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Navigation

Travelling from point A to point B in space is almost never in a straight line or at constant velocity because of the many influences on the body in motion. The basis for space navigation is inertial guidance--i.e., guidance based on the inertia of a spinning gyroscope, irrespective of any external forces and without reference to the Sun or stars. By the use of three gyroscopes and accelerometers, precise measurements may be made of any change of velocity (acceleration), either positive or negative, along any or all of the three axes. By use of a memory system in the computer it is possible to determine where the vehicle is at any time along a trajectory. By changing attitude (rotation about one or more axes) and firing a small jet or rocket motor, corrections may be made to the trajectory. Preprogrammed computers, both on the ground and in larger spacecraft, automatically or on command "keep an eye on" where the spacecraft is, where it was, and where it is supposed to be going. Direct readouts of such data from the computers are displayed to the crew in manned spacecraft.

During the launch phase, corrections to deviations in planned flight path are usually made at once by special small thrust motors, by deflection of the rocket exhaust jet, or by swinging one or more of the rocket engines in a gimbal mount.

In the case of rendezvous between two spacecraft, radar sightings fed to the computer tell the crew the corrections required along each of the three axes in metres per second. In the case of the Apollo lunar missions, deviations encountered were stored in a computer memory system until requested. One or more midcourse manoeuvres might be made. With the implementation of the Navstar Global Positioning System in the 1980s, it became possible for spacecraft to verify their locations to within a few metres, and their speeds to within a few metres per second.

The above description of navigation techniques is oversimplified; the systems used are complex, powerful tools without which manned or unmanned spacecraft would not be feasible.

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