Visions From Space
For referring text, please click here: http://deepspace.jpl.nasa.gov/dsn/.
Ever wonder how the National Aeronautics and Space Administration (NASA) brings us unforgettable images of space, planets, galaxies, and faraway stars?
Especially exciting are the pictures taken by spacecraft orbiting distant planets or flying across the solar system, such as missions to Mars and the Cassini-Huygens mission to Saturn and its large moon Titan.
This long-distance photography involves three steps - taking the picture, getting the picture, and making the picture. The long-distance photography described here is a complicated process that depends on our ability here on Earth, to communicate with spacecraft across the solar system.
This communication is the responsibility of the Deep Space Network, a global system of powerful antennas managed by the Jet Propulsion Laboratory (JPL) for NASA. JPL, a division of the California Institute of Technology, is dedicated to robotic space exploration and to the scientific study of Earth as a planet.
Taking the Picture
Taking the picture is the job of the spacecraft's imaging system: the digital camera, computer, and radio. When the Cassini camera looks at its "target" - a planet or moon, for instance - light from the target object passes through the lens and then through a color filter before falling on an electronic chip called a charge coupled device, or CCD.
The surface of the Cassini camera's CCD is divided into 1,024 parallel lines, each of which is further divided into 1,024 light-sensitive pieces - a total of 1,048,576 picture elements, or pixels. Each pixel records the scene brightness on a scale of values from 0 (black) to 4,096 (white). Pictures of the same scene are taken through various filters - red, green, blue. After all the pictures are transmitted to Earth, a color image of the scene is constructed from the red, green, and blue images.
Cassini has two digital cameras - a narrow-angle camera, which has two filter wheels with 12 filters each, and a wide-angle camera, which has two filter wheels with nine filters each. The Cassini spacecraft's onboard computer takes all 1,048,576 values recorded by the pixels - for the narrow-angle camera, that's 12,582,912 (1,048,576 x 12) pieces of information! - And converts the values into digital code, made up of 0's and 1's called "bits." The radio transmitter then relays the "bitstream" of data to Earth.
Getting the Picture
Getting the picture is the task of the DSN. The digital data bitstream can be transmitted from a spacecraft in various frequencies and at different bitrates. The bitstream is received by huge antenna receivers at any one of the three Deep Space Network sites around the globe - Goldstone, California; Canberra, Australia; and Madrid, Spain. The data are relayed using microwave links, communications satellites, landlines, and submarine cables to the Jet Propulsion Laboratory (JPL), in Pasadena, California.
Sending the Data to Earth
How fast do images get to Earth? Each spacecraft is different, but factors such as distance from Earth, data amount and sending rate, and type of antenna all play a part. Onboard data encoding reduces the amount of data and speeds up the transmission rate, and data compression allows retention of the most scientifically valuable information while minimizing or eliminating redundant or less valuable data.
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