Satellite Navigation

The most advanced navigation system uses the Global Positioning System (GPS), a network of navigation satellites orbit­ing Earth. The GPS system carried by an aircraft picks up radio signals from at least four satellites and uses them to cal­culate the aircraft’s position, altitude, heading, and ground speed. Space-based navigation systems like this are begin­ning to replace radio navigation systems because they are more accurate. In addi­tion, they do not rely on large numbers of beacons on the ground; they are not affected by bad weather; and aircraft are never out of range of the system’s signals.

Navigating Spacecraft

All the planets in the solar system are spinning as well as moving around the Sun at very high speeds. Navigating a space probe from Earth to another pla­net could be compared to sitting on a spin­ning merry-go-round and trying to throw a ball at a spinning top

О The GPS control room at Schriever Air Force Base in Colorado controls the satellites that provide navigational data to users around the world.

placed on a distant moving car. In spite of the challenges, however, space scien­tists have figured out how to send space­craft where they want them to go. Most of the work needed to guide a space probe is done before the launch.

The movements of all the planets are known, and scientists can predict exact­ly where they will be at a given point in the future. The timing of a probe’s launch, the speed it travels, and its direction are all chosen so that the probe is launched from Earth on the right flight path to reach a planet. The pull of the Sun’s gravity and that of the planets has to be taken into account when cal­culating the probe’s flight path. In fact, gravity is sometimes used to accelerate a probe or to change its direction without having to burn any fuel.

When the Space Shuttle goes to the International Space Station, its launch time is chosen to place it in orbit near
the Space Station. The Space Shuttle only has to make small adjustments to its position to rendezvous with the Space Station.