 Hello everyone, welcome back to the course. We are discussing about the topic platforms used in remote sensing and in the last lecture we discussed about sun synchronous orbits basically or polar or near polar orbits in which a special type of orbit called sun synchronous orbit. So, in polar or near polar orbit essentially the satellite will be moving in an orbit that is oriented predominantly in the north south direction. So, whenever our orbit is like having any inclination other than like 0 or exactly 90 the orbit will undergo some sort of precession I have told you in the last class. So, precession means it will rotate around the earth's polar axis. This will happen because of earth's asymmetry and we take this to our advantage in launching satellites in orbits known as sun synchronous orbits. So, what are sun synchronous orbits? Sun synchronous orbits or those orbits in which if a satellite is placed it will come over a same place in a given direction at the same local mean solar time. Also the angle between the line joining earth and the sun and the orbital plane will remain constant throughout the year. So, we are like adjusting the precession such that the orbital plane completes one rotation around itself in one year. So, that it will always be the angle between earth, sun and the orbital plane will be maintained. We will continue further in today's lecture about further interesting topics which helps us to do repeated observations and global coverage. So, this slide actually gives you like a brief overview of like what are sun synchronous orbits this angle is what will be maintained. This is the major need and as I told you orbits will basically undergo precession and we are taking advantage of this naturally occurring precession and making the orbit to be sun synchronous. Also like I told you the orbit should complete one rotation around itself in one year which implies the angular velocity at which the orbital plane is processing will be having a value of something like this. A very small value, but still having such a value alone will make the orbit process in a definite rate. I actually told you that there are like I actually told you that for a satellite in sun synchronous orbit the local mean solar time at which the satellite overpasses will be the same irrespective of the launch due. Maybe we will see that with like a single diagram. Just see this particular figure this tells us like for a given latitude and for if a satellite is placed in sun synchronous orbit then the time or the local mean solar time at which the satellite overpasses a particular location and a particular direction whether it is in ascending or descending mode will be the same. So, here we have given an example of a satellite crossing the equator in descending mode at 10 am every day. So, I am just drawing this line. So, this tells us a particular satellite will cross 0 degree latitude every day at 10 am that is irrespective of longitude say 0 degree latitude means the equator surrounding the earth. So, whenever the satellite crosses the equator it will cross the equator in descending orbit like this from north to south in descending orbit whenever it crosses the local mean solar time at the place at 0 degree equator will be 10 am. This is like an example I am telling. The time period can be adjusted for different satellites but just as an example I am telling if a satellite overpasses at 10 am what will happen. Now, this overpass time normally what we call as equatorial overpass time will vary with latitude. Say here in this self for example this satellite will overpass the equator at 10 am but any other latitudes at higher than 0 degrees let us say 20 degree north latitude the time at which it will overpass will be slightly more than that that is maybe we will take 10 10 am for example I am telling. On the other hand if it is like 20 degrees south latitude then the time period may be around say 9 50 am at that particular location. So, this suggests the time period at which the satellite will overpass will vary with latitude but at the same time it will be more or less constant. So, normally if a satellite is launched in near polar sun synchronous orbits in the orbital parameters you will find a variable called or like a parameter called equatorial overpass time where the satellite team will mention and to what time the satellite will overpass the equator in which direction in ascending mode or in descending mode that they will give. Using that information and assuming like a circular orbit and like a spherical earth we will be able to calculate at what time the satellite will overpass in a given latitude like not all places on earth or in equator at each and every place on the earth the satellite will have one definite overpass time which we can calculate knowing this particular equatorial overpass time. So, this slide gives like a very simple formula for relating the equatorial overpass time with the time at any other latitude. Say that it is like a very simple one L is the latitude of the place, I is the orbital inclination, T is the equatorial overpass time and T L is the in any latitude in which we are interested upon. So, in this particular equation we have to use plus sign if the equatorial overpass happens in ascending mode or minus sign if it happens in descending mode. So, this is like a very simple formula that helps us to know how orbits or how satellite will overpass or at what time satellite will overpass over a given location. In order to achieve this sun synchronous orbit we need to adjust the precision rate, I have already told you. The precision rate should match with the earth's revolution time that is the orbit orbital plane should complete one full rotation around itself in one year. So, that precision rate can be changed by changing the orbital inclination and also the orbital height by playing with this parameters we can change the precision rate and we will adjust the precision rate such that it matches with earth's revolution speed. So, typically for earth observation satellites like we will launch remote sensing satellites normally in the range of 500 kilometers to 1000 kilometers altitude above the earth's surface. So, just see in this particular figure. So, the normal altitude will be 500 kilometers to 1000 kilometers for that particular altitude in order to achieve the precision rate that is required that 1.991 into 10 power minus 7 radians per second. In order to achieve this precision rate there is like a definite set of inclination that we should achieve in the orbit. So, this is something like this. So, the inclination should be between say 90 degrees to 100 degrees range. Most likely it will be for most of the satellites it will be around say 98 degrees 99 degrees and so on. So, if the orbital height is between 500 to 1000 kilometers above the earth's surface then the inclination has to range between this 90 to 100 degrees in order to achieve sun synchronous orbit. Then only the orbital plane will press us with the definite rate at which we will match it with the earth's revolution. This signifies all sun synchronous orbits are near polar orbits. It cannot be polar it can only be near polar. The inclination will always be greater than 90 to 100 degrees for most of the earth remote sensing applications that is one thing. And also the satellite motion will be in retrograde direction that is say orbit will be like this let us say earth will be moving like this. Say orbit is here earth will be moving like this. So, the satellite will be moving in a net direction opposite to that of earth. If earth is rotating from west to east the satellite will appear to move from east to west in a retrograde direction then only this orbital precision will be achieved. But orbital plane itself will rotate in the same direction as that of earth like the plane in which the satellite is rotating that itself will rotate in the same direction as that of earth. But the direction in which the satellite is moving will be in a direction opposite to the tough earth. So, this is what we have to keep in mind when we learn about satellites that are in sun synchronous orbit all these things should be in our mind when we look at those satellite data and use it in our analysis. Coming back to this particular equation like the equation which relates the equator lower past time with the time taken at any latitude. Now I just I have told you like we will like further experiment with or analyze it little bit deeper let us say this is like the orbit. I told you now the orbital inclination always has to be more than 90 let us say this is now 98 degrees just for example I am telling and satellite is moving like this in ascending mode. So, this is like the normal convention for satellites in polar or near polar orbits the inclination of the orbital plane will always be measured when the satellite is in ascending mode or in the ascending mode of its overpass that is like the convention we follow. So, that is why I am marking it like this. So, here the orbital inclination is given like this. Now let us see how this particular time period will vary. So, satellite is moving in this direction from south to north let us say the equator lower past time here is for example again 10 a.m. So, on all the places in southern hemisphere the overpass time will be greater than 10 a.m maybe 10 5 10 10 and so on whereas on the other hand for places in the northern hemisphere the time of overpass will be less than 10 a.m. So, this is the case if the satellite is in ascending mode sun synchronous orbit. Let us consider about the descending mode. For satellite in descending mode orbit has to be drawn like this north south this is the equator lower past time again let us take the same example of 10 a.m. If the satellite is in descending mode then the overpass time for the places in northern hemisphere will be greater than 10 a.m and the overpass time for the places in southern hemisphere will be less than 10 a.m. So, based on this general overpass time or this general equation we can calculate at what time our or the satellite will overpass our given latitude the latitude at which we are interested upon. This will not change with longitude like whatever be the different different longitudes is there it will be constant for the time will vary only based on like a given latitude. So, for all the places present in a same latitude the overpass time will be the same that is the characteristics of sun synchronous orbit. But please remember one thing the time here we are talking about is mean solar time that is given by normally taken from the Greenwich meridian. So, we will measure it based on the longitude of the place in which we are located. So, roughly say India has like a standard longitude let us take an example of 82.5 degree east longitude. So, this is where this is a standard longitude for India. So, the time in our watches normally will represent the time taken at this particular longitude as this is like the standard longitude for Indian country. But this is like what we call the standard time. But the time what we are talking here is the local mean solar time that is we have to calculate or we need to know the longitude of our place. For that particular longitude what is like the difference in time between the Greenwich meridian and our place normally like one degree of longitude from Greenwich meridian will increment the time by 4 minutes that is our idea that is if time is say midnight 12 o'clock in Greenwich meridian at 1 degree east longitude the time will be 12 hours or 0 0 hours 4 minutes. At 2 degree east longitude it will be 0 0 hours 8 minutes and so on. So, that is like the general based on the average rotation of or average revolution average rotation speed of earth around the sun. So, this is local mean solar time we have to calculate it at all the longitudes on which we are located upon ok. We should not be seeing just the time in our watch, the time in our watch will give us like the standard time that is based on the standard meridian for each country. For example, as I already told you the standard meridian for India is 82.5 degree east. But when we want to calculate the time in which satellite will overpass first what we should do is we should first know the equatorial overpass time. From the equatorial overpass time we should adjust it to our latitude in which we are located or which we are interested upon. Then based on the longitude also like we have latitude now in order to locate a point we also need a longitude. At that longitude what will be the local mean solar time that will come to us. Let us say we are in India and I am interested in knowing the overpass time of a satellite in longitude something around say 75 degrees not in 82.5 degrees. So, the standard time which is shown in my watch I should adjust it. I should bring that to this 72 degrees or 75 degrees in which I am interested upon that time only I should take. So, there is always there may be a slight confusion between standard time and local mean solar time. Here all the time we are talking about is local mean solar time and not the time shown in our watches. The local mean solar time of a particular place should be calculated based on the longitude of the place at what time then only we will achieve that particular or we will be able to calculate the exact time in our watch at will the satellite will overpass. We always have to do this kind of minor adjustments. So, now what we have discussed is maintaining the same overpass time. But what are the main aim of launching a satellite in sun synchronous orbit? The main aim of launching a sun synchronous orbit is to get maximum coverage around the earth that is like one major goal that is from geosynchronous orbit we have already seen we will not be able to get maximum coverage around the earth. Geosynchronous orbit will be seeing only one particular area of the globe. In order to get like a global coverage naturally the satellite should rotate in the near polar orbit or polar orbit earth will be rotating based on which you will be able to cover the entire globe that is one thing. Second thing is periodic observations that is we will always want the same location on the earth to be imaged under same sensor object and sun illumination conditions that is let us say satellite is moving in one particular orbit there is a place underneath it some city or someone. We will want the place to be imaged with the same orbital parameters same look angle and everything in a repeated manner maybe once every two weeks once every one month and so on. This is to achieve like periodic observations under same conditions of achieving same geometry between satellite the target even like the sun even the sun synchronous orbit will achieve this more or less like the same conditions of solar illumination more or less it will vary with season but we will try to achieve as close as constant solar illumination condition. But this satellite overpass will vary safe example that is a place here a satellite can see this particular place from Nader or from any other orbit by treating its camera or by like having like a very wide scan angle anything is possible. Say take an example for like a satellite called MODIS which has like a very wide scanning angle of around like 2300 kilometers. So, same place can be imaged from multiple orbits when it is directly over at the place when it is in an orbit away from it by the scan or when the orbit is placed like here again by the scan. So, by increasing the scan angle also the same place can be imaged continuously from adjacent orbits that is possible. But scientist will always aim for achieving repeated orbits after every certain number of days that is the concept of getting repeated orbits. Repeated orbits means the satellite track or the ground track say the satellite is rotating in its orbital plane whenever a satellite is moving on in the space we can always create like a Nader track. Nader track means we can draw like a straight line connecting the satellite and the point on the earth surface. That will create like a Nader track. What we want is after certain number of days the Nader track should repeat say once every 16 days the satellite should exactly come to the same overhead the same ground point maybe once every 16 days once every 24 days and so on. Different different satellites have different different characteristic parameter and as I told you the main aim of doing this is to get like a repeated observations of the same place with same geometry between the satellite and the point on the earth surface. This is one of the periodic repeated coverages one of like the major aim of launching a satellite in sun synchronous orbit. So, launching a satellite in sun synchronous orbit will give us a near global coverage more or less and then it will help us to achieve like a higher spatial resolution than what is achieved by geostationary orbit that is also there definitely because of the lower altitude then it will help us to make periodic coverage in a repeated manner of the same place with same geometrical conditions. But how to achieve this repeated orbits first before going to see that we will understand how the satellite will trace the ground track. This is equator the earth is moving in this particular direction north south this is the orbit satellite is in descending mode. Let us say the satellite is starting from here it is moving in descending mode it is come going to complete one rotation when it is going to complete one orbit the earth would have moved a certain distance let us say this is point A when the satellite started. So, by the time the satellite completes one full orbit the ground point A would have moved certain distance to its east and a new point from west like the ground point B would have come here. So, the ground track of the satellite is changing because of earth's rotation underneath it orbit is there but due to earth's rotation the ground track will be keep on changing. So, by the time the satellite completes one orbit around the earth the ground track would have changed it would have come to like a new location like now the satellite will be overhead a place that is west to the to its starting location. Other than this orbit is also processing like orbit will process in this particular direction like it will process in the same direction of the earth ok. So, the net effect will be more like one due to earth rotation and the other due to orbit's precision. The net effect will be still the satellite will be now in the spot that is normally to the west of its starting location. So, between the first orbit and the second orbit the satellite would have come to the point that is to the west of its starting point. Similarly, third orbit again the new point where it is starting its new orbit will be further west of its second point and so on. It will always be proceeding like this not naturally when you look for like many satellites in sun synchronous orbits you will you can sense this particular pattern. Then let us assume it completed one full day normally a satellite will have 14 to 16 orbits normally per day ok. This will be not like integer every day the number will be 14 point some orbit 15 point some orbit it is not a integer it will vary it will have like some fraction I will tell you why later. So, let us say one full day has elapsed earth has completed one full rotation. Now normally what will happen is now the satellite is going to start its first orbit in day 2. The first orbit in day 2 will most likely will not be in the same ground point a say same ground point a is where the satellite started its first orbit in day 1. It it made like several orbits with different different starting points for each orbit. Now when comes the second day second day the first orbit it will not start with the same ground point a normally scientists will not launch a satellite in such an orbit it is possible to achieve that but normally they will not do. It will start again at some other point most likely to the west again to the west of this starting point ok. So, what they will do is every day say this is day 1 orbit 1 then on day 2 the orbit 1 orbit 1 we may be like this ok. So, day 3 orbit 1 something like this. So, every day the new starting point or the starting point of the first orbit will always be different than the starting point of the previous day this will always be proceeding like this. This is to get like a global coverage with high spatial resolution and smaller swath width maybe I will explain all these later. So, normally satellites in sun synchronous orbit the orbits will be designed in such a manner the ground coverage happens like this. So, after every time the satellite completes 1-1 rotation it will move to the west and after 1 full day also when it starts a new like orbit number 1 in day 2 also the starting point will be different than the starting point on the day 1. After a certain number of days say 16 days the satellite will start its first orbit exactly over at the point a this is what is called repeated orbits ok. So, again the entire cycle will go on again after 16 days satellite will come over at the exactly the same point a when it is starting the first orbit in day 1. So, there is like a definite number of cycles say 16 days once 24 days once even some satellite has like 90 days once when the orbit track will repeat this is this kind of orbit is being like designed or placed in order to achieve like a global coverage ok. So, in this lecture we have discussed about like again we discussed about orbital procession we also discussed about like for near polar sun synchronous orbits the inclination angles we have discussed about like the over past time conditions at different different latitude and so on. In the next lecture we will continue with this topic and finish this particular topic. Thank you very much.