Navigation

In order for a spacecraft to close in on a destination such as the International Space Station or to enable the space shuttle to retrieve the Hubble Space Telescope, scientists must do most of the groundwork prior to the launch phase. Scientists need to know the workings of the solar system well enough to predict a spacecraft's destination, when to launch, and how fast it must travel to meet the target in space.

Gravity also must be taken into account. Gravity exerted by large bodies like planets and the Sun will alter the trajectory of a spacecraft. Difficulties arise when a spacecraft is allowed to deviate too far off the intended course. If the error is realized late in the flight, the target may have moved a long distance from where the ship was originally supposed to meet it. The mistake often cannot be remedied because spacecraft do not carry enough fuel to make large course corrections. The launch vehicle pushes the spacecraft onto a heading that pushes it in the direction of a final destination. Sometimes mission planners use the gravity of a planet by swinging by that object to change the path of a spacecraft.

Spacecraft Position

Spacecraft navigation is comprised of two aspects: knowledge and prediction of spacecraft position and velocity; and firing the rocket motors to alter the spacecraft's velocity.

To determine a spacecraft's position in space, NASA generally uses a downlink, or radio signal from the spacecraft to a radio dish in the Deep Space Network (DSN) of ground receivers. The distance between Earth and the spacecraft is measured by sending a radio signal up from Earth with a time code on it. The spacecraft then sends back the signal. Because all radio waves travel at the speed of light, scientists can determine how long it took for the signal to travel and calculate the exact distance it traveled.

A more precise way of measuring distance uses two radio telescopes. Spacecraft send a signal back to Earth. Three times a day, this signal can be received by two different DSN radio telescopes at once. Researchers are able to compare how far the spacecraft is from each signal. Mission trackers can then calculate the distance to a known object in space whose location never changes, like a pulsar (pulsing star). From the three locations (two telescopes and a pulsar), scientists can use a technique called triangulation to get the ship's location.

By using a different process called Optical Navigation, some spacecraft can use imaging instruments to take pictures of a target planet or other body against a known background of stars. These pictures provide precise data needed for correcting any discrepancy in a spacecraft's path as it approaches its destination.

The exact location of the spacecraft must be determined before any course correction is made. The spacecraft will first fire small rockets to change the direction it is pointing. After that, the main thruster will give the spacecraft a push in the new direction.

During rendezvous and proximity operations, taking the space shuttle as an example, the onboard navigation system maintains the state vectors of both the orbiter and target vehicle. During close operations where separation is less than 15 miles, these two state vectors must be very accurate in order to maintain an accurate relative state vector. Rendezvous radar measurements are used for a separation of about 15 miles to 100 feet to provide the necessary relative state vector accuracy. When two vehicles are separated by less than 100 feet, the flight crew relies primarily on visual monitoring through overhead windows and closed-circuit television.

SEE ALSO GYROSCOPES (VOLUME 3); MISSION CONTROL (VOLUME 3); NAVIGATION FROM SPACE (VOLUME 1); TRACKING OF SPACECRAFT (VOLUME 3).

Lisa Klink

Bibliography

Stott, Carole. Space Exploration. New York: Dorling Kindersley Publishing, 1997.

Internet Resources

"Spacecraft Navigation." Basics of Space Flight. Jet Propulsion Laboratory, California Institute of Technology. <http://jpl.nasa.gov/basics/bsf13-1.html>.