It seems that people like to know where they are, and where their family and friends happen to be at any particular time. That is, they need location awareness. Location awareness is absolutely essential for navigation. For hundreds of years, sailors found their way by measuring the position of stars relative to time. Aircraft navigators used celestial navigation up to the middle of the 20th century. Today, navigation is also carried out by observing objects in the sky, but now those objects are artificial satellites. In the past, observations were by sight – today they are through radio, and the location accuracy obtained couldn’t be imagined only a few decades ago.