Data Handling Techniques

On-board data compression and data editing are also being used to reduce the volume of data being sent to Earth from the Galileo spacecraft while keeping as much of the interesting information as possible. Here are two simplified ways to handle data on a spacecraft BEFORE it is sent to Earth. The first is "lossless"; data are changed onboard the spacecraft before being transmitted. These data need to be reformatted back to their original state after reaching the Earth. The second is called "lossy" because some original data is lost through mathematical averaging of the data.

This technique makes use of the fact that the "asteroid" is made up of patterns of "1's" and "0's". These numbers can be easily summed up and reconstructed if you know the row length and pixel size. In this example, the first filled pixel (upper left corner) represents itself. The following pixel gives the number of pixels (8) that are identical to the first one. Instead of using 9 pixels of data you only use 2. The "X's" show unused pixels. The 10th is unfilled (equal to "0") and followed by a series of 14 filled pixels.

Continue this exercise and determine how many pixels you save using this technique (the total number of "X's").

This is a simple example of one type of on-board averaging. The original image is 16 by 16 (256 total) pixels. The "new" (reduced) image is 8 by 8 (64 total) pixels. You can mathematically average the number in 4 adjacent pixels and use one value for the "new" reduced pixel. In this example, the first group of 4 pixels had a value of "1" thus the averaged pixel also is "1" (1+1+1+1 divided by 4). The 5th pixel in the first row of the reduced image has a value less than "1" and is shaded to show this. Likewise, the next two pixels are shaded 1/2 and 3/4 as much as the filled ones.

Continue to average these pixels and shade them in a similar way. Does this look like the original image?

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