Engineers designed NASA's Mars Exploration Rovers to be exceptionally predictable when it comes to studying specific rocks and patches of soil. For example, each rover can remove the robotic arm from a target, switch scientific instruments, and resume analysis within 1 millimeter (0.04 inch) of the original position.
Most samples analyzed by the scientific instruments are large enough that a positioning accuracy of approximately 1 centimeter (0.4 inch) is acceptable. At the current winter outpost of NASA's Spirit rover, however, the science team was interested in analyzing a small, reflective spot only 1 centimeter (0.4 inch) in diameter associated with a rock target called "Halley". In this instance, a positioning error of 1 centimeter could have missed the target entirely.
To be sure this wouldn't happen, team members created a 2x1 microscopic image mosaic of the target, shown here, based on stereo imaging models of the terrain obtained by Spirit on sol 861 (June 5, 2006). The stereo images from the hazard avoidance cameras provided detailed views as well as distance to features in the field of view. The region of interest - a bit of sparkly, light-toned material - turned out to be offset slightly.
Given the exceptional repeatability in positioning the robotic arm, engineers were able to transmit the desired coordinates for further study and direct the rover to place the alpha particle X-ray spectrometer and MÃ¤ssbauer spectrometer in that particular spot, allowing analyses of the light-toned material.
A subsequent microscopic image, 32 millimeters (1.2 inches) wide, of the target known as "Halley_Brunt," acquired on sol 880 (June 24, 2006), documented that the rover did, in fact, place both spectrometers in the optimum position and analyze the desired patch of sparkly material on the Martian surface.