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

Mars Exploration Rover

NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...

s.

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

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

). The target possesses samples of primary colors

Primary Colors

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 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

Contrast (vision)

Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

. The target provides a visual reference as well, so that the initial orientation

Orientation (geometry)

In geometry the orientation, angular position, or attitude of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it is in....

 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

Sundial

A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

.