Atmospheric Errors in satellite Image
- Hello everyone, and welcome to our new discussion which is on atmospheric errors and corrections under this remote sensing essential course. Earlier we briefly touched on this issue and but today we are going to discuss in depth the reasons for atmospheric distortions and then how these are reflected in our satellite images finally we will also see what are the possibilities of getting rid of atmospheric distortions.
So as you know that you know between satellites and earth surface there is atmosphere. But if
the atmosphere is very thin or would have been thin then there should have been a delight for RCO or for any need remote sensing people. But unfortunately, it is not true in the case of earth and though it is very important for life, it is fine. But if we think about mars, then mars is having a very thin atmosphere. And therefore though it is written here the geologist delight but does not mean only for geologists for engineers or anyone who uses remote sensing data. And because there are no clouds and the atmosphere is very thin and therefore we get an almost transparent atmosphere and very thin atmosphere and very clean satellite images from morning to evening through the reflection of sunlight we get very good images. But this is not true in the case of the earth, the whole radiation spectrum of remote sensing is also available if no atmosphere or very thin atmosphere is there. As mentioned that in the case of the earth all radiation passes through or passes through a dense atmosphere relatively dense compared to mars. And because a sensor or reception or a satellite earth station is located on the ground, so this has to be also incorporated the regions or effects of the atmosphere. So the solar radiation must pass through that atmosphere and then back again to the sensor. So this creates a lot of problems in the satellite images and sometimes when it is cloudy that is also an atmospheric phenomenon. Then you clouds completely will mar the land part and do not see anything of the land. So this kind of a complication is there and as we know for a sensor measuring radiation emitted by the earth in case of thermal infrared. The path is single, not double in the case of the reflector or solar-based radiation because sunlight will come, it will interact with the atmosphere, and then when it is reflected back by the objects it goes back again. And third time also the role of the atmosphere is there when the data is being received by the earth station but at that time because the data in the microwave parts does not affect much but still there are rules are there.- And for an emitted part only the single path because whatever objects are emitting energy that is being sensed by the sensor. But nonetheless, it has to also pass through the atmosphere. This figure we have used in the very earlier stages of this course is the interaction with the atmosphere. And reach the accounting of radiation, so as we know that been earth and sensor there is the atmosphere. And there might be sometimes clouds which absorb, which also reflect and then the cloud themselves may emit energy. So when we go for emitted or thermal infrared then clouds also play an important role, thick clouds can be detected very easily. But when you are having a very thin cloud or haze or that kind of thing then it becomes very your image becomes unclear. And the removal of such effects also becomes very challenging. So as we know that if 100% of we count 100% incoming solar radiation then 47% of that is absorbed by the earth and 17% is absorbed by the water vapor which is within the atmosphere, and 4% is absorbed by the cloud. And these are just rough estimates and this is a very atmosphere is a very dynamic thing so things keep changing. But on average and then 6% is backscattered of that 100 back to the atmosphere or in space and then 19% the large part is reflected only 4% is absorbed whereas a large part that is 19% is reflected by clouds if they are there. Also, water bodies also reflect maybe lakes, maybe rivers or sea part.
- So they also reflect a lot of energy and then if we go for along with part of energy then what we see that there is longwave radiation from the earth which is going out in case of thermal infrared. So absorption by the cloud water vapor carbon dioxide here emission 20% by clouds, emission by water vapor carbon dioxide, etc. they also contribute. So a lot of longwave radiation is coming but it is getting disturbed or absorbed reflected basically absorbed emitted by that atmosphere itself. And of course, the heat which has been absorbed earlier 47% is transmitted back by the land that is latent heat transfer and which goes into the space. So this tells us how many processes are going on between a satellite and the surface of the earth and within the atmosphere. And therefore that is why I said that in order to remove these distortions it is really challenging. Because one the atmosphere is dynamic and second is so many processes are going on emission within the atmosphere, reflection within the atmosphere, and absorption within the atmosphere. And that too depends on the conditions other conditions maybe the wind speed or maybe the moisture content and temperatures also. So all these things make very complicated this part nonetheless this in percentage we have already discussed so in short this figure is in a simplified form. And before radiation especially for what we are talking about for remote sensing that reaches towards the earth, it has to travel through some distance at the earth's atmosphere. And the particles and gases which are present in the atmosphere can affect incoming light and radiation. So we are concerned mainly with the radiation directly, of course when the whatever it is going back at that time also it will have a problem. So these effects are caused by the mechanism of scattering and absorptions. And we also know that see some part of the EM spectrum is displayed here and this is spectral radiant emittance. And we find that these are the water and gases which are absorbing a lot of this radiation and therefore it becomes very difficult in that way visual part is up to only this much. So nitrogen, oxygen which is present in the atmosphere, ozone also present an atmosphere and carbon dioxide. So many of these gases will absorb and we note to provide a clear ultimately, may not provide a clear remote sensing image. So all these interactions with radiation by vibrational and rotational transition and then the net effect of which is the absorption of energy in specific wavelengths which you can see here how water if you see H 2 O it is almost in every part of the EM spectrum this part, especially which is being displayed, every part there is absorption, almost every part. So the moisture or water within the atmosphere having a lot of absorptions in different parts of the EM spectrum. So other gases but their rules are limited and not everywhere like ozone plays very large absorption in case of visible but in other parts, it is not that common.
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