The descent image motion estimation system (DIMES) is a system of hardware and software, designed for original use in estimating the horizontal velocity of a spacecraft descending toward a landing on Mars. The estimated horizontal velocity is used in generating rocket-firing commands to reduce the horizontal velocity as part of an overall control scheme to minimize the landing impact. DIMES can also be used for estimating the horizontal velocity of a remotely controlled or autonomous aircraft for purposes of navigation and control.

The DIMES Algorithm fuses images of terrain below with IMU data and radar-altimeter readings toestimate horizontal velocity.
DIMES was developed by the Mars Exploration Rover (MER) Project and was used successfully by the MER Entry Descent and Landing (EDL) system during both landings. In the original spacecraft application, when the need to determine horizontal velocity was discovered, it was too late to install traditional horizontal-velocity-measuring radar in the spacecraft. DIMES was conceived as a means of estimating the horizontal velocity by augmenting data acquired by sensors already installed in the spacecraft with data from an easy to accommodate descent imager.

The DIMES sensors include a descent imager (an electronic camera that acquires images of the approaching terrain), a radar altimeter, and an inertial measurement unit (IMU). The DIMES flight software implements an algorithm for combining measurement data from the aforementioned sensors to estimate horizontal velocity.