Navigation from Space

Navigation from Space

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 by the satellite. The scientists proposed that this process could be used in the opposite way for navigation. Specifically, using a satellite with a known orbit, one's position could be determined by observing the time delay or Doppler shift of a radio signal coming from that satellite.

The concept of being able to determine a position from satellites appealed to the U.S. Navy. To test the idea, they developed the Transit satellite navigation system. By 1964 Transit was being used by Polaris submarines to update the inertial navigation systems onboard the submarines. During roughly the same period, the U.S. Air Force also had a satellite navigation program under development. In the early 1970s, the navy and air force programs merged into one program called the Navigation Technology Program. This program evolved into the NAVSTAR (Navigation System with Timing and Ranging) GPS—the space navigation system used today.

How the Global Positioning System Works

GPS uses twenty-four satellites that circle Earth in a 20,000-kilometer high (12,400-mile high) orbit. The satellites are in orbits that are inclined at 55 degrees with respect to the equator. The satellites are in six orbital planes, each of which has four operational satellites. In March 1994 the full twenty-four-satellite constellation was in place in orbit and the network became fully functional the following year. Users of this navigational system need a GPS receiver. There are many commercial manufacturers of these devices. They are sold in most stores that sell electronic equipment and cost as little as $150.

Each satellite in the GPS transmits a signal with information about its location and the current time. Signals from all of the satellites are transmitted at the same time. These signals are received at different times by a GPS receiver because some satellites are closer than others. The distance to the satellite is determined by calculating the amount of time it takes the signal to reach the receiver. The position of the receiver is determined by triangulation, except that in this case, the distance to four GPS satellites is used to determine the receiver's position in three dimensions.

Alternatives to the Global Positioning System

The United States allows anyone around the world to use the GPS system as a free resource. For many years, however, there has been a concern in other countries that the United States could deny access to the GPS system at any time. This has led to attempts by other nations at developing alternative satellite navigation systems. The most notable of these emerging systems is a European Space Agency venture called Galileo. The European Union transport ministers approved the initial funding of 100 million euros in April 2001. Proposed as a civilian satellite navigation system, Galileo may be fully operational by 2008. One difference between Galileo and GPS is that some of the satellites in Galileo's constellation will be in orbits with greater inclination to the equatorial plane than the GPS satellites. This will give northern Europe better coverage than that provided by GPS today.

Russia has developed a military satellite navigation system called Glonass. This system, which entered service in 1993, used twenty-four satellites when it began operation. Because of the country's financial problems that began later in the 1990s, however, older satellites were not replaced. As a result, by 2001 only six of the original twenty-four satellites were still in use, although Russia had plans to launch three new satellites in the early twenty-first century.

China is also planning to develop its own satellite navigation system. In 2000 China launched two experimental navigational satellites. These satellites, called the Beidou navigation satellites, are named after the constellation the Big Dipper. They continue to be used for some limited functions. China hopes to build a more extensive satellite navigation system by around 2010.

see also Global Positioning System (volume 1); Military Customers (volume 1); Navigation (volume 3); Reconnaissance (volume 1); Satellites, Types of (volume 1).

Salvatore Salamone

Bibliography

Clarke, Bill. Aviator's Guide to GPS. New York: McGraw-Hill, 1998.

Hofmann-Wellenhof, Bernhard, Herbert Lichtenegger, and James Collins. Global Positioning System: Theory and Practice. New York: Springer-Verlag, 2001.

Stearns, Edward V. B. Navigation and Guidance in Space. Englewood Cliffs, NJ: Prentice-Hall, 1963.