The input required by the DIMES software includes three descent images. For each descent image, the software also requires the following elements of the state of the landing spacecraft at times when the images were acquired: the attitude of the spacecraft relative to the surface, the horizontal velocity estimated by the IMU, and the altitude. Using this state information, the software warps each image to the ground plane, then computes horizontal displacements between successive images by use of image correlation, applied to two locations in each of the first and second images and two locations in each of the second and third images. This process yields four image-based estimates of horizontal velocity. These estimates are compared to each other for consistency. As a further consistency check, accelerations are computed from differences of these velocities and these accelerations are compared with accelerations as measured by the IMU.

The results of the consistency checks are used, along with image-correlation metrics to decide whether the estimate of horizontal velocity is correct. In the original application, if the estimate of velocity is determined to be correct, it is sent to the rocket-firing subsystem; if the estimate of horizontal velocity is found to be incorrect, then the rocket-firing subsystem is commanded to proceed without a DIMES velocity estimate.

This work was done by Andrew Johnson, Yang Cheng, Reg Willson, Jay Goguen, Alejandro San Martin, Chris Leger, and Larry Matthies of Caltech for NASA's Jet Propulsion Laboratory.

The software used in this innovation is available for commercial licensing. Please contact Karina Edmonds of the California Institute of Technology at (626) 395-2322. Refer to NPO-40920.

NASA Tech Briefs Magazine

This article first appeared in the October, 2007 issue of NASA Tech Briefs Magazine.

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