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Navigation is a basic need for anyone who wants to move with a purpose. Navigation is the art of moving in a suitable direction in order to arrive at a desired location. Even in prehistoric times, when the most primitive form of animals started moving on Earth, the art of navigation existed in its most ancient form. Even today, when humans, the most evolved species on Earth, move by flying in the most technologically advanced aircraft, or by driving a car, or by riding a bicycle, or simply walking, with a desire to reach somewhere, we perform some sort of navigation. You may have noticed that when we move without the aid of instruments and the route to our destination is known to us, we generally use some sort of mental map, which is mostly pictorial in the form of landmarks and connected paths. On this map, we identify our positions and apply our previous experience to guide us and decide the course of our movement. However, this method does not work for a new destination or for places where such landmarks are not present. This is the reason why people get lost in new places, in deserts or in the oceans. In such situations, we use paper or digital maps, which give us necessary information to guide ourselves. However, whether paper or digital, or as mental pictures including other geographical information, these maps are aids to navigation. They only enable us to locate and relate our positions with respect to our destinations and show different possible ways to reach there. The decisions we make in choosing the course of our movement by comparing our position with the available information on these maps is actually an art, called navigation. To carry out the activity of navigation, we first need to know our position, to identify correctly where we are, and that of our destination. Only then can we make an appropriate decision about how to move to reach the latter. Satellite navigation is a technology that provides us with the correct position and its derivatives, on or off the Earth’s surface. Here, the signals transmitted from navigation satellites are used to derive the required set of position coordinates. This is done in a navigation receiver. In turn, and in conjunction with the additional information, these parameters are used to decide the further course of movement or are utilized in other positionbased applications. However, positions are not sought only for movement. Sometimes our exact position is also required to be correlated with other facts or to derive ancillary information. For example, if we know our position on the Earth’s surface, we can easily figure out the kind of climate we should expect. Knowing precise positions of a network of points on the Earth surface will also let one obtain the exact shape of the Earth. Its derivatives may provide information such as tectonic or crustal movements. There are many other interesting applications of navigation, in which the knowledge about position and its derivatives are used for useful and exciting outcomes. We shall discuss them in Chapter 11. The typical requirement of the estimation of position is global; for that, we need to represent positions uniquely. Positions are hence represented in terms of a common global reference such that the location of all the points on and near the Earth can be expressed by distinct and unique coordinates. It is like the unique identity of that position on the globe. Thus, finding the position of a person is simply a matter of determining the unique identity of the location where he or she currently is. These coordinates are hence so chosen that they can be used conveniently to specify the positions. In later subsections of this chapter, we will learn about the reference frames and different coordinate systems, which form the basis for representing positions.