Landing with time-to-contact and ventral optic flow estimates

Many recent studies on autonomous spacecraft landing use computer vision methods to improve the accuracy of the state estimates used for landing. Typically, these studies integrate the vision module with other exteroceptive sensors such as laser or radar altimeters. This is a sensible approach for the main landing system of a large spacecraft. However, for a backup emergency system or for much smaller spacecrafts, a solution entirely based on vision and proprioceptive sensors (e.g. gyros) could lead to significant mass savings. Small flying animals are capable of safe and accurate landings while relying only on proprioceptive and visual information. Since this capability holds a promise of landing safely with limited sensors and processing, it has served as inspiration for recent spacecraft landing studies. The focus of these studies has been on the use of ventral optic flow, a measure of the translational velocity divided by the height. Bees are known to use optic flow for controlling their speed and height, also when landing. In particular, when landing, they keep the ventral optic flow constant. Valette et al. study a control law that implements this strategy, simulating landings on the moon. The disadvantages of the sole use of ventral optic flow for landing are two-fold. First, the vertical dynamics of the lander is left free. The ventral flow can have the same constant value for a trajectory in which the lander ascends while accelerating and a trajectory in which the lander descends while decelerating. Thus, one has to directly or indirectly assume some type of descent profile, for example by introducing a pitch law for the spacecraft. Without the use of additional exteroceptive information to compute an optimal pitch profile, this leads to a considerable expense of propellant and to undefined final low-gate conditions. Second, in the case of a straight vertical landing the ventral flow is zero. In such a case, e.g. in the terminal phase of an asteroid landing scenario, the ventral flow does not provide any information on how to land the spacecraft. Advanced Concepts Team, European Space Agency, [email protected] Exteroceptive sensors observe entities external to the spacecraft, while proprioceptive sensors measure quantities within the spacecraft “body”.