What is Inertial Navigation System?

An Inertial Navigation System (INS) is a navigational aid that makes use of a clock, accelerometers (sensors for measuring acceleration), gyroscopes (sensors for measuring rotation) and a computer to continuously calculate the position, orientation and velocity of an object on the move without any external references. Inertial navigation is a self-contained navigation technique in which measurements provided by accelerometers and gyroscopes are used to track the position and orientation of an object relative to a known starting point, orientation and velocity. Inertial measuring units (IMUs) typically contain 3 orthogonal rate-gyroscopes and 3 orthogonal accelerometers, measuring angular velocity and linear acceleration respectively. These signals are processed with the help of the on board computer and the position and orientation of the object can be tracked.

All the IMUs fall into 1 of the 2 categories outlined below. The difference between the 2 categories is the frame of reference in which the gyroscopes and accelerometers operate. The 2 categories are stable platform systems or gimballed platform systems and the strap down systems.

In stable platform type of systems, the inertial sensors are mounted on a platform which is isolated from any external rotational motion. This is achieved by using gimbals which allow the platform freedom to move in all the 3 axes. The gyroscopes on the platform detect rotations of the platform if any and the signals are fed back to the motors which rotate the gimbals in order to cancel out such rotations and hence keeping the platform aligned with the global frame. To track the position of the object, the angles between adjacent gimbals is calculated with the help of angle pick-offs and to calculate the position, the signals from the accelerometers mounted on the platform are double integrated. To get accurate results, we need to subtract acceleration due to gravity from the vertical channel before performing integration.

In strap down systems the sensors are mounted rigidly onto the device and hence the output is measured in the body frame and not global frame as in the case of stable platform systems. The orientation of the object can be determined by keeping track of the signals from the gyroscopes and integrating them while the position is determined from the accelerometer signals which are resolved into global co-ordinates and then integrated.

Both these types of systems are based on the same principles. Strap down systems are less complex and are physically smaller compared to stable platform systems. Due to this the strap down systems are the more dominant type of INS.

INS is autonomous and does not rely on external aids or visibility conditions and can operate in tunnels or underwater and anywhere. It is immune to jamming and inherently stealthy. It neither receives nor emits detectable radiation and requires no external antenna.

Errors increase with time as they are mean-squared. Cost of acquisition is high, the maintenance costs are also higher and so are the power requirements when compared to GPS receivers. With integration with GPS, these systems have become cheaper, maintenance costs have also gone down, the sensor errors have dropped to acceptable values and when there’s loss of GPS signals this integrated INS can be used.

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How GPS works?

The NAVSTAR-GPS (NAVigation System with Timing And Ranging Global Positioning System) commonly known as GPS, is a U.S. space based radio navigation system that provides reliable positioning, navigation and timing services to users on a continuous worldwide basis, freely available to all. GPS was originally intended for military applications, but in the 1980’s the government made the system available for civilian use. GPS works in all weather conditions, anywhere in the world, 24 hours a day. GPS was created by the U.S. Department of Defence (DOD) and was originally run with 24 satellites and there are currently 28 operational satellites orbiting the earth at a height of 20,180 km on 6 different orbital planes. Their orbits are inclined at an angle of 55° to the equator, ensuring that at least 4 satellites are in radio communication with any point on the planet. Each satellite orbits the earth in approximately 12 hrs and has 4 atomic clocks on board.

Before getting into the details of how the GPS works we should first know what signal transit time is and how it is measured. At some point or the other on a stormy night, you might have wondered how far away you are from a flash of lightning. This distance can be measured quite easily:

Distance = time the lightning flash was first perceived (start time) until the thunder is heard (stop time) multiplied by the speed of sound. The difference between the start and stop time is called transit time.

Hence distance = transit time * speed of sound

The GPS system exactly works under the same principle. In order to calculate one’s position, all that needs to be measured is the signal transit time between the point of observation and four different satellites whose positions are known.

Each satellite transmits its exact position and its precise on board time to earth at a frequency of 1575.42 Mhz. These signals are transmitted at the speed of light (3,00,000 km\s) and therefore require approx. 67.3 milli seconds to reach a position on the earth’s surface. The signals require a further 3.33 micro seconds for every extra km of travel.

Measuring the signal transit time and knowing the distance from a satellite isn’t enough to calculate one’s own position in 3-D space. To achieve this, 4 independent transit time measurements are required. It is for this reason that signal communication with 4 satellites is needed to calculate one’s exact position. All this time we have been assuming that the signal transit time that we measured was precise and highly accurate, but an error of 1 micro second can produce a positional error of 300m. As the clocks on all the satellites are synchronised, the transit time in all the measurements is inaccurate by the same amount. From the basic knowledge of mathematics we know that to solve a problem with n no. of unknowns, we need n no. of equations. To precisely determine the position of a person we need 4 equations i.e. one each for longitude, latitude, height and time error hence 4 satellites are required.

The most important applications of GPS are to determine one’s exact location and the precise time anywhere on the earth. The traditional fields of application for GPS are surveying, shipping and aviation. However with the increasing demand and popularity of the GPS, now it is being used in archaeology, geology, cartography, forestry and agricultural sciences, planning and managing of plantations, fleet management, navigation systems, trekking and sports activities and most importantly military services for which it was mainly developed for.

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What is an EKRANOPLAN...?

Ekranoplan is a Russian word which means 'screen craft' or 'skimmer'. It is not a plane nor is it a ship, moreover its a mixture of both. It is a transition between a hovercraft and an aircraft. It is a vehicle that attains level flight close to the surface of the earth which is possible by the cushion of high pressure air created by the aerodynamic interaction between the wings and the surface of the earth generated by its forward movement called as ground effect. It is also known as a flarecraft, sea skimmer, wing in surface effect ship etc.

In the 1970's when this vehicle was first photographed by the American satellite, it was believed to be a normal Jumbo jet sized aircraft under construction but when the vessel was later photographed sitting in water the American’s were puzzled! These crafts are big, fast and were kept secret from the west until the fall of communism in the early 1990's, when information about them was slowly revealed.

It can quite easily be envisaged that a hundred of such crafts zooming across the ocean, flying undetected by the radar as they fly underneath it and invading a country! By the time that country realized what was going on, it would have been too late. The front of the fuselage would have opened to allow soldiers, tanks, trucks, jeeps, and armoured cars to deploy and invade. You have to imagine several hundred of these craft, all flying at over 290 knots, all under radar in a mass invasion. We could then fully realise the element of surprise that they would have had.

It’s one hell of a ship, fast, manoeuvrable, and heavily armed but maybe not so well protected. Unlike a warship that has armor plating, the Ekranoplan would be susceptible to enemy firepower, a couple of rocket or bazooka hits could seriously damage or render the Ekranoplan inoperable. As the Ekranoplan picks up speed, the air is compressed under its short stabilizing wings until enough pressure produces lift and this supports the craft. Ekranoplan's as they are bigger, have more power and travel just a few feet over the water surface. Infinitely faster than any sea vessel ever designed the Ekranoplan is a marvel to behold. The terrific speed that these crafts can reach is mainly due to the fact that there is no friction between the hull of the craft and the water. This of course is a factor that keeps normal ships at such a comparatively slow speed, as they must push themselves through the water and not over it. They have better fuel efficiency than an equivalent aircraft flying at low level due to the close proximity of the ground, reducinglift-induced drag. There are also safety benefits for the people travelling in the craft in flying close to the water as an engine failure will not result in severe ditching. However, this particular configuration is difficult to fly even with computer assistance. Flying at very low altitudes, just above the sea, is dangerous if the craft banks too far to one side while making a small radius turn.

All Ekranoplan's are amphibious and will easily fly over land just as well as the sea, and at high speed, just as long as the surface is relatively flat. The last Ekranoplan from the former Soviet Union was the 400 ton LUN. It was built in 1987 with 100% military application in mind as a missile launcher and troop carrying ship. It was equipped with six large missiles on top of the fuselage, and could carry tanks and trucks in its large cargo hold. Also it must be reiterated here that it would not have been detected by radar as it would have travelled under the radar waves but the modern radar now has the ability to scan vessels on the surface of the sea so the surprise element of the Ekranoplan can be negated.

At the time when the Soviet Union fell apart, there was a second LUN under construction, it was about 90 percent finished when the military funding stopped. This was because of Russia's poor financial situation and also the end of the cold war. Later the eventual collapse of communism was enough to create further financial upheavals in much of Russia's economy. Most countries have now built prototypes of the Ekranoplan design as it has very useful application both in the military and commercial fields. As yet no commercial flights have been instigated but there are reports that a lot of private ventures are being developed, mostly in the USA.

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