 day's class. So just to break the monotony, today let me begin by showing you the video of sea surface temperature from Earth Observatory NASA.gov website. So you see the colors here represent the sea surface temperature in degree Celsius from minus 2 to 35 for different months and for different years. You see oceans, they are a very important part and parcel of Earth's climate system because water, it stores thermal energy. And let us look at the Pacific and the North Atlantic. Water of the Pacific, the surface waters tend to be salty and warmer and hence more buoyant, okay. And when this water is lost through evaporation, it cools and it results in saline denser water that tends to sink. And when it sinks, it draws the warmer water from lower latitudes and hence this helps drive the global circulation of oceans. Now you may be wondering why am I showing you this video and why are we discussing about sea surface temperature because sea surface temperature is a key variable that is obtained by passive microwave remote sensing by satellite radiometers. So with this background let me welcome you to the fourth lecture of the fifth module wherein we shall learn the principles of measuring sea surface temperature abbreviated as SST and soil moisture and of course a few other variables like ocean winds and sea ice etc. all using passive microwave remote sensing, okay. The basic principle of measuring SST or sea surface temperature is important. So let us start by trying to understand what is the basic principle using which satellite bond radiometers measure SST. In the past you know SST could only be measured by using ships and buoys whose ranges were limited and we also saw earlier that microwave brightness temperature abbreviated as PB is directly related to the physical temperature times the emissivity. So what you see in the screen in front of you is the sea surface temperature again in degree Celsius from minus 2 to 35 from Earth Observatory NASA. In ideal cases SST or the salinity they can be inferred from microwave brightness temperature. So when I say TB I hope that you understand it refers to microwave brightness temperature. But then you know this is for the ideal case where the ocean does not contain any debris, the ocean is not affected by winds, this is ideal condition. But then realistically speaking the ocean will certainly be influenced by the effects of wind and a number of passive microwave instruments are being used to measure the sea surface temperature like the scanning multi-channel microwave radiometer that is SMMR what we saw in the last lecture as well the TRMM microwave imager that is abbreviated as TMI and the advanced microwave scanning radiometer AMSRE. So as we discussed earlier passive microwave instruments they are affected by large footprints. Footprint is the area elliptical area that a radiometer sees when it is looking down at the earth surface. Larger footprints means loss of detail, isn't it? But then the advantage gained in passive microwave remote sensing is that the measurements can be acquired even when there is cloud cover and radiation at these longer wavelengths are largely unaffected by clouds and they are generally easier to correct for atmospheric effects because let me re-itrate microwaves have the ability to penetrate through clouds because the region of wavelengths coincides with the atmospheric windows. I want you to remember that atmospheric windows, microwaves, ability to penetrate through clouds. Okay so let us try to look at now ocean salinity. At this point let me make a mention that thermal infrared remote sensing is better suited for measuring SST but then when it comes to ocean salinity microwaves dominate. So a scene here is the global map of monthly sea surface salinity from salinity.oceansciences.org. Now the CMOS mission, let me take a note to mention or introduce you to the CMOS mission which stands for Soil Moisture and Ocean Salinity. So launched in November 2009 the CMOS mission uses a frequency of 1.4 gigahertz at L band, L band microwave frequency is used to measure ocean salinity and also L band has an advantage that it is not affected much by atmospheric attenuation okay and throughout this course we now have a knowledge that longer wavelengths they require larger antennas for better spatial resolution isn't it. Now I hope you recollect the discussion we had when we were trying to understand about radars you know the length of antenna required. So throughout this course now that we are in module 5 we have a basic understanding that whenever we are using long wavelength microwave radiation it requires a larger antenna for a better spatial resolution and for any satellites say at 800 kilometers altitude it would certainly require an antenna of more than 10 meter length to provide a spatial resolution of at least 10 kilometers. Let me re-hydrate imagine a satellite that is operating at an altitude of 800 kilometers from the earth's surface it would require an antenna which is longer than 10 meters to provide a spatial resolution of at least 10 kilometers not practical at all isn't it. So CMOS uses something known as a Novel Interferometric Technique to achieve spatial resolutions better than 50 kilometers by using three small antennas together. Now you may be wondering what is interferometric technique that will be covered as part of the next module wherein we will learn in detail about radar interferometry. But to summarize both sea surface temperature as well as ocean salinity they change the emissivity values they change the emissivity and for SSD measurements frequency ranges between 5 to 10 gigahertz are better suited and for measuring salinity lower frequencies need to be used. So given here are the details of the CMOS mission which carries Miras that stands for microwave imaging radiometer using aperture synthesis passive microwave 2D interferometric radiometer operating at L band that is 1.4 gigahertz 21 centimeters and the aim of Miras is to map the soil moisture, sea surface salinity, sea ice thickness, wind speed over oceans etc. So I would urge you to visit this site to get more details about CMOS. Moving forward try to also discuss about ocean wind. Now microwave frequencies below 5 gigahertz they are not very sensitive to wind roughening and ocean winds they not only contribute to roughening but they also contribute to generate something known as a surface form which can often complicate the retrieval process. So for SSMI the wind speed information is generated from 19 gigahertz vertical polarization, 22 gigahertz vertical polarization and 37 gigahertz both in horizontal and vertical polarization and they detect ocean surface roughness which in turn can be linked to surface wind speed and this data tends to act complementary to the wind direction information which are given by ships and buoys. Okay so now with this background let us try to understand the principle of measuring sea ice. The measurement of sea ice using passive microwave remote sensing it has been a major area which was driven initially by military requirements during the Cold War and there was a need to map the sea ice for naval vessels carrying ballistic missiles which actually led to the special sensor microwave image that is SSMI instrument to be flown as part of the defense meteorological space program that is DMSP. So mapping sea ice it is also crucial to study about the earth's climate and you see frozen water it has a very high albedo in contrast to open water and when in situ measurements are difficult to acquire the use of satellites to provide coverage over the Arctic as well as the Antarctic oceans has a lot of advantages and sea ice cover it itself is very dynamic which shows large inter-annual variability which makes the choice of frequency that much more important. We cannot select any random frequency at any random polarization to measure sea ice. You know for sea ice the emissivity is different than that of ocean and the dielectric properties of ice varies as it ages which means that we need to consider the relative proportions of first year and multi-year ice which are again frequency and polarization dependent. So if you remember when we were discussing about how precipitation is estimated using microwave remote sensing there was a mention made about ratios. Here we can use polarization ratios to estimate sea ice to map sea ice and one such example is given in the screen in front of you where is where in the brightness temperature that is TB in two frequencies in two polarizations are used and you know we can also combine the information from 19 gigahertz and 37 gigahertz to create scattering index similarly we can also use polarization ratio at 19 vertical and 19 horizontal to map the sea ice. So till now we have been learning about how passive microwave remote sensing can be used to measure sea ice and ocean wind, sea surface temperature, ocean salinity etc. Now let us try to understand about measuring soil moisture. So examples of satellite missions carrying radiometers are listed here. We have already discussed about SMOS that is soil moisture and ocean salinity. We also have a mission known as MAP that is soil moisture active passive which uses an L band radiometer at 1.4 gigahertz. We also have the advanced microwave scanning radiometer 2 AMSR2. So this table is shown to make you understand that multiple frequencies can be used to study about soil moisture and each of these frequencies will result in a spatial resolution which is of the order of kilometers. For example for 7.3 gigahertz the footprint size is 62 by 35 kilometer. So I hope you have an idea of how big an area it is. Now there are many research studies which are conducted for comparative assessment of satellite derived soil moisture with that measured by institute sensors on site and this by itself is a very vast topic you know with many important facets of discussions involving temporal scales and spatial scales but of course that is outside the purview of this course. So I will just take the liberty to introduce to you that passive microwave radiometers operating at L band can be used to estimate and study about soil moisture. Okay so with this background let me now take your attention to the in-house studies which were conducted at IIT Bombay using a L band handheld radiometer which was purchased through DST center of excellence in climate studies at IIT Bombay. So if you refer literature the L band radiometers that have been used are either mounted on trucks because they are very large and heavy or they can be mounted on towers, lightweight radiometers which are handheld are rare okay. So what we did is we purchased one such radiometer the diagram of the radiometer is seen towards the right side and the specifications of the L band radiometer is tabulated on the left side. So it is operating at a central frequency of 1.41 gigahertz it has a sensitivity of 1 Kelvin and look at the antenna size very compact 160 mm by 160 mm by 10 mm. So what we did is our aim was to measure microwave brightness temperature at very fine spatial resolution and to analyze the spatial variability of observed soil temperature, emissivity and TB at different polarization. So the survey setup what was used is shown here a four wheeled platform that you see here was used having a length of 1.2 meters and 0.9 meter width and it is so designed and constructed so that we could get accurate measurements by using the handheld lightweight ground based radiometer which was used for the present study. So whenever the measurements were being taken this radiometer was mounted at one end of the platform as you can see on a horizontal bar which was inclined at 40 degree to the horizon and it was at a height of about 1.5 meters from the ground as shown in the figure. So this push cut was attached with the radiometer and the battery to carry out measurements. So what I will try to do is I will try to discuss about how we calibrated this radiometer and what all measurements were carried out with the radiometer. So we already know the importance of instrument calibration is not it? We have repeatedly discussed this throughout our lectures. In this case the calibration was carried out to estimate the radiometer response wherein the relation between voltage and radiometer antenna power was determined in terms of temperature. So the calibration method used in the study is a two point calibration hot and cold sources were used and there are two external reference sources used to calibrate the internal loads of the radiometer. The reference sources are sky pointing and a microwave absorbent that acts as a black body. So sky pointing and a microwave absorbent that acts as a black body what you see in figure B. Now these two reference loads are also referred to as cold for the sky reference what is seen in figure D and hot for the microwave absorbent since it has the value of the absorber temperature which is close to but not equal to the ambient temperature. So hot load is shown in red, cold load is shown in blue, H antenna and V antenna are shown in different colors. So hot and cold calibration was conducted to calibrate the instrument. And then for the study area we used two sites one is the Jim Khanna ground of IIT Bombay and second is the Kendriya Vidyalaya ground also within IIT Bombay campus. So these two grounds were relatively open and therefore we thought to conduct in-house preliminary studies using the radiometer instrument at these two locations. So what you see towards your left side is the Jim Khanna ground and what you see towards the right side is the ground over which the soil temperature measured by the radiometer is shown in different colors. Now different colors indicate the different values of soil temperature in Kelvin. So the interesting thing to note here is that this experiment was conducted during an afternoon. So one part of the area was relatively shaded whereas the other part was exposed to the sun which is evident by the slight difference in temperatures that you see in the diagram towards your right side. Moving on, for the results that we could get from Kendriya Vidyalaya ground you can see the temperature difference is from 305 Kelvin to 299 Kelvin. Again it was attributed to a small part which is relatively shaded to a larger part which was exposed to the sun. So these were just preliminary results for soil temperatures that was measured from the handheld radiometer on a push cart which operated at L band microwave frequency. See the major land surface variables measured from the instrument are soil temperature, emissivity and TB. The results of spatial variation of the measured land surface variables showed high spatial variability which can be due to the high number of sample measurements which were obtained at a rate of one sample per second. And unlike the satellite measurements or data from tower based radiometers where the footprint of the instrument covers a variety of soil and vegetation types, here what we did is we used a very compact very lightweight radiometer to measure TB that is microwave brightness temperature. And we assume that the knowledge gained from the present study can be useful for investigating the effect of different types of soils and passive microwave emission. So what you see here is the voltage in the first row in the horizontal and vertical polarization. You can see the legend as well. The emissivity again in the horizontal and vertical polarization and the microwave brightness temperature abbreviated as TB in the horizontal as well as vertical polarization. So this was a small case study for us to understand more about the calibration details of a radiometer as well as what are the outputs, what is the information you get by using a radiometer. So this was the preliminary results. Again, you know, there is something also known as passive polarimetry and I will take the liberty to slightly introduce it now. You see satellite based radiometers, satellite based passive microwave radiometers, they measure information at horizontal as well as vertical polarization. We know that by now, isn't it? There are microwave polarimeters which can measure the phase difference of the signal as well to derive the complete stokes vector. Just a small note on passive polarimetry. So to summarize, in this part of the lecture and throughout module 5, we have been trying to understand how passive microwave remote sensing is useful in hydrology and we started with covering the fundamental principles throughout the fifth module and slowly we try to understand the generic format of the forward model and how passive microwave remote sensing can be used to measure precipitation, soil moisture, ocean salinity, ocean wind, sea surface temperature and so on. And we also discussed on a small case study using a handheld radiometer. So let me hope that you could get an overall idea about passive microwave remote sensing and I will meet you in the next class. Thank you.