Measurements of s/c height above the Phobos surface using ShTK stereo images
During the "Phobos-Grunt" s/c descent on the Phobos surface the height of the spacecraft will be measured every 6 sec using stereo images obtained by two ShTK cameras. The algorithm for height measurements was tested on the TSNN testing equipment where TSNN cameras imaged a sequence of simulated Phobos surface stereo images with a resolution and stereo parallax corresponding to actual descent trajectory. Figure 1 shows an example of the measurements testing for a descent from 1 km to ~10 m. During this time ShTK performs 20 cycles of measurements. The dashed line shows the change in height in accordance with the actual landing trajectory, and the solid line - calculated height change in accordance with parallax change formed by simulated sequence of stereo images. Since for simulation only integer values of parallax were specified, on the upper part of the trajectory model height changes abruptly. Actual measurements of height with ShTK marked by crosses, fit a model graph of the height changes, which corresponds to the images exposed to the camera. Errors in the measurements of height shown in Fig. 2 reduced from ~100 m at an altitude of 1 km to ~10 m at 300 m and up to 1-2 cm at a height of 10 m. While measuring height the correspondence was set for 6–10 control points of stereo pair (for the maximum amount of 10 points) - Fig.3. This proves good operation stability of the algorithm.
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