Autonomous Target Ranging Techniques

For the deep space asteroid mission, Bering, the main goal is the detection and tracking of near Earth objects (NEOs) and asteroids. One of the key science instruments is the 0.3-m telescope used for imaging and tracking of the detected asteroidal objects. For efficient use of the observation time of this telescope, a fast determination of the range to and the motion of the detected targets are important. This is needed in order to prepare the future observation strategy for each target, i.e. when is the closest approach where imaging will be optimal. In order to quickly obtain such a determination two ranging strategies are presented. One is an improved laser ranger with an effective range with non-cooperative targets of at least 10,000 km, demonstrated in ground tests. The accuracy of the laser ranging will be approximately 1 m. The laser ranger may furthermore be used for trajectory determination of nano-gravity probes, which will perform direct mass measurements of selected targets. The other is triangulation from two spacecraft. For this method it is important to distinguish between detection and tracking range, which will be different for Bering since different instruments are used for detection and tracking. Also, the baseline distance between the two spacecraft will provide two different (close and far) scenarios of observation. The limiting range and the relative range accuracies of the triangulation method are discussed.