For hundreds of years, travelers have looked to the sky to help navigate their way across oceans, deserts, and land. Whether using the angle of the Sun above the horizon or the night stars, celestial bodies guided explorers to their destinations. In the twenty-first century, people still look to the sky for direction, but now they are using satellites that orbit Earth to determine their location. In fact, it is quite common to see people using what is called the Global Positioning System (GPS), which is a satellite navigation system, to answer the age-old question: Where am I?

Evolution of Satellite Navigation

The idea of using satellites for navigation was conceived when the satellite Sputnik 1 was launched in 1957. At that time, U.S. scientists developed a way to track Sputnik's orbit using the time delay or Doppler shift of the radio signal being broadcast...