A test target (for spacecraft) 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 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.