Section: Application Domains

Spatial mechanics

Participants : Ludovic Rifford, Jana Nemcova, Jean-Baptiste Pomet.

This domain is mostly related to the techniques described in section  3.2 .

Generally speaking, aerospace engineering requires sophisticated control techniques for which optimization is often crucial, due to the extreme functioning conditions. The use of satellites in telecommunication networks motivates a lot of research in the area of signal and image processing; see for instance section 4.2 . Of course, this requires that satellites be adequately controlled, both in position and orientation (the so-called attitude of the satellite). This problem and similar ones continue to motivate research in control. The team has been working for six years on control problems in orbital transfer with low thrust engines, including four years under contract with Thales Alenia Space (formerly Alcatel Space) in Cannes.

Technically, the reason for using these (ionic) low thrust engines, rather than chemical engines that deliver a much higher thrust, is that they require much less “fuel”; this is decisive because the total mass is limited by the capacity of the launchers: less fuel means more payload, while fuel represents today an impressive part of the total mass.

From the control point of view, the low thrust makes the transfer problem delicate. In principle of course, the control law leading to the right orbit in minimum time exists, but it is numerically involved, ill-conditionned and the computation is non-robust against many unmodelled phenomena. Considerable progress on the approximation of such a law by a feedback has been carried out using ad hoc Lyapunov functions. These approximate surprisingly well time-optimal trajectories. The easy implementation of such control laws makes them attractive as compared to genuine optimal control. Here the n-1 first integrals are an easy means to build control Lyapunov functions since any function of these first integrals can be made monotone decreasing by a suitable control. See [54] and the references therein.