ESA GNC Conference Papers Repository

Spacecraft trajectory determination and guidance are two GNC functions that are generally performed on ground, whereas it is mandatory to implement the trajectory control function aboard. An effective trajectory control requires selected ground guidance information to reach the spacecraft. A number of contingencies may lead to the loss of the ground trajectory navigation needed to compute a manoeuvre or the ground-spacecraft communication link needed to upload the computed manoeuvre, risking mission success or even crew lives if the failure occurs at a critical phase or lasts too long. Therefore, an autonomous GNC becomes mandatory to mitigate those risks to acceptable levels, and such a backup needs an autonomous navigation function. In a LEO (Low Earth Orbit) mission, autonomous trajectory determination may be feasible with GNSS (Global Navigation Satellite System) equipment, but the availability of GNSS signals cannot given for granted over a whole mission to the moon, and a different autonomous navigation means is required. Optical navigation is a sound candidate to autonomous navigation that is based upon the sighting of point targets (stars or far bodies) and extended targets (close bodies). Narrow field navigation cameras have been customarily employed in interplanetary navigation to reduce the navigation uncertainty in the terminal phase of the approach to a body, generally with ground processing of the images, although with some instances of onboard processing (as in [1]). In the Apollo missions (Cf. [2]), an automatic sextant looking through a window or a dome was a backup instrument for trajectory navigation, and procedures were devised to replace ground-based navigation if the necessity arose. In the frame of possible manned missions in the cis- and circumlunar space, a way to get the desired autonomous optical navigation capabilities consists in the mounting of a wide-field optical camera as navigation sensor and the onboard implementation of (i) appropriate image processing software for sight reduction and (ii) general purpose trajectory determination software. Such a navigation system is described in the paper. The choice of a wide FOV (Field of View), as baselined for Orion (see [3]), differs from the common approach for interplanetary navigation of a narrow-FOV navigation camera, the rationale being the need to image a close body and nearby stars. Incidentally, a manned spacecraft in a lunar mission is likely to mount several star sensors for attitude estimation, and these devices can be a suitable and cost effective alternative to cameras devoted exclusively to trajectory navigation. Simulation results prove that the performances of such a backup system are adequate for this purpose.