 So, let us see now the third period during Second World War what is happening. So, during Second World War aerostats were used only mainly as a balloon barrage system that means a cluster of balloons. So, one application was surveillance, protect against enemy aircraft coming into Bombay or cities. So, you can put many of them and do surveillance, but a more interesting use was runway protection. So, these aerostats they were mounted with steel wires and enemy aircraft who are coming into Bombay airports or the cities would get entangled in the wires and crash. So, what you do is you surround your runway with a barrage of balloons and when your aircraft are taking over you bring them down, launch your aircraft and bring them up it is like an iron curtain around your runway to protect your runway from enemy fire, very successful use of aerostats system. After this concept was attempted not a single runway in the Allied forces was attacked successfully by the Russians by the Germans. So, very successful and then you have heard about Normandy invasion there was recently the function to celebrate its anniversary. So, even in Normandy invasion initial surveillance was obtained using aerostats. You will see now aerostats are coming in a very big way because airships have caught fire airships have become unsafe. So, for passenger use they will not be applied, but aerostats or balloons which remain stationary with the tether for non-man carrying applications they are now being used more and more. So, from 1940 to mid-80s now this was during the world war, now after the world war there was a very very large period of almost no activity. So, between post second world war they were only used for leisure sports purposes and for anti-submarine warfare. The US Navy used blimps for surveillance in large number of airships were used by the US Navy for surveillance purposes, but they stopped the R&D effort and after that it was used only for atmospheric purposes. So, you can see this is an airship from the US Navy deployed in 56. This is an airship called Los Angeles which was used by the US Navy and then after mid-80 there was a sudden revival. Now, this is what we are most interested in, if this technology is obsolete, if this technology has been overtaken by aircraft then what happened in 1980s which suddenly made it bounce back in a way. So, in mid-80s so there was a contract or a requirement given by the US armed forces for or US Navy for very very long duration endurance. They wanted endurance of 30 days for providing the surveillance support to their ships and no aircraft actually could meet this requirement at that time. And today the maximum endurance of an aircraft without refueling is around 16-17 hours. So, at that point of time some airship enthusiasts they got together and they said this is a very good way of bringing back airships. In between we have now developed new materials, new technology therefore we are able to address some of the airship limitations and they said what is the need for us to make a rigid structure. Can we not have an airship which can be completely built by fabric and it will not have any metallic or solid parts inside. So, a project called Sentinel was launched and this project was funded development funding came from the US Navy but the project was cancelled because of financial reasons. However, one airship was made as a prototype a small scale version of Sentinel was made as skyship was made during that project and that was purchased by a company and the company said okay we will use it for commercial purposes. So with this this airship came back. So skyship 600 is one of the modern airships right we have already seen one video of Zeppelin NT. So, that will give you an idea of how these airships are and then airships began being used for commercial applications such as this example where which is used in Switzerland for tourism purposes and the latest that you have is this airship called Zeppelin NT right. Now what is happening in the present current year scenario this decade maybe 2000 onwards. So in Afghanistan in Iraq in many other applications where the US armed forces and the NATO forces are facing operation or they are carrying out their operations aerostats and airships are being used in a very big way. The main applications are surveillance but at some places they are also used for providing internet or network connectivity. These are the pictures of some applications of LTA systems in the current times which are ongoing. On the bottom right from Lockheed Martin is just a conceptual sketch and then there are many hybrid systems. We will have a special lecture on hybrid systems as part of this course and this is how a modern aerostat looks when deployed for surveillance purposes and it gives you complete surveillance. The Indian Air Force has also installed a couple of aerostats along the western border for aerial surveillance. So we have two aerostats deployed.