites are placed in near polar sun synchronous orbits. I am repeatedly telling near polar because I told you before if a satellite is placed exactly over poles with inclination is equal to 90 degrees it will not undergo precision such orbits will never be sun synchronous orbit will be fixed in initial space if you see from the outer space the orbit will never undergo rotation. If inclination is more than 90 typically it will be between 90 to 100 degrees 98 degrees 99 degrees and so on the orbit will undergo precision and the altitude will be like matched in order to achieve this sun synchronous nature. So, as like a small example we will calculate the rate of precision of a satellite orbit if it is in sun synchronous orbit that is at what speed the orbit will rotate around itself earth is spinning the orbit also will spin that is like the precision at what speed it will rotate we will calculate its angular velocity. It is very simple to imagine no need to derive anything just like I will overly tell you. For an orbit in for a satellite in sun synchronous orbit if an orbit has to be sun synchronous what it has to do? It has to match the speed of earth's revolution that is the speed at which the orbit rotates around itself like the precision should match the speed of earth's revolution around the sun correct that is let us say sun is here it is going to move in an orbit around the sun there is orbit the orbit also will rotate and that rotation should match this revolution speed. So, in one full year only it should the orbit should complete one full circle say the orbit is starting like this satellite is moving like this slowly the satellite will be moving in like a different plane like this it will start rotating in one full year this orbit will complete one full circle like 365.24 days that will be the speed that is that will be the speed in which this orbit itself will be rotating I am not talking about satellite speed satellite anyway is moving in the orbit but I am talking with the speed in which the orbit itself will rotate the plane itself will rotate okay. So, in one full year the plane will complete one full cycle around itself then that particular plane will be sun synchronous okay. So, typically the value will be if you work out we can work out the angular velocity with which such orbit rotates will be 1.991 I just write it down the angular velocity of the orbital precision of or I write it as processing orbit will be 1.991 into 10 power minus 7 radian per second extremely small value if an orbit rotates around itself with this angular velocity in one full year it will complete one full circle around itself then it will achieve the sun synchronous nature. So, if a satellite is launched in that particular orbit it will cover one full circular and itself and the satellite placed in such an orbit will maintain more or less uniform solar elimination conditions and also it will overpass a given location of earth at a same mean solar time. This is the advantage of placing a satellite in sun synchronous orbit most of the remote sensing of satellites, Landsat, Modus, SMAP everything are placed in near polar sun synchronous orbits. Also there are calculations we can actually make and show that for the typical orbital height around this 700 to 800 kilometers range normally at which we put the satellites or like 500 to 800 I will say the inclination angle should be between this above 90 90 to 100 degrees like Landsat satellites has inclination of around like 98 99 degrees there is a satellite called ISAT2 with inclination around like 91 degrees I think. So, the angle will inclination angle will always be more than 90 it will not be exactly over poles it will be slightly away from the poles then only it will undergo precision then only it will be made sun synchronous. So, this is one thing the inclination will be always above 90 less than 100 keeping in mind the altitude the range constant this is one thing we have to remember and also the direction in which the satellite moves will be in retrograde that is let us say earth is rotating around like this from west to east the satellite will appear as if it is moving from east to west it will be moving like this earth will be moving like this satellite will be moving like this. So, the net effect of motion in the east-west direction will be opposite to that of earth the satellite will is moving from north to south only but there will be like a slight component in the east-west direction also and that component will be opposite to that of earth's motion earth will be moving like this satellite will be moving like this ok. So, this is for satellite to be in sun synchronous orbit the orbit will be retrograde inclination will be greater than 0. So, as a summary in this lecture we got introduced to polar orbit or near polar orbit also we started discussing about the sun synchronous orbit and why we need sun synchronous orbit we will discuss these concepts further in the next lecture. Thank you very much.