NAVIGATION REQUIREMENTS OF A MINIMUM PROPELLANT GUIDANCE LAW FOR LOW-THRUST SPACECRAFTS.

The navigational requirements for minimum-propellant guidance of a low-thrust spacecraft are analyzed. The guidance algorithm that is studied is an implicit deterministic feedback guidance law that is developed from the theory of neighboring extremals. This guidance law has a number of interesting features resulting from its optimality; such as, not allowing throttling, and defining regions of the trajectory where no tradeoff exists between the navigation and guidance problem. The difficulty with the guidance law is that it is implicit and therefore the navigation system must be able to react to perturbations before the spacecraft strays from the linear neighborhood of the nominal trajectory. The size of this linear neighborhood is studied and the resulting implications on the navigation system are presented. Separation of the navigation problem from the guidance problem is assumed throughout. Data is presented for two test cases.