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http://en.wikipedia.org/wiki/Test_target

A test target is a common feature on interplanetary landing craft such as the Viking Lander and Mars Exploration Rovers.

The target is usually a visible marker or plate on the exterior of the vehicle within sight of the lander's imaging system (usually a CCD camera). The target possesses samples of primary colors and a grey scale. The camera uses this much like a photographer would use a chip chart on Earth; the color samples allow the camera to compensate for white balance and contrast. The target provides a visual reference as well, so that the initial orientation of the camera can be fixed.

On the Mars Exploration Rover, the target was designed with a pillar elevated above the main surface, so that it could be used as a sundial.

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Published in July 2009.

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